Syngas Purification in Based Ammonia/Urea Plants · 2018. 8. 22. · and plant performance for a typical coal-based ammonia plant using these two different processes are compared.

Syngas Purification inBased Ammonia/Urea Plants

A new syngas purification process for gasification-based ammonia/ureaplants using a UOP Selexol/PSA process sequence is presented. This new process can be

used in place of the conventional Rectisol/Nitrogen Wash process. Capital and operating costsand plant performance for a typical coal-based ammonia plant using these two different

processes are compared. These results also apply to syngas production from otherfeedstocks such as coke and residual oil, and other plant sizes.

John Y. Mak and David HeavenFluor-Daniel, Irvine, CA 92698

Introduction

Feasibility studies of a coal-to-ammonia/ureafertilizer complex demonstrated that the use ofthe UOP Selexol/PSA process technology for

synthesis gas purification is superior to the conven-tional route of Rectisol/Nitrogen Wash. To completelyevaluate these two gas processing routes, the differ-ences in capital and operating costs, the consequencesof varying operating parameters, and the impacts onsurrounding units were investigated. Four cases wereevaluated using Texaco Quench Gasification technologyto gasify coal, considering the additional CÜ2 co-pro-duction for urea manufacture. Material balances weredeveloped for each case on the basis of producing2,000 MTD of ammonia and, as an option, 1,475MTD of CO2 for urea manufacture. Capital and oper-ating costs were developed for the entire plant so thatthe overall effects of the two different synthesis gaspurification technology routes could be fully evaluat-ed. The results show that the Selexol/PSA process islower in both capital and operating costs than theRectisol/Nitrogen Wash process. The results of this

study apply to various feedstocks (coal, petroleumcoke, and refinery heavy residues) and to differentplant sizes and gasification technologies.

Independent of this work, a 1,000 MTDammonia/urea fertilizer based upon Texaco gasifica-tion of refinery petroleum coke using the UOPSelexol/PSA technology for synthesis gas purificationis currently under engineering/construction in NorthAmerica. The low value of refinery residue feedstocksand this gasification/syngas purification route to fertil-izer can be competitive with the conventional naturalgas reforming processes.

Case Studies

This section describes the fertilizer complex config-urations for the four cases of syngas production.Overall block flow diagrams, heat and material bal-ances and process flow diagrams are shown in the sub-sequent sections. The gasification complex is designedto produce a syngas that is further processed to pro-duce 2,000 MTD ammonia. Illinois No. 6 coal wasused as the feed basis for these cases.

AMMONIA TECHNICAL MANUAL 319 1999

Case 1: Selexol/PSA without CO2 Production

Case 1 is the base case to which all other configura-tions are compared. This case uses Selexol/PSA foracid gas removal and hydrogen purification. The coalfeed to the plant is 2,593 MID to produce 2,000 MTDof ammonia. The overall block flow diagram for thiscase is depicted in Figure 1. The overall heat andmaterial balance is summarized in Table 1. Theprocess units are briefly described below.

A single train Air Separation Unit (2,340 MTD) isused to supply high-pressure oxygen at 99.5% purityto the gasification unit plus low-pressure oxygen to theoxygen blown Claus Unit. The use of oxygen in theClaus Unit reduces the sulfur plant size and improvesits operating efficiency. The Air Separation Unit alsosupplies high-pressure gaseous nitrogen which ismixed with the hydrogen from the HydrogenRecovery Unit (PSA) to make up a stoichiometric feedto the ammonia plant. The PSA unit, designed byUOP, uses low-pressure nitrogen for purging, resultingin a substantial improvement in hydrogen recovery.

Coal from storage is conveyed to the coal grinding andslurry preparation system at a rate of 2,593 MTD. Coaland recycle carbon is wet ground with water to producea coal-slurry. The slurry and a 99.5% purity oxygenstream from the air separation plant are fed to the TexacoQuench Gasifier operating at 1,000 psig for the produc-tion of a raw syngas stream. The particulate-ladenquench and scrubber water stream is further treated forsoot removal and recovery of unconverted carbon.

The CO in the syngas from the gasification unit isconverted to hydrogen using two stages of sour shiftreactors via the water-shift reaction.

