Removal and Use of Ammonia in Gas Purification

A recent trend in gas purification has been to use dryers containing a molecular sieve to remove water from the synthesis gas before addition to the loop. This has the advantage that the water level can be reduced to a very low level (0.1 ppm v/v) and this enables the synthesis gas to be added to the loop downstream of the ammonia separator. This has a process advantage in that the concentration of ammonia in the gas returning to the converter is lower, because the concentration of ammonia at equilibrium at the separator temperature is diluted by the fresh synthesis gas which is, of course, free from ammonia. 

Ammonia, hydrogen sulfide, and carbon dioxide are major impurities in coke oven gas (COG). In addition to these three components, COG often contains caibon disulfide, carbonyl sulfide, hydrogen cyanide, organic acids, pyridine, phenol, and other impurities, which can cause problems with conventional amine plants. The presence of large quantities of ammonia in these gases naturally led to consideration of its use for removal of HjS and CO2, and several ammonia-based coke oven gas purification processes have been developed and commercialized. 

Gas Reduction. The use of a gaseous reduciag agent is attractive because the metal is produced as a powder that can easily be separated from the solution. Carbon dioxide, sulfur dioxide, and hydrogen can be used to precipitate copper, nickel, and cobalt, but only hydrogen reduction is appHed on an iadustrial scale. In the Sherritt-Gordon process, the excess ammonia is removed duting the purification to achieve a 2 1 ratio of NH iNi ia solution. Nickel powder is then precipitated by... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Purification of Synthesis Gas. This involves the removal of carbon oxides to prevent poisoning of the NIT3 catalyst. An absorption process is used to remove the bulk of the C02, followed by methanation of the residual carbon oxides in the methanator, Modern ammonia plants use a variety of C02-removal processes with effective absorbent solutions. The principal absorbent solutions currently in use are hot carbonates and cthanolamincs. Other solutions used include methanol, acetone, liquid nitrogen, glycols, and other organic solvents. 

Conventional reforming with methanation as the final purification step produces a synthesis gas that contains inerts (CH4 and argon) in quantities that do not dissolve in the condensed ammonia. Most of the inerts are removed by taking a purge stream out of the synthesis loop. The size of this purge stream controls the level of inerts in the loop at about 10% to 15%. The purge gas is scrubbed with water to remove ammonia and then it can be used as fuel or sent to hydrogen recovery. 

This process for production of synthetic ammonia by catalytic steam reforming of natural gas is a relatively clean process and presents no unique environmental problems. To assess the environmental impacts of a modem ammonia plant on air, water, and soil, each step in the ammonia synthesis namely, desulfurization, reforming, shift conversion, carbon dioxide removal, final purification, ammonia synthesis, and refrigeration should be examined. The sources of pollutants need to be identified and matched with cost-effective solutions for minimization/elimination by using the best available pollution control measure. 

In some forms of generators for the gas, which are used commercially, the finely powdered carbide runs slowly into a reservoir of water. A better regulation of the gas supply is effected by using this method, and overheating, which may lead to an explosion, is avoided. Acetylene prepared from commercial calcium carbide contains ammonia, hydrogen sulphide, and phosphine. These substances must be removed when the gas is to be used for indoor illumination. The purification is effected by washing the gas with water and a mixture of slaked lime and bleaching powder, or with a solution of chromic acid in hydrochloric acid. In the laboratory a solution of mercuric chloride in dilute hydrochloric acid can be conveniently used for the purpose. 

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