advanced steady-state modelling and optimisation of lng

Advanced steady-state modelling and optimisation of LNG

production

Pedro Daniel Dias Chainho

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Examination Committee

Chairperson: Prof. Doutor José Madeira Lopes

Supervisors: Prof. Doutor Henrique Aníbal Santos de Matos

Doutor Maarten Nauta

Members of the Committee: Doutor Diogo Alexandre Cipriano Narciso

Doutor Ricardo Manuel Pinto de Lima

October 2013

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“It is hard to fail, but it is worse never to have tried to succeed”

Theodore Roosevelt

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v

Acknowledgements

First of all, I would like to express my gratitude to Professor Henrique Matos and especially Professor

Costas Pantelides for the opportunity to work at Process Systems Enterprise.

A huge word of appreciation goes to all the PSE employees for all the kindness, willingness and

sympathy during all the internship, enhancing the role of Diogo Narciso, Filipe Calado, Pablo Rolandi

and specially Maarten Nauta for all the patience and endless support.

I would also like to thank my colleagues and housemates Francisco, Vasco and Tiago for all the

companionship and confidence.

A word of gratitude goes to all my friends, the old ones from Portugal and the recent ones from

London for the friendship and good mood shown during the time in London.

A huge thanks go to all my family members, specially to my brave mother and to my beloved father

that not being here today, will be always on my mind.

A last word of love and appreciation goes to Patrícia for all the love and support shown during the last

few years.

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Abstract

The main goal of the work was the development of a mathematical model of a typical liquefied natural

gas (LNG) cycle, and to demonstrate the use of the model for steady-state optimisation.

The liquefaction of the natural gas (NG) is a common procedure that allows a volume reduction by a

factor of 600, resulting in several advantages in terms of transportation and storage. A typical LNG

liquefaction process consists in series of refrigeration cycles, using both pure and mixed refrigerants.

Two advanced modelling approaches (simple and complex) were taken for the following LNG

processes: (i) C3/MR, (ii) AP-X™ and (iii) AP-X™ alternative, using both GERG and PR property

methods. Both property packages affect the results in a negligible difference of less than 0.1%.

A steady-state optimisation procedure was also carried out for the AP-X™ process using PR physical

properties.

The simple approach comes down to the composition of a top-level LNG flowsheet using models from

the PML:SS library. Specific power consumption (SPC) of 46, 111 and 18 [kW-day/tonne LNG] was

respectively observed for AP-X™, AP-X™ alternative and C3/MR processes.

In the complex approach it was observed a SPC of 40, 120 and 19 [kW-day/tonne LNG], respectively

for AP-X™, AP-X™ alternative and C3/MR processes. A deviation of 20% in terms of power

consumption was noted for the C3/MR model in relation to the APCI®-OMAN plant.

The optimisation improved the total cost of the AP-X™ process in 58%, ending up with a value of

around 83 M$/year and a SPC of 15.8 [kW-day/tonne LNG], fitting up the standard industrial values.

Keywords

LNG, AP-X, C3/MR, gPROMS, refrigeration

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Resumo

O objectivo principal da realização do trabalho foi o desenvolvimento de um modelo matemático de

um processo típico de produção de LNG e a sua utilização numa optimização em estado

estacionário.

A liquefacção do gás natural é uma prática comum que permite uma redução de volume por um

factor de 600, permitindo inúmeras vantagens em termos de armazenamento e transporte. Um

processo típico de LNG é constituído por uma série de ciclos de refrigeração cujos refrigerantes

podem ser puros ou mistos.

Duas abordagens de modelação (simples e complexa) foram tomadas para os seguintes processos:

(i) C3/MR, (ii) AP-X™ e (iii) AP-X™ alternativo, usando GERG e PR como métodos de propriedades.

Para ambos os métodos de propriedades verificou-se uma discrepância negligenciável de 0.1% para

os resultados.

Foi também realizada uma optimização em estado estacionário para o processo AP-X™ usando PR

como método de propriedades.

A abordagem simples reside na composição de um modelo/flowsheet de nível superior constituído

por modelos da biblioteca PML:SS. Um SPC de 46, 111 e 18 [kW-dia/tonelada LNG] foi observado

respectivamente para os processos AP-X™, AP-X™ alternativo e C3/MR.

Na abordagem complexa foi observado um SPC de 40, 120 e 19 [kW-dia/tonelada LNG],

respectivamente para os processos AP-X™, AP-X™ alternativo e C3/MR. Em comparação com a

fábrica APCI®-OMAN foi verificado um desvio de 20% em termos de SPC para o processo C3/MR.

A optimização reduziu o custo tal do processo AP-X™ em cerca de 58%, culminando num valor de

83 M$/ano e um SPC de 15.8 [kW-dia/tonelada LNG], enquadrando-se um valores industriais típicos.

Palavras-chave

LNG, AP-X, C3/MR, gPROMS, refrigeração

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Contents

1. Introduction ...................................................................................................................................... 1

1.1. Physical properties [3]

............................................................................................................... 1

1.1.1. GERG equation of state (EOS) ....................................................................................... 2

1.1.2. Peng-Robinson EOS ....................................................................................................... 2

1.2. The LNG plant [2]

...................................................................................................................... 3

2. Background ...................................................................................................................................... 7

2.1. LNG liquefaction stage [2]

......................................................................................................... 7

2.1.1. Liquefaction technologies evolution [6]

............................................................................. 8

2.2. LNG liquefaction technologies ................................................................................................ 9

2.2.1. Conoco-Phillips optimised cascade [2]

........................................................................... 10

2.2.2. PRICO® process

[2] ........................................................................................................ 11

2.2.3. APCI® propane pre-cooled MR process (C3/MR) [2]

..................................................... 11

2.2.4. APCI® AP-X™ process [6]

............................................................................................. 12

2.2.5. Shell® dual mixed refrigerant process (DMR) [6]

........................................................... 12

2.2.6. Axens/IFP Liquefin™ process [8]

.................................................................................... 13

2.2.7. Statoil®/Linde® mixed cascade process [2]

................................................................... 15

2.3. APCI® – AP-X™ process ...................................................................................................... 15

2.3.1. Process AP-X™, alternative scenario ........................................................................... 17

2.3.2. AP-X™ process ............................................................................................................. 20

2.4. Main cryogenic heat exchanger [11]

........................................................................................ 20

2.4.1. Wound Coil heat exchanger .......................................................................................... 21

2.4.2. Plate and Fin HX ........................................................................................................... 21

3. Materials and methodology ............................................................................................................ 23

3.1. gPROMS model builder ........................................................................................................ 23

3.2. Methodology .......................................................................................................................... 23

4. gPROMS process model library – PML:SS ................................................................................... 25

4.1. Source ................................................................................................................................... 25

4.2. Sink ....................................................................................................................................... 25

4.3. Compressor_centrifugal ........................................................................................................ 26

4.4. Cooler .................................................................................................................................... 26

xii

4.5. Heat_exchanger .................................................................................................................... 26

4.6. MSHX_simple ........................................................................................................................ 27

4.7. PFHX ..................................................................................................................................... 27

4.8. Expander ............................................................................................................................... 27

4.9. JT_valve ................................................................................................................................ 28

4.10. Separator ............................................................................................................................... 28

4.11. Mixer ...................................................................................................................................... 28

4.12. Splitter ................................................................................................................................... 28

4.13. Stream_analyser ................................................................................................................... 29

4.14. Loop_breaker ........................................................................................................................ 29

5. LNG process modelling – simple approach ................................................................................... 31

5.1. AP-X™ process [10] [9]

............................................................................................................. 31

5.1.1. AP-X™ process - flowsheet assembly .......................................................................... 32

5.2. AP-X™ alternative process [10] [9]

........................................................................................... 37

5.2.1. AP-X™ alternative process - flowsheet assembly ........................................................ 37

5.3. C3/MR process [13] [14]

............................................................................................................ 40

5.3.1. C3/MR process flowsheet assembly ............................................................................. 40

5.4. Results .................................................................................................................................. 43

6. Plate and Fin HX Modelling ........................................................................................................... 45

6.1. Model Overview..................................................................................................................... 45

6.2. Model Description.................................................................................................................. 46

6.3. Composite Models and Topology.......................................................................................... 47

6.3.1. Mixer_array and Splitter_array ...................................................................................... 47

6.3.2. Channel_1D .................................................................................................................. 48

6.3.3. Wall_1D ......................................................................................................................... 49

6.3.4. Heat_Losses_1D ........................................................................................................... 50

6.3.5. Auxiliary Models ............................................................................................................ 51

6.4. Model dialog boxes ............................................................................................................... 52

6.4.1. Wall specifications tab ................................................................................................... 52

6.4.2. Hot stream specifications tab ........................................................................................ 52

6.4.3. Cold stream specification tab ........................................................................................ 53

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6.4.4. Heat losses tab .............................................................................................................. 53

6.4.5. Numerics tab ................................................................................................................. 54

6.4.6. Costing tab .................................................................................................................... 55

7. LNG process modelling – complex approach ................................................................................ 57

7.1. AP-X™ Process .................................................................................................................... 57

7.1.1. PFHX-1 .......................................................................................................................... 58

7.1.2. PFHX-2 .......................................................................................................................... 61

7.1.3. PFHX-3 .......................................................................................................................... 63

7.2. AP-X™ alternative Process ................................................................................................... 64

7.2.1. PFHX-1 .......................................................................................................................... 65

7.2.2. PFHX-2 .......................................................................................................................... 67

7.2.3. PFHX-3 .......................................................................................................................... 67

7.3. C3/MR Process ..................................................................................................................... 68

7.3.1. PFHX-1 .......................................................................................................................... 69

7.4. Results .................................................................................................................................. 70

8. AP-X™ Process NLP optimisation ................................................................................................. 73

8.1. Methodology .......................................................................................................................... 73

8.2. AP-X™ process – base case ................................................................................................ 74

8.3. Sensitive analyses ................................................................................................................ 75

8.4. Refrigerants flow optimisation ............................................................................................... 76

8.5. Performance optimisation ..................................................................................................... 77

8.6. Overall optimisation ............................................................................................................... 79

8.7. Results .................................................................................................................................. 80

9. AP-X™ Process Results ................................................................................................................ 83

10. Conclusions and Future work .................................................................................................... 85

11. Bibliography ............................................................................................................................... 87

Appendices ........................................................................................................................................... 91

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List of Figures

Figure 1: Visual representation of the vapour phase volume and liquid phase volume of NG. [1]

........... 1

Figure 2: LNG plant simplified diagram. [6]

.............................................................................................. 3

Figure 3: LNG chain steps from the plant to the final consumer. [6]

........................................................ 4

Figure 4: Representation of the LNG and MR cooling curves. [2]

............................................................ 7

Figure 5: Simplified representation of the Conoco-Phillips optimised cascade LNG process. [8]

.......... 10

Figure 6: Simplified diagram of the Prico process. [8]

............................................................................ 11

Figure 7: Simplified representation of the C3/MR process. [2]

............................................................... 12

Figure 8: LNG APX™ process diagram. [8]

............................................................................................ 12

Figure 9: Simplified representation of the Shell® DMR process. [8]

...................................................... 13

Figure 10: Simplified represention of the Liquefin™ process. [2]

........................................................... 14

Figure 11: Simplified representation of the mixed cascade process. [8]

................................................ 15

Figure 12: Liquefaction capacity of the APCI® technologies. [9]

........................................................... 16

Figure 13: Representation of process diagram from the patent that preceded the AP-X™ process. [10]

.............................................................................................................................................................. 17

Figure 14: Description of the AP-X™ process. [10]

................................................................................. 20

Figure 15: Photo of an unfinished wound coil HX. [11]

........................................................................... 21

Figure 16: Schematic representation of a plate and fin HX. [11]

............................................................. 22

Figure 17: Topology representation of the source model. .................................................................... 25

Figure 18: Topology representation of the sink model. ......................................................................... 26

Figure 19: Topology representation of the compressor_centrifugal model. ......................................... 26

Figure 20: Topology representation of the cooler model. ..................................................................... 26

Figure 21: Topology representation of the heat_exchanger model. ..................................................... 27

Figure 22: Topology representation of the MSHX_simple model. ........................................................ 27

Figure 23: Topology representation of the PFHX model. ..................................................................... 27

Figure 24: Topology representation of the expander model. ................................................................ 28

Figure 25: Topology representation of the JT_valve model .................................................................. 28

Figure 26: Topology representation of the separator model. ................................................................ 28

Figure 27: Topology representation of the mixer model. ...................................................................... 28

Figure 28: Topology representation of the splitter model. .................................................................... 28

Figure 29: Topology representation of the stream_analyser model. .................................................... 29

Figure 30: Topology representation of the loop_breaker model. .......................................................... 29

Figure 31: Topology representation of the AP-X™ process model – simple approach. ....................... 32

Figure 32: Topology representation of the NG-LNG line - AP-X™ process. ........................................ 32

Figure 33: Topology representation of the nitrogen cycle - AP-X™ process. ....................................... 33

Figure 34: Topology representation of the MR cycle - AP-X™ process. .............................................. 35

Figure 35: Topology representation of the alternative AP-X™ process model – simple approach. ..... 37

Figure 36: Topology representation of the NG-LNG line - AP-X™ alternative process. ...................... 37

Figure 37: Topology representation of the nitrogen cycle - AP-X™ alternative process. ..................... 38

Figure 38: Topology representation of the C3/MR process model – simple approach. ....................... 40

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Figure 39: Topology representation of the NG-LNG line – C3/MR process. ........................................ 41

Figure 40: Topology representation of the MR cycle – C3/MR process. .............................................. 42

Figure 41: Overview of the PFHX model. ............................................................................................. 45

Figure 42: Topology representation of the PFHX model. ..................................................................... 46

Figure 43: Topology representation of the PFHX model - Mixer_array and Splitter_array. ................. 48

Figure 44: Topology representation of the PFHX model – Channel_1D. ............................................. 49

Figure 45: Topology representation of the PFHX model – Wall_1D. .................................................... 50

Figure 46: Topology representation of the PFHX model – Heat_Losses_1D. ...................................... 51

Figure 47: Dialog box of the PFHX model – Wall specifications. .......................................................... 52

Figure 48: Dialog box of the PFHX model – Hot stream specifications. ............................................... 53

Figure 49: Dialog box of the PFHX model – Cold stream specifications. ............................................. 53

Figure 50: Dialog box of the PFHX model – Heat losses. .................................................................... 54

Figure 51: Dialog box of the PFHX model – Numerics. ........................................................................ 54

Figure 52: Dialog box of the PFHX model – Costing. ........................................................................... 55

Figure 53: Topology representation of the AP-X™ process model – complex approach. .................... 57

Figure 54: Temperature profile - PFHX-1. ............................................................................................ 60

Figure 55: LP-MR pressure profile - PFHX-1. ....................................................................................... 60

Figure 56: Vapour-fraction profile - PFHX-1 ......................................................................................... 61


Figure 58: LP-nitrogen pressure profile - PFHX-2. ............................................................................... 62



Figure 61: Topology representation of the AP-X™ alternative process model – complex approach. .. 64



Figure 64: Vapour-fraction profile - PFHX-1. ........................................................................................ 67

Figure 65: Topology representation of the C3/MR process model – complex approach ..................... 68



Figure 68: Vapour-fraction profile - PFHX-1. ........................................................................................ 70

Figure 69: Schematic representation of the optimisation procedure. ................................................... 74

Figure 70: Sensitive analyses results. .................................................................................................. 76

Figure 71: Refrigerants flow optimisation results. ................................................................................. 77

Figure 72: Performance optimisation results – Flows. .......................................................................... 78