CO + H2O->H2 + CO2

Since the raw syngas has been saturated with waterin the scrubbing process, it contains sufficient steamfor the shift reaction and no additional steam isrequired. The heat contained hi the reactor effluent isused to generate various levels of steam.

A side reaction of the CO shift catalyst convertsmost of the COS to CO2 and H2S by the catalytichydrolysis reaction, which also occurs in the presenceof steam.

COS + H2O -> H2S+ CO2

The syngas is then fed to the UOP Selexol unit thatis designed to remove 99.5% of the H2S and to pro-duce a CO2 stream suitable for urea manufacture.

The process flow diagram for the Selexol unit isdepicted in Figure 2.

Selexol Unit

The HP Absorber is designed with an inter-coolerfor controlling the absorption temperature and maxi-mizing the acid gas loading of the rich solvent. Thisexchanger results in reducing the lean solvent circula-tion. The Selexol solvent reduces the H2S content ofthe svngas from 0.92 mol. % to 18 ppmv while alsoremoving about 50% of COS.

Table 1. Case 1: Material Balance for Selexol/PSA Without CO2 ProductionStream Number

Description

Component

COHydrogen

CO2Methane

Argon

Nitrogen

H2SCOS

H20Oxygen

Sulfur

Coal

Total

MolWt

1Coal Food

Ib/hr

238,167

238,167

2Coal Slimy

ToGaaftara

Mir

145,973

238,167

384,140

3Oxygen ToGaslftets

Lbmol/hr

25

9

6,685

6.718

32

4

Raw SyngasTo SyngasScrubbing

Lbmol/hr

9,722

8,065

3,751

22

33

97

286

7

5,777

27,761

21

5ScrubbedSyngas

Lbmol/hr

9,722

8,065

3,751

22

33

97

286

7

34,169

56,153

19

6Shifted

Syngas ToSetexolLbmol/hr

445

17,330

12,941

22

33

97

293

0

65

31,228

20

7

Sulfur FromOxygen

Claus UnitLbmol/hr

293

293

32

8

TreatedSyngas To

PSALbmol/hr

444

17,315

9,522

22

33

97

1

0

64

27,498

17

9Tola! Purge

Gas

Lbmol/hr445

1.053

12,941

1922

4,168

10

64

18,714

36

10LP NitrogenFromASU

LbmoMir

4,240

4,240

28

11HP NitrogenFromASU

Lbmol/hr

5,216

5,216

28

12HydrogenFrom PSA

Lbmol/hr

16,276

3

12

169

16,460

2

13Syngas ToAmmoniaSynthesis

Lbmol/hr

16,276

3

125,385

21,876

9


Figure 1. Case 1 (base case): coal to ammonia complex selexol case without C(>2 production.

The rich Selexol solvent from the HP Absorber is letdown to 450 psia, and the flash gas is recycled back tothe absorber so that its hydrogen content is recovered.The solvent is then heated in the lean/rich exchangers to244°F and is then flashed to 200 psia. The LP flashvapor is sent to the H2S Concentrator, which is designedto reject the bulk of the CO2 thereby concentrating theH2S content of the acid gas feed to the Claus unit

The H2S Concentrator uses a pre-saturator to pro-duce a CO2 stream with a very low H2S content. Thelean Selexol is first mixed with the overhead vaporand is cooled to 0°F in an overhead heat exchanger.Most of the CO2 absorption occurs in this exchangerand the solvent becomes saturated with CO2. The CO2

saturated liquid is pumped back to the column for H2Sabsorption.

PSA Unit

The PSA unit is designed to purify the treated gasfrom the Selexol unit and produce 99% purity hydrogenfor the ammonia plant. This PSA unit is a new designoffered by UOP that improves the overall hydrogenrecovery in the gasification-to-ammonia plants.

