Operational Flexibility of theLCA Process

The LCA concept uses an innovative approach to decoupling theprocess steps and eliminating the interactions and interdependen-cies. It enhances plant safety and reliability.

T.C. Hicks and A. PintoICI Chemicals & Polymers Ltd., Billingham, Cleveland TS23 1LB, England

M.J.S. MossICI Chemicals & Polymers, Hallen, Bristol BS10 7SJ, England

Ammonia production from hydrocarbon feedstocksinvolves four major processing steps :

1 Feed pretreatment

2 Steam and air reforming of hydrocarbons

3 Gas purification

4 Ammonia synthesis

With conventional ammonia technology,high energy efficiency is only achievable bycomplex and integrated heat recovery betweenthese processing steps. This energyintegration leads to extended plant start-uptimes and major operational and controlproblems.

The LCA concept on the other hand usesan innovative and imaginative approach todecouple the process steps and eliminate theinteractions and interdependencies.

This paper describes in detail the LCAconcept for ammonia production anddemonstrates how this novel design approachresults in considerable benefits andenhanced operational flexibility.* rapid start-up

* reduced start-up energy requirements andcosts

* faster, more efficient shut-downprocedures

* minimised gaseous venting during start-upor under upset conditions

* flexibility to operate efficiently atreduced rates

* considerably reduced effluents

* simplified control

By separating into a 'core' unit onlythose key operations essential to makingammonia and incorporating steam/powergeneration, refrigeration and C02 recoveryinto a separate 'utilities', the LCA conceptoffers not only excellent opportunities forefficient site integration of a new plant, butalso effective expansion and modernisation ofexisting ammonia plants.

PROCESS DESCRIPTION

A flowsheet of the ICI Severnside LCAplant is shown in Figure (1).

Natural gas feed is mixed with recyclehydrogen, heated and desulphurised. It isthen cooled by preheating the feed to thedesulphuriser before passing to a feed gassaturator where it is contacted withcirculating hot process condensate supplying90% of the process steam. The feed gas fromthe saturator is mixed with a further quantityof steam to give a steam to carbon ratio ofabout 2.5 and preheated by the reformed gas

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stream. Reactants enter the primary reformerwhich operates with an exit temperature of7ÛÛ to 75CTC (1300 to 1400°F) at a pressure of30 to 45 bar. The gas mixture is then fed toa secondary reformer for further reformingwith an excess of process air. Effluent gasis cooled by providing the heat for theprimary reforming reaction and preheating thereactants. The cooled reformed gas is shiftedin a single, isothermal, water cooled, lowtemperature shift converter.

After shift conversion, the gas is cooledby direct contact with circulating processcondensate and then fed to a pressure swingseparation unit to remove the excess ofnitrogen, C0?and part of the i nerts. Aproportion of the available COp is recoveredfrom the pressure swing separation waste gasusing an aqueous solution of tertiary amine.At Severnside the C0„ is sold by ICI as anindustrial gas.

The gas leaving the pressure swingseparation unit is methanated, cooled anddried before entering the ammonia synthesisloop at the circulator suction. In thesynthesis loop, gas from the circulator isheated and passed over low pressure ammoniasynthesis catalyst to produce ammonia.

The hot gas leaving the ammoniaconverter is cooled by generating 60 barsteam (the first point in the process wheresteam is raised) and by heating the feed gasto the converter. Ammonia is separated fromthe partially cooled gas by vaporisingproduct ammonia. The unreacted gas isreturned to the circulator.

Argon and methane are removed from thesynthesis loop by taking a purge andrecycling back to the synthesis gas generationsection as feed.

MAJOR PROCESSING STEPS

More detailed consideration will now begiven to the individual processing steps andin particular the benefits accruing from theLCA concept approach.

Feed Pretreatment

Hydrodesulphurisation of the feednatural gas is carried out at a temperaturebelow 250°C (500°F), which is readilyachievable using process steam.

