Syngas Purification in Based Ammonia/Urea Plants

A new syngas purification process for gasification-based ammonia/urea plants 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 costs and plant performance for a typical coal-based ammonia plant using these two different processes are compared. These results also apply to syngas production from other feedstocks such as coke and residual oil, and other plant sizes.

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

Introduction study apply to various feedstocks (coal, petroleum coke, and refinery heavy residues) and to different easibility studies of a coal-to-ammonia/urea plant sizes and gasification technologies. fertilizer complex demonstrated that the use of Independent of this work, a 1,000 MTD Fthe UOP Selexol/PSA process technology for ammonia/urea fertilizer based upon Texaco gasifica- synthesis gas purification is superior to the conven- tion of refinery petroleum coke using the UOP tional route of Rectisol/Nitrogen Wash. To completely Selexol/PSA technology for synthesis gas purification evaluate these two gas processing routes, the differ- is currently under engineering/construction in North ences in capital and operating costs, the consequences America. The low value of refinery residue feedstocks of varying operating parameters, and the impacts on and this gasification/syngas purification route to fertil- surrounding units were investigated. Four cases were izer can be competitive with the conventional natural evaluated using Texaco Quench Gasification technology gas reforming processes. to gasify coal, considering the additional CÜ2 co-pro- duction for urea manufacture. Material balances were Case Studies developed for each case on the basis of producing 2,000 MTD of ammonia and, as an option, 1,475 This section describes the fertilizer complex config- MTD of CO2 for urea manufacture. Capital and oper- urations for the four cases of syngas production. ating costs were developed for the entire plant so that Overall block flow diagrams, heat and material bal- the overall effects of the two different synthesis gas ances and process flow diagrams are shown in the sub- purification technology routes could be fully evaluat- sequent sections. The gasification complex is designed ed. The results show that the Selexol/PSA process is to produce a syngas that is further processed to pro- lower in both capital and operating costs than the duce 2,000 MTD ammonia. Illinois No. 6 coal was Rectisol/Nitrogen Wash process. The results of this used as the feed basis for these cases.

AMMONIA TECHNICAL MANUAL 319 1999 Case 1: Selexol/PSA without CO2 Production The CO in the syngas from the gasification unit is converted to hydrogen using two stages of sour shift Case 1 is the base case to which all other configura- reactors via the water-shift reaction. tions are compared. This case uses Selexol/PSA for CO + H O->H + CO acid gas removal and hydrogen purification. The coal 2 2 2 feed to the plant is 2,593 MID to produce 2,000 MTD Since the raw syngas has been saturated with water of ammonia. The overall block flow diagram for this in the scrubbing process, it contains sufficient steam case is depicted in Figure 1. The overall heat and for the shift reaction and no additional steam is material balance is summarized in Table 1. The required. The heat contained hi the reactor effluent is process units are briefly described below. used to generate various levels of steam. A single train Air Separation Unit (2,340 MTD) is A side reaction of the CO shift catalyst converts used to supply high-pressure oxygen at 99.5% purity most of the COS to CO2 and H2S by the catalytic to the gasification unit plus low-pressure oxygen to the hydrolysis reaction, which also occurs in the presence oxygen blown Claus Unit. The use of oxygen in the of steam. Claus Unit reduces the sulfur plant size and improves COS + H2O -> H2S+ CO2 its operating efficiency. The Air Separation Unit also The syngas is then fed to the UOP Selexol unit that supplies high-pressure gaseous nitrogen which is is designed to remove 99.5% of the H S and to pro- mixed with the hydrogen from the Hydrogen 2 duce a CO2 stream suitable for urea manufacture. Recovery Unit (PSA) to make up a stoichiometric feed The process flow diagram for the Selexol unit is to the ammonia plant. The PSA unit, designed by depicted in Figure 2. UOP, uses low-pressure nitrogen for purging, resulting in a substantial improvement in hydrogen recovery. Selexol Unit Coal from storage is conveyed to the coal grinding and slurry preparation system at a rate of 2,593 MTD. Coal The HP Absorber is designed with an inter-cooler and recycle carbon is wet ground with water to produce for controlling the absorption temperature and maxi- a coal-slurry. The slurry and a 99.5% purity oxygen mizing the acid gas loading of the rich solvent. This stream from the air separation plant are fed to the Texaco exchanger results in reducing the lean solvent circula- Quench Gasifier operating at 1,000 psig for the produc- tion. The Selexol solvent reduces the H S content of tion of a raw syngas stream. The particulate-laden 2 the svngas from 0.92 mol. % to 18 ppmv while also quench and scrubber water stream is further treated for removing about 50% of COS. soot removal and recovery of unconverted carbon.

