Am monia Con verter verter Operation Operat ion Makoto Shimagaki an d Kenjiro Kenjiro M iyashita iyashita
oration , Funabashi, Funabashi, Chiba, Chiba, Japa Toyo Engineering Corp oration, and
Alita Alita llyas an d Aslam Kalyubi
P.T. Asean Aceh Fertilizer, Aceh, Sumatra, Indonesia
Optimization the operating conditions Kellogg four-stage adiabatic quench converter was achieved in a 1000 M T P D plant Asean Aceh by catalyst catalyst beds wi h adjustments quench flows.
INTRODUCTION
P.T. Asean Asean Aceh Ferti lize r (P.T. AAF) AAF) operates 1,000 MTPD Ammonia Plant in its complex at Aceh, Sumatra, Indonesia. The Ammonia Ammonia Plant is large single train train un it based on an M.W. Kellogg Kellogg steam reforming reforming pro cess, and which had been designed in detail and constructed by Toyo Engineering Corp. (TEC). Th plant acceptance had been c omplet ed in December, 1983 and since then, the pla nt has continued the commercial operation satissatisfactoril factorily y with 102-108% 102-108% produ ction capacity. P.T. AAF AAF and TE C found that the operating temperature profile profile of catalyst beds in Kellogg's Kellogg's four stage adiabatic quen ch typ e ammonia converter was far far different from an optimized temperature profile t h e c o m p u te te r simulation and carried out the optimization of of ammonia converter operation jointly in November, 1985. Under th e conditions of constant ammonia production production rate, the adjustment of the tem perature profile through through the catalyst beds had been done by controlling the quen ch rate to each bed, which results in the synthesis pressure drop by approximately Kg/cm2and Kg/cm2and accordingl the spee d decrease of the Syn. Gas Compressor Turb ine by 80 r.p.m. Thi s is eq uiv alen t to 0.02-0.03 MMK MMKca ca Ton-NH, saving energy of natural gas. Furthermore, longer catalyst life will be expected due to relatively lower operating temperat ure of catalyst catalyst beds. In this article, an example of the op timization proceprocedure of the quen ch type Ammonia Ammonia Converter and the results are introduc ed. Als Also o inc luded are several several p oints for the optimization of the ammonia converter operation which has cont ributed to th e good performance of of the ammonia plant. PLANT OUTLINE Process Flow
The Mock flow diagram of overall process flow is shown in in Figu re 1. he process flow flow for for ammo nia synthesis loop and th e configuration configuration of Ammonia Ammonia Syn thesis Converter is shown in in Figure and Figure 3 respectivel respectively. y.
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e synthesis gas is is compressed in a centrifiig, compressor. Recycle gas from the synthesis section is admitted to an interstage wheel in the high pressure casing and goes through the final stages of compression nirxed with the synthesis loop fresh fresh feed. Th e total compressor discharge is first first water cooled and then split into two parallel streams. One stream passe through two ammonia refrigerated refrigerated chillers operating at successively lower temperature s while the other stream is heat exchanged against against t he ammonia separator vapor vapor to recover refrigeration. refrigeration. Th e two streams are then r ec on bine d a nd further cooled in a third ammonia refrigera refrigerated ted chiller. Th e chilled vapor-liquid stream then flow flow the ammonia separator where condensed ammonia product is disengage d from from the converter feed gas. This separation at -23°C remove s any traces traces of water of saturatio n and the remainin g carbon oxides oxides entering the synthesis toop toop with fresh fee d. It, in effect, serves to purify the fee d gas to th converter which in turn provides for long catalyst catalyst life, life, since all oxygen compounds, including w' er capor, are deleterio us to th e synthesis catalys catalyst. t. Th e gas from from the separator is heated against against portion of th e compressor discharge and then against converter effluent. Converter feed then enters the ammonia synthesis converter. The ammonia synthesis converter consists of a high pressure shell containing a catalyst catalyst section section and ii heat chan ger. Th e catalyst catalyst section is a cylindrical cylindrical sh ell which fits fits inside the pressure sh ell of the vessel, leaving an an nulus between the two. Th e catalys catalystt basket conlains four four catalyst beds. I n order to maint ain all the catalyst at an optimum t emp erat ure for maximum yield, provision is made to inject feed gas as quench in the space between the beds. The catalyst beds are arranged so that the top contai ns the sma llest quantity of catalyst catalyst to limit the teinperature rise before the first first quench point. Since the temperature g radient is smaller in succe eding beds, the bed sizes are graduated with the largest bed at the bottom. bottom. Located above th e catalyst catalyst section is a heat exchanger which preheats part of the fresh inlet gas against hot reacted ga
Plant/OperationsProgress Plant/OperationsProgress (Vol. 6, No. 2)
OUTLET NTER CHANGER
QUENCH
SYNTHESIS GAS
COMPRESSOR
AMMONIA SEPARAIOR
A*l*ONiA RODJCT
Figure 2. Process flow for ammonia synthesis loop.
