Ammonia Manufacture

Introduction Ammonia is compound of nitrogen and hydrogen with formula NH 3. At standard temp. (25 o C) and pressure (1 atm) ammonia is a gas. It is toxic and corrosive to some materials and has a characteristic pungent odour. Ammonia used commercially can be anhydrous ammonia or an aqueous solution of ammonia and water referred to as ammonium hydoxide. Anhydrous ammonia must be stored under pressure or at low temp. to remain a liquid.

Uses of Ammonia In addition to serving as a fertilizer ingredient like in urea, ammonium nitrate, it can also be used directly as a fertilizer by forming a solution with irrigation water, without additional chemical processing. The second important single use of ammonia is in the production of nitric acid.

Uses of Ammonia Ammonia has good thermodynamics properties that make it very well suited as a refrigerant. Since it liquefies readily under pressure and was used in virtually all refrigeration units prior to the advent of haloalkane. It also some times added to drinking water along with chlorine to form chloramines a disinfectant. it used to manufacture a lot of other chemicals compounds like HCN and explosives etc.

Block Diagram Of Steam Reforming Process

Overall Conversion The theoretical Process conversion, based on methane feedstock are given in the following approximate formulae 0.88CH Air H 2 O 0.88CO N 2 +3H 2 N 2 + 3H 2 2NH 3

Feedstock Desulphurization Most of the catalysts used in the process are sensitive to sulfur and sulfur compounds. The feedstock normally contains up to 5mg/Nm 3 (normal m 3 ) as sulfur compounds. The feedstock is pre heated to o C and then treated in a desulphurization vessel, where the sulfur compounds are hydrogenated to H 2 S, Typically using a Cobalt molybdenum catalyst and also commonly used catalyst ZnO.

Cont… R-SH + H 2 H 2 S + RH H 2 S + ZnO ZnS + H2O In this way sulfur is remove to less than 0.1ppm sulfur in the gas feed. The ZnS remains in the adsorption bed. The hydrogen for the reaction is usually recycled from the synthesis reaction.

Steam Reforming Primary Reforming The gas from desulphurizer is mixed with process steam. Mixture is then heated further to o C before entering the primary reformer. The primary reformer consist of a large number of high nickel chromium alloy tubes filled with nickel containing reforming Catalyst. The overall reaction is highly endothermic. The composition of the gas leaving the primary reformer is given by following chemical equilibria CH 4 + H 2 O CO + 3 H 2 ΔH=206kJ/mol CO + H 2 O CO 2 + H 2 ΔH= - 41kJ/mol The heat for the primary reforming process is supplied by natural gas or other fuel.

Secondary Reforming Upto 85% hydrocarbon feed is reformed in the primary reformer because of the chemical equilibria at the actual operating Conditions. The temp. must be raised to increase the conversion. This is done in the secondary reformer by internal combustion of the part of gas with process air, which also provides the N 2 for the final synthesis gas H 2 + Air N 2 + H 2 O The process gas is cooled o C in a waste heat boiler.

Shift Conversion Process gas form the secondary reformer contains 12-15% CO (dry gas bases) and most of the CO is converted in the shift section according to the reaction CO + H 2 O CO 2 + H 2 ΔH= - 41kJ/mol In the High Temp. Shift Conversion (HTS), the gas is passed through a bed of iron oxide/ chromium oxide catalyst at around 400 o C, where the CO content are reduced to 3%(dry gas base), limited by shift equilibrium at the actual operating Temp. The gas from HTS is further cooled to increase the conversion and through the Low Temp. Shift (LTS) converter. The LTS converter is filled with a zinc oxide based catalyst and operated at about o C. The residual CO content in converted gas is %(dry gas base).

CO 2 Removal The process gas from LTS converter contains mainly H 2, N 2, CO 2 and the excess process steam. The gas is cooled and most of the excess steam is condensed before it enters the CO 2 removal System. The CO 2 is removed in a chemical or physical absorption process. The solvent used in chemical absorption processes are mainly aqueous amnie solution ( Mono Ethanolamine (MEA) ), Activated Methyl Di-Ethanolamine (AMDEA) or hot potassium carbonate solutions. Physical solvents are glycol, di-methyl ethers (selexol), Propylene carbonate and other.

Absorption & Stripping

Methanation The small amount of CO and CO 2, remaining in the synthesis gas, are poisonous for the ammonia synthesis catalyst and must be removed by conversion of CH 4 in the methanation CO + 3H 2 CH 4 +H 2 O CO 2 + 4H 2 CH 4 + 2H 2 O The reaction take place at around 300 o C in the reactor filled with a nickel containing catalyst. Methane is an inert gas in the synthesis Reaction, but the water must be removed before entering the convertor. This is done by cooling and condensation downstream of the methanator.

Synthesis Gas Compression After mathanation the gas pressure is 27kg/cm 2 ( atm) and temp. is 364 o C. This gas is pressurized to 151Kg/cm 2 ( atm) as required by the synthesis reaction. Modern Ammonia plants use centrifugal compressor for synthesis gas compression, usually driven by steam turbines, with the steam being produced in the ammonia plant.

Ammonia Synthesis The synthesis of ammonia is takes place on an iron catalyst at pressure usually in the range bar and temp o C. N 2 + 3H 2 2NH 3 ΔH = - 46 kJ/mol Only 20-30% is reacted per pass in the reactor due to the Unfavorable equilibrium conditions. The ammonia that is formed is separated from the recycle gas by cooling/ condensation, and un- reacted is substituted by the fresh make up synthesis gas, thus maintaing the pressure. In addition, extensive heat exchanger is required due to exothermic reaction.

NH 3 Purification Conventional reforming with methanation as the final purification step produces a synthesis gas containing inerts ( methane and argon) in quantities that do not dissolve in the condensed Ammonia. The major part of these inerts are removed by taking out a purge stream from the loop. The purge gas is scrubbed with water to remove ammonia before being used as fuel or before being sent for hydrogen recovery.

Energy Conservation Conservation and reuse of heat is so vital that it has been said that an ammonia plant is really a steam generating plant that incidentally produces ammonia. Much more steam than ammonia is made 4:1.