Figure 73: Performance optimisation results – MR composition. ......................................................... 79

Figure 74: Representation of the optimisation results in absolute and relative terms. ......................... 80

Figure 75: Representation of the contributions for total cost. ............................................................... 81

Figure 76: Representation of the contributions for capital cost............................................................. 81

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List of Tables

Table 1: Range of composition of the LNG components. ....................................................................... 2

Table 2: Comparison between PFHX and WCHX. ............................................................................... 20

Table 3: Plant equipment mapping to PML:SS library. ......................................................................... 25

Table 4: Caracterisation of the stream conditions downstream NG-1 and upstream LNG-2. .............. 32

Table 5: Caracterisation of the stream conditions downstream N-1, downstream N-2 and upstream N-

4. ........................................................................................................................................................... 34

Table 6: Caracterisation of the stream conditions downstream MR-1, downstream MR-3 and upstream

MR-5. ..................................................................................................................................................... 35

Table 7: Main characteristics of the propane cycle. .............................................................................. 36

Table 8: Caracterisation of the stream conditions downstream NG-1 and upstream LNG-2. .............. 37

Table 9: Caracterisation of the stream conditions downstream N-1, downstream N-2 and upstream N-

4. ........................................................................................................................................................... 39

Table 10: Caracterisation of the stream conditions downstream MR-1, downstream MR-3 and

upstream MR-5. .................................................................................................................................... 39

Table 11: Caracterisation of the stream conditions downstream NG-1 and upstream LNG-2. ............ 41


upstream MR-4 ..................................................................................................................................... 42

Table 13: Main characteristics of the propane cycle. ............................................................................ 43

Table 14: Simple approach – main results. ........................................................................................... 43

Table 15: Caracterisation of the stream conditions upstream LNG-2. .................................................. 58

Table 16: Wall specifications considered for the PFHX model. ............................................................ 58

Table 17: Hot streams specifications considered for the PFHX model ................................................ 59

Table 18: Cold stream specifications considered for the PFHX model ................................................ 59

Table 19: Other specifications considered for the PFHX model ........................................................... 59

Table 20: Caracterisation of the stream conditions upstream LNG-2. .................................................. 65

Table 21: Caracterisation of the stream conditions upstream LNG-2 ................................................... 68

Table 22: Complex approach – main results ........................................................................................ 71

Table 23: Base case results – refrigerants flow. ................................................................................... 75

Table 24: Base case results – MR composition. ................................................................................... 75

Table 25: Base case results – pressures (see figure 53). .................................................................... 75

Table 26: Base case results – areas (see figure 53). ........................................................................... 75

Table 27: Sensitive analyses results – refrigerants flow. ...................................................................... 75

Table 28: Refrigerants flow optimisation results. .................................................................................. 76

Table 29: Performance optimisation results – refrigerants flow. ........................................................... 77

Table 30: Performance optimisation results – MR composition. ........................................................... 78

Table 31: Overall optimisation results – refrigerants flow. .................................................................... 79

Table 32: Overall optimisation results – MR composition. .................................................................... 79

Table 33: Overall optimisation results – pressures (see figure 53)....................................................... 80

Table 34: Overall optimisation results – areas (see figure 53). ............................................................ 80

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Table 35: AP-X™ Process – final results. ............................................................................................. 83

Table 36: Specific power consumption values associated to LNG technologies. ................................. 83

Table 37: Standard values of specific power consumption ................................................................... 84

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Abbreviations

APCI® – Air Products and Chemicals International

BAHX – Brazed Aluminium Heat Exchanger

CPU – Central Processing Unit

DMR – Dual Mixed Refrigerant

DOF – Degrees Of Freedom

EOS – Equation Of State

GERG – Group Européen de Recherches Gaziéres

HP – High Pressure

HTC – Heat Transfer Coefficient

HX – Heat Exchanger

IFP – Institut Français du Petroil

J-T – Joule-Thompson

LNG – Liquefied Natural Gas

LP – Low Pressure

LPG – Liquefied Petroleum Gas

MCHX – Main Cryogenic Heat Exchanger

MFC – Mixed Fluid Cascade

MFCP – Multi Fluid Cascade Process

MTPA – Million Tonnes Per Anum

MR – Mixed Refrigerant

MSHX – Multi Stream Heat Exchanger

NG – Natural Gas

NGL – Natural Gas liquids

OCLP – Optimised Cascade Loop Process

PFHX – Plate and Fin Heat Exchanger

PML:SS – Proceess Model Library : Steady-State

PMR – Pre Mixed Refrigerant

PR – Peng-Robinson

SEM – Stream Evolution Models

SMR – Single Mixed Refrigerant

SPC – Specific Power Consumption

SSML – Steady-State Model Library

SWHX – Spiral Wound Heat Exchanger

WCHX – Wound Coil Heat Exchanger

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1

1. Introduction

In its naturally occurring vapour state, NG is a bulky energy source. Transporting NG from production

sources to points of consumption requires large pipeline networks. Thus, only overland or somewhat

shorter undersea routes can be considered.

The main reason for liquefying NG is a 600-fold reduction in volume that occurs with the vapour-to-

liquid phase. The alternative to this possibility is the storage of gas phase NG in huge underground

caverns or in large telescoping storage tanks.

Figure 1: Visual representation of the vapour phase volume and liquid phase volume of NG. [1]

The storage of LNG in near urban areas allows the satisfaction of seasonal peak demands without the

need of building additional pipelines that would be underutilized the most of times. Compressed NG is

a feasible transportation fuel for truck and bus fleets. However more recent aircraft and railroad

applications require the higher energy density of LNG to be commercially viable [2]

.

1.1. Physical properties [3]

LNG is on the light end of the liquefied gas spectrum:

LNG (primarily methane with some ethane)

NGL (primarily ethane with some propane)

LPG (primarily propane with some butane)

The mentioned products are difficult to standardly characterize since their compositions may change,

however certain characterisations of LNG are possible and useful.

Due to the large fraction of methane, its boiling point is ca. -160ºC. LNG is odourless and colourless.

Trace contaminants as mercury can render LNG potentially corrosive, if not removed until sufficient

low levels during pre-purification and liquefaction stages. Other contaminants as water, carbon

dioxide and sulphur compounds usually do not exist in LNG (liquid phase). Similarly the relative low

boiling point of LNG limits the maximum concentration of pentane and heavier hydrocarbons that

could exist in liquid phase.

2

Trade Name: LNG

General: Clear, Colourless Cryogenic Liquid

Molecular Weight: 16

Freezing Point: -183°C

Boiling Point: -162 °C

Liquid Density: 450 kg/m3

Gas Specific Gravity: 0.6

Liquid-to-gas Expansion Ratio: 1:600

Vapour Flammable Range in Air: 5% to 15% by volume

Auto-ignition Temperature: 540°C

Table 1: Range of composition of the LNG components.

1.1.1. GERG equation of state (EOS)

Two properties methods are used to predict the physical properties of the compounds used in the

LNG processes. The first is based on the GERG EOS [4]

and the second is based on the Peng-

Robinson EOS [5]

. All the processes were simulated using both methods. The optimisation procedure

was only done using Peng-Robinson method.

The GERG EOS is a wide-range EOS for NG and other mixtures developed by Kunz et al. is based

on a multi-fluid approximation. The mixture model uses accurate equations of state in the form of

fundamental equations for each component along with functions developed for the binary mixtures of

the components to take into account the residual mixture behaviour.

1.1.2. Peng-Robinson EOS

The Peng-Robinson EOS has become the most popular EOS for NG systems in the petroleum

industry. During the decade of the 1970’s, D. Peng was a phD student of Prof. D.B. Robinson at

University of Alberta (Edmonton, Canada). The Canadian Energy Board sponsored them to develop

an EOS specifically focused on NG systems. Comparing the performance of the PR EOS and similar

others as SRK EOS, they were pretty close to a tie except for a slightly better behaviour by the PR

EOS at the critical point. A better performance around critical conditions makes the PR EOS

somewhat better suited to gas/condensate systems.

3

1.2. The LNG plant [2]

In a typical scheme, the feed gas is delivered at high pressure (for example, up to 90 bar) from

upstream gas fields via trunk lines where any associated condensate is removed. The gas is metered

and pressure-controlled to the plant’s design operating pressure.

Figure 2: LNG plant simplified diagram. [6]

The gas is first pre-treated to remove impurities that interfere with processing or are undesirable in

final products. These include acid gases and sulphur compounds (e.g., CO2, H2S and mercaptans),

water and mercury.

Dehydration step is one of the most important phases of the treatment, since it prevents the formation

of gas hydrates and reduces corrosion. If the gases were not dehydrated, liquid water could condense

in pipelines and accumulate at low points along the line, reducing its flow capacity.

Heavier hydrocarbons are also removed from dry and sweet NG, using high-level refrigerant to

provide the required cooling.

As shown on the diagram above, NG from wells is sent to LNG plant reception. The first pre-treatment

stage regards the removal of acid gases as H2S and CO2, using for instance an absorption-desorption

process with amines. After the previous step the removal of mercury takes place, using an adsorption

process with a particular selective absorbent. As said before, dehydration is a major role stage of the

pre-treatment process and is done using first direct cooling, followed by an adsorption and desorption

procedure. After treatment the dried and sweetened NG is ready for liquefaction.

During the liquefaction process a heavier end of the NG (NGL) might be collected and fractionated.

The vapour phase light end might as well be collected, compressed and used as expansion fluid for

electricity production or used directly as plant fuel.

Once the NG is liquefied (LNG) the product is ready for storage or shipping.

4

Figure 3: LNG chain steps from the plant to the final consumer. [6]

LNG has become an increasingly important supply source in meeting the world’s energy needs. Of all

NG consumed in 2011, 10% was transported in the form of LNG.

Such scenario was unimaginable twenty years ago. However, the past two decades have been

characterized above all by the development and diversification of gas markets worldwide. While it

may be too early to talk about gas market maturity on a par with a globally fungible commodity such

as oil, the profile of gas in the fuel mix has changed, reshaping the way that gas is marketed on a

fundamental level. LNG has been at the heart of this evolution.

Is estimated an annual market growth of 9.5% until the year of 2020. This LNG demanding must be

fulfilled by the current and future LNG technologies that after 30 years of stagnation must evolve to

more efficient levels and to higher capacities.

A proper prediction of the industrial behaviour of an LNG plant is becoming more and more important

to improve the exiting plants’ results and to optimise others in project phase. When talking about

processes with huge capacities of flammable components, operating at high pressure and cryogenic

conditions, a rigorous and complex modelling is a great challenge.

The main goals to achieve with this project are the developing of a mathematical model of a typical

LNG cycle, and to demonstrate the use of the model for steady-state optimisation.

The literature review showed that the APCI® processes are still the leaders in terms of LNG industry

with more than 80% in terms of production capacity of the LNG global trades. The C3/MR process

ruled the industry for more than 30 years being now upgraded and gradually replaced for the new

APCI® technology: the AP-X™ process.

In this thesis is presented a general LNG review that presents all the available technologies and key

aspects related to the subject, see section 2.

Then, is presented a brief introduction of gPROMS and PML:SS library that were the tool to build up

the LNG models, see section 3 and 4.

5

The section 5 regards a simplified model approach of three APCI® LNG processes, based in pre-

made models from PML:SS library.

To upgrade the level of the LNG models presented in section 5, a distributed model of a plate and fin

heat exchanger (PFHX) was done, the results are present in section 6.

In the section 7 is presented all the work related to the upgrade of the simplified models presented in

section 5, with the introduction of the new PFHX model.

The optimisation procedure followed for the AP-X™ model and the final results are presented in

section 8 and 9.

Note that during the entire thesis, brief discussions after the presentation of relevant results are done.

In section 9, is made a general discussion of the work that fact are a digest of all the comments made

during the document.

6


7

2. Background

In this section is presented a general review of a LNG liquefaction plant and all its relevant aspects. It

is presented key issues related to an LNG liquefaction stage as well as its evolution during the past

few decades.

It was also done a brief description of all the LNG technologies and a detailed presentation of the

relevant process for this work: the AP-X™ process.

Other aspects relevant for the work are also presented as the introduction to cryogenics heat

exchangers (HXs).

2.1. LNG liquefaction stage [2]

The liquefaction section is the key element of a LNG plant. Liquefaction technology is based on

refrigeration cycles, where a refrigerant by means of successive expansion and compression

transports heat from a lower to a higher temperature. LNG plants consist of parallel units, called

trains. Liquefaction train capacity is primarily determined by the liquefaction process, refrigerant used,

HXs capacity and largest available size of the compressor/ driver combination that drives the cycle.

Figure 4: Representation of the LNG and MR cooling curves. [2]

The liquefaction cooling curve is another benchmark usually reviewed at LNG analysis and often

misunderstood or incorrectly applied when considering energy performance relative to lifecycle cost.

Caution should be used with this type of comparison. Detailed knowledge of each process design and

options they can achieve at different performance levels along this curve is required for a valid

comparison.

The liquefaction section typically accounts for 30% to 40% of the capital cost for the overall plant,

which in turns accounts for 25% to 35% of total project costs. Key equipment items include

compressors used to circulate the refrigerants, compressor drivers and HXs used to cool and liquefy

the gas and exchange heat between refrigerants. At a baseload level, in recent LNG plants, this

equipment is among the largest of its type and at the leading edge of technology.

For many years, there was absolutely no problem to choose the process of a new liquefaction plant:

the C3/MR process was the only choice. The same process was implemented again and again, with

small improvements until nowadays.

8

C3/MR process is now reaching its technology limits, e.g. maximum mach number on the propane

compressor and spiral wound exchanger becoming enormous, and due to that, now many new

processes are appearing: Air Products and Chemicals International (APCI®) has launched the APX™

process (C3/MR/NITROGEN cycles), Shell® a DMR process, Linde® a process with three mixed

refrigerant cycles and IFP/Axens another DMR with PFHXs.

2.1.1. Liquefaction technologies evolution [6]

First generation (1960’s):

The world’s first large-scale plant, CAMEL, started-up in 1964. Technip/Air Liquide provided the

design of the cascade liquefaction process. This process used three separate cooling cycles: first the

propane cycle cooling the gas to -30°C, followed by the ethylene cycle taking the gas down to about -

100°C and finally the methane cycle cooling the NG to a liquid ready for transport at -160°C. This first

plant used steam turbines to drive the compressors for each refrigerant cycle and seawater cooling

for the condensers. HXs were of kettle type for the propane cycle and coil wound HXs for the ethylene

and methane cycles. The plant is still in operation today.

Later in the decade, Alaska applies the three refrigerant Phillips Cascade cycle similar to that used in

the CAMEL plant but with much larger, with a single train capacity 50% greater than the total of the

three trains at CAMEL. This plant was also the first to use gas turbines (single shaft) as the prime

movers. HXs were of a Phillips designed plate and fin variety.

Second generation (1970’s):

The 70’s opened with the start-up of the Exxon-led Marsa el-Brega plant in Libya. This plant was the

first to use the much simpler but less efficient single-mixed refrigerant cycle (SMR), designed by

APCI®, known as Prico process. The objective in this case was to reduction of the number of

compressors and HXs. Instead of the classic refrigeration approach using separate cycles, the SMR

solution employs a single cycle with a mixed refrigerant (methane, ethane, propane, among several

others) where the condensation and evaporation take place in only one cycle over a wide temperature

range down to the required -160°C.

Starting up in the Asia Pacific was the Brunei LNG plant (BLNG) where Shell® is one of the partners.