from th e last catalyst bed. by-pass lin e is provided to permit introduction of feed gas without preheatin g an provides temperatu re control to th e top catalyst bed. Th e normal point of entry for the feed is at the bottom of the converter. Th e gas flows upward between the pressure she ll and the wall of the catalyst section. It serves as a cooling medium for the s hell and thus receives prehea prior to entering the exchan ger. It ente rs the exchanger at the top of the converter and is preheated against hot ef fluent circulating downward around ex changer tubes. For temperatu re control to the to p bed, a portion of the feed gas will be introduced directly to the converter where it by-passes the exchanger and meets the preheated feed. This gas passes downward through th e catalyst with rapid temperature rise as the ammonia reac tion proceeds. It passes through a grid supporting the catalyst into a space betw een the bottom of the first and second beds. At this point, the temperature is redu ced and t he ammonia content dilute the injection ofa portion of cold feed gas. In like manner, the gas flows downward through the four beds. In the presence of the iron catalyst, a portion of the total hydro gen a nd nitro gen combines at temperature of approximately 400°C to 480°C and a pres sure of 140-147 Kg/cm2G o yield ammonia i n a concentrati on of abou t 12% in the effluent from the last catalyst bed. Th e hot effluent from the bottom be d passes u p through a cen ter return pipe into the tubes of the exchanger giving up heat to the incoming fresh feed on the shell side. From th e exchanger inside of the converter shell, the converter effluent flows to the boiler feed water heater w here the gas is cooled. Then t he converter effluent undergoes heat exchange with the feed to the co nverter, lowering the converter effluent temperature. This cooled gas passes to the interstage of the second casing of the centriftigal compressor for recycle back to the converter, thus comple ting the synthesis loop. A portion of recycle gas is vented to the fuel system through ammonia recovery unit as continuous purge to control the concentration of methane and argon inerts in the synthesis loop.
MAIN I N L E T Figure 3. Configuration of ammonia synthesis converter.
Ammonia Synthesis Converter Operating Conditions
Table shows the ammo nia converter operating conditions before optimization tests. Th e ammonia converter was operated steadily in about 103% production rate with 139 Kg/cm2G synthesis pressu re at the inlet of the converter. The converter feed gas was preheated to about 432°C by t he h ot effluent from the last catalyst bed at the intercha nger provided at the top of the converter and then red to th e first catalyst bed directl y withou t mixing with by-pass flow of intercha nger. Accordingly all exit tem perature of four catalyst beds lay around 480°C to 495°C which se emed to b e relatively high and close to the equilibrium li ne except the first catalyst bed.