Conventional HDS systems requiretemperatures in excess of 370°C (700 F),which is not possible with steam heating and

has to be carried out using a source of heatat elevated temperatures. The most commonapproach is to use the primary reformer fluegas and this energy integration introducesconstraints on plant start-up which causesboth delays and energy wastage. Using thePURASPEC low temperature desulphurisationprocess gives significant capital costsavings but also allows a fast and energyefficient start-up of the HDS independentlyof the reforming section. (PURASPEC is anICI trademark.)

Steam and Air Reforming of Hydrocarbons

Primary reforming of steam andhydrocarbon is an endothermic reaction withthe heat requirement traditionally beingprovided by the thermodynamically inefficientcombustion process. Here typically only 50%of the heat generated is transferred toreaction, leaving the remainder as wasteheat which has to be recovered through a costintensive and thermodynamically inefficientsystem.

Air addition and secondary reforming bycontrast is an extremely exothermic reactionoverall and subsequent waste heat recoveryfrom the effluent gas at around 1000 C(1830 F)is also achieved by similarly expensive andinefficient power steam generation.

In the LCA concept, by contrast, the heatgenerated in the secondary reforming reactionis utilised to provide the endothermic heatrequirement of the primary reforming reactionby direct heat exchange in a tubular gasheated reformer (GHR). By changing thedominant heat transfer mechanism from radiantto convective, a compact pressurised GHRprovides the optimum capital and energyutilisation solution to what was previouslythe most cost intensive and energy inefficientprocessing step in ammonia production.

Start-up of a conventional reformingsection is a lengthy process requiring a numberof steps, many linked to commissioning of theplant steam system, which result in excessivefeed and fuel wastage. A LCA plantGHR/secondary reforming section can be broughton line in a single step operation, and withprocess air the only source of heat for theGHR, feed gas, air and steam are introducedtogether to the secondary reformer to providethe GHR heat requirements.

A further feature is that the absence ofthe fired heater multi-burner reformereliminates the need to stabilise operation at

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high throughputs during start-up to achievegood feed distribution between the tubes andavoid overheating by careful burner control.There is thus no possibility of thecatastrophic melt-downs experienced in anumber of tubular reformers on start-up and aGHR can be safely operated at as low as 15%of flowsheet rates while remaining processequipment is commissioned.

This simultaneous GHR/secondary reformerstart-up method combined with much reducedrates greatly reduces the time to come online and the energy wasted by venting.

There are also improved safety aspectsaround design and operation of the LCAreforming section. Because the GHR operateswith a very low differential pressure acrossthe tubes, stresses are very low compared tothose in a conventional reformer and creeplife of the tubes is probably longer than theplant design life. In the very unlikely eventof a tube failure, some gas will pass directlyto the secondary reformer effluent resultingin a small increase in methane slip from thereforming section. As the shellside gas isoxygen free, there is no danger of a leakfiring in the GHR and a tube failure will havelittle effect compared with a leak in a firedfurnace.

Loss of hydrocarbon feed to a conventionalreformer requires a trip of process air andreduction in primary firing. In the LCAprocess feed and air supply are interlinkedand failure of either automatically trips theother, resulting in a safe plant shut down.With hot re-start achieved in only 2 to 4hours, the LCA process provides safer andfaster recovery from plant upsets than anyother operating process.

Gas Purification

Use of robust and self-adjusting processunits in this section of a LCA plant makes itmuch less sensitive to catalyst performanceand process upsets.

The single isothermal shift convertereliminates the need for an expensive controlsystem to cope with plant disturbances byensuring the catalyst temperature is alwaysabove the raw syngas dew-point, whilesimultaneously providing total protectionagainst excessive temperatures. The reactionheat is transferred directly to the saturatorcircuit, contributing to the provision ofprocess steam for reforming. This singlestage efficient isothermal shift concept

greatly reduces start-up time requiredcompared with conventional two (andoccasionally even three) stage shift systems.

Also the ability to utilise a coppercatalyst in this duty enables energy efficientreforming at low steam : carbon ratio (2.5),without formation of hydrocarbons by Fischer-Tropsch reactions which can occur overconventional iron-based high temperatureshift catalysts.