Shell®’s approach since the beginning has been to take one or two steps out from proven technology

with each new plant design. In the case of Brunei, the major innovations were the first application of

the APCI® propane pre-cooled mixed refrigerant process (C3/MR), which had a thermal efficiency of

over 90%, dramatic improvement over the SMR process.

While the various projects in the 70’s used numerous process licensors, engineering contractors and

equipment vendors, the APCI® C3-MR liquefaction process would be to dominate the industry for

thirty years.

9

Third generation (1980’s):

The main technological innovation during the 1980s was the full air cooling system used in the Shell®

designed NWS plant. Air-cooling was applied due to environmental restrictions in the use of water-

cooling in this location, a concept which has been followed elsewhere later on.

This was also Shell®’s first plant fully driven by gas turbines. The NWS project employed dual shaft

gas turbines, allowing compressor speed control and easy start-up.

Fourth generation (1990’s):

One of the main challenges of the early 90s was to maximise the shaft power from the gas turbines to

compressors and push the trains’ size boundaries even further.

The multi-train approach had become the norm for green field projects, supported by the arguments

for reliability of supply and lowest unit cost, the Atlantic LNG project in Trinidad demonstrated that an

alternative one train approach is possible and can be economically attractive under the right

circumstances. The approach was focused on cost and schedule, not in the choice of technology.

Fifth generation (2000’s):

By the new century the liquefaction technology developments accelerated again and diversified. At

one hand existing projects continued to further exploit the use of the C3/MR process. On the other

hand, processes other than APCI® C3/MR, were selected for projects in the new century.

Linde® developed their own spool-wound HXs for the liquefaction process, which were used in the

NWS trains 4 & 5. They also introduced an own multi-fluid cascade cycle which has been applied in

the Norwegian Snohvit plant.

Shell® selected the double mixed refrigerant liquefaction process for the green field Sahkalin II project

of 2 x 4.8 MTPA capacities. This process is very much suited for artic conditions, allowing operating

flexibility and the optimum power balance between pre-cool and mixed refrigerant drivers.

APCI® announced the new “AP-X™” liquefaction technology in 2002 following a patent [7]

of 2001.

This process line-up combines a C3/MR process with a closed nitrogen expansion cycle, driven by

large Frame 9E gas turbines and 35 MW helper/starters. The innovative use of an extra refrigerant

cycle and large refrigerant compressor allow a major step up in capacity. The LNG production in the

world will see a significant increase with the start-up of six, 7.8 MTPA trains in RasGas and Qatar Gas

from 2008 to 2010.

Current technology innovation focuses therefore on better efficiency, fuel economy, reducing

emissions improving current capacities.

2.2. LNG liquefaction technologies

In this sub-section all the LNG technologies with market impact are presented.

However, a simplified and complex modelling approach are only done for the C3/MR process, AP-X™

process and an alternative scenario of the AP-X™ process due to their market share (more than

80%), liability and production capacity.

10

2.2.1. Conoco-Phillips optimised cascade [2]

Phillips Petroleum Company developed the original Optimised Cascade LNG Process (OCLP) in the

1960s.

This process uses two pure refrigerants – propane and ethylene circuits, and a methane flash circuit –

cascaded to provide maximum LNG production by using the horsepower available from six Frame 5D

gas turbines. Each circuit uses two 50% compressors with common process equipment. All HXs, with

the exception of the propane chillers, are housed in two “cold boxes”. The compressor inter-cooling,

after-cooling and propane refrigerant condensing are provided by fin-fan HXs. The LNG from the last-

stage flash drum is sent to the LNG tanks where is stored at about at 70 bar and – 161°C.

The refrigeration is done by discrete temperature levels inside each refrigeration loop. As more

refrigerants and pressure levels the process has more efficient it becomes, although the increase of

components and pressure levels would also increase the fixed cost of the process.

Figure 5: Simplified representation of the Conoco-Phillips optimised cascade LNG process. [8]

In the figure 5, is possible to observe the NG-LNG line being cooled down first in the propane

(refrigeration cycle represented in red) heat-exchanger, then the light end of the line is sent to the

ethylene (refrigeration cycle represented in yellow) cold-box where the NG is further cooled before

being liquefied and sub-cooled in the methane (refrigeration cycle represented in green) cold-box and

sent to storage.

Note that all the ethylene and propane streams are pre-cooled by their upstream refrigeration cycle

before performing their main cooling in their respective cold-boxes.

11

2.2.2. PRICO® process [2]

Black & Veatch has developed a mixed refrigerant process, PRICO, which has been successfully

used in baseload and peak shaving applications. This process has a single mixed refrigeration cycle

and a single compression system used in an earlier baseload plant in Algeria.

Train capacity has been up-rated to 1.3 MTPA. The mixed refrigerant is a mixture of nitrogen,

methane, ethane, propane and iso-pentane. The component ratio is chosen to closely match its

boiling curve with the respective of the gas feed. The closer the curves match, more efficient the

process becomes. The mixed refrigerant is compressed and partially condensed before entering the

insulated enclosure for the highly efficient plate and fin HXs, collectively known as the “cold box.”

Figure 6: Simplified diagram of the Prico process. [8]

2.2.3. APCI® propane pre-cooled MR process (C3/MR) [2]

APCI® began to dominate the industry from the late 1970s on. This process accounts for a very

significant proportion of the world’s LNG production capacity. Train capacities up to 4.7 MTPA have

been built or are under construction.

The C3/MR process uses a MR mainly composed of nitrogen, methane, ethane and propane. The NG

feed is initially cooled by a separate set of propane chillers to an intermediate temperature of about -

35°C, at which the heavier components in the feed gas condense and are sent to fractionation.

The NG is then sent to the MCHX (wound coil or plate and fin HXs). These allow very close

temperature approaches between the condensing and boiling streams.

The MR refrigerant is partially condensed in the propane chillers before entering the cold box. The

separate liquid and vapour streams are then further chilled before being flashed across Joule-

Thompson valves providing then the cooling for the final gas liquefaction.

12

Figure 7: Simplified representation of the C3/MR process. [2]

2.2.4. APCI® AP-X™ process [6]

The AP-X™ process is essentially an upgrade of the C3/MR process, with the inclusion of one extra

refrigeration cycle: propane pre-cooling cycle, mixed refrigerant cycle and nitrogen expander cycle.

The HXs used in the propane cycle are kettle type HXs, while the MCHX contained in the cold boxes

are of the type plate and fin or wound coil. The process is analogous to the C3/MR process however

the new third cycle allows the decreasing the propane and MR flowrates which allows a better match

of the MR composition as well as smaller quantities of flammable compounds on the plant. Detailed

description of the process will be done further on.

Figure 8: LNG APX™ process diagram. [8]

2.2.5. Shell® dual mixed refrigerant process (DMR) [6]

The Shell® DMR process is a dual mixed refrigerant process, with different refrigeration duty on the

two cooling cycles.

The first refrigeration cycle does the pre-cooling of the gas to about – 50°C while the other cycle does

the final cooling and liquefaction of the NG.

This concept allows the designer to choose the load on each cycle. It also uses proven equipment,

e.g. spiral-wound HXs, throughout the process. DMR process is the basis of the Sakhalin LNG plant,

with a capacity of 4.8 MTPA.

13

Process configuration is similar to the C3/MR process, with the pre-cooling being conducted by a

mixed refrigerant (made up mainly of ethane and propane-PMR) rather than pure propane. PMR

vapour from pre-cool exchangers is routed via knock-out vessels to a two-stage centrifugal

compressor. De-superheating, condensation and sub-cooling of the PMR is achieved by using

induced-draft air coolers.

Using a mixed refrigerant, with a lower molecular weight, allows pushing further the limit of the mach

number of the compressor.

The PMR compressor is driven by a single gas turbine, equipped with an electric starter motor.

Another main difference is that the pre-cooling is carried out in WCHX, rather than kettles.

The refrigerant compressors are driven by two Frame 7 gas turbines, equipped with a separate

variable speed starter/ helper motor. An axial compressor is also used as part of the cold refrigerant

compression stages.

The cooling for liquefaction of the NG is provided by a second mixed refrigerant cooling cycle (MR

cycle). The mixed refrigerant is a mixture of nitrogen, methane, ethane and propane.

Mixed refrigerant vapour from the Shell® side of the main cryogenic HX is compressed in an axial

compressor, followed by a two-stage centrifugal compressor. Inter-cooling and initial de-superheating

is achieved by air-coolers.

The mixed refrigerant vapour and liquid are separated, and further cooled in the main cryogenic heat.

Figure 9: Simplified representation of the Shell® DMR process. [8]

In the figure 9, is possible to observe the NG-LNG line, in green, being cooled down first in the first

MR (refrigeration cycle represented in blue) cold-box, then the line is sent to the second MR

(refrigeration cycle represented in red) cold-box where the NG is further cooled and liquefied before

being sent to storage.

Note that the second MR refrigeration cycle, in red, is pre-cooled by the first MR cycle, in blue.

2.2.6. Axens/IFP Liquefin™ process [8]

IFP and Axens have developed the Liquefin™ process that aims to produce cheaper LNG than with

any other process.

All cooling and liquefaction activity is conducted in a PFHX, arranged in cold boxes.

14

This process is a DMR process. Plate and fin HXs are used for the whole exchange line. As for all

other processes with mixed refrigerant on the first refrigeration cycle, the main condenser is smaller

which means that the respective compressor has a lower mach number.

The lower amount of mixed refrigerant on the cold cycle allows reaching much higher LNG capacities

with the existing axial compressors.

The first mixed refrigerant is used at three different pressure levels to pre-cool the process gas and to

pre-cool and liquefy the second mixed refrigerant. The second mixed refrigerant is then used to

liquefy and sub-cool the process gas.

Using a mixed refrigerant for the pre-cooling stage decreases the temperature down to a range of –

50°C to – 80°C depending on refrigerant composition. At these temperatures, the cryogenic mixed

refrigerant can be completely condensed, avoiding any phase separation and reducing its flow.

A very significant advantage of this new scheme is the possibility to adjust the power balance

between the two cycles, making possible the use of full power provided by two identical gas drivers.

The Liquefin™ process is very flexible. The process represents a real breakthrough – the plant

capacity can be chosen by considering the economics and marketing possibilities without being

bothered by technical hindrances. A total cost reduction per ton of LNG is reported to be 20% when

compared to the C3/MR process. The cost reductions arrive from:

increasing the plant capacity

reducing the HX costs

all-over plate and fin HXs

The Liquefin™ process has all the positive features of MFCP, with much better efficiency and a

smaller amount of rotating equipment. It is particularly well-adapted to the range of 4 to 8 MTPA of

LNG, per train, with many open options for designing and erecting a plant fully responding to the

client’s needs.

Note that despite all the above cited advantages, the Liquefin™ process is new, and is not yet

implemented at industrial scale.

Figure 10: Simplified represention of the Liquefin™ process. [2]

15

In general, this process is an agglutination of single MR refrigeration cycles with cascade type cycles

in a single MSHX.

2.2.7. Statoil®/Linde® mixed cascade process [2]

The Statoil®/Linde® LNG technology alliance were established to develop alternative LNG baseload

plants for the North Sea. Besides other innovative procedures and concepts, this work resulted in a

new LNG baseload process, the so-called Mixed Fluid Cascade Process (MFCP).

This process is a three cycle process, like the cascade process, but with mixed refrigerants.

Compared to the simple cascade process, the efficiency is better, since that mixed refrigerants allow

a closer approach to the cooling curve of the LNG.

Plate and fin exchangers are used on the first cycle while WCHX are used on the other two cycles.

The NG is sequentially cooled down to its final conditions along the three HXs. The refrigerants used

at the second and third refrigeration step are first cooled down in the previous by the others

refrigerants.

The MFC process is a new technology, without any industrial references. However, the concept is

built up by well-known elements. Additionally, the size and complexity of the separate SWHX applied

in the MFCP are considerably smaller when compared with today’s single unit used in dual-flow LNG

plants.

Figure 11: Simplified representation of the mixed cascade process. [8]

2.3. APCI® – AP-X™ process

As mentioned before, the APCI® processes rule more than 80% of worldwide LNG trades in terms of

volume. The C3/MR process has dominated the industry since the seventies until the 21st century,

being even today the market leader.

Is unanimous by LNG producers that the robustness, simplicity, capacity and invested know-how in all

the APCI® processes are well above all the other licensors. Being the AP-X™ an upgrade in terms of

capacity and efficiency of the old C3/MR process this is indeed the process of the future due to the

LNG demanding of the next few years.

The information gathered about the LNG processes shown that the APCI® AP-X™ process was a

new, but reliable process with a growing market share with some industrial applications. It had all the

advantages of the leader process APCI® C3/MR with a larger capacity. This was the process chosen

for the modelling and optimisation stages, being this, the primary goal to achieve during the period of

the internship.

16

The modelling of the C3/MR process and an alternative scenario of the AP-X™ process were also

performed as complement for the thesis.

Figure 12: Liquefaction capacity of the APCI® technologies. [9]

It was also clear that all the LNG modelling work relies on the PR and GERG property methods,

therefore the modelling will be done using both, as individual models.

Due to the large variety of NG sources and operational conditions, the prediction of NG-LNG physical

properties is still nowadays a big challenge for the LNG producers. Some EOS can predict under

some restrictions the properties of the components present in LNG processes.

This field is an uncertainty of the process but most of the modelling papers reviewed during the

literature review recommended the PR and GERG EOS as property methods, due to the accuracy in

predict the high non-idealness of the NG systems as well as the binary interactions of the

hydrocarbons present in the mixtures.

17

2.3.1. Process AP-X™, alternative scenario

Figure 13: Representation of process diagram from the patent that preceded the AP-X™

process. [10]

This sub-section is based in the document that preceded the LNG process whose commercial name

is AP-X™. Several embodiments might be associated to the process: (i) inclusion of a different type of

MCHX (ii) other mixed refrigerants as fluid for the gas expander refrigeration cycle (iii) implementation

of a propane pre-cool cycle. The description of process is shown below, the streams and units

present in the figure 13 are explained there.

The process AP-X™ is analogous to the process which is represented in the figure above with the

particularity of having a propane pre-cooling stage to cool down the NG, MR and nitrogen streams.

Considering the AP-X™ process, the NG feed is initially cooled by a separate set of propane chillers

to an intermediate temperature, about – 35°C. The MR refrigeration stage has to cool down the

process gas to around -100ºC [2]

. The final liquefaction step and sub-cooling is done using a nitrogen

expansion cycle.

Detailed description of the embodiment

Nowadays, most LNG plants use refrigeration produced by compressing a gas fluid to high pressure,

liquefying the gas against a cooling source, expanding the resulting liquid to a low pressure stage and

vaporizing the resulting fluid to provide refrigeration. The vaporized fluid is recompressed again

returning to the circuit.

This type of refrigeration process can use a multi-component mixed refrigerant or a cascaded single

component refrigeration cycle. The type of cascade, used at the propane cycle, is very efficient

providing cooling at ambient conditions.

18

For instance, a two-fluid cascade can be used in which a heavier fluid provides warmer refrigeration

while the lighter one provides cooler refrigeration. Instead of rejecting heat to an ambient temperature

sink, the lighter fluid rejects heat to the boiling heavier one while it condenses. The main

disadvantage of a cascade process comparing to a MR cycle is the necessity to have individual

compressors and HXs for each fluid for different pressure stages.

Another type of providing refrigeration is through a gas expander cycle. In this cycle the working fluid

is compressed, cooled sensibly, work expanded as vapour in a turbine and warmed providing

refrigeration to the process gas. The gas expander cycle isentropically provides refrigeration and is

especially useful in providing it for very low temperature situations such as the required in producing

liquid nitrogen and hydrogen. In opposition it is relatively inefficient at providing warm refrigeration.

