Reactors ammonia production

The technology of urea production is highly advanced. The raw materials requited ate ammonia and carbon dioxide. Invariably, urea plants ate located adjacent to ammonia production faciUties which conveniently furnish not only the ammonia but also the carbon dioxide, because carbon dioxide is a by-product of synthesis gas production and purification. The ammonia and carbon dioxide ate fed to a high pressure (up to 30 MPa (300 atm)) reactor at temperatures of about 200°C where ammonium carbamate [111-78-0] CH N202, urea, and water ate formed. 

In the production of ammonia from hydrogen and nitrogen the conversion, based on either raw material, is limited to 15 per cent. The ammonia produced is condensed from the reactor (converter) product stream and the unreacted material recycled. If the feed contains 0.2 per cent argon (from the nitrogen separation process), calculate the purge rate required to hold the argon in the recycle stream below 5.0 per cent. Percentages are by volume. 

When produced from natural gas the synthesis gas will be impure, containing up to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate are favoured by high pressure. The conversion is low, about 15 per cent and so, after removal of the ammonia produced, the gas is recycled to the converter inlet. A typical process would consist of a converter (reactor) operating at 350 bar a refrigerated system to condense out the ammonia product from the recycle loop and compressors to compress the feed and recycle gas. A purge is taken from the recycle loop to keep the inert concentration in the recycle gas at an acceptable level. 

Ammonia production in a biophotoreactor. Initial experiments showed the requirements for optimal continuous production of a flow-through reactor packed with foam immobilized cyanobacteria. Results indicated that at the dilution rate of 0.4 h -1 and in the presence of MSX, ammonia production ceased after... 

The purified gas is fed into the Synthol and fixed-bed reactors. The products from the reactors are cooied and separated in a water phase, oil phase and tail gas. The + Ca olefinic products from the tail gas are separated in an oil absorption tower and oligomerized over an acidic catalyst to gasoline. Tite remaining tali gas can be treated in a cryogenic unit to provide methane and hydrogen, which is partly used as fuel gas or feedstock for ammonia synthesis. The remainder is steam-reformed over nickel catalysts to give CO/H3. 

Converters with indirect cooling have been known since the early days of ammonia production, for example, the Fauser-Montecatini reactor [843], [844], [848], [867], [891]-[893], In this converter, tube coils between catalyst beds transfer the reaction heat to a closed hot water cycle under pressure, operating by natural draft. The hot water releases the absorbed heat in an external steam boiler generating about 0.8 t of steam per tonne of ammonia at about 45 bar (ca. 250 °C). 

Figure 116. Ammonia production based on heavy fuel oil (l.inde flow scheme with Texaco gasification) a) Air separation unit h) Soot extraction c) C02 absorption d) Methanol/H,() distillation e) Stripper f) Mol regenerator g) Refrigerant h) Dryer i) liquid N2 scrubber j) Syngas compressor k) Nil, reactor Material Balance...

Summarize the effects on ammonia production (/ip) and reactor throughput (nr) of changing each of the three input variables. 

All ammonia production plants in the world operate according to the same basic principles i.e. reaction of nitrogen and hydrogen in a catalyst-filled pressure reactor at temperatures between 400 and 500°C, pressures between 100 and 1000 bar (depending upon the plant) and removal of the ammonia formed from the reaction gas. The plants differ in their design, catalyst composition and production and purification of the synthesis gas. 

Figure E5.5a shows a simplified flowsheet. All the units except the separator and lines are adiabatic. The liquid ammonia product is essentially free of Nz, Hz, and A, and assume that the purge gas is free of NH3. Treat the process as four separate units for a degree-of-freedom analysis, and then remove redundant variables and add redundant constraints to obtain the degrees of freedom for the overall process. The fraction conversion in the reactor is 25%.

The working principal of a Haber-Bosch reactor is sketched in Fig. 1. Reactors actually used by the commercial processes, e.g., the Haber-Bosch reactor at BASF, the radial fiow converter at Haldor-Topsoe bear the same working principal. This is a typical tube-bundle of fixed-bed reactors. The production volume is typically 1500 tons of ammonia per day. The feedstocks are air containing N2 (and enriched O2 to reach a high catalyst activity) H2 is made from... 

The principal conclusion, then is that attempts to make the ammonia production more efficient should start with improvement of the reactors. 

The forward reactions also are favored at higher pressures. However, the space velocity becomes high with increased pressures, and contact time becomes shorter, decreasing the yield. The actual process conditions of pressure, temperature, and space velocity are practically a compromise of several factors. Raney nickel is the preferred catalyst. Typical methanation reactor operating conditions are 200—300°C and approximately 10 atm. The product is a gas mixture of hydrogen and nitrogen having an approximate ratio of 3 1 for ammonia production. 

The conventional ammonia production line consists of seven gas-solid catalytic reactors, namely desulfurization unit, primary reformer, secondary reformer, high temperature shift, low temperature shift, methanator and finally the ammonia converter. In addition the production line includes an absorption-stripping unit for the removal of CO2 from the gas stream leaving the low temperature shift converter. The ammonia converter is certainly the heart of the process with all the other units serving to prepare the gases for the ammonia synthesis reaction which takes place over an iron promoted catalyst under conditions of high temperature and pressure. 

In the following sections, we will analyse the dynamic behaviour of a gas-fed, adiabatic, catalytic-bed reactor, of the sort commonly used for methanol and ammonia production. Figure 20.4 gives a schematic representation of an element from such a... 

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