Syngas coke formation

Freni et al 2 and Galvita et al 1 have employed a two-layer catalyst bed for the SRE reaction. Ethanol is dehydrogenated to acetaldehyde over a Cu/Si02 catalyst72 or decomposed to a mixture of methane, CO and H2 over a supported Pd catalyst87 in the first layer. A supported Ni catalyst in the second layer reformed the acetaldehyde or methane into syngas. The use of two-layer catalyst bed was reported to reduce the coke formation significantly. 

Burke, N. and David, T. Coke formation during high pressure catalytic partial oxidation of methane to syngas. Reaction Kinetics and Catalysis Letters, 2005, 84, 137. 

Syngas can be obtained as equilibrium product of reaction (1). The main requirement for a syngas catalyst is therefore to activate methane and oxygen under conditions where the equilibrium composition is favorable, and where the catalyst is not active for total oxidation (reaction 2) or coke formation. 

Augustine et al. [28] reported durability of porous stainless steel supported Pd tubular membranes for WGS reaction. The synthesized membrane is very stable in the presence of H2/H2O mixture. No significant change in the H2 permeance is observed for 2000 h. However, under WGS conditions when the H20 C0 ratio was 2 1, a reduction in H2 recovery was observed over 65 h due to coke formation on the membrane surface. They conducted another WGS experiment for 1000 h with a higher H20 CO ratio of 3 1 and stable behaviour was observed. They achieved 97% CO conversion and 85% H2 recovery from a simulated syngas mixture for 900 h. 

Kiylov et aZ. studied the catalytic reaction of methane with carbon dioxide over MnO -containing catalysts for syngas production. The effect of the modification of nickel by different additives, such as copper, chromium, iron and manganese oxides, was evaluated and the best effect was obtained with manganese oxide an increase of manganese content originated an increase of the carbon dioxide conversion and a decrease of coke formation. 

In the preparation of syngas from methane, there is always some total oxidation of methane to CO2 and H2O as well as coke formation. Coke formation can be reduced by improving the catalytic selectivity, while CO2 and H2O can be reused in reforming reactions ... 

Vernon et al tested a series of noble metal catalysts, either Ru, Rh, Pt, Pd or Ir supported on AI2O3, or rare earth ruthenium pyrochlore materials. All materials yielded the equilibrium CPO conversion of methane to syngas at IITC and 1 atm. After the reaction, it was found that Ru had been reduced out of the pyrochlore structures. Claridge et al. studied coke formation over alumina-supported or pyrochlore-derived catalysts and reported that the coke forming rates decreased in the order Ni > Pd > Rh > Ir, as illustrated in Fig. 8.3. ... 

Burke, N. and Trimm, D. (2005). Coke Formation during High Pressure Catalytic Partial Oxidation ofMethane to Syngas, React. Kinet. Catal. Lett., 84, pp. 137-142. 

The capital cost of POX can be high because of the need for post treatment of the raw syngas to remove carbon and acid gases. There are also issues of coke and soot formation if the oxidation temperature becomes too low or the mixing of the feed components is incomplete. The addition of steam to the process allows for greater flame temperature control and suppression of carbon however, the hydrogen production efficiency is reduced due to more fuel being consumed in the combustion... 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

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