The objective of this process is to exploit the benefits of the gas expander cycle in providing cold

refrigeration while using the benefits of pure or multi-component vapour recompression refrigeration

cycles in providing warm refrigeration.

Patent description [7]

The gas is first cleaned and dried in a pre-treatment on the section 172 for the removal of CO2 and

H2S. The pre-treated stream 100 enters the HX 106 (wound coil HX, base part) where is cooled to

approximately -30oC. The cooled stream 102 is partially condensed and flows until scrub column 108.

The cooling in the exchanger 106 is carried out by the warming of mixed refrigerant stream 125 in the

interior of the HX

In the column 108, it is made a separation of the pentane and heavier components from the process

gas stream.

The bottom product of the scrubber 108 then enters the fractionator, section 112 (appears to be a

block of distillation columns), where the heavy components are recovered as stream 114. The

propane and lighters from stream 118 pass through HX 106 and are cooled until around -30oC before

being recombined with the overhead product from scrubber 108, stream 120.

The stream 120 is then further cooled to a typical temperature of -100oC, on the HX 122 (it appears

to be the top part of the wound coil HX 106).

The resulting cooled stream 126 is cooled to a temperature of about -166oC in the HX 128 (wound

coil HX). The refrigeration duty in the HX 128 is provided by the stream 130 that comes from the

expander 166.

The refrigerant fluid of stream 130 is mostly nitrogen vapour containing less than 20% liquid, in a

typical pressure of 11 bara and a typical temperature of -168oC.

Further cooled stream 132 is flashed adiabatically to a pressure of 1.05 bara across throttling valve

134. The LNG flows to the separator 136 and is collected in stream 142.

In some cases a significant quantity of light gas evolved as stream 138 can be warmed in HXs 128

and 150 and compressed to be used as fuel gas in the facility.

MR cooling stage

The refrigeration to cool the process gas from ambient temperature to -100oC is provided by a multi-

component refrigerant. The stream 146 is the high pressure mixed refrigerant which enters the HX at

19

about ambient temperature and 38 bara. The MR is cooled until about -100oC on the HX 106, exiting

as stream 148.

The stream 148 is divided into two portions. A smaller one (about 4%) is reduced in pressure

adiabatically to around 10 bara before being introduced as stream 149 into HX 150 to provide extra

refrigeration. The other part of stream 148 is also reduced to 10 bara and is sent to HX 106.

The refrigerant flows downwards and vaporizes in the interior of the HX, before exiting on the bottom

at a temperature slight below ambient on stream 152. Stream 152 is mixed with stream 154 that has

been vaporized on HX 150 forming the combined low pressure stream 156. The stream 156 is

compressed until the pressure of 38 bara on the multistage compressor 158.

Nitrogen cooling stage

The final cooling of the process gas to -166oC is accomplished using a gas expander cycle employing

nitrogen has fluid. High pressure nitrogen, stream 162 enters the HX 150 typically at ambient

temperature and a pressure of 67 bara and is cooled to about -110oC. The cooled vapour is

isentropically expanded in expander 166, to a pressure of 11 bara and a temperature of about -168oC.

Ideally the exit pressure should be at or slightly below the dew point of nitrogen with a temperature

cold enough to liquefy the LNG.

The expanded stream 130 is warmed to near ambient temperature in HX 128 and 150. Extra

refrigeration is done by the small stream of MR 149. This step is done to reduce the irreversibility in

the process closely aligning the cooling curves HX 150.

From HX 150, warmed low pressure nitrogen, stream 170, is compressed in the multistage

compressor 168 back to about 67 bara.

20

2.3.2. AP-X™ process

Figure 14: Description of the AP-X™ process. [10]

The only difference between this process and the previous one described is the Kettle type [6]

HX

402, 401 and 400 that respectively pre-cool the feed, the nitrogen and MR streams.

The units represented by the number 106, 128 and 150, are MSHXs. In other words they represent a

HX with several inlets and outlets. These types of HXs are the core ingredient of liquefaction units,

being usually known as MCHX (main cryogenic HXs) insulated on what in LNG jargon is called as

“cold-box” of the plant.

2.4. Main cryogenic heat exchanger [11]

The Main Cryogenic HX is the heart of the LNG liquefaction plant and is where NG will be liquefied

and sub-cooled. The Main Cryogenic HX will convert the gas temperatures up to -166oC.

For LNG facilities are used two types of MCHX: The WCHX or PFHX.

Table 2: Comparison between PFHX and WCHX.

PFHX WCHX

Features Extremely compact

Up to ~10 streams

Extremely robust

Compact

Fluids Very clean

Non-corrosive

Heating Surface 300 - 1000 m²/m³ 50 - 150 m²/m³

Materials Al Al, SS, CS etc.

Design Temperatures -269°C to +65°C All

Applications smooth operation High temperature gradients

21

limited installation space High temperature differences

Cost ~25 - 35 % 100%

2.4.1. Wound Coil heat exchanger

A wound coil HX is, in general, a tubular HX. However, the bundle consists of not straight tubes.

Tubes of relatively long length and small diameter are wound in alternating directions around a central

pipe. In parallel, a pressure vessel shell is prepared to contain the complete tube bundle. All single

tubes start and terminate in tube sheets which are integral parts of the pressure vessel shell.

Due to the flexible tube bundle arrangement these HX can bear temperature gradients clearly

exceeding the limits of any other HX type (e.g. plate and fin HXs). Optimal liquid distribution of the

shell side two-phase stream over the whole cross section of the bundle is achieved by internal phase

separation and special liquid distribution systems.

The tube bundles are designed and fabricated to be vibration-proof and self-draining.

According to the literature, there is only one SEM (stream evolution models) powerful computational

tool for the WCHX, the GENIUS®, developed by Linde®.

Figure 15: Photo of an unfinished wound coil HX. [11]

2.4.2. Plate and Fin HX

The PFHX (eg. brazed plate and fin HX) is a stack of alternating flat and corrugated plates. The

corrugations (fins) form the flow channels for the diverse process fluids. Each process stream

occupies a certain number of passages within the stack. These are collected by half pipe headers and

nozzles to single point connections on the inlet and the outlet of the respective process stream. In this

way, up to ten process fluids can exchange heat in only one block.

They are extremely compact due to the use of aluminium and highly efficient fins. The heating surface

density can be greater than 1000 m²/m³. Thus this type of HX is perfectly suitable for installations

which require compact design. The wide selection of heat transfer fins combines high heat transfer

rates with low pressure drops (i.e. low energy consumption). The ability to combine up to ten process

streams in only one HX system can eliminate the need for multiple HX arrangements and

22

interconnecting piping. The use of high strength aluminium alloy results in light weight units reducing

drastically the foundation and support requirements.

This type of HX cannot be recommended for cases of operation such as: (i) high temperature

gradients (i.e. thermal shocks), (ii) high temperature differences between the cold and the warm

process streams and (iii) process streams containing particles susceptible to cause severe fouling.

According to the literature, there is only one SEM (stream evolution models) powerful computational

tool for the PFHX, the Aspen plate fin exchanger®.

Figure 16: Schematic representation of a plate and fin HX. [11]

In the figure16 can be observe a PFHX with three stream represented in blue, green and red. In

contrast a typical shell and tube HX is not possible to characterise the streams of a PFHX as cold and

hot streams because all the streams exchange heat between them.

23

3. Materials and methodology

3.1. gPROMS model builder

The software used for the implementation and simulation of all the models was gPROMS, a platform

for high-fidelity predictive modelling for the process industries. At the heart of the gPROMS platform is

the gPROMS ModelBuilder, which is used to build, validate and execute models, using an equation

oriented language.

A gPROMS model features an equation system that involves both variables and parameters. The

parameters can be defined as integer, real or foreign objects (eg. multiflash files for physical

properties). A default value of these parameters can be defined in the model. As for the variables, a

variable type has to be created and attributed for each of the variables in the model. Variable types

represent a separate entity in ModelBuilder and both bounds and default value have to be defined in

the VariableType section.

In order to connect the different component models in a topology environment, a port connection has

to be defined in the ConnectionType section and added to the models, becoming this one a top level

model (also known as flowsheeting level model). The top level model is often composed by several

sub-models (composite or component models). The sub-models have the same structure as the

previous ones but they have to be defined in the Unit section.

To run the simulations, a Process has to be created, where the remaining of Degrees of Freedom

(DOF) are specified and solver is selected. For the optimisations, an Optimisation entity must be

created, where both objective function and Process of the problem are specified. Also, all decision

variables (including bounds and initial guesses) and constraints (equalities and inequalities) are

specified by the user in the Optimisation environment.

For more information on how the software works consider the reference [12]

.

3.2. Methodology

The first part of the work, after the literature review was the modelling of the process C3/MR, AP-X™

and AP-X™ alternative, using both PR and GERG properties package, following a simple approach.

The simple approach relies on the flowsheet assembling on the topology environment of a top level

model using PML:SS models. This approach was all about to get good initial values to be applied in a

more complex approach.

A notorious gap of the PML:SS library was the lack of a proper MSHX. In combination with a

consulting project for Proctor and Gamble®, it was developt a model of a distributed PFHX to be

applied to liquefaction units. This was the second part of the project.

The third part of the work was all about the introduction of the new PFHX model in the LNG models

developed in the simple approach. Using the same type of assignments and the initial values got in

the simple approach it was possible to initialise and converge the models, for both PR and GERG

methods.

The last part of the work was a steady-state optimisation of the AP-X™ model using PR method.

24


25

4. gPROMS process model library – PML:SS

As said before, models from the PML:SS library were used as composite models for the modelling of

LNG processes. It would be too exhaustive to explain the detail and the maths behind each model

used, but it is important to understand its basic functionality.

From table 3 is possible to see the models used for equipment of the processes.

Table 3: Plant equipment mapping to PML:SS library.

LNG plant equipment PML:SS model(s) used

Upstream and downstream conditions Source; Sink

Compression train Compressor_centrifugal; Cooler; Heat_exchanger

Kettle type HXs Heat_exchanger

MCHX MSHX_simple; PFHX

Expander Expander

Expansion valves JT_valve

Auxiliary units Separator; Mixer; Splitter

Flowsheet blocks Stream_analyser; Loop_breaker

4.1. Source

The Source model represents the conditions of the connected stream, is used to represent the NG

conditions after the pre-treatment stage. Assignment of temperature, pressure, flow, composition and

the property method to be used along that stream is required.

Figure 17: Topology representation of the source model.

4.2. Sink

The Sink model represents the opposite of the source. If the source represents the upstream of the

process, the sink represents the downstream.

The sink is the model where a stream should be connected after living the last equipment or unit. No

assignments are required.

26

Figure 18: Topology representation of the sink model.

4.3. Compressor_centrifugal

The Compressor_centrifugal model is used to simulate a single stage or multistage centrifugal

compressor.

To fulfil the DOF of this model several variables should be assigned:

Pressure specifications

Efficiencies specifications

Number of stages

The design and costing mode of the model are also an option. For LNG processes, outlet pressures,

single stage mode and adiabatic efficiency were always the inputs used.

Figure 19: Topology representation of the compressor_centrifugal model.

4.4. Cooler

The Cooler model is a simplified model of a HX. This model was used to represent the intercooling of

the compressors on the simple approach. The user may define the heat duty involved on the unit or

the outlet temperature of the stream, the last option was the case for the present project.

Figure 20: Topology representation of the cooler model.

4.5. Heat_exchanger

The Heat_exchanger model is used to represent a two stream HX. In the particular case of propane

cycles those were assumed as kettle type for both simple and complex approach. In the complex

approach, for the compression train intercoolers were assumed as being of Shell® and tube type.

The assignment of this model might be done in several ways:

Outlet temperature

Heat duty

Vapour fraction

Among others other process variables

27

For the complex approach that will be presented further on, the model is working in performance

mode which means that only the area and design characteristics are assigned. For the simple

approach the outlet temperature of the process stream was the chosen input.

Figure 21: Topology representation of the heat_exchanger model.

4.6. MSHX_simple

The MSHX_simple is a simplified version of a multistream HX. This model is used to simulate the

MCHX of the LNG processes. The outlet temperatures of the streams based on value from the patent

were considered as assignments.

Figure 22: Topology representation of the MSHX_simple model.

4.7. PFHX

This PFHX model allows the simulation of a multistream 1D transfer phenomena. This model was only

used in the complex approach. For better understanding see section 6.

Figure 23: Topology representation of the PFHX model.

4.8. Expander

The Expander model was used to simulate an isenthalpic expansion. Several variables might be

chose as assignments, namely:

Pressure specifications

Efficiencies

Power consumption

Outlet temperatures

For the LNG processes the outlet pressure and adiabatic efficiency were the chosen assignments.

28

Figure 24: Topology representation of the expander model.

4.9. JT_valve

The JT_valve model represents a Joule-Thompson or expansion valve. It is possible to assignment

pressure specifications, outlet temperatures and vapour pressures. The outlet pressure was always

the chosen assignment.

Figure 25: Topology representation of the JT_valve model

4.10. Separator

The Separator model represents vapour-liquid separating equipment. The user as the option to

define removed or added heat, pressure drop and design mode.

Figure 26: Topology representation of the separator model.

4.11. Mixer

The Mixer model is used to simulate the mixing of two or more streams. It is not need the assign of

any variable.

Figure 27: Topology representation of the mixer model.

4.12. Splitter

The Splitter model is used to represent a stream splitting. There is an option to define splitting flows or

splitting fractions as inputs. The last was the chosen variable to assignment.

Figure 28: Topology representation of the splitter model.

29

4.13. Stream_analyser

The Stream_analyser model is an informative model that when connected to a stream reports its

conditions. Several information details might be requested.

Figure 29: Topology representation of the stream_analyser model.

4.14. Loop_breaker

The Loop_breaker model is essentially used to solve numerical issues, only applicable to closed

loops with fixed composition. The model is similar to the source model but its information is used to

initiate the simulation.

Figure 30: Topology representation of the loop_breaker model.

30


31

5. LNG process modelling – simple approach

In the simple approach, the idea was to build up the LNG flowsheet in the topology environment, get a

simple and quick simulation to obtain some good first results to be used on a more complex modelling

methodology which includes design mode in the models.

For both property methods used, GERG EOS and PR EOS, the inputs used were exactly the same.

The resulting variables of the models show less than 0,1% difference for both property packages.

Note that throughout this approach all possible heat losses and pressure drops on the units were

neglected.

For a better resolution of the flowsheet, see appendix B-1 to B-3.

5.1. AP-X™ process [10] [9]

The figure 31 represents the topology representation at gPROMS model builder of the AP-X™

process model.

From the figures shown in sub-section 4.1 to 4.14, it is possible to understand the models used at any

point of the flowsheet.

Note that only some models are identified. These models are the key elements to understand the

global flowsheet network. The non-identified models are not purposely identified to simplify and

smooth the topology environment, being anyway as all the other models an indispensable part of the

overall work.

For an easier comprehension, the following notation was adopted to identify models/equipment of the

process streams:

MR-x for the MR cycle

P-x for the Propane cycle

N-x for the Nitrogen cycle

NG-x or LNG-x for the NG-LNG line

.

32

Figure 31: Topology representation of the AP-X™ process model – simple approach.

5.1.1. AP-X™ process - flowsheet assembly

The flowsheet assembly was done in stages. In particular, the model was assembled by stream type,

starting up with the NG-LNG line alone. Step by step (with the respective assignments and testing

simulation) units of each refrigeration cycle were added until the final version of the model shown

above.

NG-LNG Line

Figure 32: Topology representation of the NG-LNG line - AP-X™ process.

