1996: linde ammonia concept - iffcokandla.in

Linde Ammonia Concept

The Linde Ammonia Concept (LAC) can achieve 20% cost savings compared to the conventionalprocess. It requires less equipment, less instruments, and less foundation materials.

J. lig and Bernd KaedzioraLinde AG, Wiesbaden, Germany

Introduction

The LAC plant (Figure 1) consists essentially ofa hydrogen unit with only a PSA unit for purifi-cation of the syngas, a standard nitrogen unit

adding nitrogen where it is needed and an ammoniasynthesis unit.

Compared with the conventional process route, theLAC process has the following important features(Figure 2):

• Elimination of three catalytic process steps, reduc-ing the total catalyst volume to approximately 50% ofthat in a conventional plant.

• Provision of a reformed gas purification system bypressure swing adsorption (PSA), which has a provenand unmatched reliability.

• The generation of inert-free synthesis gas, resultingin significant savings in the synthesis loop, and elimi-nating a purge gas purification step.

• Recovery of CO2 is possible through an additionalwashing unit.

• The process condensate treatment is eliminated by

routing the condensate directly back to the isothermalshift reactor for production of process steam.

9 The simplified flow sheet also results in an overallreduced pressure drop.

• The LAC is a much more direct route to ammoniaresulting in a reduced startup time and important sav-ings in feedstock consumption.

9 An overall simplification of the classical processroute, resulting in savings in investment costs, con-struction time, site area, maintenance and spare partscosts as well as catalyst replacement costs.

® Pure hydrogen and pure nitrogen are directly avail-able from process streams. Other potential byproductssuch as oxygen, argon, carbon dioxide, carbon monox-ide, and methanol can be easily integrated (Figure 2).

Concept

The LAC (Figure 3) consists essentially of a modernhydrogen plant, a standard nitrogen unit, and a highefficiency ammonia synthesis loop. The basic gas gen-eration unit is a modern hydrogen plant with proven

AMMONIA TECHNICAL MANUAL 341 1997

and outstanding reliability. The success of this hydro-gen process can be judged by the fact that today prac-tically no new hydrogen plant is built with the conven-tional route including HT- and LT-shift, CO2 wash andraethanation.

Yet, in the modern conventional ammonia processthe classical hydrogen generation route is employedwith the addition of a secondary reformer. It is inter-esting to consider why modern hydrogen technologyhas not been applied more often for the production ofammonia.

Up to now, the major application for PSA basedhydrogen plants is in refineries. It seems that mostengineering contractors active in the field of ammoniatechnology tried rather to improve the classical routethan to go for completely new solutions.

The pure hydrogen from the PSA unit is mixed withpure nitrogen from a standard air separation unit(ASU) to give inert-free ammonia synthesis gas. Thesynthesis gas is fed to a high efficiency ammonia syn-thesis loop based on the Ammonia Casale axial-radialflow converter.

The comparison of the process steps in the LAC andin the conventional process shows clearly the reduc-tion in process steps which is achieved by LAC.

The comparison of the temperature profiles throughthe process steps (Figure 4) demonstrates even to agreater extent why there are savings in investment costwith LAC. The number of temperature changes is lessand the temperature levels are lower than in the con-ventional process. Furthermore, the flow in the con-ventional plant through the secondary reformer and allthe downstream steps is considerabley higher than inthe LAC plant due to the addition of process air to thesecondary reformer.

The trend in the modern conventional process is toincrease the duty of the secondary reformer, whichincreases the flow rate due to the excess air and thusincreases the size of downstream equipment. In theLAC process, the nitrogen is introduced where it isneeded: just upstream of the ammonia synthesis.

Process Description

The numbers in the following paragraphs refer to thenumbers shown in Figure 5.

Desulfurization

The feedstock is heated and flows through a desulfu-rization (1) reactor. If nonreactive sulfur compoundsare present in the feed, these are first hydrogenatedusing a small stream of recycle hydrogen. The hydro-gen sulfide is removed from the feed by passingthrough a bed of zinc oxide catalyst.

Primary reformer

Reformer Feed Preheating (2). The desulfurizedfeedstock is mixed with steam produced in the isother-mal shift reactor (5), supplemented by makeup fromthe steam and power system (17) and preheated in themixed feed preheater.

Reforming (3) (and Figure 6). In the top-firedreformer, the feedstock is reacted with steam over anickel-based catalyst in high-alloy reformer tubes.

The steam reforming reactions produce a reformedgas consisting largely of hydrogen and carbon monox-ide.

Process Gas Cooler (4). The reformed gas leavesthe reformer at about 850°C and is cooled before inletto the isothermal shift reactor by generation of saturat-ed steam. Potential problems in steam superheating byprocess gas are avoided.

