Reforming mechanisms

Trinuclear carbonyls have been studied with the anticipation that the retention would prove to be in some way inversely related to the molecular complexity. The values obtained were surprisingly high, despite careful chemical purification, as is shown in Table 10. It was suggested that the reformation mechanism must involve exchange reactions during and after the hot zone, starting with M(CO)4, as building blocks . 

To discriminate between the combustion-reforming mechanism and the pyrolysis mechanism, one must clarify whether CO or C02 is the primary product (or whether both are). Typically, to discriminate between primary and secondary... 

Catalyst thiotolerance for the n-hexane conversion followed the order Pt = Pt-Cre > Pt-Re > Pt-Sn (Fig. 2). However, the thiotolerance trend is different when the individual reactions involved in the n-hexane reforming mechanism are analyzed separately. In fact, TIJ,, TTg and TTJj values decreased in the order Pt-Ge > Pt > Pt-Re Pt-Sn whereas in the case of T I .c,... 

Li, C. and Suzuki, K. (2009) No TitlTar property, analysis, reforming mechanism and model for biomass gasification-an overview. Renewable and Sustainable Energy Reviews, 13, 594-604. 

In all cases the experiments were carried out at 800°C and the flow rates were so chosen to visit the kinetic regimes rather than equilibrium. For all the cases presented here no carbon deposition was observed due to the high dilution and high concentration of reforming agents present. This possibly increased the accuracy of the calculation since the heterogeneous reforming mechanism was not developed for carbon deposition conditions. 

Catalysts for partial oxidation consist mostly of a noble metal supported on porous ceramic monoliths [113]. Two mechanisms have been proposed the direct partial oxidation mechanism, where hydrocarbon fuel and oxygen are adsorbed and react directly on the catalyst surface to give H2 and CO, and the combustion reforming mechanism, which assumes that the reaction is initiated near the entrance of... 

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. 

Steam Reformings of Natural Gas. This route accounts for at least 80% of the world s methanol capacity. A steam reformer is essentially a process furnace in which the endothermic heat of reaction is provided by firing across tubes filled with a nickel-based catalyst through which the reactants flow. Several mechanical variants are available (see Ammonia). 

Gas-Heated Reforming. Gas-heated reforming is an extension of the combined reforming concept where the primary reformer is replaced by a heat-transfer device in which heat for the primary reforming reaction is recovered from the secondary reformer effluent. Various mechanical designs have been proposed which are variants of a shell-and-tube heat exchanger (12,13). 

Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

The primary reformer is essentially a process furnace in which fuel is burned with air to indirectiy provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because this is the primary mechanism for heat transfer at the high (750—850°C) temperatures required by the process. Reforming pressures in the range 3 —4 MPa (30,000—40,000 atm) represent a reasonable compromise between cost and downstream compression requirements. 

Thermal cracking tends to deposit carbon on the catalyst surface which can be removed by steaming. Carbon deposition by this mechanism tends to occur near the entrance of the catalyst tubes before sufficient hydrogen has been produced by the reforming reactions to suppress the right hand side of the reaction. Promoters, such as potash, are used to help suppress cracking in natural gas feedstocks containing heavier hydrocarbons. Carbon may also be formed by both the disproportionation and the reduction of carbon monoxide... 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

Reflux overhead vapor recompression, staged crude pre-heat, mechanical vacuum pumps Fluid coking to gasification, turbine power recovery train at the FCC, hydraulic turbine power recovery, membrane hydrogen purification, unit to hydrocracker recycle loop Improved catalysts (reforming), and hydraulic turbine power recovery Process management and integration... 

Used anodically it readily forms a protective film which is reformed if removed mechanically. This is grey-white in appearance and has a tendency to flake under the compressive stress produced at thickened areas. The film... 

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. 

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