The source NG-1 simulates the downstream conditions of a pre-treatment unit. The incoming NG, see

table 4, is cooled down until 242K, using a series of five kettle type HXs, at different pressure and

temperature stages, using saturated propane as cold fluid.

Table 4: Caracterisation of the stream conditions downstream NG-1 and upstream LNG-2.

NG-1 LNG-2

Flow (kg/s) 118.3 109.4

Temperature (K) 305.15 111.6

Pressure (bar) 66.5 1.05

Composition (kg/kg)

33

"NITROGEN" 0.014 0.006

"METHANE" 0.867 0.872

"ETHANE" 0.059 0.057

"PROPANE" 0.033 0.034

"2-METHYLPROPANE" 0.001 0.010

"BUTANE" 0.013 0.013

"2-METHYLBUTANE" 0.003 0.003

"PENTANE" 0.002 0.002

"HEXANE" 0.005 0.003

"HEPTANE" 0.003 0.001

After the propane pre-cooling stage, the NG enters a gas-liquid separator to remove the condensates,

before being further cooled by the MR and the nitrogen, at the MCHXs.

The liquid outlet of the separator is normally sent to a fractionating plant where the heavy compounds

as propane, butane and C5 plus, might be fractionated. The gas outlet, is sent to the MSHX_simple-1

where is further cooled until 171K using expanded MR as cold fluid.

After that stage the NG is then totally liquefied and sub-cooled until 117K at the MSHX_simple-2

using expanded nitrogen as cold fluid, becoming LNG.

At the final stage, the LNG is expanded from its inlet pressure, until 1.05 bar at the J-T valve LNG-1,

sent to a gas-liquid separator, where the outlet liquid stream might be dispatched to the commercial

pipeline or storage, sink LNG-2. Check the produced LNG characteristics at the table 4.

Nitrogen refrigeration cycle

Figure 33: Topology representation of the nitrogen cycle - AP-X™ process.

The nitrogen refrigeration cycle is a closed loop stream. The ending point of a refrigeration cycle,

considering no losses, should fit the exact same conditions of the cycle’s starting point.

34

Consider the stream analyser N-1. At this point the nitrogen line has the conditions presented in table

5 for N-1.

Table 5: Caracterisation of the stream conditions downstream N-1, downstream N-2 and

upstream N-4.

N-1 N-2 N-4

Nitrogen Flow (kg/s) 313.76 313.76 313.76

Nitrogen Temperature (K) 300 111 240

Nitrogen Pressure (bar) 67.1 10.5 10.5

The nitrogen is then cooled down to 242K, using a series of five kettle type HX, at different pressure

and temperature stages, using saturated propane as cold fluid.

After the pre-cooling stage, the nitrogen is further cooled at the MSHX_simple-3, using an expanded

MR stream, and itself (after expansion and warming at the MSHX_simple-2) as cold fluids, before

being isentropically expanded (at the expander N-2) and used for the NG liquefaction and sub-

cooling.

At the expander N-2 the nitrogen passes from 67.1 to10.5 bar, cooling down from 170 to 111K. After

the expansion the line has the conditions presented in table 5 for N-2.

The expanded cold nitrogen (at loop_breaker N-3) is used as cold fluid at the MSHX_simple-2 to cool

down the NG. The resulting warm stream is again used as cold fluid to cool down the high pressure

nitrogen coming from the propane pre-cooling cycle.

Before compression, the warm expanded nitrogen (at loop_breaker N-4) has the conditions presented

in table 5 for N-4.

The nitrogen is then compressed at three stage intercooled compression train until the pressure of

67.1 bar and 300K, ending up with the conditions of the starting point presented at table 5 for N-1.

35

MR refrigeration cycle

Figure 34: Topology representation of the MR cycle - AP-X™ process.

Analogously to the nitrogen line the MR refrigeration loop was built in the same way.

Consider the stream analyser MR-1. At that point the MR line has the conditions presented in table 6

for MR-1.


upstream MR-5.

MR-1 MR-3 MR-5

MR Flow (kg/s) 1254 1203 1254

MR Pressure (bar) 38.6 9.8 9.8

MR Temperature (K) 305.1 167.6 220.1

Composition (kg/kg)

"NITROGEN" 0.038 0.038 0.038

"METHANE" 0.479 0.479 0.479

"ETHANE" 0.413 0.413 0.413

"PROPANE" 0.002 0.002 0.002

"2-METHYLPROPANE" 0.027 0.027 0.027

"BUTANE" 0.040 0.040 0.040

The MR is then cooled down until 242K, using a series of five kettle type HX, at different pressure and


36

After the pre-cooling stage, the MR is further cooled at the MSHX_simple-1, using an expanded MR

stream (major split of the main line), before being isentalpically expanded (at J-T valve MR-3) and

used as cold fluid at the same MSHX_simple-1.

At the J-T valve MR-3 the MR passes from 38.6 to9.8 bar, cooling down from 200.3 to 167.6K. After

the expansion the stream has the conditions presented in table 6 for MR-3.

The minor split of the MR main line, is used as cold fluid to cold down the nitrogen stream (at

MSHX_simple-3), as explained in the previous point. The stream has the exact conditions (except

flow) presented at table 6 for MR-3.

After the MSHX_simple-1, both splits of the MR line are mixed before compression. At this point the

stream has conditions presented in table 6 for MR-5.

The MR is then compressed at a two stage intercooled compression train until the pressure of 38.6

bar and 305.1K, ending up at the conditions of the starting point presented at table 6 for MR-1.

Propane pre-cooling cycle

The propane pre-cooling cycle is a five stages cascade refrigeration cycle. The flowsheet with the

propane cycle inclusion is represented at figure 31.

The cascade pre-cooling intends to pre-cool the NG-LNG, MR and nitrogen streams. At each

pressure step the saturated propane is directed to a set of three kettle type HXs in series, working as

cold fluid on the shell-side, while the NG, MR and nitrogen are cooled down on the tube-side,

vaporizing part of the propane boiling pool.

After each set of kettles, the warm propane is redirected to a gas-liquid separator, where the outlet

gas stream is sent to compression while the outlet liquid stream is expanded (respectively at J-T

valves P-1, P-2, P-3, P-4 and P-5) and sent to the next set of HXs.

The following table presents the propane flow and the five pressure steps of the propane cascade.

Table 7: Main characteristics of the propane cycle.

Propane Flow (kg/s) 1175.6

Initial Pressure (bar) 14.33

Pressure let-down P-1 (bar) 8.47





Note that the evaporated propane in each pressure stage is sent to a compressor where is

compressed until the initial pressure of 14.33bar.

37

5.2. AP-X™ alternative process [10] [9]

The figure 35 represents the topology representation of the AP-X™ alternative process model at

gPROMS model builder. Note that the difference to the AP-X™ process shown in sub-section 5.1 is

the removal of the propane pre-cooling cycle.

Figure 35: Topology representation of the alternative AP-X™ process model – simple

approach.

5.2.1. AP-X™ alternative process - flowsheet assembly

The methodology used is analogous to the one explained in sub-section 5.1.1.

NG-LNG Line

Figure 36: Topology representation of the NG-LNG line - AP-X™ alternative process.

Similarly to the AP-X™ process the source NG-1 simulates the downstream conditions of a pre-

treatment unit. See table 8.


NG-1 LNG-2

Flow (kg/s) 118.3 111


38


Composition (kg/kg)

"NITROGEN" 0.014 0.006

"METHANE" 0.867 0.863

"ETHANE" 0.059 0.057

"PROPANE" 0.033 0.035


"BUTANE" 0.013 0.014


"PENTANE" 0.002 0.002

"HEXANE" 0.005 0.005

"HEPTANE" 0.003 0.004

From the source, the NG is sent to the MSHX_simple-1 where is further cooled until 171K using

expanded MR as cold fluid.

After that stage the NG is then totally liquefied and sub-cooled until 118K at the MSHX_simple-2

using expanded nitrogen as cold fluid, becoming LNG.

At the final stage, the LNG is expanded from its inlet pressure, to1.05 bar at J-T valve LNG-1, sent to

a gas-liquid separator, where the outlet liquid stream might be dispatched to the commercial pipeline

or storage, sink LNG-2. Check the produced LNG characteristics in table 8.

Nitrogen refrigeration cycle

Figure 37: Topology representation of the nitrogen cycle - AP-X™ alternative process.

Consider the stream analyser N-1. At that point the nitrogen line has the following conditions, see

table 9.

39

Table 9: Caracterisation of the stream conditions downstream N-1, downstream N-2 and

upstream N-4.

N-1 N-2 N-4

Nitrogen Flow (kg/s) 313.76 313.76 313.76

Nitrogen Temperature (K) 305.1 111 303.5

Nitrogen Pressure (bar) 67.1 10.5 10.5

The nitrogen is further cooled at the MSHX_simple-3, using an expanded MR stream, and itself (after

expansion and warming at the MSHX_simple-2) as cold fluids, before being isentropically expanded

(at the expander N-2) and used for the NG liquefaction and sub-cooling.

At the expander N-2 the nitrogen passes from 67.1 to10.5 bar, cooling down from 170 to 111K. At this

stage the nitrogen has conditions presented in table 9 for N-2.

The expanded cold nitrogen (at loop_breaker N-3) is used as cold fluid at the MSHX_simple-2 to cool

down the NG. The resulting warm stream is again used as cold fluid to cool down the high pressure

nitrogen.

Before compression, the warm expanded nitrogen (at loop_breaker N-4) has conditions presented in

table 9 for N-4.

The nitrogen is then compressed at a 3 stage intercooled compression train until the pressure of 67.1

bar and 305.1K, ending up with the conditions of the starting point presented at table 9 for N-1.


The flowsheet with the MR cycle inclusion is represented at figure 35.

Consider the stream analyser MR-1. At that point the MR line has the following conditions, see table

10.


upstream MR-5.

MR-1 MR-3 MR-5

MR Flow (kg/s) 4940 4737.5 4940

MR Pressure (bar) 38.6 9.8 9.8

MR Temperature (K) 305.1 168 293.3

Composition (kg/kg)

"NITROGEN" 0.038 0.038 0.038

"METHANE" 0.479 0.479 0.479

"ETHANE" 0.413 0.413 0.413

"PROPANE" 0.002 0.002 0.002

"2-METHYLPROPANE" 0.027 0.027 0.027

40

"BUTANE" 0.040 0.040 0.040

The MR is cooled at the MSHX_simple-1, using an expanded MR stream (major split of the main

line), before being isentalpically expanded (at J-T valve MR-3) and used as cold fluid at the same

MSHX_simple-1.

At the J-T valve MR-3 the MR passes from 38.6 to 9.8 bar, cooling down from 200.7 to 168K. After

the expansion the stream has the conditions presented in table 10 for MR-3.

The minor split of the MR main line, is used as cold fluid to cold down the nitrogen stream (at

MSHX_simple-3), as explained on the previous point. The stream has the exact conditions (except

flow) presented at table 10 for MR-3.

After the MSHX_simple-1, both splits of the MR line are mixed before compression. At this point the

stream has the conditions presented in table 10 for MR-5.

The MR is then compressed at a two stage intercooled compression train until the pressure of 38.6

bar and 305.1K, ending up with the conditions of the starting point presented at table 10 for MR-1.

5.3. C3/MR process [13] [14]

The figure 38 represents the topology representation at gPROMS Model builder of the C3/MR

process model. The process is analogous to the AP-X™ process, shown in sub-section 5.1, without

the nitrogen refrigeration cycle.

Figure 38: Topology representation of the C3/MR process model – simple approach.

5.3.1. C3/MR process flowsheet assembly

The methodology used is analogous to the one explained in sub-section 5.1.1.

41

NG-LNG Line

Figure 39: Topology representation of the NG-LNG line – C3/MR process.

The source NG-1 simulates the downstream conditions of a pre-treatment unit. The incoming NG, see

table 11, is cooled down until 242K, using a series of five kettle type HXs, at different pressure and



NG-1 LNG-2

Flow (kg/s) 98.89 83.7



Composition (kg/kg)

"NITROGEN" 0.014 0.003

"METHANE" 0.862 0.871

"ETHANE" 0.053 0.061

"PROPANE" 0.033 0.036


"BUTANE" 0.013 0.013


"PENTANE" 0.002 0.001

"HEXANE" 0.005 0.002

"HEPTANE" 0.003 0.001

After the propane pre-cooling stage, the NG enters a gas-liquid separator to remove the condensates,

before being further cooled by the MR and the nitrogen, at the MCHXs.

The liquid outlet of the separator is normally sent to a fractionating plant where the heavy compounds

as propane, butane and C5 plus, might be fractionated. The gas outlet, is sent to the MSHX_simple-1

where is further cooled until 171K using expanded MR as cold fluid, becoming LNG.

At the final stage, the LNG is expanded from its inlet pressure, until 1.05 bar at the J-T valve LNG-1,

sent to a gas-liquid separator, where the outlet liquid stream might be dispatched to the commercial

pipeline or storage, sink LNG-2. Check the produced LNG characteristics at the table 11.

42


Figure 40: Topology representation of the MR cycle – C3/MR process.

Consider the stream analyser MR-1. At that point the MR line has the following conditions, see table

12.


upstream MR-4

MR-1 MR-2 MR-4

MR Flow (kg/s) 267 267 267

MR Pressure (bar) 52 4.4 4.4

MR Temperature (K) 303.1 127 210

Composition (kg/kg)

"NITROGEN" 0.07 0.07 0.07

"METHANE" 0.38 0.38 0.38

"ETHANE" 0.41 0.41 0.41

"PROPANE" 0.14 0.14 0.14

The MR is then is cooled down until 242K, using a series of five kettle type HXs, at different pressure

and temperature stages, using saturated propane as cold fluid.

After the pre-cooling stage, the MR is further cooled at the MSHX_simple-1, using an expanded MR

stream, before being isentalpically expanded (at J-T valve MR-2) and used as cold fluid at the same

MSHX_simple-1.

At the J-T valve MR-2 the MR passes from 52 to 4.4 bar, cooling down from 130 to 127K. After the

expansion the stream has the conditions presented in table 12 for MR-2.

After the MSHX_simple-1 the stream has conditions presented in table 12 for MR-4.

43

The MR is then compressed at a two stage intercooled compression train until the pressure of 52 bar

and 303.1K, ending up with the conditions of the starting point presented at table 12 for MR-1.

Propane pre-cooling cycle

The flowsheet with the Propane pre-cooling cycle inclusion is represented at figure 40.

The propane cascade cycle, is absolutely identical to the one described at sub-section 5.1.1.

The following table presents the propane flow and the five pressure steps of the propane cascade.

Table 13: Main characteristics of the propane cycle.

Propane Flow (kg/s) 465

Initial Pressure (bar) 14.33






Note that the evaporated propane in each pressure stage is sent to a compressor where is

compressed until the initial pressure of 14.33bar.

5.4. Results

As said before, this step of the project was about getting good initial values for the complex approach

presented at section 7. For all the situations the relevant results differs less than 0.1% running with

the PR or GERG EOS and due to that only the results with PR physical properties will be shown and

analysed.

Because liquefaction units are very energy demanding processes, the analysis of the energy

consumption is a benchmark in the LNG industry. Some other relevant results are presented in the

table 14.

The main equipment considered was all the relevant units involved in compressions, expansions or

heat transfer.

Table 14: Simple approach – main results.

