Shift conversion

The gaseous product from a gasifier generally contains large amounts of carbon monoxide and hydrogen, plus lesser amounts of other gases. Carbon monoxide and hydrogen (if they are present in the mole ratio of 1 3) can be reacted in the presence of a catalyst to produce methane (Cusumano 

However, some adjustment to the ideal (1 3) is usually required and, to accomplish this, all or part of the stream is treated according to the waste gas shift (shift conversion) reaction. This involves reacting carbon monoxide with steam to produce carbon dioxide and hydrogen whereby the desired 1 3 mole ratio of carbon monoxide to hydrogen may be obtained  

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Ammonia production by partial oxidation of hydrocarbon feeds depends to some degree on the gasification step. The clean raw synthesis gas from a Shell partial oxidation system is first treated for sulfur removal, then passed through shift conversion. A Hquid nitrogen scmbbiag step follows. 

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

The second step after secondary reforming is removing carbon monoxide, which poisons the catalyst used for ammonia synthesis. This is done in three further steps, shift conversion, carbon dioxide removal, and methanation of the remaining CO and CO2. 

The product gas mixture from the secondary reformer is cooled then subjected to shift conversion. 

Exit gases from the shift conversion are treated to remove carbon dioxide. This may be done by absorbing carbon dioxide in a physical or chemical absorption solvent or by adsorbing it using a special type of molecular sieves. Carbon dioxide, recovered from the treatment agent as a byproduct, is mainly used with ammonia to produce urea. The product is a pure hydrogen gas containing small amounts of carbon monoxide and carbon dioxide, which are further removed by methanation. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

As particle size decreases, hydrogen leakage decreases and hot spot temperature in the bed is higher. Thus the smaller particle size has greater activity (see Table VI). A kinetic system which defines the reaction in terms of CO and C02 methanation and CO shift conversion was used to determine the activity (see last column of Table VI). 

Significant differences were observed when S/G was varied from 0.15 to 0.40. At the lower S/G ratios there is no CO shift conversion whereas there is CO shift conversion at the higher S/G ratios. When the data are evaluated and activity constants for CO and C02 methanation and CO shift conversion are determined, the activity for methanation remains the same regardless of the S/G. However, with high S/G, shift conversion occurs at about 25% of the rate of CO methanation. At low S/G, no shift conversion is observed. 

Consequently, two semicommercial pilot plants have been operated for 1.5 years. One plant, designed and erected by Lurgi and South African Coal, Oil, and Gas Corp. (SASOL), Sasolburg, South Africa, was operated as a sidestream plant to a commercial Fischer-Tropsch synthesis plant. Synthesis gas is produced in a commercial coal pressure gasification plant which includes Rectisol gas purification and shift conversion so the overall process scheme for producing SNG from coal could be demonstrated successfully. The other plant, a joint effort of Lurgi and El Paso Natural Gas Corp., was operated at the same time at Petrochemie Schwechat, near Vienna, Austria. Since the starting material was synthesis gas produced from naphtha, different reaction conditions from those of the SASOL plant have also been operated successfully. 

H2/CO Ratio. In a commercial shift conversion plant, a change in throughput and conversion must be taken into account since it will affect... 

Residual C02 Content. The feed gas to Rectisol gas purification contains 29-36 vol % C02 depending on the rate of shift conversion. The rate of C02 to be washed out will be determined by the requirements of methane synthesis and by the need to minimize the cost of Rectisol purification. 

The series of reactors and exchangers which methanates a raw syngas without pretreatment other than desulfurization is collectively termed bulk methanation. The chemical reactions which occur in bulk methana-tion, including both shift conversion and methanation, are moderated by the addition of steam which establishes the thermodynamic limits for these reactions and thereby controls operating temperatures. The flow sequence through bulk methanation is shown in Figure 1. 

There is no separate shift conversion system and no recycle of product gas for temperature control (see Figure 1). Rather, this system is designed to operate adiabatically at elevated temperatures with sufficient steam addition to cause the shift reaction to occur over a nickel catalyst while avoiding carbon formation. The refractory lined reactors contain fixed catalyst beds and are of conventional design. The reactors can be of the minimum diameter for a given plant capacity since the process gas passes through once only with no recycle. Less steam is used than is conventional for shift conversion alone, and the catalyst is of standard ring size (% X %= in). 

Shift Conversion. The shift reaction and methanation proceed concurrently without interference over bulk methanation catalyst thereby eliminating the need for a separate shift conversion operation. 

Steam Utilization. Less steam is used in the RMProcess than is required for conventional shift conversion even though in other methanation processes as little as one-half of the total syngas is processed through shift conversion in order to achieve a near-stoichiometric balance of hydrogen and carbon monoxide for methanation. 

Reforming 573 K Shift-Conversion Phosphoric Acid, 473 K or Proton Exchange Membrane Fuel Cells, 363 K... 

The next two stages in the process carry out shift conversion at lower temperatures in which the second reaction above is used to convert CO to H2 to higher conversion. 

Pr q/Pro = 5,0 are relevant sorption effects of CO- but not of H2 thus only the 1 wavefronts represent rather tne shift conversion). Therefore it seems conceivable that there are two different mechanisms which participate in the CO shift conversion which is also in agreement with the established two different sorption mechanisms for 1 0 and with the transient behavior, depicted on Figure 6. 

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