The pressure swing separation unitperforms a number of functions

COp removal

control of N2:H2 ratio by removal ofexcess nitrogen

removal of a proportion of CO, CH», Ar

and overall this makes the process very muchless sensitive to performance of the up-streamcatalysts. It can also be fully automated andrequires much less time to bring on line thanthe wet CO2 removal systems and cryogenicexcess nitrogen rejection systems which havebeen eliminated. It also cushions thesynthesis area of the plant from feed gasdisturbances.

Ammoni a Synthes i s

ICI has considerable experience with lowpressure (80 bar) ammonia synthesis and whenapplied in the LCA process the main additionaladvantage is the reduced start-up powerrequirement. In this case the synthesis loopcan be started in parallel with synthesis gasgeneration, contributing very significantlyto reduced start-up time.

PROCESS CONTROL SYSTEM

There are three fundamental controlsinherent within the LCA process scheme

steanr.carbon ratio is closely controlledby adding 90% of the process steamthrough natural gas saturation andsubsequent trim addition to about 2.5

process air rate is directly linked to thesecondary reformer exit temperature

strict control of the H2:N2 ratio in themake-up gas is achieved by adjustingpressure swing separation cycle times onan individual bed basis

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The simple, decoupled nature of the LCAprocess scheme makes it possible to automatestart-up, normal operation and shut-downsextensively.

The control system on the ICI SevernsideLCA plant consists of a Taylor Mod 300 systema Sattcom computer and PLC's. A hardwiredsystem, it's reliability independent of themain control system, provides safety tripprotection. Figure (2) shows a schematiclayout.

Non-safety trips are programmed into thecontrol system. These software trips caninitiate hardwired trips if required. Thealarm system is designed to be intelligent;for example the hydrodesulphuriser lowtemperature alarm will only initiate if thereis a low temperature and a flow of naturalgas to the plant. This system reduces thenumber of unnecessary alarms that areinitiated, especially during shut down. Thealarm system has been significantlysimplified since start-up by using ourexperience of plant operation.

Historical recording is a key functionof the control system. Most plantconditions, including temperatures, flows,levels, pressures, vibration and controlloop output, are recorded every few seconds.All alarms and computer messages are printedand recorded. A first-up event recorderdetails the sequence of events after a trip.With this information available plant tripshave been quickly diagnosed, allowing quickrestart times.

The system is designed to give reliable,repeatable operation, similar to thatprovided by a good operator and to provide agood overall, annual plant efficiency, closeto the best spot efficiencies.

This is achieved with computer controlthrough about 70 control loops and automatedstart-up and shut down sequences.

Automatic start-up sequences have beenwritten for a number of plant operations (seeFigure (3)). The structure of a typicalsequence is shown in Figure (4).

Once commissioned, automatic start-upsequences provide a fully developed, reliableand repeatable plant start-up, leading toless chance of mal operation. They ensureprestart checks are done and they willalways leave equipment in a stable holdpoint, ready for the operator or anothersequence to continue the start-up. These

sequences will also switch on intelligentalarms. For example they may switch onalarms on a compressor that need only bevalid once the run command has been given.

Cost savings can be significant as, notonly is there the saving against maloperation,but they will also allow faster, smootherstart-ups and reduced manpower.

Automatic shutdown sequences were writtenfor all equipment with start-up sequences, aswell as key plant trips. An example of thestructure for the shutdown sequence for theSevernside turbo alternator is shown inFigure (5).

The advantages of such sequences are thesame as those for start-up i.e. reliable,repeatable and cost effective operation. Inaddition they will switch off certain alarmsthat are not required after the equipment isshutdown. This reduces the number ofunnecessary, distracting alarms the operatorhandles at shutdown.

Use of this high level of automation notonly allows rapid and cost effective start-upsand subsequent optimised operation butsignificantly enhances manipulation of theplant, a maximum of 3 operators per shift isrequired for the complete 900 tpd complex atSevernside and these manning levels do notrequire supplementing for start-ups orshutdowns.