Processes

AP-X™ Process AP-X™ alternative Process C3/MR Process

No of variables 2186 1040 1645

No of equations 1858 919 1413

Total CPU time (s) 8 3 4

No of main equipment 33 12 20

NG inlet (kg/s) 118.3 118.3 98.9

LNG Produced (kg/s) 109.7 111 83.7

44

Methane yield (%) 93.8% 93.8% 84.4%

Power consumption (MW) 438 1062 128

SPC (kW-day/tonne LNG) 46 110 17

The simulations for the three LNG processes take less than 10s both with PR or GERG property

methods.

There is a big difference between the AP-X™ processes and the C3/MR process. This is due to the

fact that the inputs for the first were based on their patent [10]

information, while the inputs of the

C3/MR process were based at optimal values from a similar model described at [15]

[16]

[14]

[17]. It is

suspected that the assignments are not adequate to due possible misrepresented information in the

patent.

45

6. Plate and Fin HX Modelling

The models regarding the fluid and materials properties still need to be finished and the geometry

models need to be done fitting the commercial technology of several suppliers.

However, at this point all the major components of the sub-modelling structure is complete, which

means that model is good enough to be applied to liquefaction processes, although, being the

geometry and fluid properties asked as user assignment instead of modelled.

6.1. Model Overview

Figure 41, shows the structure of the PFHX (plate and fin HX) model.

Figure 41: Overview of the PFHX model.

From a gPROMS user point of view, only the top-level model PFHX will be shown. This model is

connected to a fluid property model, which basically allows the choice of the property package to use.

It is clear that the PFHX sub-model decomposition represents the major difficulty of the model, not

only in terms of intrinsic modelling code, but also in terms of connections and connection types.

The connection between models was done by the means of ports. The boxes with grey background

represent the composite models of the PFHX, while the boxes with white background represent

auxiliary models.

The PFHX could be roughly described in the following way:

Drag-and-drop flowsheeting model

Requires specification:

46

Inlet stream properties (hot and cold)

PFHX design and operating conditions

Calculates:

Outlet stream properties (hot and cold)

PFHX performance and operating conditions

Sizing parameters

Multistream/single pass HX

Nc cold, Nh hot streams

One pass per stream

Streams flow co/counter-currently through multiple parallel channels

6.2. Model Description

The figure below represents the topology environment of the PFHX model.

Figure 42: Topology representation of the PFHX model.

In terms of sequence a stream (hot or cold) connected to the model at the flowsheet environment is

also connected to the PFHX model trough a gMLMaterial port. Is then redirected through the

splitter_array to its flow_multiplier, where the flow is divided equally by its number of channels.

After leaving the channels, the divided stream is collected again into a single stream, using the other

flow_multiplier, that multiplies the flow by its number of channels and send the stream to the

mixer_array, where is directed to its respective outlet port.

Note that this procedure is done for all hot and cold streams.

The number of the channels is defined by the user and represents the number of in-between plates

layers related to a certain stream. Assuming all the channels equal among its stream, only one

47

channel per stream must be simulated. The flow_multiplier is used to reproduce the effect of the total

number of channels per stream.

To set the model in a general way that could deal with all the possible hot and cold streams

combination with all the wanted channels per stream, some of the models were grouped in array form

(e.g. Flow_multipers and channel_1D are defined as arrays of cold and hot streams).

At figure 44, all the blue lines represent connections between gMLMaterial ports, while the green lines

represent connections between gMLThermalContact_1D ports. These connections are of the major

important because information about related entities of different models must be fulfilled to ensure the

complete definition of the overall model.

Further information about the composite models will be shown on the following sections.

6.3. Composite Models and Topology

The general topology form of the PFHX model is shown at figure 42, however for a better

comprehension of the models interactivity, is useful to consider a particular case of the PFHX

topology.

Remember that at the general form, to deal with all the combinations of streams and channels, some

models were defined as arrays. This example is a particular form of those arrays (two hot and one

cold streams, with only one channel per stream). So in fact for this case, the flow multiplier model is

not needed because all the streams are sent to singular channels. The channel model doesn’t need to

be written in array form because the number of hot and cold streams are also known and fixed.

The first two left channels of the model represent the hot channels, while the right channel of the

model represents the cold one, see figures 43 to 46.

6.3.1. Mixer_array and Splitter_array

The mixer_array and splitter_array models were created to avoid the definition of the Mixer and

Splitter models in array form. So, in fact, these models represent respectively an array of mixers and

an array of splitters.

48

Splitter_array

represents an array of splitters

each with several inlet streams

several outlets per inlet stream

As specification the splitter_array model allows the definition in mass or mole basis as working mode.

The user should also define split fractions or split flows per inlet stream in relation to different outlet

streams.

Mixer_array

represents an array of mixers

each with several inlet streams

several outlet streams

each being a mixture of a subset of inlet streams

As specification the user should define what inlet streams are mixed together in relation to different

outlet streams.

6.3.2. Channel_1D

Along with the model wall_1D, this is the core of the PFHX model. This model describes one

dimensional mass, momentum intrinsic phenomena, as well as one dimensional heat transfer with the

wall_1D model.

mixer_array

splitter_array

Figure 43: Topology representation of the PFHX model - Mixer_array and Splitter_array.

49

In the 1D heat balance the following factors were considered:

Fluid convection in axial direction

Fluid conduction in axial direction

Fluid to wall convection

In the 1D momentum balance the following factors were considered:

Inlet and outlet fluid momentum

Forces on fluid

Friction on the wall

Note that channel_1D and wall_1D connection is used to pass common information between models

needed to describe the fluid to wall convection term of the heat balance.

This is an internal sub-model of the PFHX, and due to that no specification is required.

6.3.3. Wall_1D

The model wall_1D describes the heat transfer that occurs between channels.

Due to the PFHX compactness and construction is assumed that the heat balance on the

perpendicular direction of the fluid is fast enough so that the profiles of the wall/plates are the same

along all the PFHX.

The wall_1D represents a model of all the walls of the HX. The model describes the heat balance as a

combined effect of all the walls.

Channel_1D

Figure 44: Topology representation of the PFHX model – Channel_1D.

50

In the 1D heat balance the following factor were considered:

Fluid conduction in axial direction

Wall to fluid convection

Thermal inertia effects along axial direction

This is an internal sub-model of the PFHX, and due to that no specification is required.

6.3.4. Heat_Losses_1D

The heat_losses_1D model describes the 1D heat losses through the wall.

Wall_1D

Figure 45: Topology representation of the PFHX model – Wall_1D.

51

6.3.5. Auxiliary Models

Some of these models are in fact the work in progress part of the PFHX model. The following

explanations represent the goals to achieve and not the already implemented work.

Channel_geometry model

The channel_geometry model is a model used to define the geometry of the channel.

Physical_properties model

The physical_properties model determines physical properties of each stream in a distributed way

along the axial direction of the fluid.

Material_properties model

The material_properties model determines physical properties of the wall in a distributed mode along

the axial direction.

Wall_geometry model

The wall_geometry model is the model used to define the plate and fin characteristics, with supplier’s

information.

Flow_multiplier model

The flow_multiplier model is used to multiply or divide an inlet flow by a required input number.

heat_losses_1D

Figure 46: Topology representation of the PFHX model – Heat_Losses_1D.

52

Transport_properties

The transport properties model determines transport properties of each stream in a distributed way

along the axial direction of the fluid.

6.4. Model dialog boxes

One of the most important parts of a model is its dialog box. The dialog box is the interface that the

user sees when doing flowsheet assembling. The dialog is the face of the model.

The PFHX model has a long dialog box, divided into six tabs.

Common to all the tabs is the performance mode of the PFHX. In this dropdown list the user may

choose performance or design mode.

6.4.1. Wall specifications tab

In this tab the user might define the simulation mode, geometric parameters and physical properties.

Figure 47: Dialog box of the PFHX model – Wall specifications.

6.4.2. Hot stream specifications tab

In this tab the user might define the operation mode, geometric parameters, transfer phenomena

coefficients, number of channels per stream and change the available transfer area per channel.

53

Figure 48: Dialog box of the PFHX model – Hot stream specifications.

6.4.3. Cold stream specification tab

This tab is similar to the cold stream specifications tab.

Figure 49: Dialog box of the PFHX model – Cold stream specifications.

6.4.4. Heat losses tab

In this tab the user can define whether assume heat losses or adiabatic operation. Assuming the first,

environmental conditions should be defined.

54

Figure 50: Dialog box of the PFHX model – Heat losses.

6.4.5. Numerics tab

In this tab, the user can select the adequate initialisation procedure for the present working mode.

This issue is particular relevant when the model cannot initialise.

When working in performance mode, the user should choose the “Performance” or “Performance

Robust” initialisation procedures. When working in design mode, the user should choose “Design” or

“Design Robust” initialisation procedures. The robust version of the procedures initialise the model

considering mass balance alone – level one, then energy balance – level two and finally momentum

balance – level three, using a “MOVE_TO” strategy to gradually move between the levels. The default

version of the procedures is similar to the robust however it uses a “JUMP_TO” strategy instead of a

“MOVE_TO”.

The robust version of the procedures is much robust then the default version but also much slower.

Figure 51: Dialog box of the PFHX model – Numerics.

55

6.4.6. Costing tab

In this tab is possible to activate or deactivate the costing mode. If costing mode is activated it is

possible to assume a specific cost value or select a cost function.

Figure 52: Dialog box of the PFHX model – Costing.

Note that this subject is yet a work in progress, in other words, the final structure is not concluded.

Some problems related to initialisation procedures will be solved in a new version of the model.

56


57

7. LNG process modelling – complex approach

The general description presented in the sub-section 5.1 to 5.3 is applicable for this situation. The

only differences to the simplified approach were the introduction of PFHX models as replacement of

MSHX_simple models and the introduction of heat_exchanger models to replace the cooling

equipment models used at compression trains. This last modification was only applied to the AP-X™

process because this issue is only relevant during optimisation where the models should work in

performance mode, which in contrast to the cooler model the heat_exchanger has.

For a better resolution of the flowsheet, see appendix C-1 to C-3.

7.1. AP-X™ Process

As said before, the description presented in sub-section 5.1 is analogous to this case.

In this situation, all the heat transfer models are used in performance mode, i.e. instead of the

assignment of outlet temperatures the user specifies HX areas, heat transfer coefficients [18] [19]

and

flow direction. The outlet temperature of the streams is calculated using the LMTD method.

Figure 53: Topology representation of the AP-X™ process model – complex approach.

For the heat_exchanger models, the input area was obtained in the following way:

1. Define the required outlet temperatures as general input for heat transfer models

2. Run the model in design mode with the previous specification

3. Use the calculated area as new input in performance mode

4. For the first initialisation use the initial outlet temperatures as initial guess

Although being the same process and have a similar flowsheet, the modifications described above

change the results shown at section 5.1. Small changes were observed at the propane, MR and

nitrogen lines, however relevant aspects as refrigerants flows and compositions stay the same.

Regarding the NG-LNG line, the inlet NG has the same conditions presented at table 4.

The final LNG stream has the following characteristics:

58

Table 15: Caracterisation of the stream conditions upstream LNG-2.

LNG outlet Flow (kg/s) 110.04

LNG outlet Temperature (K) 111.50

LNG outlet Pressure (bar) 1.05

Composition (kg/kg)

"NITROGEN" 0.006

"METHANE" 0.872

"ETHANE" 0.057

"PROPANE" 0.034

"2-METHYLPROPANE" 0.010

"BUTANE" 0.013

"2-METHYLBUTANE" 0.003

"PENTANE" 0.002

"HEXANE" 0.003

"HEPTANE" 0.001

The modifications were not significant in terms of LNG production. That is due to the inputs chosen for

the PFHX model. Those were done in a certain way to match the results observed in the simple

approach.

7.1.1. PFHX-1

See in sections below the assignments assumed for the PFHX models. The wall of the PFHX is

assumed to be made of aluminium. Other geometrical characteristics were based on the Linde®’s

brazed-aluminium HX [11]

.

The assignments below described were inserted at the dialog boxes shown at 6.4.

Main inputs

For the wall tab, the inputs are presented in the following table.

Table 16: Wall specifications considered for the PFHX model.

Wall Specifications

Simulation mode Steady-state

Heat capacity 500 J/kg.K [18]

Density 8000 kg/m3 [18]

Thermal conductivity 50 W/K.m [18]

Thickness 5 mm

No of grids 50

59

For the hot streams tab, the inputs are presented on the following table.

Table 17: Hot streams specifications considered for the PFHX model

Hot streams specification

Operation mode Co-current mode

Specification Type Geometry specifications

Cross section 0.1 m

Length 10 m

HTC 4000 W/m2.K

[18]

Fanning factor 0.0001

Number of channels (1) 136


Area per channel 120 m2

For the cold streams tab, the inputs are presented on the following table.

Table 18: Cold stream specifications considered for the PFHX model

Cold streams specification

Operation mode Counter-current mode

Specification Type Geometry specifications

Cross section 0.1 m

Length 10 m

HTC 4000 W/m2.K

[18]

Fanning factor 0.0001


Area per channel 120 m2

Other specifications used in other parts of the model.

Table 19: Other specifications considered for the PFHX model

Other specifications

Mode Performance calculation

Adiabatic mode On

Costing Pre-defined cost estimation

Price per sq. meter 12 US$/m2 [20]

Main results

It is important to understand the capabilities of the PFHX models. The following representations are

some, among other results that can be achieved using the 1D model. It is possible to know, at the

exact point of the HX, pressure, temperature or vapour fraction conditions of fluids inside the

channels.

The following profile is a temperature profile inside the PFHX-1.

60

Figure 54: Temperature profile - PFHX-1.

Note that the cooling curve of the NG-LNG stream matches the warming curve of the LP (low-

pressure)-MR, due to a high HTC and a right composition used for the refrigerant.

It is possible to observe that the major part of the transferred phenomena happens between the NG-

LNG and LP-MR streams. That is due to the higher number of channels, therefore area, of those two

streams compared to the HP (high pressure)-MR stream.

The following profile represents the pressure profile of the LP-MR stream.

Figure 55: LP-MR pressure profile - PFHX-1.

Note that the curve assumes a linear profile. Remember that the flow enters at position 1, which

means that the stream is in fact losing pressure. The pressure drop is a consequence of the fluid

partial evaporation.

The amount of momentum lost is however negligible due to the low speed of the fluid inside the

channels and also to a low Fanning factor coefficient assumed for the case.

Different work conditions might result on a similar profile with a higher order of magnitude.

168

178

188

198

208

218

228

238

0 0,2 0,4 0,6 0,8 1

Te

mp

era

ture

(K

)

Channels axial position (z)

LP-MR

NG-LNG

HP-MR

Wall

0,979992

0,979993

0,979994

0,979995

0,979996

0,979997

0,979998

0,979999

0,98

0,980001

0 0,2 0,4 0,6 0,8 1

Pre

ss

ure

(M

Pa

)

Channel axial position (z)

61

The following profile represents the vapour-fraction change of the two MR streams inside the PFHX-1

Figure 56: Vapour-fraction profile - PFHX-1

Analogously to the previous cases, that the LP-MR enters at the position 1 of the channels, which

means that is evaporating along the channels while the HP-MR is condensing. As predicted, it is

possible to observe that most of the heat transferred in the HX is latent heat.

The NG-LNG stream is supercritical at the working conditions of the HX, therefore, its vapour-fraction

has no physical meaning and cannot be represented.

7.1.2. PFHX-2

The inputs assumed for the PFHX-1, see sub-section 7.1.1, were also used in this case. The changes

are presented below.

Main inputs

For the hot streams tab, the inputs are the following.

Number of channels (1) = 10

For the cold streams tab, the inputs are the following.


There are only a cold and a hot stream entering this HX.