The PSA unit uses conventional multibed adsorp-tion/desorption cycle design, with nitrogen being usedfor purging the bed in the desorption cycle.Conventional design uses hydrogen for purging duringthe desorption cycle, which results in loss of hydrogenin the purge gas. In a gasification/ammonia plant, verypure nitrogen is available from the Air SeparationPlant and purging the beds using nitrogen improvesthe overall hydrogen recovery, typically from88%-90% to 92%-94%. The tail gas from the PSAunit contains a substantial amount of nitrogen and


Table 2. Case 2: Material Balance for Selexol/PSA With CO2 Production

Stream Number

Description

COMPONENT

COHydrogen

CO2Methane

Arg«)

Nitrogen

H2S

COSH20Oxygen

Sulfur

Coal

TOTALMoiwt

1Coal Fond

Ib/tu

238,167

238.167

2

Coal SlunyToGasHai»

Ib/hr

145,973

238,167

384,1«)

3

Oxygen ToQasHera

Lbmot/hr

25S

6,685

8,718

32

4

RawSyngas To

SyngasScrubbingLbmol/hr

9,722

8.065

3,751

22

33

97

286

7

5,777

27,761

21

5

ScrubbedSyngas

Lbmol/hr

9,722

8,065

3.751

22

33

97

286

7

34,169

56,153

19

6

ShiftedSyngaa ToSeta)

Lbmol/hr

44517,330

12.S41

22

33

87

293

0

65

31,226

20

7

SuftirFromOxygan

Claus Unil

Lbmol/hr

293

293

32

8

TreatedSyngas To

PSA

Lbmol/hr

44417,315

9,522

22

33

97

1

0

64

27,498

17

9

Total Purg»Gas

LbrnoVhr

4441,039

9,861

19

22

4,168

1

0

64

15,634

36

10

LP tttraginFran ASU

LbmoVhr

4,240

4,240

28

11HP

NitrogenFromASU

Lbmol/hr

5,216

5,216

28

12HydrogenFrom PSA

LbmoWir

18,276

312

169

16,460

2

13Syngas ToAmmoniaSynUunlt

Lbmol/hr

18.276

3

12

5,385

21,676

9

14

LP(Qas)COZTo

UnaPtM*

LbrnoVhr

1,232

00

1,232

44

15

HP(Uqu«i)COZTo

Una Plant

Lbmol/hr

1,848

00

1,848

44

requires supplementary natural gas for combustion.

Case 2: Selexol/PSA With CO2 production

Case 2 is similar to Case 1 with the exception that1,475 MTD CO2 is recovered and purified for the ureaplant. In this case, the CO2 waste stream is furthercompressed from 185 psia to the CO2

Compression/Liquefaction unit. The overall block dia-gram for this case is depicted in Figure 3 and the massbalance is shown in Table 2.

The CO2 Compression/Liquefaction unit is shown in

Figure 4. The CO2 stream from the Selexol unit iscompressed to 425 psia, and steam is injected into theCO2 to hydrolyze the residual COS. The effluent issent to a ZnO bed that removes its sulfur content downto 1 ppm to meet the CO2 specification for the ureaplant.

The sulfur-free CO2 stream is cooled by coolingwater and sent to a knockout drum where the con-densed liquid, mostly water, is removed. The CO2

stream is further dried using a molecular sieve dryer hiorder to avoid water freezing in the downstreamexchangers. A feed/effluent first cools the CO2, and a

Table 3. Case 3: Material Balance for Rectisol/N2 Wash Without CO2 ProductionStream Number

Description

COMPONENT

COHydrogen

CO2Methane

Argon

Nitrogen

H2SCOSH2OOxygen

Sulfur

Coal

Total

MOLWT

1Coal FeedTo Slurry

Preparation

Ib/hr

225,083

225,083

2

Coal SlurryTo Cashiers

Ib/hr

137,954

225,083

363,038

3

Oxygen ToGas Idem

Lbmol/hr

S

23

6,326

6,358

32

4

RawSyngas ToSyngas

Scrubbing

Lbmol/hr

7,574

7,621

3,319

78

22

157

247

7

5.452

24,477

19

5

ScrubbedSyngas

Lbmol/hr

7,574

7,621

3,319

78

22

157

247

7

25,012

44,037

19

6

ShittedSyngas ToRecllsol

Lbmol/hr

347

16,375

11,456

78

22

157

254

0

47

28,735

20

7

Sulfur FromOxygen

Claus Unit

Lbraol/hr

254

25432

8Treated

Syngas ToNitrogenWash

Lbmol/hr

33916,387

7121

174

16,993

3

9Total Purge

Gas

Lbmol/hr

099

11.456

7822

1,541

47

13,589

42

10StrippingNitrogen

From ASU

Lbmol/hr

1,082

1.082

28

11

HP NitrogenFrom ASU

Lbmol/hr

1

5,688

5,688

28

12

RecycleCas FromN2 Wash To

ReclisolUnitLbmol/hr

3

74

0

5

82

5

13

Syngas ToAmmoniaSynthesis

Lbmol/hr

16,276

1

5,385

21,662

9


ftAWCO, j.