OPERATIONAL AND DESIGtl FLEXIBILITY

The overall result from the many LCAprocess features descrioed earlier is e.smaller scale plant design, achieving capitaland energy efficiencies previously onlyachieved in a world scale unit. In botiioperation and design the LCA concept is muchmore flexible and able to meet therequirements of the ammonia industry of the1990's than any other process available.

Operation

Previous mention has been made of therapid, low cost start-up capability of the LCAunits from cold. The second Severnside plantwas commissioned in a world record time,making ammonia just 19 hours afterintroduction of feed gas. Subsequent coldrestart has been achieved in only 12 hours,with a future target to reduce this to 8hours. Following a trip, production can bere-established in only 3 to 4 hours for a hotrestart and in future it is expected this time

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can be trimmed to only 2 hours.

This experience compares with typicalstart-up times for conventional units ofaround 25 hours (accelerated low energystart-up) - 50 hours (conventional). Withthe reduced venting rates during the LCAstart-up cycle, savings in gas wastage alonecompared with a world scale unit start-upcycle, savings in gas wastage alone comparedwith a world scale unit start-up areestimated at 1475 GO (31,000 million BTU),without taking into account the added value ofadditional beneficial ammonia productionachieved by an LCA plant,

The ability to start-up rapidly andcheaply also allows adoption of a differentphilosophy opposite operating problems andwith LCA it will often be more cost effectiveto shut-down and repair equipment, ratherthan limp along accepting process limitations.

A further feature in normal operation isthe high turn-down ratio. LCA units can beefficiently operated down to 70% of designrates, enabling plant output to be adjustedto market demand where required. Furtherturn-dov;n to as low as 15% of flowsheet ispossible, with only a small penalty on theoverall gas efficiency.

Design

By separating the basic steps forammonia manufacture into the reproducible'core' unit, the 'utilities' section canthen be adapted to meet the specificindividual customer requirements. Thisallows an excellent opportunity to optimisea complex site operation and integrate withdownstream processes such as urea in themost effective manner.

Total process feed and fuel energyrequirement for the LCA process is less than24.4GJ/Tonne (21 million BTU/ton) of productand this offers the flexibility to choose themost cost and energy efficient power cycleand fuel for on site generation, or,alternatively the option to import.

CO, recovery is not essential for ammoniaproduction and the LCA concept allows thefreedom to match this to specific customerrequirements.

As the main processing steps aredecoupled, they can be used separately or inappropriate combinations for debottleneckingand improving efficiency on existing units, or

for stage wise modernisation and upgrading ofthe facilities.

EFFLUENTS

As appropriate to novel technologyexpected to represent a major component, of theworld fertiliser production during the next 20years, the LCA technology has been designedto meet the highest environmental standards.

By eliminating the atmospheric combustionof fuel in the primary reformer, the only NOproduced is where waste gas is burnt at low xtemperatures in the process air preheater andutilities boiler and the converter start-upheater which only operates infrequently.

Net reduction is around 87% as shown inthe comparative figures below for UK naturalgas based processes.

Conventional

LCA Units

Kg NO/tonne NH,X j

0.56 - 0.66

0.08

Similarly since all hydrocarbon feed tothe core plant is desulphurised there isessentially no emission of S0?, compared withtypical levels for UK based conventionalplants around 1.5 tonnes/year/1000 tpd capacityof ammonia. With C0? recovered in an LCAplant, there will also be a very significantreduction in COp emissions to atmosphere, afurther cause for concern to many environment-alists today.

There are also major improvements in thelevels of liquid effluents, with particularbenefit being obtained by recycling processcondensate to the natural gas saturator.Specifically the ammonia level discharged isless than 25% that from UK conventional unitsand with similar reductions in BOD andchromate levels from water treatment.

Reduced noise levels and visible impactprovide further contributions, while theextensive use of modules, smaller items ofequipment and shorter project timetable alsominimise local environmental impact duringthe construction phase.