Main results

The following chart represents the temperature profile inside the PFHX-2.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,2 0,4 0,6 0,8 1

Va

po

ur-

fra

cti

on


Cold-MR

warm-MR

LP-MR

HP-MR

62


The fluids exchange heat in a counter-current distribution. At this stage of the process, there is only

sensitive heat being exchange. In the case of the nitrogen, the fluid is in vapour state since its inlet

position (position 1) until its outlet position (position 0), while he NG-LNG stream enters at 0, in liquid

form and is sub-cooled until its outlet position 1.

Note that the curves follow a non-straight typical counter-current behaviour due to the non-linearity of

the Cp function.

The following profile shows the pressure drop of the LP-nitrogen inside the cold channels.

Figure 58: LP-nitrogen pressure profile - PFHX-2.

The nitrogen is dropping momentum because is warming, hence decreasing density and therefore its

volumetric flow is higher. Since the cross section by channels is constant the velocity inside the

channels is also increasing. The wall friction factor is negligible.

110

120

130

140

150

160

170

180

0 0,2 0,4 0,6 0,8 1

Te

mp

era

ture

(K

)


Nitrogen

NG-LNG

Wall

1,047

1,0475

1,048

1,0485

1,049

1,0495

1,05

1,0505

0 0,2 0,4 0,6 0,8 1

Pre

ss

ure

(M

Pa

)

Channel axial position

63

7.1.3. PFHX-3



Main inputs






This HX is prepared to work with two cold and one hot stream.

Main results

The following chart represents the temperature profile inside the PFHX-3.


The chart above shows that the cooling curve of the nitrogen assumes a parabolic behaviour.

This behaviour is intrinsically related to the enthalpy function, predicted by the PR method.

It seems that the increase of channels, and therefore available transfer area, would not influence the

outlet temperature of the HP-nitrogen stream since the cooling curve is approaching a temperature

step, near the outlet temperature.

The following profile shows the pressure drop of the LP-MR inside the cold channels.

167

177

187

197

207

217

227

237

0 0,2 0,4 0,6 0,8 1

Te

mp

era

ture

(K

)


LP-MR

LP-nitrogen

Wall

HP-nitrogen

64


Note that the streams enters at position one and loses pressure until its outlet position. The pressure

drop is a result of the stream partial evaporation. Note that the Fanning factor is assumed as a

constant value of 1E-4, resulting in negligible effects due to friction in the wall.

Other aspects as inlet-outlet Δz, or other gravitational effects were not considered.

7.2. AP-X™ alternative Process

As for the previous case, the description presented in sub-section 5.2 is analogous to this case.

Except the for the PFHX models, all the assignments of the simple approach were also used for this

case.

Figure 61: Topology representation of the AP-X™ alternative process model – complex

approach.

0,97986

0,97988

0,9799

0,97992

0,97994

0,97996

0,97998

0,98

0,98002

0 0,2 0,4 0,6 0,8 1

Pre

ss

ure

(M

Pa

)


65

Again, the assignments of the PFHX model were done in a way to match the results of the simple

approach.

Regarding the NG-LNG line, the inlet NG as the same conditions presented at table 8.

The final LNG stream has the following characteristics:

Table 20: Caracterisation of the stream conditions upstream LNG-2.

LNG outlet Flow (kg/s) 110.10

LNG outlet Temperature (K) 111.7

LNG outlet Pressure (bar) 1.05

Composition (kg/kg)

"NITROGEN" 0.005

"METHANE" 0.862

"ETHANE" 0.057

"PROPANE" 0.035


"BUTANE" 0.014


"PENTANE" 0.002

"HEXANE" 0.005

"HEPTANE" 0.004

7.2.1. PFHX-1

The inputs assumed for the PFHX-1, see sub-section 7.1.1 were also used in this case. The changes


Main inputs






Main results


66


Several effects mentioned on previous points are also valid for this analysis. It should be pointed out

the fact that the cooling curves appear to show slight temperature steps. This is due to the

predominance of latent heat into the overall heat transferred. This might also indicate that a cascade

cooling technology would perform better at some parts of the temperature range.



Note that some points of the curve reveal some disturbance effects. This is only a numerical issue

that would be solved with the increase of number of grids used into the model discretisation. Previous

pressure drops effects explained above, are analogous for this situation.

The following profile represents the vapour-fraction change of the two MR streams inside the PFHX.

9,799965

9,79997

9,799975

9,79998

9,799985

9,79999

9,799995

9,8

9,800005

0 0,2 0,4 0,6 0,8 1

Pre

ss

ure

(P

a)

x 1

00

00

0


167

187

207

227

247

267

287

0 0,2 0,4 0,6 0,8 1

Te

mp

era

ture

(K

)


MR-expanded

NG-LNG

Wall

MR-warm

LP-MR

NG-LNG

Wall

HP-MR

67

Figure 64: Vapour-fraction profile - PFHX-1.

The vapour-fraction profiles show that from position 0 to position 0.6 of the channels, no latent heat is

being transferred. This means that or MR composition or MR flow or both are not adequate for the

cooling range of the HX.

7.2.2. PFHX-2



Main inputs





There are only a cold and a hot stream entering this HX.

Main results

The profiles shown at 7.1.2 regarding the AP-X™ process are identical to their similar of the present

process. The conclusions are also the same.

7.2.3. PFHX-3

This HX is composed by two cold and one hot stream.

Main inputs





0

0,2

0,4

0,6

0,8

1

1,2

0 0,2 0,4 0,6 0,8 1

Va

po

ur-

fra

cti

on


LP-MR

HP-MR

68


Main results

The profiles shown at 7.1.3 regarding the AP-X™ process are identical to their similar of the present

process. The conclusions are also the same.

7.3. C3/MR Process

As for the previous cases, the description presented in sub-section 5.3 is analogous to this case.

The guidelines explained in 7.2 are the same for the present situation.

Figure 65: Topology representation of the C3/MR process model – complex approach

Regarding the NG-LNG line, the inlet NG has the same conditions presented at table 11.

Due to the modifications explained above, and assuming the PFHX inputs described below, the final

LNG stream has the following characteristics:

Table 21: Caracterisation of the stream conditions upstream LNG-2

NG outlet Flow (kg/s) 81.02

NG outlet Temperature (K) 112.23

NG outlet Pressure (bar) 1.05

Inlet Composition (kg/kg)

"NITROGEN" 0.003

"METHANE" 0.867

"ETHANE" 0.063

"PROPANE" 0.037


"BUTANE" 0.013


69

"PENTANE" 0.001

"HEXANE" 0.002

"HEPTANE" 0.001

7.3.1. PFHX-1



Main inputs






Main results



The temperature profile presented above shows some slight temperature steps. This might represent

a bad MR composition. Note that the MR used for the present LNG process is a mixture of four

components, while the MR used for the AP-X™ processes contained six components.

The increase of components with different saturation conditions inside the NG-LNG cooling range

would reduce the bumps of the temperature profile and make the cooling and liquefaction steps

smoother.


129

149

169

189

209

229

0 0,2 0,4 0,6 0,8 1

Tem

pe

ratu

re (

K)


MR-expanded

NG-LNG

Wall

MR-warm

LP-MR

NG-LNG

Wall

HP-MR

70


The pressure drop of the fluid is explained by the friction in the wall and its partial evaporation.

The following profile represents the vapour-fraction profile inside PFHX-1.

Figure 68: Vapour-fraction profile - PFHX-1.

The flat profile of the HP-MR may suggest that the refrigerant flow used for the cooling is too high.

However most of the heat transferred in the HX is not removed from the HP-MR but from the NG-LNG

stream, not represented in the chart. In fact, the LP-MR is losing latent heat from its inlet until its

outlet, which shows that the flow is not abusively high.

7.4. Results

For all the situations the relevant results differ less than 0.1% running with the PR or GERG EOS,

therefore, only the results with PR physical properties will be shown and analysed.

0,43995

0,43996

0,43997

0,43998

0,43999

0,44

0,44001

0 0,2 0,4 0,6 0,8 1

Pre

ss

ure

(M

Pa

)

Channel axial position

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,2 0,4 0,6 0,8 1

Va

po

ur-

fra

cti

on


Cold-MR

warm-MR

LP-MR

HP-MR

71

Table 22: Complex approach – main results

Processes

AP-X™ Process AP-X™ alternative Process C3/MR Process

No of variables 10134 8588 4446

No of equations 8228 6955 3672

Total CPU time (s) 50 44 158

No of main equipment 33 12 20

NG inlet (kg/s) 118.3 118.3 98.9

LNG Produced (kg/s) 110.4 110.1 81

Methane yield (%) 94.4% 93.0% 82.3%

Power consumption (MW) 381 1146 130

SPC (kW-day/tonne LNG) 40 120 19

Compared to the approach described in section 5, the CPU effort is higher due to the complexity of

the models. Note that AP-X™ is a recent process and unfortunately its modelling activity among the

scientific community is still very scarce. In consequence the assignments used for the different

processes are based in different literature sources, complicating their comparison.

Regarding the C3/MR process, results are an outcome of a mix between the PML:SS models (used

as composited models of the top-level flowsheet) with results from [15]

(used as assignments of the

process). In fact, they represent a 20% deviation (in terms of specific power consumption - SPC) in

relation to the OMAN-LNG plant [21]

. This value could potentially be reduced with the performing of a

steady-state optimisation.

Regarding the AP-X™ and the AP-X™ alternative process, the results observed are too distant from

industrial reports. It is not possible to compare this model to any industrial case. Possibly patent

information used as assignments in the models was very inadequate for the situation.

Due to this is useless to compare them with any other industrial records. The AP-X™ detailed

analyses will be done further on, see section 8 and 9.

72


73

8. AP-X™ Process NLP optimisation

Before optimisation takes place some decisions have to be made:

Costs to consider for the overall capital cost of the plant.

Capital costs amortisation type and duration.

Operational costs to include into the analysis.

Decision factors as plant location and year of operation.

After solving the above describe issues, it is necessary to find that information in the literature and

add that data to the costing section of the models whose capital and operational costs should be

considered.

In terms of capital costs, the following equipment costs were considered for the overall fixed cost of

the plant:

Kettle HX cost

Shell and tube HX cost

PFHX cost

Centrifugal compressors capital costs

Expander capital cost

For the equipment amortization it was considered a linear amortisation during the 10 years life-time

for the plant. The costs were update to the year of 2012.

As highly dependent plant of refrigeration cycles, the power consumption cost has a major role for the

operational cost of the plant. Electricity cost is the variable considered for this case.

It was assumed that the plant was located in the south part of the state of Texas near the Gulf of

Mexico. The operation year is 2012.

All the data related to capital costs were taken from the following references:

PFHX: [22] [20] and [19]

Other equipment: [19]

The operational cost information was assumed as an average electricity price from several US

companies.

Note that some extra variables and equations were added to the model. These variables will be used

as constrains or objective functions in each optimisation case. One of those variables is for the total

cost, which adds the operational cost of the plant and the depreciated value of the capital cost in

relation to 2012, in an hourly base status. Variables were also created to control and avoid the cross-

over inside heat transfer equipment. See in the appendix D-1 the relevant equations created for the

optimisation procedure.

8.1. Methodology

As told in 7.1 all the AP-X™ process models are defined in performance mode. This mode allows the

definition of design entities as heat transfer area.

Performing the optimisation stage with the models in design mode, would allow the change of design

entities of the model during the procedure. This is not correct when assuming the optimisation of an

74

existing plant. Design characteristics of equipment are fixed and are not replaceable, which means

that the performance mode assignment in the models is a mandatory condition.

Further on in the optimisation stage, the design variables of the equipment will also be changed, but

keeping the performance mode of the models activated and assuming those as control variables.

The procedure taken follows an initial assumption that the optimisation is being applied to an existing

plant, which means that only operational conditions should be improved.

After finding the optimal performance solution that leads to the minimum total cost of the plant

controlling only operational variables, an overall optimisation case will be done to find the optimal

solution controlling operational and design variables of the LNG model.

See below the schematic representation of the optimisation stage:

Figure 69: Schematic representation of the optimisation procedure.

The details of each optimisation will be presented on the following sections. For all cases, the

objective function is the plant total cost, analysed in an hourly base.

The cases are constrained in the following way:

Minimum LNG production obtained on the base case

No cross-over in the heat exchange equipment

Minimum vapour fraction of 99% at the entrance of the compressors

8.2. AP-X™ process – base case

The base case is the starting point of the optimisation stage. This model is the exact model described

in point 7.1, with the inclusion of the equations and variables mentioned above.

The process presents a total hourly based cost of 24980 $/h.

Overall Optimisation

Refrigerants flow + composition + design and operational specifications

Performance optimisation

Refrigerants flow + composition

Refrigerants flow optimisation

MR, N2 and C3 flow

Sensitive analyses

AP-X™ process - base case

Performance and costing mode on

75

In the following table is possible to observe the base case values of the variables that were optimised

in the further steps.

Table 23: Base case results – refrigerants flow.

Refrigeration cycle Propane MR Nitrogen

Flow (kg/s) 1176 1254 314

Table 24: Base case results – MR composition.

Component Composition (%m/m)

"NITROGEN" 0.038

"METHANE" 0.479

"ETHANE" 0.413

"PROPANE" 0.002


"BUTANE" 0.040

Table 25: Base case results – pressures (see figure 53).

Expansion device MR-3 MR-4 N-2

Outlet Pressure (bar) 9.8 9.8 10.5

Table 26: Base case results – areas (see figure 53).

Intercooler Area (sq. m)

MR compression train intercooler 760

Nitrogen compression train intercooler 176



Propane compression train intercooler 8971

8.3. Sensitive analyses

The sensitive analyses step was result of a handmade simplified sensitive analyses, regarding a

combination of Propane and MR flows that would improve the total cost of the plant, meeting the

constrains mentioned above using a trial and error approach. Note that from this case and beyond, an

extra HX will be added for the nitrogen compression train.

The following results were achieved.

Table 27: Sensitive analyses results – refrigerants flow.

Refrigeration cycle Propane MR

Flow (kg/s) 815 800

In the figure bellow it can be observe the evolution of the control variables from the base case to the

present optimisation stage.

76

Figure 70: Sensitive analyses results.

As result the total cost was reduced to 19059 $/h.

Only with a sensitive analyses a 24% reduction was observed in the total cost of the plant, showing

that the base case conditions have room for improvement.

8.4. Refrigerants flow optimisation

From this step, all the optimisations were done using the optimiser tool of the gPROMS model builder.

The optimisation interface is a user-friendly tool that requires the definition of an objective function (to

maximise or minimise), control variables and a set of constrains.

Note that the optimisation steps were made in sequence, this means that the previous optimisation

case is the starting point for the next one.

For the present situation, the control variables are the flows of the three refrigeration cycles use in the

process.


Table 28: Refrigerants flow optimisation results.


Flow (kg/s) 500 507 310

In the figure bellow it can be observe the evolution of the control variables from the base case to the

present optimisation stage.

800

850

900

950

1000

1050

1100

1150

1200

1250

Base case Sensitive analyses

Flo

w (

kg/s

)

Propane

MR

77

Figure 71: Refrigerants flow optimisation results.


The flow optimisation represents a 50% cost reduction of the base case. This is indeed the major

portion of the optimisation stage. This is expected because the total cost major quote is due to energy

consumption of the compression trains which is closely related to the refrigerants flows.

8.5. Performance optimisation

Starting from the previous case, the total cost function is minimised, controlling the refrigerant flows

and the MR composition.

This is the standard optimisation procedure used for LNG refrigeration cycles, especially the ones

with mixed refrigerants.

The ideal composition has an important role with regards to the efficiency of the process, namely to

deal with the required cooling range and to match the LNG cooling curve.


Table 29: Performance optimisation results – refrigerants flow.