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

Stream Number 1 2 3 4 5 6 7 8 9 10 11 12 13 Description Coal Food Coal Slimy Oxygen To Raw Syngas Scrubbed Shifted Sulfur From Treated Tola! Purge LP Nitrogen HP Nitrogen Hydrogen Syngas To ToGaaftara Gaslftets To Syngas Syngas Syngas To Oxygen Syngas To Gas FromASU FromASU From PSA Ammonia Scrubbing Setexol Claus Unit PSA Synthesis Component Ib/hr Mir Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr LbmoMir Lbmol/hr Lbmol/hr Lbmol/hr CO 9,722 9,722 445 444 445 Hydrogen 8,065 8,065 17,330 17,315 1.053 16,276 16,276 CO2 3,751 3,751 12,941 9,522 12,941 Methane 22 22 22 22 19 3 3 Argon 25 33 33 33 33 22 12 12 Nitrogen 9 97 97 97 97 4,168 4,240 5,216 169 5,385 H2S 286 286 293 1 1 COS 7 7 0 0 0 H20 145,973 5,777 34,169 65 64 64 Oxygen 6,685 Sulfur 293 Coal 238,167 238,167 Total 238,167 384,140 6.718 27,761 56,153 31,228 293 27,498 18,714 4,240 5,216 16,460 21,876 MolWt 32 21 19 20 32 17 36 28 28 2 9

AMMONIA TECHNICAL MANUAL 320 1999 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 let PSA Unit down to 450 psia, and the flash gas is recycled back to the absorber so that its hydrogen content is recovered. The PSA unit is designed to purify the treated gas The solvent is then heated in the lean/rich exchangers to from the Selexol unit and produce 99% purity hydrogen 244°F and is then flashed to 200 psia. The LP flash for the ammonia plant. This PSA unit is a new design vapor is sent to the H2S Concentrator, which is designed offered by UOP that improves the overall hydrogen to reject the bulk of the CO2 thereby concentrating the recovery in the gasification-to-ammonia plants. H2S content of the acid gas feed to the Claus unit The PSA unit uses conventional multibed adsorp- The H2S Concentrator uses a pre-saturator to pro- tion/desorption cycle design, with nitrogen being used duce a CO2 stream with a very low H2S content. The for purging the bed in the desorption cycle. lean Selexol is first mixed with the overhead vapor Conventional design uses hydrogen for purging during and is cooled to 0°F in an overhead heat exchanger. the desorption cycle, which results in loss of hydrogen Most of the CO2 absorption occurs in this exchanger in the purge gas. In a gasification/ammonia plant, very and the solvent becomes saturated with CO2. The CO2 pure nitrogen is available from the Air Separation saturated liquid is pumped back to the column for H2S Plant and purging the beds using nitrogen improves absorption. the overall hydrogen recovery, typically from 88%-90% to 92%-94%. The tail gas from the PSA unit contains a substantial amount of nitrogen and

AMMONIA TECHNICAL MANUAL 321 1999 Table 2. Case 2: Material Balance for Selexol/PSA With CO2 Production