Figure 2. Typical selexol unit.

BFW HP MC IfSUPPIY STEAUfiTEAM STOM

Figure 3. Case 2: coal to ammonia complex selexol case with CO2 production.


Table 4. Case 4: Material Balance for RectisoI/N2 Wash With CO2 Production

Stream Number

Description

COMPONENT

COHydrogen

CO2Methane

Argon

Nitrogen

H2SCOS

H2O

Oxygen

Sulfur

Coal

Total

MolWt

1Cial Prod

Ib/hr

225,083

225,083

2Coal SlimyTo Gasllfens

IWhr

137,954

225,083

363,038

3

Oxygen ToGasifiofs

Ibmol/hr

9

23

6,326

6,358

32

4Raw

Syngas ToSyngas

Scrubbing

Lbmol/hr

7,574

7,621

3,319

78

22

157

247

7

5,452

24,477

19

5

ScnibbadSynga»

Lbmol/hr

7,574

7,621

3,319

78

22

157

247

7

25,012

44,037

19

6

ShfflBdSyngas ToRectisot

Lbmol/hr

347

16.375

11,458

78

22

157

254

1

47

28,735

20

7

Sulfur FromOxygen

Ctaus Unil

Lbmol/hr

254

254

32

8

TreatedSyngas ToNitrogenWach

Lbmol/hr

339

16,387

71

21

174

16,993

3

9

TotalPurgeGas

Lbmol/hr

0

99

8,376

78

22

1,541

47

10,509

42

10

SW»lnoMfrogan

FramASU

Lbmol/hr

1,082

1,082

28

11

HPNitrosen

FramASU

Lbmol/hr

1

5,688

5,688

28

12

RecycleGas FromN2Wash

ToRecllaol

UnitLbmol/hr

374

0

5

825

13Syngas ToAmmoniaSynthesis

Lbmol/hr

16,276

1

5,385

21.662

9

14

U>(Ga«)CO2TO

Un» Plant

Lbmol/hr

1,232

1232

44

15

HP (Liquid)COZTo

Uva Plant

Lbmol/hr

1,848

4848

44

chiller using propane refrigeration at 5°F liquefiesmost of the CO2. The cooled stream is flashed in theCO2 Flash Drum, and is sent to a stripper where theresidual hydrogen and CO are removed down to a verylow level. The liquid CO2 product is then pumped tothe pressure required by the urea plant.

Case 3: Rectiso/N2 wash without CO2 production

The Case 3 process flow scheme is similar to thebase case except that the Rectisol/N2 Wash process isused as the acid gas removal /hydrogen purificationstep. Because of the higher hydrogen recovery in thiscase, the coal feed to the plant is lower (2,450 MTDvs. 2,593 MTD in the base case). The overall blockdiagram for this case is depicted hi Figure 5, The over-all heat and material balance is shown hi Table 3.

The Rectisol process uses methanol as the solventfor acid gas removal and has been used for acid gasremoval in conventional "Coal to Ammonia" plantapplications. The process operates at cryogenic tem-peratures and when used in combination with the N2

Wash Unit, produces syngas with a very low levels ofimpurities.

The N2 Wash unit is designed as an integrated unitwith the Rectisol unit, which improves its overallenergy consumption and hydrogen recovery. The flashgas from the N2 Flash Drum downstream of the N2

Wash Column is recycled back to the Rectisolabsorber, which eliminates hydrogen loss from theWash Unit.

Similar to the UOP Selexol unit design, the acid gasfrom the Rectisol unit is sent to the oxygen-blownClaus unit, and the tail gas from the Claus unit ishydrogenated and recycled back to the acid gasremoval unit so that SOX emissions are eliminated.

The mam differences between the Rectisol unit andthe Selexol unit are summarized as follows:

• The Rectisol unit removes all the CO2 and H2Sfrom the syngas, while Selexol selectively removesH2S, and the PSA unit removes the residual CO2.