Flue Gas Duct (9). The flue gas from the radiantsection of the reformer furnace is used to preheatreformer feed, generate superheated HP steam andpreheat combustion air, before being passed by aninduced draft fan via a stack to the atmosphere.

Isothermal shift

Isothermal Shift Reactor (5). The carbon monoxidein the reformed gas is converted to hydrogen by theshift reaction:

The isothermal shift reactor is a Linde developmentin which the catalyst bed is kept at a constant tempera-ture of about 250°C by a spiral- wound cooling bundle.Inside the tubes of the bundle, process condensate cir-culates producing steam for the reforming process.


HydrocarbonFeed

H2 Unitwith

PSA Purification

N2-Unit

CM

1£L

CM

£I

AmmoniaSynthesis

Loop

Atmosph.Air

Ammonia

Figure 1. Combination of proven technologies.

Linde Ammonia Concept (LAC)Comparison of LAC process with conventional scheme

Feed

Air

Feed

CO2

IsothermalShift

NH3

Air

Figure 2. Possible valuable byproducts.


Heat Recovery and Condensate Separation (6).Further cooling of the process gas takes place withheat recovery by various streams including processcondensate. The separated process condensate is recy-cled to the isothermal shift reactor to generate processsteam. No process condensate treatment unit isrequired.

Pressure swing adsorption (PSA)

Pressure Swing Adsorption (7). The PSA unit con-sists of a number of adsorbers between 4 and 12,depending on the size of the plant and other opera-tional aspects. The process gas passes through theadsorbers, thus being purified up to 99.999 mol%hydrogen. Meanwhile, the loaded adsorbers are regen-erated using a controlled sequence of depressurizationand purging steps.

Fuel System (8). Swings in the composition andpressure of the tail gas produced in the PSA unit areleveled out by means of a buffer drum, thus allowing asteady supply of fuel gas to the reformer furnace.

Nitrogen unit

Nitrogen Production (10). High purity nitrogen (1ppm O2) is produced by low temperature separation inthe air separation unit (ASU). Air is filtered, com-pressed and purified before being fed to the ASU cold-box. The pure nitrogen product is further compressedand mixed with hydrogen to give ammonia synthesisgas of stoichiometric composition.

Ammonia synthesis

Synthesis Gas Compressor (11). Synthesis gas andrecycle gas are compressed to synthesis pressure forsupply to the ammonia converter.

Gas-Gas Interchanger (12). Synthesis gas is pre-heated to the ammonia converter inlet temperature byheat exchange with converter exit gas.

Ammonia Converter (13) (and Figure 7). Theammonia converter is the high efficiency AmmoniaCasale design, with axial-radial flow beds and inter-bed exchangers.

Heat Recovery (14). The hot gas leaving the ammo-nia converter is used to generate HP steam.

Product Separation (15). By cooling and refrigera-tion of the converter exit gas, the ammonia product isliquified and separated. Unconverted gas is recycledby the synthesis gas compressor (11). Traces of impu-rities in the synthesis gas (e.g., argon) leave the loopdissolved in the liquid product. No purge gas needs tobe taken from the loop.

Utility systems

Product Chilling (16). The ammonia product isflashed to low pressure and pumped to consumers orstorage. The flash vapors are condensed in the refrig-eration unit (19).

Power System (17). Superheated HP steam is used togenerate power for drives in the ammonia plant. Somesteam is extracted as makeup to the process. All steamcondensate is sent to the condensate system (18).

Condensate System (18). Steam condensate from theplant is collected and, together with makeup BFW, isdeaerated for supply to steam generators.

Ammonia Refrigeration Unit (19). The refrigerationunit is powered by energy generated within the plant.

Special Features

Three sections of the LAC are exclusive Lindedeveloped technology, and are introduced to the fertil-izer industry through the LAC plant for GSFC with1,350 mtd capacity of ammonia (Figure 11). These arethe Linde isothermal shift reactor, the Linde PSA unit,and the Linde air separation unit.

The Ammonia Casale Converter represents an equal-ly important contribution to this advanced technology.

Linde isothermal reactor

This reactor (Figure 8) is used for the CO shift reac-tion, and allows conversion to below 0.7% CO (drybasis) in a single step. Thus, the conventional series ofHT and LT shift reactors with heat recovery betweenthe beds is replaced by a single reactor with built-insteam generation.