Stream Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Description Coal Fond Coal Sluny Oxygen To Raw Scrubbed Shifted SuftirFrom Treated Total Purg» LP tttragin HP Hydrogen Syngas To LP(Qas) HP(Uqu«i) ToGasHai» QasHera Syngas To Syngas Syngaa To Oxygan Syngas To Gas Fran ASU Nitrogen From PSA Ammonia COZTo COZTo Syngas Seta) Claus Unil PSA FromASU SynUunlt UnaPtM* Una Plant Scrubbing COMPONENT Ib/tu Ib/hr Lbmot/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr LbrnoVhr LbmoVhr Lbmol/hr LbmoWir Lbmol/hr LbrnoVhr Lbmol/hr

CO 9,722 9,722 445 444 444 Hydrogen 8.065 8,065 17,330 17,315 1,039 18,276 18.276 CO2 3,751 3.751 12.S41 9,522 9,861 1,232 1,848 Methane 22 22 22 22 19 3 3 Arg«) 25 33 33 33 33 22 12 12 0 0 Nitrogen S 97 97 87 97 4,168 4,240 5,216 169 5,385 0 0 H2S 286 286 293 1 1 COS 7 7 0 0 0 H20 145,973 5,777 34,169 65 64 64 Oxygen 6,685 Sulfur 293 Coal 238,167 238,167 TOTAL 238.167 384,1«) 8,718 27,761 56,153 31,226 293 27,498 15,634 4,240 5,216 16,460 21,676 1,232 1,848 Moiwt 32 21 19 20 32 17 36 28 28 2 9 44 44

requires supplementary natural gas for combustion. Figure 4. The CO2 stream from the Selexol unit is compressed to 425 psia, and steam is injected into the Case 2: Selexol/PSA With CO2 production CO2 to hydrolyze the residual COS. The effluent is sent to a ZnO bed that removes its sulfur content down Case 2 is similar to Case 1 with the exception that to 1 ppm to meet the CO2 specification for the urea 1,475 MTD CO2 is recovered and purified for the urea plant. plant. In this case, the CO2 waste stream is further The sulfur-free CO2 stream is cooled by cooling compressed from 185 psia to the CO2 water and sent to a knockout drum where the con- Compression/Liquefaction unit. The overall block dia- densed liquid, mostly water, is removed. The CO2 gram for this case is depicted in Figure 3 and the mass stream is further dried using a molecular sieve dryer hi balance is shown in Table 2. order to avoid water freezing in the downstream The CO2 Compression/Liquefaction unit is shown in exchangers. A feed/effluent first cools the CO2, and a

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

Stream Number 1 2 3 4 5 6 7 8 9 10 11 12 13 Description Coal Feed Coal Slurry Oxygen To Raw Scrubbed Shitted Sulfur From Treated Total Purge Stripping HP Nitrogen Recycle Syngas To To Slurry To Cashiers Gas Idem Syngas To Syngas Syngas To Oxygen Syngas To Gas Nitrogen From ASU Cas From Ammonia Preparation Syngas Recllsol Claus Unit Nitrogen From ASU N2 Wash To Synthesis Scrubbing Wash Reclisol Unit COMPONENT Ib/hr Ib/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbraol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr CO 7,574 7,574 347 339 0 3 Hydrogen 7,621 7,621 16,375 16,387 99 74 16,276 CO2 3,319 3,319 11,456 11.456 Methane S 78 78 78 71 78 Argon 23 22 22 22 21 22 1 0 1 Nitrogen 157 157 157 174 1,541 1,082 5,688 5 5,385 H2S 247 247 254 COS 7 7 0 H2O 137,954 5.452 25,012 47 47 Oxygen 6,326 Sulfur 254 Coal 225,083 225,083 Total 225,083 363,038 6,358 24,477 44,037 28,735 254 16,993 13,589 1.082 5,688 82 21,662 MOLWT 32 19 19 20 32 3 42 28 28 5 9

AMMONIA TECHNICAL MANUAL 322 1999 ftAWCO, j.

