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NATURAL GAS DEHYDRATION IN OFFSHORE RIGS : COMPARISON BETWEEN TRADITIONAL GLYCOL PLANTS AND

INNOVATIVE MEMBRANE SYSTEMS

F.Binci1,a*#, F.E. Ciarapica1,b, G.Giacchetta1,c

1Department of Energetics, Faculty of Engineering, University of Ancona via Brecce Bianche, 60131, Ancona, Italy

aTel. +39 338 3149012; Fax +39 71 2804239; email: [email protected] bTel. +39 (0) 712204435; Fax +39 71 2804239; email: [email protected]

cTel. +39(0) 71 220 4763; Fax +39 71 2804239; email: [email protected] Abstract

Natural gas dehydration is a treatment that removes most of the water vapor content before forwarding the gas to pipelines. For decades, the most widely-used technology has been to absorb the water with a liquid solvent flowing in countercurrent inside bubble cap trays columns. In the last ten years, research has been developing new dehydration systems based on selective membranes in order to lower plant costs, increase separation efficiency and reduce emissions. The present work draws a technical and economic comparison between traditional glycol absorption plants and an innovative system using polymer membranes. The membrane system proved more cost-effective for low feed gas flow rates and more environment-friendly. 1. Introduction

Dehydration treatment to remove water vapor from natural gas is needed to prevent hydrate formation and pipeline acid corrosion, and to guarantee compliance with the dew-point required by the standard specifications.

About 95% of existing offshore installations currently use TEG (triethylene glycol) technology, where dehydration is achieved inside an absorber column where natural gas bubbles in countercurrent through a highly hygroscopic solvent (i.e. triethylene glycol) capable of absorbing the water molecules contained in the gas.

The use of membranes for natural gas dehydration began only ten years ago and these systems are still in the experimental stage. For the time being, there are just a few onshore installations, while nothing is known about their use offshore: all the information available come from experimental data and from the few pilot plants working onshore.

Such installations are nonetheless likely to be the next step in natural gas dehydration technology. Their large-scale application will solve most of the old problems related to TEG plants, particularly reducing the hazardous environmental effects of BTEX (benzene, toluene, ethyl benzene, xylene) and VOCs (volatile organic compounds) emissions. The experimental results obtained make this technology in the field of offshore plants even more attractive and cost-effective than for onshore plants because membrane modules enable major savings in terms of the weight and size of the installations, leading to considerable platform CAPEX (capital expenditure) savings as a result.

2. Plant description

For the analysis of traditional dehydration plants, we considered a TEG system with bubble cap trays column.

The novel membrane plant taken for comparison is based on the use of PRISM® polymer membranes made into hollow fiber modules by AirProducts.

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The gas transport performances of this membranes, in terms of permeability and stability, are shown in fig.1 and fig.2 [1].

These devices have been used at the ENI’s Hera Lacinia onshore gas field (CR – Italy) working on a feed gas rate of 600,000 Sm3/day1 and the plant layout at said installation was considered for the present analysis (fig.3)[2].

The most widely-known advantages of the membrane technology over TEG plants are as follows:

− no emissions are released into the atmosphere and no solvents are used; − a reduction in the size and weight of the installations. 3. Background hypothesis for the comparison

The economic assessment compared traditional TEG plants with bubble cap trays versus PRISM® polymer membrane plants on the assumption of the following: − feed gas rates varying between 500,000 and 4,500,000 Sm3/day; − 20-year plant lifetime; − upstream conditions: water-saturated methane gas at a pressure of 30 bar and a

temperature of 30°C; − downstream conditions: dehydrated methane gas at a pressure of 70 bar and a dew point

of -10°C. The TEG plants considered were classified on the strength of the operating conditions

shown in Table 1. TEG reference scheme Case Description

TEG HPC (High pressure column)

HIGH-PRESSURE (70 bar) COLUMN: the gas has to be compressed and cooled before entering the column.

LT LOW-PRESSURE COLUMN (30 bar) and LOW-TEMPERATURE (30 °C) FEED GAS : the gas directly enters the column.

TEG LPC (Low pressure column)

HT LOW-PRESSURE (30 bar) COLUMN and HIGH-TEMPERATURE (50 °C) FEED GAS: the gas is cooled to 30°C, then enters the column.

Table 1 TEG plant reference schemes

1 Sm3/day = Standard Cubic Meters per day. (Standard Conditions : T = 15°C and P = 101325 Pa.)

10001000 10,00010,0001001001010110.10.10.010.01

HH22OOHH22COCO22OO22NN22CHCH44

Fig. 2 PRISM® membrane permeability Fig. 3 Membrane plant layout

DDrryy NNaattuurraall GGaass

PPrriissmm®® DDeehhyyddrraattoorr

SSwweeeepp

PPeerrmmeeaattee

NNaattuurraall GGaass FFeeeedd

CCoooolleerr

LLiiqquuiiddss

HHeeaatteerr

PPeerrmmeeaattee ccoommpprreessssoorr

FFeeeedd GGaass CCoommpprreessssoorr

Fig. 1 PRISM® membrane stability

94,00

96,00

98,00

0 200 400 600 800Days of Operation

H2O

% R

emov

al

3

All the TEG plants considered operate at an internal temperature of 30°C. In the only case LPC we examined the further condition of a feed gas temperature of 50°C with the aim to evaluate the economic effects of a cooling cycle before the dehydration column.

