Carbon shift conversion

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

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. 

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. 

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). 

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. 

Thus the conversion of cis-6, 9 dimethylspiro [4, 4] nona-1, 3 diene to dimethyl bicyclo nona-diene is an example. The first product is a suprafacial [1, 5] carbon shift with retention of configuration at the migrating carbon. This is then followed by a [1, 5] hydrogen shift which becomes the major product. 

The gases from the furnace are cooled by the addition of condensate and steam, and then passed through a converter containing a high or low temperature shift catalyst, depending on the degree of carbon monoxide conversion desired. Carbon dioxide and hydrogen are produced by the reaction of the carbon monoxide with steam. 

Note that in the second step, the shift conversion process (also known as the carbon monoxide or water gas shift reaction), more hydrogen is formed along with the other product, carbon dioxide. A variety of methods is used to make carbon dioxide, but this process is the leading method. 

C 0 and H O, unavoidable by-products of alcohols synthesis. Considering chemical reactions of table H, water and carbon dioxide appear as equiva-lentby-products due to shift conversion equilibrium, equation (1). Most other low temperature alcohol synthesis catalysts have a rather high shift activity as well. CO removal fhom reacted syngas of synthesis loop, before recycling to reactor, leads to a significant decrease of water formation which, in turn, results in a lower water content in the raw alcohols, leading to simplified fhactionation-dehydration processes. 

A thermal sigmatropic 1,7-carbon shift has been ruled out as the mechanism for the conversion of l-phenylspiro[2.6]nona-4,6,8-triene (9) to 8-phenylbicyclo[5.2.0]nona-l,3,5-triene (10). Most... 

This system includes several mixing and heat exchange units. A concept for an integrated, microtechnology-based fuel processor was proposed by PNNF [8]. As examples for unit operations which may be included in future integrated systems the same publication mentions reactors for steam reforming and/or partial oxidation, water-gas shift reactors and preferential oxidation reactors for carbon monoxide conversions, heat exchangers, membranes or other separation components. 

The shift conversion involves two stages. The first stage employs a high-temperature catalyst, and the second uses a low-temperature catalyst. The shift converters remove the carbon monoxide produced in the reforming stage by converting it to carbon dioxide by the reaction... 

As shown in Figure 5.4 and Figure 5.17 and as described for each of the major processes that produce synthesis gas, the Water Gas Shift Conversion or the Carbon Monoxide Shift reaction is one of the traditional purification steps that will still be found in many ammonia plants. The CO must be removed because it acts as a poison to the catalyst that is used in ammonia synthesis. 

The shift from carbon monoxide to carbon dioxide generally occurs in two steps - first a High Temperature Shift Conversion and then a Low Temperature shift conversion. In some cases the two steps may be combined in one isothermal or adiabatic step called Medium Temperature Shift Conversion. When the feed gas to the CO conversion is not desulfurized, the CO conversion is called Sour Gas Shift and a special type of sulfur-resistant catalyst is used166. 

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