PROCEDURE FOR OPTIMIZATION OF AMMONIA CONVERTER OPERATION Approach for Optimization and Analysis
Ammonia Synthesis by
Computer Simulotion
For the temperature profile optimization of the ammonia converter and the resulting analysis, the computer
TABLE
OPERATINC ONDITIONEFORETEST
Production Hate (%
103
Pressure (KG/cm2G) ACV Inlet
139
ACV AP Quench Flow Rate (%) of Total Feed Flow (Estiniatrtl) Bed #2 Bed
#3 #4 Bed
10.7 11.6
simulation program ow ned TE C was utilized. The ki netic system of TE C simulation program is based on the kinetic equation advanced Temkin and Pyzhev. Thoug h there are many factors involved in the ammon ia synthesis reaction, the simplified equation (1 or the reaction rate per unit catalyst volume derived from the basic Tenikin and Pyzhev kinetic equation.
A F U ) (F(Zecl)
first catalyst b ed inlet tempe rature which is indicated in Figure n this manner, the optimized ttbmperature profile prepared the computer simulatiori aided th smooth approach of the optimization test. The optimization test was cariied out r r he follow ing conditions: Make-up gas flow rate to the synthesis gas com(1 pressor and purge gas flow rate were kept unchanged and the ammonia production rate wa kept constant accordingly. Que nch valve o pen ing ratio for second, third arid (2 fourth catalyst beds was remained unchanged, because each quen ch valve in th e existing oper ating condition had b een already opcmed to the deg ree of the ope ning ratio giving the. optimized que nch flow Th e following shows the p rocedure for the tcmperatrire
(1
F(z)) F(P, Z, 10
Where reaction ra te p er u nit catiilyst volriine constant fiunction total pressure temperature mole fraction NH,, in gas equilibrium mole fi-action NII, mole fraction inerts coefficient measuring de viation from stoiIN,) chiom etric composition (3yH,
ze I"
F ( T ) a n d ( F ( Z e q ) F ( z ) ) in (1 are function oftein perature and have general tendency against temperature at some NI-I,?concentration as lielow.
(2
By-pass valve of the interchanger \ \ . a s opened stepwise to get 400°C inlet temperatiire of the first catalyst bed. T he temp erature decrease at the inlet of the first catalyst I l e d with on action was aimed at about 5°C. Once action was taken, the conditionr were kept at.least 30 minutes confirming the steady ammonia synt hesis reaction in o rder to ;disolutely prevent reaction failure which results ti synthesis loop shut-down.
OPTI MIZA TION RESULT Tendency of Temperature Profile Movement on Optimization Test
In th e course of the test to decrease thc teniperature of the first catalyst bed, the operating conditions of th imonia converter indicated the phenomena such When t he i nlet tempera ture of the first catalyst (1 bed was decreased up to 420°C, aboirt Kg/cin, reduction of th e syn thesis pressni-e was served. On the way to decrease the inlet tcmperatiire (2 from 430 °C to 415"C, the o utlet temperature the first catalyst be d d ecreased to a same de gree,
HIGH TEMP Accordingly the reaction rate per unit catalyst volume, V, has an optimum point at a temperatu re point. Test Procedure
The existing operation conditions of the ainrnoiiia converter were simulated and aiialyzed the computer prior to th e optimization test. The best way to get an optimized tem perature profile through th e catalyst beds was discussed and a decision was made to decrease the inlet tem pera ture of the first catalyst bed to aroiiii 400°C from existing 432°C by feeding onIy first bed quench flow. Then, the predicted quench flow for each catalyst bed was determined to achieve an optimized temperature profile by the compute r simulation on basis of 400°C
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April, 1987
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BEFORE
EST
SIMULATED AFTER TEST 1
400
1
1
1
1
1
1
,
1
1
1
450 TEMP.
("C)
Figure 4. Temperature p rofile through catalyst beds.
Plant/Ope ration s Progres (Vol. 6, No.