Figure 2. Typical selexol unit.

BFW HP MC If SUPPIY STEAUfiTEAM STOM

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

AMMONIA TECHNICAL MANUAL 323 1999 Table 4. Case 4: Material Balance for RectisoI/N2 Wash With CO2 Production

Stream Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Description Cial Prod Coal Slimy Oxygen To Raw Scnibbad ShfflBd Sulfur From Treated Total SW»lno HP Recycle Syngas To U>(Ga«) HP (Liquid) To Gasllfens Gasifiofs Syngas To Synga» Syngas To Oxygen Syngas To Purge Mfrogan Nitrosen Gas From Ammonia CO2TO COZTo Syngas Rectisot Ctaus Unil Nitrogen Gas FramASU FramASU N2Wash Synthesis Un» Plant Uva Plant Scrubbing Wach To Recllaol Unit COMPONENT Ib/hr IWhr Ibmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr Lbmol/hr CO 7,574 7,574 347 339 0 3 Hydrogen 7,621 7,621 16.375 16,387 99 74 16,276 CO2 3,319 3,319 11,458 8,376 1,232 1,848 Methane 9 78 78 78 71 78 Argon 23 22 22 22 21 22 1 0 1 Nitrogen 157 157 157 174 1,541 1,082 5,688 5 5,385 H2S 247 247 254 COS 7 7 1 H2O 137,954 5,452 25,012 47 47 Oxygen 6,326 254 Sulfur Coal 225,083 225,083

Total 225,083 363,038 6,358 24,477 44,037 28,735 254 16,993 10,509 1,082 5,688 82 21.662 1232 4848 MolWt 32 19 19 20 32 3 42 28 28 5 9 44 44 chiller using propane refrigeration at 5°F liquefies Wash Column is recycled back to the Rectisol most of the CO2. The cooled stream is flashed in the absorber, which eliminates hydrogen loss from the CO2 Flash Drum, and is sent to a stripper where the Wash Unit. residual hydrogen and CO are removed down to a very Similar to the UOP Selexol unit design, the acid gas low level. The liquid CO2 product is then pumped to from the Rectisol unit is sent to the oxygen-blown the pressure required by the urea plant. Claus unit, and the tail gas from the Claus unit is hydrogenated and recycled back to the acid gas

Case 3: Rectiso/N2 wash without CO2 production removal unit so that SOX emissions are eliminated. The mam differences between the Rectisol unit and The Case 3 process flow scheme is similar to the the Selexol unit are summarized as follows: base case except that the Rectisol/N Wash process is 2 • The Rectisol unit removes all the CO2 and H2S used as the acid gas removal /hydrogen purification from the syngas, while Selexol selectively removes step. Because of the higher hydrogen recovery in this H2S, and the PSA unit removes the residual CO2. case, the coal feed to the plant is lower (2,450 MTD • The Rectisol unit operates under cryogenic temper- vs. 2,593 MTD in the base case). The overall block atures (typical, -80°F), while Selexol operates at diagram for this case is depicted hi Figure 5, The over- all heat and material balance is shown hi Table 3. warmer temperatures (0 to 40°F). The Rectisol process uses methanol as the solvent • Low-pressure nitrogen is required by the Rectisol for acid gas removal and has been used for acid gas unit for CO2 stripping in producing a concentrated removal in conventional "Coal to Ammonia" plant H2S feed to the Claus unit and hi the regeneration applications. The process operates at cryogenic tem- of the solvent. The Selexol solvent is regenerated peratures and when used in combination with the N2 by steam and H2S is concentrated by rejecting CO2 Wash Unit, produces syngas with a very low levels of in the Pre-saturator. impurities. • The Rectisol/N2 Wash design produces a very pure The N2 Wash unit is designed as an integrated unit syngas feed which results in a very minimal purge with the Rectisol unit, which improves its overall from the ammonia synthesis loop. The PSA design energy consumption and hydrogen recovery. The flash allows a residual amount of argon and methane gas from the N2 Flash Drum downstream of the N2 (200 to 700 ppmv) which requires a slightly higher