• The Rectisol unit operates under cryogenic temper-atures (typical, -80°F), while Selexol operates atwarmer temperatures (0 to 40°F).

• Low-pressure nitrogen is required by the Rectisolunit for CO2 stripping in producing a concentratedH2S feed to the Claus unit and hi the regenerationof the solvent. The Selexol solvent is regeneratedby steam and H2S is concentrated by rejecting CO2

in the Pre-saturator.

• The Rectisol/N2 Wash design produces a very puresyngas feed which results in a very minimal purgefrom the ammonia synthesis loop. The PSA designallows a residual amount of argon and methane(200 to 700 ppmv) which requires a slightly higher


Figure 4. Typical CO2 compression/liquefaction unit.

purge from the ammonia plant. In addition, if theoxygen content of the nitrogen from the air separa-tion is greater than the limit specified by the ammo-nia converter design, an oxygen removal bed isrequired upstream of the ammonia plant.

The process flow diagram of a typical Rectisol unitis depicted in Figure 6. The methanol scrubber con-sists of an upper bulk CO2 removal section and alower H2S removal section. A portion of the CO2 richmethanol is drawn from the CO2 removal section andletdown hi pressure to the CO2 flash drum. The H2Ssaturated methanol from the bottom of the H2Sremoval section is let down to the H2S flash drum.The flash gas from both drums is recompressed andrecycled to the inlet of the unit. The gas stream fromthe top of this column is warmed by the incoming syn-gas and is then sent to the CO2 methanol adsorptionunit.

The methanol from the bottom of the CO2 Stripperis sent to the H2S Concentrator, which uses low-pressurenitrogen for stripping the bulk of CO2 from themethanol, thereby concentrating its H2S content. Thesolvent is finally regenerated in the methanol regener-ator, which produces a concentrated H2S feed to theClaus unit.

The N2 Wash Unit consists of a cold box, a N2 Washcolumn, and a flash drum. The process is depicted inFigure 7.

The feed gas originates at the top of the methanolscrubber column and is sent to the CO2/methanoladsorber. Traces of CO2 and methanol are removedhere to avoid the formation of solids inside the cryo-genic section of the unit. The treated gas from theadsorber is cooled down against product streams andis then sent to the N2 Wash column. The impuritiesstill remaining in the hydrogen syngas (Argon, CO


and methane) are removed by means of liquid nitro-gen. Cooling is accomplished by the Joule-Thompsoneffect. Purified gas leaves the top of the column and issent to ammonia synthesis. The liquid from the bottomof the column is expanded into a flash drum. The flashgas is recycled back as feed to the Rectisol unit, whilethe bottoms is vaporized against incoming feedstreams, providing cooling for the process before it issent to the waste fuel boiler.

The nitrogen required for the wash process and formaking up the stoichiometric feed to the ammoniaplant enters the cold box at an anbient temperature andis cooled against product streams. The nitrogen is splitinto two streams. One stream is further cooled andsent to the top of the column to perform the absorp-tion. The other stream is added to the purified hydro-gen to maintain the required stoichiometric ratio forthe ammonia plant.

Case 4: Rectisol/N2 Wash With CO2 Production

This case is the same as Case 3 with the exceptionthat a CO2 stream is produced for the Urea Plant. Aportion of the CO2 stream from the top of the CO2

stripper is sent to the CO2 compression/liquefactionunit while the remainder is sent to the waste fuel burn-er. The overall block diagram for this case is depictedin Figure 8. The heat and material balance is shown inTable 4.

The Rectisol process produces CO2 at a lower pres-

sure (33 psia) than the Selexol process (185 psia) and,consequently, the CO2 Compression/liquefaction unitrequires additional compression stages.

Cost Evaluation

Capital cost

Capital cost estimates were prepared for each of thefour cases using equipment and unit capacity factoringtechniques and licensor quotations. The capital costsare based on a U.S. location and third-quarter 1996time frame. The capital cost estimation results aresummarized in Table 5.