Furthermore, the process condensate is far less


Conventional Ammonia Plant

Feed

AirNH3

Downstream this pointthe flowrate of a conventionalplant is 30 to 80% highercompared to the LAC-prooess

Cost related facts:• number of temperature changes• temperture levels• flowrate• number of equipment and catalyst

Efficiency related facts:• heat exchange losses• pressure drops

N2-PioducllonPurgegas Separation

Figure 3. Comparison of LAC process withconventional scheme.

f«El

till

:> UWQKU

Figure 4. Comparison of LAC process with conventional scheme.


contaminated than in the conventional process withsecondary reformer and HT shift. A small amount ofmethanol is present, but after degassing, the conden-sate is used to generate process steam. Thus, theprocess condensate treatment unit in the conventionalplant is eliminated.

The Linde isothermal reactor is now proven in tenplants worldwide for which the following characteris-tics have been demonstrated:

• The temperature profile in the reactor correspondsto kinetic ideal conditions and gives optimum catalystperformance.

« The isothermal mode of operation produces theleast possible stress on the catalyst and increases itslifetime.

• The operation temperature of the reactor is regulat-ed by simple control of the steam pressure.

• Under all operating conditions including startupand partial load, the water cycle ensures absolute tem-perature stabilization.

• Simple and rapid catalyst reduction without dangerof overheating.

• A startup heater is not necessary. The catalyst bedis brought to temperature by steam injection to thewater circuit.

• The reactor concept enables the largest possibleamount of catalyst to be accommodated per unit reac-tor volume.

Linde pressure swing adsorption (PSA)

Linde PSA technology is widely used for hydrogenpurification duties. Capacities of about 112,000 Nm3/hhydrogen product at 1 ppm CO purity, suitable for a1,350 MTD ammonia plant, require a 12 bed system.Systems of this type, such as the UKW plant inGermany (Figure 9), have now demonstrated over 10years of trouble-free operation.

The extremely high reliability of the PSA systemresults from the use of high quality equipment and thecompletely automatic switching and monitoring of theunit performance by a programmable logic controller(PLC). Should a disturbance occur in any of the oper-ating sequences, the faulty item is identified and takenoff-line, without interrupting the supply of product. A12 bed system can switch automatically to operation

on a lower number of beds with only a slight reductionin efficiency, but maintaining quality of hydrogen.This proven automatic control system gives 100%availability in Linde PS A units.

Furthermore, the PSA unit is able to produce purehydrogen directly from reformed gas, so that distur-bances in the front-end steps would not interrupt pro-duction. This is in sharp contrast to the conventionalplant where failure of the CO shift or CO2 removalcause a high temperature trip on the methanator andcomplete plant shutdown.

Equally, the startup is much faster, since pure hydro-gen is produced as soon as reformed gas is available.

Linde air separation unit

The Linde air separation unit is a state-of-the-art lowtemperature separation process such as is used world-wide for the production of industrial gases. As theinventor of the air separation process, Linde does notneed to emphasize its expertise in this field. Figure 10shows a simplified flow sheet for such a unit.

Ammonia Casale axial-radial flow converter

This converter is a development of the classical radi-al flow type (Figure 7).

In the Ammonia Casale converter, as opposed to thepure radial flow converter, the catalyst bed has no topcover, and some gas enters axially. The amount of gasflowing axially is controlled by proper design of theperforation pattern, and is such that the "seal" catalystworks under similar conditions to the catalyst in theradial flow part of the bed (Figure 7).

The Casale design therefore obtains the maximumperformance from the catalyst volume charged in theconverter.

In addition to the process design advantages of theCasale converter, the mechanical design features givethe following advantages for operation and mainte-nance compared with competitive designs:

9 All proven catalysts can be used.* High thermodynamic efficiency due to: the pres-

ence of three beds; optimal gas distribution in catalystbeds; maximum utilization of catalyst volumes.

• Maximum utilization of converter vessel volume.


Linde Ammonia Concept (LAC)Linde Isothermal Reactor

Steam

Circulating Water

f) O O. «Ü U, " "' • . . . ~' ' / • ' / - ' s '

~*^ "<^ l ' il .

Boiler Feed Water

Gas Entry

Circulating Water | N Gas Exit

Figure 5. Simplified process flow diagram.

Figure 6. Section view of top-fired reformer.


ITable 1. LAC Comparison of Catalyst Volumes 1,350 MTPD NH3

DesulfurizatiçnPrimary ReformerSecondary ReformerHT ShiftLT ShiftMethanationAmmonia Synthesis

Total

Conventional Plant

24.836.035.170.596.527.092.3

382.2

LAC Plant

28.037.0

52.0—

83.9

200.9

Table 2. LAC Comparison of Equipment Items

ExchangersVesselsColumnsReactorsBig Machines(Compressors, TurbinesGenerator)PumpsOther MachinesAir Separation UnitInert Gas UnitPurge Gas SeparatorPSA Unit

Totals

Conventional Plant

6025886

3511—11

—

155

LAC Plant

3419454

25121

——1

105


Linde Ammonia Concept (LAC)Nitrogen Generation Unit (Cryogenic)

:.̂ ï...

iTrif

Figure 7. Ammonia Casale 3 bed converter.