AMMONIA TECHNICAL MANUAL 324 1999 Figure 4. Typical CO2 compression/liquefaction unit.

purge from the ammonia plant. In addition, if the The methanol from the bottom of the CO2 Stripper oxygen content of the nitrogen from the air separa- is sent to the H2S Concentrator, which uses low-pressure tion is greater than the limit specified by the ammo- nitrogen for stripping the bulk of CO2 from the nia converter design, an oxygen removal bed is methanol, thereby concentrating its H2S content. The required upstream of the ammonia plant. solvent is finally regenerated in the methanol regener- The process flow diagram of a typical Rectisol unit ator, which produces a concentrated H2S feed to the is depicted in Figure 6. The methanol scrubber con- Claus unit. The N Wash Unit consists of a cold box, a N Wash sists of an upper bulk CO2 removal section and a 2 2 column, and a flash drum. The process is depicted in lower H2S removal section. A portion of the CO2 rich Figure 7. methanol is drawn from the CO2 removal section and The feed gas originates at the top of the methanol letdown hi pressure to the CO2 flash drum. The H2S scrubber column and is sent to the CO /methanol saturated methanol from the bottom of the H2S 2 adsorber. Traces of CO and methanol are removed removal section is let down to the H2S flash drum. 2 The flash gas from both drums is recompressed and here to avoid the formation of solids inside the cryo- recycled to the inlet of the unit. The gas stream from genic section of the unit. The treated gas from the the top of this column is warmed by the incoming syn- adsorber is cooled down against product streams and is then sent to the N Wash column. The impurities gas and is then sent to the CO2 methanol adsorption 2 unit. still remaining in the hydrogen syngas (Argon, CO

AMMONIA TECHNICAL MANUAL 325 1999 and methane) are removed by means of liquid nitro- sure (33 psia) than the Selexol process (185 psia) and, gen. Cooling is accomplished by the Joule-Thompson consequently, the CO2 Compression/liquefaction unit effect. Purified gas leaves the top of the column and is requires additional compression stages. sent to ammonia synthesis. The liquid from the bottom of the column is expanded into a flash drum. The flash Cost Evaluation gas is recycled back as feed to the Rectisol unit, while the bottoms is vaporized against incoming feed Capital cost streams, providing cooling for the process before it is sent to the waste fuel boiler. Capital cost estimates were prepared for each of the The nitrogen required for the wash process and for four cases using equipment and unit capacity factoring making up the stoichiometric feed to the ammonia techniques and licensor quotations. The capital costs plant enters the cold box at an anbient temperature and are based on a U.S. location and third-quarter 1996 is cooled against product streams. The nitrogen is split time frame. The capital cost estimation results are into two streams. One stream is further cooled and summarized in Table 5. sent to the top of the column to perform the absorp- The capital costs for the units surrounding the tion. The other stream is added to the purified hydro- Selexol/PSA Unit (or Rectisol/N2 Wash process), that gen to maintain the required stoichiometric ratio for is, the balance of plant (BOP), are determined using the ammonia plant. the overall unit capacity-factoring method. The bal- ance of plant represents the sum of the front-end units Case 4: Rectisol/N2 Wash With CO2 Production in the plant and does not reflect the cost of any unit downstream of the purification unit, as the costs of the This case is the same as Case 3 with the exception downstream units are the same for all the cases. that a CO2 stream is produced for the Urea Plant. A As seen in Table 5, the capital cost for the gas purifi- portion of the CO2 stream from the top of the CO2 cation unit is lower for the UOP Selexol/PSA case stripper is sent to the CO2 compression/liquefaction because of less processing equipment and less expen- unit while the remainder is sent to the waste fuel burn- sive material of construction. On the other hand, the er. The overall block diagram for this case is depicted Rectisol/N2 Wash unit is lower in the BOP cost, in Figure 8. The heat and material balance is shown in because of its higher hydrogen recovery. The net over- Table 4. all cost savings for selecting Selexol/PSA over The Rectisol process produces CO2 at a lower pres- Rectisol/N2 Wash amount to approximately $18 MM.