The membrane system operates at 70 bar and 50°C, with a permeate gas flow rate of 7% of feed gas flow rate at a pressure of 2,5 bar. Depending on the predicted membrane module lifetime, three alternatives were considered : − Alternative 1 : no membrane replacement during the 20-year lifetime; − Alternative 2 : one membrane replacement after 10 years; − Alternative 3 : two membrane replacements, after 7 and 14 years.

All the typical expenditures were analyzed for the above-described systems, defining the CAPEX and operating expenditure (OPEX) of each installation. Then the items were used as input data for the economic evaluation of the various technologies using the NPV (Net Present Value) and a 5% discount rate. 4. CAPEX comparison between TEG and membrane plants

In comparing the CAPEX, we analyzed the expenses incurred for the construction and installation of the various systems. We studied the direct cost items (costs of separation equipment, ancillary equipment and installation) and the indirect cost items (engineering and allocated platform cost). The cost of separation and ancillary equipment was obtained directly from manufacturers. The installation costs were evaluated as a percentage of the amount of separation and ancillary equipment costs (35% and 25% for TEG and membrane plants respectively). The engineering cost was evaluated as a percentage of 15% of the amount of direct costs [8]. The results for direct CAPEX are shown in fig. 4.

The analysis attributed fundamental importance to the assessment of the allocated platform cost, which represents the cost of the part of platform occupied by the dehydration plants, because the sizes and weights of the systems being compared were very different and their reduction should always be pursued. The coefficients used in the analysis are shown in Table 2. The results, processed on the strength of data from specialist companies and the existing literature, are shown in fig. 5 [3] [4] [5] [6] [7].

The analysis of the allocated platform cost (depending on the feed gas rate) shows that membrane plants enable a considerable saving in terms of the weight and size of the installation for low feed gas rates. As the plant’s size grows, there is a corresponding growth in the number of membrane modules that have to be installed in parallel, leading to a greater increase in the size and weight of the

Platform Cost Weighting Factor

Area 7310 $/m2 70% Weight 3215 $/1000Kg 30%

Table 2 Platform cost coefficients

Fig. 5 Allocated platform cost

0,00,10,20,30,40,50,60,70,80,9

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Alloc

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MEMBRANETEG

Fig. 4 Direct CAPEX

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TEG LPC-HT MEMBRANE

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membrane skid than in the case of a TEG plant. So, in terms of allocated platform cost, the advantage of the membrane plant is considerable for small platforms, where the space available for dehydration systems is limited. This advantage decreases as the feed gas flow rate grows until it reaches the break even point for a feed gas flow rate of 4.200.000 Sm3. Over this value of feed gas flow rate TEG plants become advantageous (fig. 5).

As for the total CAPEX, by comparison with the traditional TEG plant, the initial outlay for a membrane plant is (fig. 6): − lower only for feed gas rates up to 850,000 Sm3/day; − comparable, depending on the operating conditions, between 850,000 and 1,600,000

Sm3/day; − higher for feed gas rates over 1,600,000 Sm3/day.

The greater rise in the CAPEX for larger membrane systems versus TEG plants is due to the need to buy larger numbers of dehydration modules the higher rate of the feed gas being processed. Direct and indirect cost items, for a feed gas flow rate of 1,500,000 Sm3/day are shown in fig. 7.

5. OPEX comparison between TEG and membrane plants

As for the OPEX, we analyzed the expenditure involved in operating the plant properly throughout its lifetime. We studied the following cost items: − manpower; − chemical agents; − routine maintenance; − extraordinary maintenance; − energy.

The calculation of the first three items was made with reference to the hypothesis shown in Table 3 and the results obtained are shown together in fig. 8 [8]. Membrane systems were found to enable an evident saving on the whole range of feed gas flow rates considered, with a greater absolute advantage the higher the feed gas flow rate.

Fig. 6 CAPEX

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CAPE

X [M

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TEG HPCTEG LPC-LTTEG LPC-HTMEMBRANE

Fig. 7 CAPEX cost items

Feed gas flow rate : 1.500.000Sm3/day

02468

101214

TE

G H

PC

TE

G L

PC-L

T

TE

G L

PC-

HT

ME

MB

RA

NE

CA

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Allocated platform costEngineering costInstallation costSeparation and ancillary equipment cost

Fig. 8 Manpower, current maintenance and chemical agents costs

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Man

pow

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TEG HPC

TEG LPC-LT

TEG LPC-HT

M EM BRANE

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(Manpower + Chemical Agents + Routine Maintenance)TEG 10% of (Direct CAPEX + Engineering) (Manpower + Chemical Agents + Routine Maintenance)MEMBRANE 4% of (Direct CAPEX + Engineering)

Table 3 Manpower, chemical agents and routine maintenance costs calculation

Conversely, for the cost of the energy needed to work the compressors, the analysis showed that the presence of the permeate compressor makes the new membrane systems clearly more expensive than TEG plants over the whole feed gas flow rate range considered, with a gap that widens as the feed flow rate increases (fig. 9). The energy cost was calculated assuming to use gas-driven type compressors and evaluating their natural gas consumption.