The capital costs for the units surrounding theSelexol/PSA Unit (or Rectisol/N2 Wash process), thatis, the balance of plant (BOP), are determined usingthe overall unit capacity-factoring method. The bal-ance of plant represents the sum of the front-end unitsin the plant and does not reflect the cost of any unitdownstream of the purification unit, as the costs of thedownstream units are the same for all the cases.

As seen in Table 5, the capital cost for the gas purifi-cation unit is lower for the UOP Selexol/PSA casebecause of less processing equipment and less expen-sive material of construction. On the other hand, theRectisol/N2 Wash unit is lower in the BOP cost,because of its higher hydrogen recovery. The net over-all cost savings for selecting Selexol/PSA overRectisol/N2 Wash amount to approximately $18 MM.

Table 5. Selexol/PSA vs. Rectisol/N2 Wash Capital Cost Comparison

Capital Cost ($1,000)Selexol UnitPSA UnitRectisol/N2 Wash UnitsBalance of Plant (Front End Units) (BOP)Incremental Gas Purification Capital CostsIncremental BOP Costs

Incremental Overall Plant Capital Costs

Case 1(Selexol/

PSA)NoC02

16,33024, 530

N/A307,200

BaseBase

Base

CaseS(Rectisol/N2Wash)

Production

N/AN/A

72,000294,30031,140

(12,900)

18,240

Case 2(Selexol/

PSA)WithCOo

16,33024,530N/A

310,0100

2,810

2,180

Case 4(RectisoyN2Wash)

Production

N/AN/A

72,000297,85031,140(9,350)

21,790


Figure 5. Case 3: coal to ammonia complex rectisol case without CO2 production.

Table 6. SelexoI/PSA vs. Rectisol/N2 Wash Annual Operating Cost Comparison

Annual Operating Cost ($1,000)Annual Fixed Operating CostsAnnual Utilities and Feed CostsAnnual Catalyst and Chemical Costs

Total Annual Operating Costs

Incremental Annual Fixed Op. CostsIncremental Annual Utilities and Feed CostsIncremental Annual Cat. and Chem. Costs

Total Incremental Annual Operating Costs

Casel(Selexol/

PSA)NoCO2

14,20025,7001,280

41,180

BaseBaseBase

Base

CaseS(Rectisol/N2Wash)

Production

14,54026,5401,430

42,510

340840150

1,330

Case 2(Selexol/

PSA)WithC02

14,26027,3501,570

43,180

60,1,650290

2,000

Case 4(Rectisol/N2Wash)

Production

14,74028,7801,710

45,230

5403,080430

4,050


WASTEWATER

Figure 6. Typical rectisol wash unit.

Table 7. Selexol/PSA vs. Rectiso!/N2 Wash Required Feedstock Comparison to Produce 2,000 MTDAmmonia

Purification UnitsUOP Selexol/PSA

Rectisol/Nitrogen Wash

Incremental Feedstock RequirementsUOP Selexol/PSA

Rectisol/Nitrogen Wash

Required Tar Feedstock (MTD)

Coal Coke Residual Oil (tar)

2,600

2,450

Base

Base

2,230

2,100

(370)

(350)

1,610

1,520

(990)

(930)


CO,/METHANOLADSORBERS

t

SYNTHESISOAS

MTROGEMWASH

COLUMN

Figure 7. Typical nitrogen wash unit.

CO2 co-production adds to the savings of the Selexolcase because of its less compression requirement.

Operating Costs

The operating costs for the overall plant are dividedinto fixed and variable components. The fixed costsare composed of operating labor, maintenance labor,and administration, support labor and maintenancematerials. The variable operating costs depend uponthe capacity and the configuration of the plant and arecomposed of utility costs, feed costs, and catalyst andchemical costs.

A summary of the annual operating costs can befound hi Table 6. Selexol/PSA requires less utility tooperate because less refrigeration is required and itspurge gas is used to generate power/steam to supply theprocess. The net savings by the UOP Selexol/PSA case

are about $1.3 MM per year. CO2 coproduction furtheradds to the operating cost savings because of the lessCO2 compression requirement by the Selexol unit.

Effect of Operating Parameters on PlantPerformance and Costs

The following parameters have been studied to eval-uate then" effects on the acid gas removal and hydro-gen recovery design. These parameters and theireffects are discussed qualitatively below.