Table 5. Selexol/PSA vs. Rectisol/N2 Wash Capital Cost Comparison Case 1 CaseS Case 2 Case 4 (Selexol/ (Rectisol/ (Selexol/ (Rectisoy PSA) N2Wash) PSA) N2Wash) NoC02 Production WithCOo Production Capital Cost ($1,000) Selexol Unit 16,330 N/A 16,330 N/A PSA Unit 24, 530 N/A 24,530 N/A Rectisol/N2 Wash Units N/A 72,000 N/A 72,000 Balance of Plant (Front End Units) (BOP) 307,200 294,300 310,010 297,850 Incremental Gas Purification Capital Costs Base 31,140 0 31,140 Incremental BOP Costs Base (12,900) 2,810 (9,350)

Incremental Overall Plant Capital Costs Base 18,240 2,180 21,790

AMMONIA TECHNICAL MANUAL 326 1999 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 Casel CaseS Case 2 Case 4 (Selexol/ (Rectisol/ (Selexol/ (Rectisol/ PSA) N2Wash) PSA) N2Wash) NoCO2 Production WithC02 Production Annual Operating Cost ($1,000) Annual Fixed Operating Costs 14,200 14,540 14,260 14,740 Annual Utilities and Feed Costs 25,700 26,540 27,350 28,780 Annual Catalyst and Chemical Costs 1,280 1,430 1,570 1,710 Total Annual Operating Costs 41,180 42,510 43,180 45,230 Incremental Annual Fixed Op. Costs Base 340 60, 540 Incremental Annual Utilities and Feed Costs Base 840 1,650 3,080 Incremental Annual Cat. and Chem. Costs Base 150 290 430 Total Incremental Annual Operating Costs Base 1,330 2,000 4,050

AMMONIA TECHNICAL MANUAL 327 1999 WASTE WATER

Figure 6. Typical rectisol wash unit.

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

Required Tar Feedstock (MTD) Coal Coke Residual Oil (tar) Purification Units UOP Selexol/PSA 2,600 2,230 1,610 Rectisol/Nitrogen Wash 2,450 2,100 1,520

Incremental Feedstock Requirements UOP Selexol/PSA Base (370) (990) Rectisol/Nitrogen Wash Base (350) (930)

AMMONIA TECHNICAL MANUAL 328 1999 SYNTHESIS OAS

CO,/METHANOL MTROGEM ADSORBERS WASH COLUMN t

Figure 7. Typical nitrogen wash unit.

CO2 co-production adds to the savings of the Selexol are about $1.3 MM per year. CO2 coproduction further case because of its less compression requirement. adds to the operating cost savings because of the less CO2 compression requirement by the Selexol unit. Operating Costs Effect of Operating Parameters on Plant The operating costs for the overall plant are divided Performance and Costs into fixed and variable components. The fixed costs are composed of operating labor, maintenance labor, The following parameters have been studied to eval- and administration, support labor and maintenance uate then" effects on the acid gas removal and hydro- materials. The variable operating costs depend upon gen recovery design. These parameters and their the capacity and the configuration of the plant and are effects are discussed qualitatively below. composed of utility costs, feed costs, and catalyst and chemical costs. Evaluation of different feedstocks A summary of the annual operating costs can be found hi Table 6. Selexol/PSA requires less utility to Heat and material balances were developed and ana- operate because less refrigeration is required and its lyzed for the Selexol/PSA and the Rectisol/N2 Wash purge gas is used to generate power/steam to supply the processes for two other feedstocks: coke and residual process. The net savings by the UOP Selexol/PSA case oil. The comparison results are discussed below.