The extraordinary maintenance costs were assessed by calculating the expenses incurred at regular intervals for absorber and still column inspections by auditing authorities (for the TEG plants) and for the periodical replacement of the dehydration modules assuming the progressive loss of efficiency declared by the manufacturer (for the membrane plants). The extraordinary maintenance of membrane modules was assumed to be negligible during the whole lifetime of the module. 6. Economic evaluation of the alternatives through NPV method (Net Present Value)

Applying the NPV method to the above-illustrated costs enabled us to compare traditional TEG plants and new membrane systems on the assumption of different membrane module lifetimes. We analyzed the economic advantage of each of the three previously-mentioned membrane alternatives vis-à-vis each TEG configuration variable with the operating conditions of the plant. Membrane - Alternative 1

This always proved better than all possible TEG configurations for feed gas flow rates varying between 500,000 and 2,200,000 Sm3/day. Between 2,200,000 and 3,350,000 Sm3/day it appears to be less cost-effective than the TEG LPC-LT, but more so than the TEG HPC or TEG LPC-HT. Between 3,350,000 and 4,350,000 Sm3/day it was only better than the TEG LPC-HT. Over 4,350,000 Sm3/day it was never more cost-effective than any of the TEG system configurations (fig. 10). Membrane - Alternative 2

This was always a good choice vis-à-vis any of the TEG configurations for feed gas flow rates varying between 500,000 and 1,600,000 Sm3/day. Between 1,600,000 and 2,500,000 Sm3/day it was less cost-effective than the TEG

Fig. 10 NPV membrane alternative 1

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Fig. 9 Energy cost

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LPC-LT, but more so than the TEG HPC or TEG LPC-HT. Between 2,500,000 and 3,250,000 Sm3/day it was only better than the TEG LPC-HT. Over 3,250,000 Sm3/day it was never better than any of the TEG plants (fig.11). Membrane - Alternative 3

This alternative was always more cost-effective than the TEG configurations for feed gas flow rates varying between 500,000 and 1,000,000 Sm3/day. From 1,000,000 to 1,900,000 Sm3/day it lost its edge over the TEG LPC-LT, but was still better than the TEG HPC and TEG LPC-HT. Between 1,900,000 and 2,400,000 Sm3/day it still performed better than the TEG LPC-HT. Over 2,400,000 Sm3/day it was always less cost-effective than any of the TEG plants (fig.12). 7. Conclusions

The economic assessment showed that membrane dehydration plants could be better value

for money than the TEG only depending on predicted membrane lifetimes, on feed gas flow rates and on the operating conditions of the plant.

Assuming the most likely membrane life span is 10 years, selective membrane systems can only claim to be the most cost-effective technology for offshore rigs of limited dimensions (processing up to 1,600,000 Sm3/day), while they definitely offer no economic advantage for medium to large rigs (over 3,250,000 Sm3/day). As for all the possible situations falling within these limit conditions, the advisability of installing a dehydration plant based on selective membrane modules needs to be evaluated case by case, in the light of the operating conditions that the system will be required to work with.

References [1] WILLIAM M. POPE Jr. : “AirProducts – Natural Gas Dehydration Sales Presentation

January 2001” – personal correspondence. [2] ENI S.p.a. − DIVISIONE AGIP : “Impianto per la disidratazione del gas naturale

mediante membrane polimeriche da installarsi presso la centrale gas “Hera Lacinia” nel comune di Crotone – Relazione tecnico-illustrativa e prevenzione incendio” , 2001.

[3] S. De Donno, S. Biagi, G. De Ghetto : “Disidratazione gas naturale con sistemi a membrane selettive – Studio di pre-fattibilità per un impianto da 600.000 Sm3/giorno per la Centrale di Hera Lacinia” – AGIP-RIIN, 1997.

[4] C. Richard Sivalls : “Glicol dehydration design” – Laurance Reid Gas Conditioning Conference, University of Oklahoma, 25-28 February 2001.

[5] ENI – AGIP E&P Division Research & Development Department : personal correspondence.

[6] SiiRTEC NIGI S.p.a. : personal correspondence. [7] Michael P. Quinland, Linda W. Echterhoff, Dennis Leppin, Howard S. Meyer : “Cost-

cutting for offshore sulfur recovery process studied” – Oil & Gas Journal, 21 July 1997. [8] R. D’Agostini, M. Calvarano, L. Ciccarelli : Progetto M.E.D.E.A. – MEmbrane di

Disidratazione Estensione Applicabilità − “Comparazione tecnico economica tra impianti di disidratazione gas naturale con TEG e membrane” Relazione N° TEIM-052-20, 1998.


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