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1、Steam Production in Ammonia Synthesis LoopsThe heat of reaction in ammonia synthesis is 46.22 kJ/mol ammonia at standard temperature and pressure. It is of utmost importance to integrate the ammonia synthesis process with the steam generation system. The waste heat not u5ed for preheating the conver
2、ter feed gas and for preheating the inlet gas streams for each catalyst bed is most efficiently used for production of high pressure steam. Normally waste heat down to around 80*100aC can be utilised for these purposes.In large ammonia plants, steam is most often produced at 10- 12 MPa. The waste he
3、at downstream the synthesis converter is typically available for steam production between 450C and down to about 260C with the exact value depending on the layout and the conversion within the converter system (39). This means that the waste heat must be recovered partly in a boiler (down to about 3
4、40C)t and partly in a boiler feed water preheater. In low itiergy plants, the tendency is to increase the conversion by increasing the outlet concentration of ammonia from the last catalyst bed, resulting, in a reduced converter outlet temperalure (at constant pressure). At the same time, maximum hi
5、gh pressure st-eam production in The synthesis loop is desired in order to satisfy the overall rtergyin tht pkntLn some cases it may be necessary to recover pan of lhe heat before the read ions in the converter system have been completed. This may be accomplished in a converter system with two or mo
6、re converters, where： cooling, not only after th卑 Ust converter, but dso between, the converters, is done by rGCOvring lhe waste heat A)r steam production. Such a converter configuration will allow for more duty available tor boiling or superheating of steam, and might be a way to shift the waste he
7、at available tor boiling and boiler feed water prehwtmg in the loop.Ammonia Product RecoveryAmmonia is without exception recovered as a product from synthesis loops by cooling at synthesis pressure to condensation, fblhwed by separation of the liquid product from the gas, which is recirculated to th
8、e converter. In high- pressure synthesis loops, the cooling or part of it can be done by water cooling and/or air cooling. In modem plants, this is in most cases supplemented by cooling from a refrigeration unit. The refrigeration is typically supplied from a mechanical ammonia refrigeration cycle w
9、ith one, two or several refrigeration levels. Refrigeration down to -25C has been used, which corresponds to cooling by evaporation of ammonia at almost atmospheric pressure.The liquid rmnonia from theCQHUins a Small amount of dS-|vc：dgasiffs- Th 飾耳 are partly released by pressure reduction in a let
10、-down vesitT normally to about 2.5 MPa. After thevessel the ammonia is turtherflashed u almost stm.o-spfiric pressure before t)ein sem to the mmoniistorage tEk一 In order to redutt the risk pf 曲耳豁 qattpspoti Jcking in the ammonia, storage tanks the ammonia product should contain minimum 0.2% of water
11、.In order to increase th-e producL yield, the ammonia vapour from the 1 el-down, gas is recovered by washing with water, and this ammonia water 恆 distilled (oecher with ammonja waler obtained from, washing of the purg gas. Rimainirtg off-gas is nortnaily used as 他时 somewhere in the compk?i. The ammo
12、nia vapour &om th-e ast flash vessel is normally sent to the refrigeration compressor.Purge Gas RecoveryTn gasificatian based plants, or in plants using liquid nitrogen wash or PSA (Pressure Swing Adsorption) for final purification, th edn-terilration inerts in the make up gas to the syntheiis loop
13、my be so lew 一 inerts free loop with 100 vol ppm &r less 亠 lhat they 蛊te dissolved in ths liquid ammonia leaving, th匕 separator Jn such cases, rto contiiruous purge stream i$ taken from the loopIn the inerts-containing loop, the synthesis gas will nonnally contain certain coft ten (rat ions ofi Hert
14、s, Le, compounds tht are not consumed by the amm-onia synthesis reaction and that do not int&rfer-e with the catalyst performanceL The ineTts are lypically CHi, Art Eincl traces of other rare gases. In special cases, significant amounts of lieliiim may b已 present originating (rom He-containing natur
15、al gas. In all inQrts-irontwining hops, it is necessary to purge 4 弈 stream from the synthesis loop in order to prevent excessive bkiild-u p of the inerts,Along with, the inerts in the purge gas. stream also some Iiydrogen and nitragen is present, which represents potenthl raw materials for ammonia
16、produedon- And since energy has been invested in the production of cs-p-Ghlly the hydrogen and an bringing the gases to th七 synthesis pressure, it is obvio口5 to do an effbn to recover Lhe (lost hydrogen 孑甲tin It is r of the nitrogen is kss attractive, spwially in plwt? based on steam reforming! because prefercmini r