AMMONIA TECHNICAL MANUAL 329 1999 BFW HP MP IP MAKEUP SUPPLY STEM« STEAM STEAM Hfl

SULFUR PLANT

(0, BLOWN CLAUS)

& TAIL GAS HYDROGENATION

1 t i ' T LP STEAM f

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) Residual oils such as vacuum residue, tar, and can be used as a feedstock to the gasification unit for asphalt can also be used as a gasifier feedstock. Tar hydrogen production. The overall plant requires less feed is used in this sensitivity analysis. When other coke than coal as feed to produce the same amount of types of residual oil are used, the amount of feedstock ammonia. This is mainly due to the higher heating and utility consumption will vary, and, in general, the value of petroleum coke as compared to that of coal. feedstock requirement increases in the order of vacu- Coal contains inert materials such as sulfur and ash, um residue, tar, and asphalt. and is usually at least 15% lower in heating value than The overall plant requires less residual oil (38%) coke. As a result, the front-end of the plant is propor- than coal feed to produce the same amount of ammo- tionally reduced hi size. Once again, it is observed that nia. The front-end solid handling equipment in a less petroleum coke (about 6%) is required for the coal/coke gasification plant is not required hi an oil- Rectisol/N2 Wash case than for the Selexol/PSA case fed plant, and this results in a considerable size reduc- due to its higher hydrogen recovery efficiency. tion. Once again, it is observed that less residual oil (about 6%) is required for the Rectisol/N2 Wash case than for the Selexol/PSA case due to the higher hydro-

AMMONIA TECHNICAL MANUAL 330 1999 gen recovery efficiency. The results of this evaluation be affected by the variation in plant sizes. are summarized in Table 7. Conclusion Evaluation of a Different Gasification Technology This feasibility shows that the UOP Selexol/PSA technology is clearly an attractive alternate to the con- The Shell Gasification technology was evaluated ventional Rectisol/N2 cryogenic route, both in capital using coal as the feedstock to determine the applicabil- and operating costs. With the advent of UOP's new ity of the Selexol/PSA process to this technology. The PSA technology and with its improvement hi hydro- Shell Coal or Coke Gasification technology is differ- gen recovery, the UOP Selexol/PSA technology offers ent than the Texaco process in that the Shell process an economic and a simple plant in terms of design and uses nitrogen as a "carrier," or transport fluid, rather operation when compared to the conventional than water as in the Texaco Quench process. When Rectisol/N2 Wash design. gasification is considered by itself, the Shell process produces a syngas higher in CO and H2 than the Acknowledgments Texaco process because less CO shifting occurs hi the gasifier. The UOP Selexol/PSA process is applicable Flour-Daniel wishes to express their gratitude to to this gasification technology, as the only difference UOP/Equipment and Systems Dept. for their sponsor- is the higher nitrogen content in the syngas. ship and support of this project.

Evaluation of Varying Plant Sizes Literature Cited Kohl, A, and R. Nielsen, Gas Purification, 5th Ed., The reduction in plant costs for small plant sizes can Gulf Publishing Company, Houston, TX, pp. 1202- be estimated by capacity factoring from the base case. 1223 (1997). The exponent factor is expected to be slightly lower in Mak, Y. J., "Gasification to Ammonia Feasibility the Rectisol/N2 Wash case than the Selexol/PSA case because of the complexity and more equipment counts Studies" Fluor-Daniel internal reports (1995). in the Rectisol/N2 Wash design. The operating costs Mak, Y. J., "Study on Selexol/ PSA vs can be estimated by proportioning based on the base Rectisol/Nitrogen Wash in Coal to Ammonia plant capacity. The conclusions of this study will not Plant", Fluor Daniel/UOP Report (Dec. 1996).

AMMONIA TECHNICAL MANUAL 331 1999