CONCLUSIONS

The ICI LCA Process achieves world-scalecapital and energy efficiencies in smallercapacity plants by decoupling processing

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steps and eliminating interaction andinterdependency between these. Key enablingfeatures include use of low temperaturecatalysts for feed pretreatment andmethanation, total process waste heat recyclethrough the saturator and gas heated reformer(GHR), single stage isothermal shift andsyngas purification.

Total process feed and fuel energyrequirement is less than 24.4 GJ/Tonne (21million BTU/ton) of ammonia, which ensureshigh overall efficiency by meeting processpower and steam requirements using a higherefficiency combined gas turbine power cycle.

Rapid start-up and shutdown greatlyreduce waste energy. Gaseous and liquideffluents are drastically decreased byrecycling process condensate through thesaturator and eliminating the fired primaryreformer.

Plant safety is vastly improved byadopting inherently safe processing methods,a sophisticated control system and eliminatingpotential dangerous occurrences associatedwith conventional tubular reformers.

Plant reliability is enhanced byreducing the number of process steps andinteractions between them, using milderoperating conditions and enhanced controlautomation.

Finally, by eliminating the hightemperature shift duty there are no problemsassociated with disposal of chrome containingcatalysts.

CONCLUSIONS

The ICI LCA Process achieves world-scalecapital and energy efficiencies in smallercapacity plants by decoupling processing stepsand eliminating interaction andinterdependency between these. Key enablingfeatures include use of low temperaturecatalysts for feed pretreatment andmethanation, total process waste heat recyclethrough the saturator and gas heated reformer(GHR), single stage isothermal shift andsyngas purification.

Total process feed and fuel energyrequirement is less than 24.4 GJ/Tonne (21million BTU/ton) of ammonia, which ensureshigh overall efficiency by meeting processpower and steam requirements using a higherefficiency combined gas turbine power cycle.

Rapid start-up and shut-down greatlyreduce waste energy. Gaseous and liquideffluents are drastically decreased byrecycling process condensate through thesaturator and eliminating the fired primaryreformer.

Plant safety is vastly improved byadopting inherently safe processing methods, asophisticated control system and eliminatingpotential dangerous occurrences associatedwith conventional tubular reformers.

Plant reliability is enhanced by reducingthe number of process steps and interactionsbetween them, using milder operatingconditions and enhanced control automation.

T.C. Hicks A. Pinto M.J.S. Moss

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Fuel Gas Fuel Gas

j To Air S _ 3-0 b»'X Heater V^ "" I

JL Secondary 3.5 bar j" "[. Reloimer ~ \ *

_L S.TOTl .XBOJU

Fuel NCV12.6 MW

J J Offsite CO; Product

Recovery

Ammonia GasTo Refrigeration

Compressor2.8 bar

and 5.7 bar

Figure 1. LCA process.

MOD3QO

SATT

HardwiredTrip System

PLC

— Air Compressors

— Synthesis Gas Compressors

— Refrigeration Compressors

Turbo-Alternator

— Fired Heaters

Fired Boiler/Superheater

Pressure Swing Adsorption Units

— Cooling Water

— Demin Water Plant

— Syn Gas Driers

2

Figure 2. Control system layout. Figure 3. Automatic start-up: equipment.

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EXAMPLE: COMPRESSOImpulse lines open IPTW/cleanliness. ivalves open/closed ,

Permissives & tripsSets up controllersAvailability of motorsInstruments

Switch on heatersStart lub. oil pumpsWarm up

Check permissivesStart-upConfigure control

R:

MANUAL PRESTART *"!CHECKLIST COMPLETE j

4PRESTART CHECKS

1MESSAGES

*PRESTART ACTIONS

1START-UP

4HOLD POINT

]

EXAMPLE: TURBO-ALTERNATOR:

Puts Governor onminimumCloses valves

Defeats alarmsStops Start-upSequencesPuts controllers onmanual 0%

Lub. oil runningWarming steam onBars machine

Lub. oil offAll steam offBars machine fora time

SHUTDOWN

1GIVE EQUIPMENT /

PLANT SOFT LANDING

CHECKS/CHANGESSTATE OF EQUIPMENT

/PLANT

HOLD POINT 2

Figure 4. Automatic start-up: sequence content. Figure 5. Automatic shutdown: sequence content.

