Primary reformer catalysts Composition

The catalyst used was Katalco 23-1 Primary Reforming Catalyst, a commercial nickel reforming catalyst supported on alumina. Its chemical composition was reported as 10-14% NIO, 0.2% SIO2 balance AI2O3. It was supplied as hollow cylinders of size 5%-in O.D., 5/16-in 1.0. and 3/8-1n long, and had an apparent bulk density of 66 5 lb/ft. The rings were crushed and sieved to obtain the 24/32 mesh cut used In all of the experiments. 

Natural gas is reacted with steam on an Ni-based catalyst in a primary reformer to produce syngas at a residence time of several seconds, with an H2 CO ratio of 3 according to reaction (9.1). Reformed gas is obtained at about 930 °C and pressures of 15-30 bar. The CH4 conversion is typically 90-92% and the composition of the primary reformer outlet stream approaches that predicted by thermodynamic equilibrium for a CH4 H20 = 1 3 feed. A secondary autothermal reformer is placed just at the exit of the primary reformer in which the unconverted CH4 is reacted with O2 at the top of a refractory lined tube. The mixture is then equilibrated on an Ni catalyst located below the oxidation zone [21]. The main limit of the SR reaction is thermodynamics, which determines very high conversions only at temperatures above 900 °C. The catalyst activity is important but not decisive, with the heat transfer coefficient of the internal tube wall being the rate-limiting parameter [19, 20]. 

Synthetic gas can be produced from a variety of feedstocks. Natural gas is the preferred feedstock when it is available from gas fields (nonassociated gas) or from oil wells (associated gas). The first step in the production of synthesis gas is to treat natural gas to remove hydrogen sulfide. The purified gas is then mixed with steam and introduced to the first reactor (primary reformer). The reactor is constructed from vertical stainless steel tubes lined in a refractory furnace. The steam to natural gas ratio is 4—5 depending on natural gas composition (natural gas may contain ethane and heavier hydrocarbons) and the pressure used. A promoted nickel-type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700 800°C and 30—50 atm, respectively. The product gas from the primary reformer is a mixture of H2, CO, C02, unreacted CH4, and steam. The main reforming reactions are ... 

It therefore appears preferable to convert the methane by air, so as to introduce the nitrogen required. This operation is performed at a comparable temperature, in order to maintain the required thermal levels of the successive operating sequences and to avoid excessively disturbing the stream compositions. This is done in the presence of nickel-, based catalysts similar to those employed in the primary reforming reactor, to guarantee the conversion of low hydrocarbon contents in a dilute medium. Post-combustion is thus carried out adiabatically, between 8S0 and 1000 C, at a pressure that is also close to that of the initial steam reforming. 

The sulfur-free natural gas is then mixed with steam and sent to a pre-reformer where a nickel-based catalyst decomposes complex hydrocarbons into methane in order to avoid cracking in the primary reformer and to feed it with a uniform stream whose composition is independent of variations in the composition of the original feedstock. 

Desulfurized natural gas or naphtha is then mixed with process steam and preheated before passing to the primary reformer. The steam ratio, which is the molar ratio of steam to carbon, is typically between 3.0 and 4.0 moles of steam per atom of carbon in the hydrocarbon feedstock. An excess of steam over the stoichiometric quantity is required to suppress carbon-forming reactions and to provide a favorable equilibrium composition for the reaction of methane. The primary reformer consists of a large number of tubes packed with supported nickel oxide catalyst and contained in a furnace. The purpose of>the furnace is to heat the reactants to... 

In another process, tested by Uhde, reforming of hydrocarbons in catalyst-filled tubes was combined with the partial oxidation of the reformed gas at 1300°C within a single vessel (Figure 9.10). The primary reforming reaction was similar to that of the CRG process and operated endothermically at a low steam/ratio. The composition of the product gas and control of the reaction was... 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

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