Purification of Synthesis Gas

If sulfur is present as H2S or COS, it is a poison for many catalysts and will partly or completely inhibit the catalyst activity46. Carbon monoxide (CO) and carbon dioxide (CO2) can poison the ammonia synthesis catalyst so both of these compounds must be removed53. 

Fi(wt 5J3. Alternative process steps for generation and purification of synthesis gas 

Hydrogen Product Pressure Feed Pressure Feed Pressure Feed / Low 

Scale Economics (Ease of Expansion) Modular Moderate Good 

Besides the economic benefits, the PSA-based hydrogen plant has a number of technical advantages compared to traditional Steam Methane Reforming with solvent extraction and methanation. These advantages include172  

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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. 

Refineries and petrochemical industry Paraffins, olefins, acetylenes, reformer gas, hydrocracking gas, solvents Sweetening of liquid petrol gas and aromatics, removal of CO2 from olefin containing gases, purification of synthesis gas Normal and branched-chain alkanes... 

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. 

Methanol synthesis plants utilizing the low-pressure process currently operate at capacities of 2 x 105 to 2 x 106 metric tons per year [15]. Such installations are composed of a synthesis gas production unit, the actual methanol synthesis reactor, and a separation and purification section. The production and purification of synthesis gas accounts for 50%-80% of the total cost of methanol production, with the remaining cost associated with the actual synthesis and purification of methanol [2, 8], Although a variety of carbonaceous feedstocks can be transformed into synthesis gas, the steam reforming of natural gas (Equation [4]) is by far the most common option, especially for large plants [2, 15-16] ... 

Fig. 22.8. Alternative process steps for generation and purification of synthesis gas. (Courtesy of Wiley-VCH. Bakemeier, H., Huberich.T., et al. "Ammonia" in Ullmann s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 2, VCH Verlagsgesellschaft, Weinheim 1985, pp. 143-242.

Final purification of synthesis gas by removal of oxygen-containing impurities... 

Purification of Synthesis Gas. The raw synthesis gas must be purified to remove solids such as unbumed carbon and fine ash. This is generally done by water washing the gas in packed or spray towers, cooling as well as cleaning the gas. 

Purification of synthesis gas. The sulfur- and carbon-containing compounds in synthesis gas must be removed in order to avoid the poisoning of catalysts in the following processes. Sulfur and carbon containing compounds are absorbed by different solvents. The used solvents are regenerated by desorption and H2S (or element S) and carbon dioxide are recovered. The trace amounts of carbon monoxide and carbon dioxide which remained in synthesis gas is removed via the reaction of methanation or other methods. After a series of purifications, the content of carbon monoxide and carbon dioxide in the synthesis gas are on the levels of ppm (1 ppm = 1 ml m ). 

During ammonia synthesis, the major reactions of production and purification of synthesis gas and the synthesis of ammonia, all are carried out over different catalysts. At least eight kinds of catalysts are used in the whole process, where natural gas or naphtha is used as feedstock and steam reforming is used to produce synthesis gas. These catalysts are Co-Mo hydrogenation catalyst, zinc oxide desulfurizer, primary- and secondary-steam reforming catalysts, high- and low-temperature shift catalysts, methanation catalyst and ammonia synthesis catalyst etc (Table 1.1). 

The absorption of carbon dioxide in water at elevated pressure was formerly an important industrial process, particularly for the purification of synthesis gas for ammonia production. The process has now generally been replaced by more efficient systems which employ chemical or physical solvents with much higher capacities for carbon dioxide than water. Such systems are described in Chapters 2, 3, 3, and 14. A description of the water wash process for carbon dioxide removal is included in this chapter because of its historical interest, its technical value as a classical liquid film-controlled operation, and the hope that the extensive work done on the process will prove usefril in the development of new processes or applications. 

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