Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 22.2.3.8 NH 3 synthesisThe synthesis of ammonia takes place on an iron catalyst at pressures usually in the range of100 – 250 bar and at temperatures of between 350 and 550 °C:N 2 + 3H 2 d 2NH 3 gH 0 = -46 kJ/molOnly 20 – 30 % of the synthesis gas is converted per pass to ammonia, due to unfavourableequilibrium conditions. The unreacted gas is recycled after removing the ammonia formed.Fresh synthesis gas is supplemented in the loop.As the exothermic synthesis reaction proceeds, there is a reduction in volume and so a higherpressure and lower temperature favours the reaction. The temperature of the catalyst needs to becontrolled, as the heat of reaction at the necessary equilibrium and reaction rate produces a risein temperature. Subdividing the catalyst into several layers is one technique which can achievethis temperature control. In this technique, between the layers, the gases are cooled eitherdirectly by adding cooled synthesis gas or indirectly by generating steam. Various converterdesigns can be utilised for this purpose.For ammonia condensation from the loop, cooling with just water or air is insufficient toachieve low ammonia concentration at the inlet. For this reason, vaporising ammonia is used tochill the gas. The ammonia vapours are liquefied by means of a refrigeration compressor. Thevarious synthesis configurations may differ with respect to the point where the make-up gas isadded or where liquefied ammonia and purge gas is withdrawn. New developments refer to theuse of more active catalysts such as cobalt-promoted iron and ruthenium. These catalysts allowa lower synthesis pressure and a lower energy consumption to be achieved (see Section 2.4.17).Conventional reforming with methanation as the final purification step produces a synthesis gascontaining unreacted gases and inerts (methane and argon). In order to prevent the accumulationof these inerts, a continuous purge gas stream has to be applied. Purge gas basically containsammonia, nitrogen, hydrogen, inerts and unreacted gases. The size of this purge stream controlsthe level of inerts in the loop, keeping these to approximately 10 – 15 %. The purge gas isscrubbed with water to remove ammonia, before then being used as fuel or before being sent forhydrogen recovery.2.2.3.9 Steam and energy systemThe high amount of surplus heat available from the flue-gas of the primary reformer, secondaryreformer, shift conversion and the ammonia synthesis requires the design of an efficient overallsteam system in which high pressure steam, usually in excess of 100 bar, is generated.Generally, all the high pressure steam will be fed to steam turbines driving the synthesis gascompressor. At an intermediate pressure level, a part of the steam is extracted from this turbineto supply the process steam for the reforming reaction and to drive other compressors, pumpsand fans. The rest of the steam in the main turbine is condensed. Modern ammonia plants do notimport energy to drive mechanical equipment, but in fact in most cases energy is exported toother consumers either as steam or as electricity. A way to improve the plant efficiency is to usea gas turbine to drive the air compressor and to use the hot exhaust gases as preheatedcombustion air for the reformer. In this case, the energy loss encountered by steam condensationin the usually employed steam turbine is avoided.The same approach is also applied for the refrigeration compressor, which is needed forcondensation of the ammonia product, and for the compressor used for pressurising process airin the secondary reforming step. A special advantage of these machines is that they can bedriven directly by steam turbines, using steam generated in the plant mainly from waste heat.This allows an efficient integration into the energy system of the whole plant. In addition, thehigh degree of reliability and the lower investment and maintenance costs, compared toreciprocating compressors, improve the plant economics.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 43