DISCUSSIONMAX APPL, BASF: You mentioned that the consump-tion for feed and fuel was 24.4 GJ/tonne. Would youmind giving us the total energy consumption per tonne?

PINTO: The total energy consumption is 25 million Btu/ton (26 GJ/ton). I'm not talking about the total gas energyconsumption. The gas energy consumption must beseparated from the total energy consumption. The gasenergy consumption of this process is 24.2 GJ/ton. Thetotal energy to meet all the power requirements of theammonia process, including the cooling water pumps andthe power required for the mechanical draft coolingtowers, is 25 million Btu/ton.

APPL: What site conditions does that include? Carbondioxide removal as well as refrigeration of ammonia?

PINTO: That is correct. That is the total power requiredto recover all the CÛ2 needed to convert all the ammoniainto urea to meet all the power requirements of theprocess, and to produce liquid ammonia at 10°C.

APPL: So this is 25 million Btu per short ton?

PINTO: Yes.

RAY LeBLANC, M.W. Ke/fogg: One of your slidesshowed a worldwide increase of approximately 2 to 4%in demand for nitrogen fertilizers in the future. We allwould have to agree that most of that's going to endup in urea, at least for the new installations which arebeing built today. How does this fit into a urea processcomplex, in which the CÛ2 from the PSA unit probablycannot be used as is?

PINTO: First of all, I do not agree that all the ammoniais going to end up as urea. I'll answer that separately.As to how this process couples with a urea process,we are producing about 10 to 15% more COa than requiredfor total conversion of the ammonia into urea. You canremove the COz either before or after the PSA. Afterthe PSA, the CO2 content of the PSA waste gas is 50%.The energy requirement for 90% COz removal from thePSA waste gas is not all that high. It is because youdo not have to reduce the CÛ2 down to methanationlevel. The inlet CO2 content is 50% and the CÛ2 contentof the gas leaving the COa absorber will be approximately10%. This is enough (X>2 needed to convert all theammonia into urea. If it is economically feasible, the same

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amount of COz can be removed before the PSA. In thatcase, you do not have to get the COz down to thehundreds of PPM level. Therefore, the option is availableto recover the CÛ2 either before the PSA or from thePSA waste gas, whichever is more economical.

Regarding the question of all the ammonia going intourea. The roundtable meeting in Atlanta gave me theimpression that too much nitrogen is used without P &K and nitrogen utilization is poor. Countries like Chinaand India have put in quite a lot of nitrogen in their soilsand there isn't enough P & K. So the emphasis will beon more NP fertilizers.

LeBLANC: Regarding urea being the future nitrogenfertilizer, I guess that's something we have to wait andsee.

If I understand you correctly, the options you havefor CÛ2 recovery for urea production are upstream ofthe PSA or on the waste gas stream. I presume eitherlocation would involve conventional CÛ2 recoveryprocesses.

PINTO: CÛ2 recovery, if required, would involveconventional processes. However, if you do not requireCO2, you do not need to install this conventionalrecovery equipment.

COOK SONG, Esso Chemical, Canada: How do youstart up reforming sections? Do you start up thesecondary first? Would you elaborate on that?

PINTO: The primary and secondary reformer can bestarted up at the same time. One must warm it up initiallyto keep the temperature above the dew point, and thensimultaneously introduce steam, natural gas and air.Without air you can't produce the heat energy whichis required to do primary reforming.

SONG: Do you have any fired heater to heat up thesecondary reformer?

PINTO: The only fired heater we have is the one topreheat the process air. But you use it for warming upthe primary and secondary reformer.

OBDULIO FANTI, Ultrafertil, Brazil: We are nowstudying the total integration of first and second reformerinto a single vessel. What is your opinion on this?

PINTO: We looked at it and concluded that there isno incentive to combine the primary and seconaryreformer for a simple reason. The cost saving is not greatenough to justify the added complexity of the combined,integrated primary and secondary reformer.

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