Linde Ammonia Concept (LAC)Possible valuable By-Products

CO2 CO H2

HydrocarbonFeed

Methanol

| CO-Recovery

H2 Unitwith

PSA Purification

t

AmmoniaSynthesis

Loop

Atmosph.Air

Rare Ar O2Gases

N2

Figure 8. Linde isothermal reactor.


Axial-Radial Flow Concept

Figure 9. PSA plant for the production of hydrogen.

Figure 10. Nitrogen generation unit (cryogenic).


Figure 11. General overview (1,350 MTPD NH3).

• Maximum mechanical reliability of cartridges.• Use of "rod-baffle" internal heat exchangers to

improve heat transfer and to eliminate vibration prob-lems.

Valuable byproducts

Valuable byproducts can be produced either directlyfrom process streams in the LAC plant, or by simpleintegration of the necessary facilities (Figure 2).

High purity hydrogen and nitrogen are producedwithin the LAC process, and flows from both thesestreams can be made available for other consumers ifrequired.

The potential for production of oxygen, argon, andrare gases from the air separation unit can be easilyintegrated at the design stage.

For CO2 byproduct, Linde has selected the MDEAwash process licensed by BASF for several recentplants. This high efficiency CO2 wash step is includedin the LAC plant being built for GSFC in India.

According to the CO2 demand of downstream con-

sumers, the gasflow to the absorber column is con-trolled with a bypass.

The installation of a cold box for CO product, or ofa methanol synthesis and distillation unit, can be easi-ly integrated with the LAC process.

Operating Aspects

The LAC meets the target of 7 Gcal/NH3 for modernammonia plants in steady operation. This figure, validfor moderate climates, includes utilities and also theenergy required for the N2 unit, which representsapprox. 0,33 Gcal/t (less than 0,1 Gcal/t NH3 on top ofthe power for conventional secondary air compres-sion).

The LAC is a much more direct route to ammoniathan the conventional process (Figure 3). This resultsin rapid startup and consequently important savings infeedstock consumption. The elimination of manyhours of unproductive feedstock consumption per yearhas an important benefit for the actual achieved con-sumption per ton of product. Further, the annual pro-


auction capacity is increased by rapid startup.The advantages of LAC from the operators point of

view arise from the reduction in the number of processsteps, and the ability of individual parts of the plant tooperate independently. For example:

• The nitrogen unit (ASU) can be operated to gener-ate inert gas (and other products) for other consumers,when the ammonia plant is shutdown.

» The isothermal shift reactor can be preheated bysteam injection to the water circulating through thecoils.

• The PSA unit can produce pure hydrogen directlyfrom reformed gas, without the CO shift or the CO2-wash. The startup of the PSA unit is automatically per-formed by the PLC.

• The startup of the ammonia synthesis loop cantherefore proceed about 2 h after feed is introduced tothe reformer, a time which is unmatched by any otherammonia process.

Cost Savings

Detailed studies made during the proposal stage ofthe LAC process for GSFC in India showed clearly thecost savings which are achieved with the LACprocess.

The absolute figures of this project are, however, notsufficiently representative for the U.S. market condi-tions to be presented with this article.

Comparisons with the conventional process showedcost reductions of approximately 20% in the LACprocess for

• Fewer catalytic steps resulting in 50% less totalcatalyst volume (Table 1).

9 Reduced number of equipment items (Table 2).• Reduced piping quantity.• Reduced number of control loops and instruments.• Less structural steel.• Less foundation materials.The total weight of the plant, and the site area

required (Figure 11), show similar reductions com-pared to the conventional process.

Note: Table 2 is simplified such that each caseincludes two packages not detailed in the items.

Conclusions

Linde has already earned a special position in thefertilizer industry through the introduction of the highpressure gasification route to ammonia, designed andbuilt by Linde for the first time in the world at theGNFC site hi India.

The GNFC 1,350 mtd ammonia plant based onheavy residue has now achieved over ten years ofrecord production, and the process is recognizedworldwide as the accepted route to ammonia fromheavy feedstocks.

In comparison to the LAC process, the high pressuregasification process introduced challenges of anextremely high technical nature. It is confidentlyexpected that, within the decade, the LAC process,which is a combination of proven process steps, willbecome the established route to ammonia from lighthydrocarbon feedstocks.


1996: linde ammonia concept - iffcokandla.in

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