Flow (kg/s) 464 429 295

In the figure bellow it can be observe the evolution of the flow control variables from the base case to

the present optimisation stage.

0

200

400

600

800

1000

1200

Base case Refrigerants flow optimisation

Flo

w (

kg

/s)

Propane

MR

Nitrogen

78

Figure 72: Performance optimisation results – Flows.

This optimisation stage also included the MR composition as control variable beside the refrigerants’

flows.

Table 30: Performance optimisation results – MR composition.


"NITROGEN" 0.1199

"METHANE" 0.3181

"ETHANE" 0.5333

"PROPANE" 0.0125


"BUTANE" 0.0002

In the figure bellow it can be observe the evolution of the composition control variables from the base

case to the present optimisation stage.

0

200

400

600

800

1000

1200

Base case Performance optimisation

Flo

w (

kg

/s)

Propane

MR

Nitrogen

79

Figure 73: Performance optimisation results – MR composition.

As results the total cost was reduced to 10740 $/h.

The present optimisation represents a 57% cost reduction of the base case.

8.6. Overall optimisation

The last optimisation step regards an overall optimisation of the main variables associated to the

refrigeration cycles.

Beside the flow and composition of the refrigerants, the following operational and design variables are

optimised:

Intercooler’s area for the compression trains

Expansion pressure of nitrogen and MR cycles


Table 31: Overall optimisation results – refrigerants flow.


Flow (kg/s) 451 424 294

Table 32: Overall optimisation results – MR composition.


"NITROGEN" 0.1177

"METHANE" 0.3115

"ETHANE" 0.5384

"PROPANE" 0.0159


"BUTANE" 0.0003

0

0,1

0,2

0,3

0,4

0,5

0,6

Base case Performance optimisation

Co

mp

osi

tio

n (

kg/k

g)

"NITROGEN"

"METHANE"

"ETHANE"

"PROPANE"

"2-METHYLPROPANE"

"BUTANE"

80

Table 33: Overall optimisation results – pressures (see figure 53).

Expansion device MR-3 MR-4 N-2

Outlet Pressure (bar) 10.2 10.2 10.05

Table 34: Overall optimisation results – areas (see figure 53).

Intercooler Area (sq. m)

MR compression train intercooler 751




Propane compression train intercooler 8964


The overall optimisation represents a 58% cost reduction of the base case.

8.7. Results

The optimisation results show a clear positive evolution of the objective function. The overall

optimisation step show that the major optimisation parcel was already achieved in the previous steps,

namely during flow optimisation.

Figure 74: Representation of the optimisation results in absolute and relative terms.

The total cost of the optimised case has the following distribution:

0

10

20

30

40

50

10000

12000

14000

16000

18000

20000

22000

24000

base case sensitiveanalyses

flowoptimisation

performanceoptimisation

overalloptimisation

To

tal

Co

st

red

uc

tio

n (

%)

To

tal

Co

st

($

/h)

Plant Total Cost

Total Cost reduction

81

Figure 75: Representation of the contributions for total cost.

It is observed that the major part of the cost associated to the plant is due to energy consumption, as

expected for a liquefaction plant.

The operational cost assumes an even major role because some factors associated to the capital cost

depreciation might be undervalued, namely, the investment amortisation type, interest rate, fabrication

type, among others. The capital cost is the sum of all equipment costs, excluding transportation.

From the chart below is possible to see the capital costs distribution by equipment.

Figure 76: Representation of the contributions for capital cost.

In resemblance to the operational costs distribution, the major portion is associated to the

compression train capital costs. Note that this value would be reduced with optimisation of the

compression train. Issues like number of compressors, optimal pressure rations and number of

parallel compression lines were not evaluated, the information used was based in [7].

Operational Cost 94%

Capital Cost depreciation

6%

Expander 0%

Kettle-HX 3%

Centrifugal Compressor

92%

Intercoolers 2%

PFHX 3%

82


83

9. AP-X™ Process Results

The following table presents the main results associated to the AP-X™ optimised process.

Table 35: AP-X™ Process – final results.

AP-X™ Process

No of variables 10127

No of equations 8224

Total CPU time (s) 22 (with Variable Set)

NG inlet (kg/s) 118.3

LNG Produced (kg/s) 116.3

Methane yield (%) 99.3

Power consumption (MW) 158.8

Specific power consumption (kW-day/tonne LNG)

15.8

Total Cost ($/h) 10543

From the values shown, it is clear the improvement of the results from the point 5.4, to 7.4 until the

final optimised model.

The optimisation not only improved the energy consumption of the plant, but also improved the LNG

produced flow, 3.66 MTPA, as well as its richness in methane. It is possible to observe that 99.3% of

the inlet methane was recovered as LNG.

One of benchmark of the LNG industry is the analysis of the specific power consumption associated

to a process.

See below typical values for different industrial applications of the LNG processes [21]

.

Table 36: Specific power consumption values associated to LNG technologies.

year Technology Specific power consumption (kW-day/tonne LNG)

1963 Cascade Process 20.8


1970 C3/MR 21.6

1975 C3/MR 19.2

1982 Prico 14.6

1995 C3/MR 12.9

1996 C3/MR 15.8


2003 C3/MR 12.5


It is possible to observe a clear efficiency improvement during the last years. However, the specific

power consumption is closely related with NG composition, environmental conditions, inlet NG

conditions and LNG required composition, making LNG processes’ comparison, often a rough task.

The lack of information about AP-X™ industrial cases added to the issues cited above limit a proper

comparison. Therefore, the optimised results can only be compared to C3/MR values.

84

According to Foster Wheeler [23]

, is possible to standardise the specific power consumption by LNG

technology in the following way:

Table 37: Standard values of specific power consumption

Process SMR C3/MR DMR

Specific power consumption (kW-day/tonne LNG) >13.5 >12.2 >12.8

Being, SMR a single mixed refrigerant process and DMR a double mixed refrigerant process.

It is possible to see that the specific power consumption of the optimised AP-X™ process fits the

usual values observed in industrial cases and show in the tables above.

Note that the execution of an optimisation procedure and posterior discussion for the C3/MR instead

of the AP-X™ would make the thesis richer, since the C3/MR is a consolidated process with several

modeling and optimising procedures with a lot of comparison models. However from my personal and

PSE’s point of view it was better to perform the project in this way because it was a pioneer approach

benefic in terms of business perspective.

85

10. Conclusions and Future work

The literature review showed that the LNG universe is a fast growing world that represents a major

role in terms of worldwide energy consumption. An old, present and future problem related to the NG-

LNG chain is the physical properties prediction. Note that there are endless NG sources with several

compositions, which according to each industrial environment will end up as LNG products with

unique characteristics. This unpredictability associated to the NG-LNG chain make the physical

properties prediction a task almost impossible to achieve in a general and overall way.

During decades the APCI®-C3/MR process has dominated the LNG industry due to its simplicity and

high production capacity. Since the new millennium the liquefaction technologies just step-up, keeping

up the LNG growing rhythm. The struggle for the best technology is being intense but the APCI®

processes are still, by far, in the lead.

For both simple and complex approach, the easy and user-friendly process assembling allows a fast

top-level composition of the LNG process models, giving the user the opportunity of caring only with

the engineering part of the work and letting all the mathematical work for the software and model

developers.

In the simple approach the C3/MR process fits the typical values for industrial cases. This is due to

the inputs of the C3/MR process that were based in optimal values from a similar model described at

[15], [16], [14] and [17].

The advanced modelling of the 1D PFHX model is a stepped-up for PSE’s liquefaction processes

including the present ones. During all the literature review related to the PFHX model, it was clear that

most part of the models published in the general literature were too simplistic: (i) assume no phase

change either for hot and cold fluids, (ii) assume constant physical properties along exchanger length.

In general it was observed simplified modelling approaches for easier optimisations. A SEM type

model was mandatory for this project, similar to the ones developed by Aspen® (Aspen plate fin

exchanger™) and Linde® (GENIUS™).

The complex approach with the inclusion of the new PFHX increases the complexity of LNG models,

becoming this approach a reliable representation of industrial environments with small computational

effort. In terms of flowsheeting assembling the difficulty was nil, since the MSHX_simple model and

the new PFHX had the exact same port structure, in opposition almost 90% of the time was spent in

initialisation procedures to face the endless numerical issues related to the models’ swap.

Unfortunately, the results observed are still far from the industrial standard values due to the lack of

good industrial data and to the uncertainty of the patent information often used as assignments.

After optimisation the AP-X™ process presents a SPC that fits the industrially observed values,

showing that with the right combination of correct assignments the model would be validated.

Personally, this project was a very rich and embracing moment of my short professional life that

included; (i) Advanced flowsheeting of LNG processes, (ii) Advanced modelling of PFHX model and

(iii) Contribution for training courses material presented in the company. It was clear that the main

difficulty of the modeling approaches taken was related to numerical issues and lack of data, since

75% of the time spent was related to initialisation procedures, physical properties, recycle/loop

breaking and lack of process information.

86

For the future, great part of the work will be related to the development and upgrade of the PFHX

model: (i) Generalisation of the PFHX initialisation procedure, (ii) Inclusion of property and geometry

models for the PFHX (iii) Improvement of the PFHX design mode.

After the conclusion of the PFHX model, an industrial cooperation should be made in order to get

correct and rigorous assigns to validate the LNG process models.

87

11. Bibliography

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for the Snøhvit LNG Plant", LNG 13 Conference, Seoul, May 2001.

[2] T. Shukri, “LNG Technologies and the important criteria for selection", Hydrocarbon Engeering,

Vol. 9, No. 2, Feb. 2004.

[3] The International Group of Liquefied Natural Gas, “Information Paper No. 1 — Basic Properties of

LNG”, The International Group of Liquefied Natural Gas, 2009.

[4] O. Kunz and W. Wagner, “The GERG-2008 Wide-Range Equation of State for Natural Gases

and”, Journal of chemical & Engineering Data, Vol. 57, pp. 3032-3091, 2012.

[5] D. Y. Peng and D. B. and Robinson, “A New Two-Constant Equation of State”, Industrial and

Engineering Chemistry: Fundamentals, Vol. 15, p. 59–64, 1976.

[6] M. J. Saeid Mokhatab, "Process selection is critical to onshore LNG economics", World Oil, Vol.

227, No. 2, pp. 95-99, 2006.

[7] R. A. Mark Julian Roberts, "Hybrid cycle for the production of LNG", US Patent No 6,308,531,

2011.

[8] J. P. Pierre-Yves Martin, "NATURAL GAS LIQUEFACTION PROCESSES COMPARISON",

AICHE Spring National Meeting, 2003.

[9] S. A. Nezhad, B. Shabani and M. Soleimani, “Thermodynamics Analysis of Liquified Natural Gas

Production Cycle in APCI Process”, Journal of Thermal Science, Vol. 21, pp. 564-571, 2012.

[10] M. J. Roberts, “Large Capacity Single Train AP-X Hybrid LNG Process”, in Gastech 2002 - Qatar.

[11] AG, Linde, “Plate-Fin versus Wound-Coil heat exchangers”, Linde AG, Pullach, Germany, 2009,

from http://www.linde-engineering.com/, [Accessed 02 09 2013].

[12] “http://www.psenterprise.com/”, Process Systems Enterprise. [Online]. [Accessed 17 09 2013].

[13] F. D. Nogal, J.-K. Kim, S. Perry and R. Smith, “Optimal Design of Mixed refrigerant cycles”, Ind.

Eng. Chem. Res., Vol. 47, pp. 8724-8740, 2008.

88

[14] H. Saffar and M. Faihbeiki, “Modelling and Optimization of a C3MR LNG Plant efficiency by

change of mixed refrigerants' components”, International Journal of Heat and technology, Vol. 28,

No. 2, pp. 69-76, 2010.

[15] M. Wang, J. Zhang and Q. Xu, “Optimal design and operation of a C3MR refrigeration system for

natural gas liquefaction”, Computers and Chemical Engineering, Vol. 39, pp. 84-95, 2012.

[16] P. Hatcher, R. Khalilpour and A. Abbas, “Optimisation of LNG mixed-refrigerant processes

considering operation and design objectives”, Computers and Chemical Engineering, Vol. 41, pp.

123-133, 2012.

[17] A. Alabdulkarem, A. Mortazavi, Y. Hwang, R. Radermacher and P. Rogers, “Optimization of

propane-precooled mixed refrigerant LNG plant”, Applied Thermal Engineering, Vol. 31, pp.

1091-1098, 2011.

[18] R. K. Sinnott, Coulson and Richardson's Chemical Engineering, Oxford:Elsevier, 2005.

[19] W. D. Seider, J. D. Seader, D. R. Lewin and S. Widadgo, Product and Process Design Principles,

Synthesis, Analysis, and Evaluation, Asia: John Wiley & Sons, Inc., 2010.

[20] Bryan Research and Engineering, Inc., “Process simulation and optimisation of cryogenic

operations using multi-stream aluminium exchangers”, 2006.

[21] Foster Wheeler Energy Limited, “Selecting offshore LNG processes”, LNG journal, pp. 34-36,

2005.

[22] Bryan Research and Engineering Inc., “Advantage of brazed heat exchangers in the gas

processing industry”, 2006.

[23] Foster Wheeler, “Enhanced single mixed refrigerant process for stranded gasliquefaction”, Foster

Wheeler Energy Limited, Reading.

[24] D. L. Katz, Handbook of Natural Gas engineering, New york, Toronto, London: McGraw-Hill book

company inc. , 1959.

[25] L. L. Faulkner, Fundamentals of Natural Gas Processing, Taylor and Francis Group, LLC, 2006.

89

[26] “World LNG report 2011”, International Gas Union (IGU), 2011.

[27] “World LNG report 2010”, International Gas Union (IGU), 2010.

[28] Michael Barclay, Noel Denton , “Selecting offshore LNG processes”, LNG journal (Foster

Wheeler Energy Limited), pp. 34-36, 2005.

[29] Ullmans, Section 3 - liquefaction part, 2012.

[30] L. Castillo, M. M. Dahouk, S. D. Scipio and C. Dorao, "Conceptual analysis of the precooling

stage for LNG processes", Energy Conversion and Management, Vol. 66, Feb. 2013, pp. 41–47.

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91

Appendices

Appendix A - 1: Representation of process diagram from the patent that preceded the AP-X™ process.

92


93

Appendix B - 1: Topology representation of the AP-X™ process model – simple approach.

94


95

Appendix B - 2: Topology representation of the alternative AP-X™ process model – simple approach.

96


97

Appendix B - 3: Topology representation of the C3/MR process model – simple approach.

98


99

Appendix C - 1: Topology representation of the AP-X™ process model – complex approach.

100


101

Appendix C - 2: Topology representation of the AP-X™ alternative process model – complex approach.

102


103

Appendix C - 3: Topology representation of the C3/MR process model – complex approach.

104


105

Appendix D – 1: List of equations used in the optimisation

Objective Function

( )

Being the totalcost the variable that describes the hourly based total cost of the plant, considering

operational costs and hourly based capital costs.

totalcost

Being the Opercost and the Capcost respectively the variables that describe the operational cost and

hourly based capital cost of the plant.

Opercost

∑ ( )

Being the electricityconsumption the electricity consumed in each centrifugal compressor of the plant.

Capcost

∑ ( )

The Capcost represents the sum of the cost of the main equipment presented in the plant. The value is

amortised to an hourly base, considering 10 years of lifetime and 330 working days per year.

ΔTcontrol

( ( ) )

Being the ΔT(i) an array of all the HX’s ΔT present in the plant. The ΔTmin should be defined by the

user.

advanced steady-state modelling and optimisation of lng

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