2)
SUMMARY Before Test ~
Production Rate (%) Pressure (KGicm'C) Synthesis ga compressor discharge AC Inlet Loop
AC
l'
After
-rest
_
103
_
10
145 139
15.2 5.9
134 14.7
Quench Flow Hate (70) f Total Feed Flow (Estiniated) 8. 10.7 #2 Bed 11.6 10.0 Bed 8.5 8.2 #4 Bed Pcr Pass Conversion (%) 9.74 9.85 Calciilated Power (KW) Of. synthesi s gas compresso r 15,020 14,530 490 Difference 10,160 Turbine Speed (RPM) 10,080 80 Difference 0.02-0.03 M M K c d l T - N I I , Expected Energy Savin
whereas the outle t temperature of the succ eeding catalyst beds remained as they were. On t he way from 415°C to 400"C, the synthes is pressure was kept as low a 134 Kg/cm2Gand the outlet tempera ture for each catalyst bed was d ecreased according to inlet temperatu re decrease of the first catalyst bed. Th e comparison of operating condition after the test with that before test is shown in Table Figure hows the temperature profile through th e catalyst beds with each inlet temperature estimated the computer using the measured data.
Th e optimization test of the ammon ia converter opera tion was carried out during th e normal operation, ac_ cordance with the tem perature profile through the cata lyst bed optimized computer. Th e results, the effects and th e expectation in future obtaine d by this test can be summarized as below. Results: Temperature decrease of catalyst beds by (1) 10-40°C (2 Synthesis pressure decre ase al)out Kg/cmz (3) Synthesis loop pressure drop ahoiit 0.5 Kg/cm2
(4)
Conv ersion per pass im provem ent from 9.74% to 9.85%
Effects
0.02-0.03 MMKcaliTon-NH, energy saving (1 Expectation in future: Longer catalyst life due to relatively lower op er(1 ating temperature Production rate increase with same energy consumption In short, significant power savings and an expected longe r catalyst life.
Shimagaki holds B . E . in cheniical engineer ing from Kyoto University in Japan. He has over fifteen years exper ienc e in animonia, inethano1 and fertilizer process desig n. Hr is c u r r e n t l y s e n i o r e n g i n e e r o f p r o c es s e n g i n e e r i n g d e p a r t m e n t of Toyo Engineering Corp. Makoto
Optimization Test Results
Th e optimization test results under the condition of consta nt producti on rate are ta1)ulated in Talile The remarkable improvement of the ammonia converter operation are: Temperature of th e catalyst beds went down to (1 440-470°C by 10-40°C (2 Synthesis pressure dec reased about Kg/cm2. (3 Pressure drop through the synthesis loop decreased by ab out 0 .5 Kgicm'. Per pass conversion in th e Ammonia converter was imp roved fro 9.74% to 9.85%.
Kenjiro Miyashita holds
h1.E. in chem ical engineei-ing from Kyushn University in Japan. H e has ten years ex peri ence in aniinonia and metharrol procevs design. He currently lead enginee r of p r o ce s s e n g i n e e r i n g d e p a r t m e n t Toyo Engineering Corp.
Consideration
The optimized temperature profile could be established d uring the normal operation by t he adjustment of the quench valve. This results in the improvement of per pass conversion. Th e effect is equiv alent to the power reduction of out 490 KW o e synthesis gas compressor which corresponds to 0.02-0.03 MMKcal/Ton-NH, energ y saving of natural gas. And further the Ammonia Plant in P.T. AAF will be able to expect the fo!lowing from the plant operation point of view: (1 Longe r catalyst life du e to relatively lower operating temperature (2 Production rate increase with same natural ga consumption by reducin g the purge gas rate This test may make a suggestion to th e ammonia manufacturers:
(1) (2
It is significant to grope for an op timize d temp erature profile of the ammonia converter dependin g on plant by plant operating conditions. Tem pera ture control of the first catalyst I)ed is most important for the optimized operation.
Alita Ilyas j o i n e d i n P . T . . 4 se a A c e h F e r t i l i z e r (P.T. A A F ) in 1980. e is cnrrently th e opcratioir manager of P.T. A A F .
Aslam Kalyubi h o l d s R . E . i n c h e n r i c d e n g i n e e r i n g fr o m In s ti t ot c , o f T e c h n ( ~ l ( ~ gaiidnng y in Indoncsia. He is currently developnrent engin eer of Ascan Aceh Fei-tiIizt>r.