Evaluation of different feedstocks

Heat and material balances were developed and ana-lyzed for the Selexol/PSA and the Rectisol/N2 Washprocesses for two other feedstocks: coke and residualoil. The comparison results are discussed below.


BFW HP MP IP MAKEUPSUPPLY STEM« STEAM STEAM Hfl

LP STEAM f

SULFUR PLANT

(0, BLOWN CLAUS)

& TAIL GASHYDROGENATION

i ' T 1 t

Figure 8. Case 4: coal to ammonia complex rectisol case with CO2 production.

Evaluation of coke as a feedstock Evaluation of residual oil as a feedstock

Petroleum coke (usually from a delayed coker unit)can be used as a feedstock to the gasification unit forhydrogen production. The overall plant requires lesscoke than coal as feed to produce the same amount ofammonia. This is mainly due to the higher heatingvalue of petroleum coke as compared to that of coal.Coal contains inert materials such as sulfur and ash,and is usually at least 15% lower in heating value thancoke. As a result, the front-end of the plant is propor-tionally reduced hi size. Once again, it is observed thatless petroleum coke (about 6%) is required for theRectisol/N2 Wash case than for the Selexol/PSA casedue to its higher hydrogen recovery efficiency.

Residual oils such as vacuum residue, tar, andasphalt can also be used as a gasifier feedstock. Tarfeed is used in this sensitivity analysis. When othertypes of residual oil are used, the amount of feedstockand utility consumption will vary, and, in general, thefeedstock requirement increases in the order of vacu-um residue, tar, and asphalt.

The overall plant requires less residual oil (38%)than coal feed to produce the same amount of ammo-nia. The front-end solid handling equipment in acoal/coke gasification plant is not required hi an oil-fed plant, and this results in a considerable size reduc-tion. Once again, it is observed that less residual oil(about 6%) is required for the Rectisol/N2 Wash casethan for the Selexol/PSA case due to the higher hydro-


gen recovery efficiency. The results of this evaluationare summarized in Table 7.

Evaluation of a Different GasificationTechnology

The Shell Gasification technology was evaluatedusing coal as the feedstock to determine the applicabil-ity of the Selexol/PSA process to this technology. TheShell Coal or Coke Gasification technology is differ-ent than the Texaco process in that the Shell processuses nitrogen as a "carrier," or transport fluid, ratherthan water as in the Texaco Quench process. Whengasification is considered by itself, the Shell processproduces a syngas higher in CO and H2 than theTexaco process because less CO shifting occurs hi thegasifier. The UOP Selexol/PSA process is applicableto this gasification technology, as the only differenceis the higher nitrogen content in the syngas.

Evaluation of Varying Plant Sizes

The reduction in plant costs for small plant sizes canbe estimated by capacity factoring from the base case.The exponent factor is expected to be slightly lower inthe Rectisol/N2 Wash case than the Selexol/PSA casebecause of the complexity and more equipment countsin the Rectisol/N2 Wash design. The operating costscan be estimated by proportioning based on the baseplant capacity. The conclusions of this study will not

be affected by the variation in plant sizes.

Conclusion

This feasibility shows that the UOP Selexol/PSAtechnology is clearly an attractive alternate to the con-ventional Rectisol/N2 cryogenic route, both in capitaland operating costs. With the advent of UOP's newPSA technology and with its improvement hi hydro-gen recovery, the UOP Selexol/PSA technology offersan economic and a simple plant in terms of design andoperation when compared to the conventionalRectisol/N2 Wash design.

Acknowledgments

Flour-Daniel wishes to express their gratitude toUOP/Equipment and Systems Dept. for their sponsor-ship and support of this project.

Literature Cited

Kohl, A, and R. Nielsen, Gas Purification, 5th Ed.,Gulf Publishing Company, Houston, TX, pp. 1202-1223 (1997).

Mak, Y. J., "Gasification to Ammonia FeasibilityStudies" Fluor-Daniel internal reports (1995).

Mak, Y. J., "Study on Selexol/ PSA vsRectisol/Nitrogen Wash in Coal to AmmoniaPlant", Fluor Daniel/UOP Report (Dec. 1996).


Syngas Purification in Based Ammonia/Urea Plants · 2018. 8. 22. · and plant performance for a typical coal-based ammonia plant using these two different processes are compared.

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