Methanation high exothermicity

The processes that have been developed for the production of synthetic natural gas are often configured to produce as much methane in the gasification step as possible thereby minimizing the need for a methanation step. In addition, methane formation is highly exothermic which contributes to process efficiency by the production of heat in the gasifier, where the heat can be used for the endothermic steam—carbon reaction to produce carbon monoxide and hydrogen. 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

The highly exothermic reaction has already been mentioned. It is particularly important to realise that at the elevated temperatures employed other reactions can occur leading to the formation of hydrogen, methane and graphite. These reactions are also exothermic and it is not at all difficult for the reaction to get out of hand. It is necessary to select conditions favourable to polymer formation and which allow a controlled reaction. 

Chlorine gas reacts directly and highly exothermically with alkanes, giving rise to alkyl chlorides and hydrogen chloride, e.g., for addition to methane. 

These reactions are highly exothermic, for example one mole of -CH2- units generates 165 kj. The hydrocarbon distribution ranges from methane up to heavy waxes, depending on the nature of the catalyst and the reaction conditions. 

Methanation of CO requires removal of C02 due to the highly exothermic competitive methanation. 

Methanation Reactions While carrying out the WGS reaction, methane can be formed in the reactor through the methanation reaction, which is the reverse methane SR reaction and is highly exothermal. 

These values of A Hr are standard state enthalpies of reaction (aU gases in ideal-gas states) evaluated at 1 atm and 298 K. 7VU values of A are in kilojoules per mole of the first species in the equation. When A Hr is negative, the reaction hberates heat, and we say it is exothermic, while, when A Hr is positive, the reaction absorbs heat, and we say it is endothermic. Tks Table 2-2 indicates, some reactions such as isomerizations do not absorb or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation reactions are fairly exothermic. Note, for example, that combustion or total oxidation of ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + H2) or ethylene (C2H4) are only slightly exothermic. 

Attention also must be given to the explosion and fire hazards presented by combustible organic vapors and combustible gases such as hydrogen and methane. These vapors are readily ignited by static electricity, electrical sparks from most laboratory appliances, open flames, and other highly exothermic reactions. Thus appreciable atmospheric concentrations of combustible vapors should be avoided. 

This reaction equation describes the combustion of methane, a reaction you might expect to release heat. The enthalpy change listed for the reaction confirms this expectation For each mole of methane that combusts, 802 kJ of heat is released. The reaction is highly exothermic. Based on the stoichiometry of the equation, you can also say that 802 kJ of heat is released for every 2 mol of water produced. (Flip to Chapter 9 for the scoop on stoichiometry.)... 

We have chosen to concentrate on a specific system throughout the chapter, the methanation reaction system. Thus, although our development is intended to be generally applicable to packed bed reactor modeling, all numerical results will be obtained for the methanation system. As a result, some approximations that we will find to apply in the methanation system may not in other reaction systems, and, where possible, we will point this out. The methanation system was chosen in part due to its industrial importance, to the existence of multiple reactions, and to its high exothermicity. 

To retain consistency throughout this presentation, we will consider a general nonadiabatic, packed bed reactor, as shown in Fig. 1, with a central axial thermal well and countercurrent flow of cooling fluid in an exterior jacket.1 We focus on the methanation reaction since methanation is a reaction of industrial importance and since methanation exhibits many common difficulties such as high exothermicity and undesirable side reactions. 

The methanation reaction is a highly exothermic process (AH = —49.2 kcal/ mol). The high reaction heat does not cause problems in the purification of hydrogen for ammonia synthesis since only low amounts of residual CO is involved. In methanation of synthesis gas, however, specially designed reactors, cooling systems and highly diluted reactants must be applied. In adiabatic operation less than 3% of CO is allowed in the feed.214 Temperature control is also important to prevent carbon deposition and catalyst sintering. The mechanism of methanation is believed to follow the same pathway as that of Fischer-Tropsch synthesis. 

Tile partially purilied synthesis gas leaves the C02 absorber containing approximately 0.1% CO2 and 0.5% CO. This gas is preheated at the methanator inlet by heat exchange with the synthesis-gas compressor interstage cooler and the primary-shift converter effluent and reacted over a nickel oxide catalyst bed in the methanator. The methanation reactions are highly exothermic and are equilibrium favored by low temperatures and high pressures. 

The direct chlorination of methane is carried out as a radical reaction in the gas phase and the highly exothermal reaction produces a mixture of chlorinated methanes (Table 1, entry 6). Higher chlorination is achieved by recycling lower... 

Alkylalumoxanes are prepared by a variety of methods, with the preferred being the reaction of trimethylaluminum with water (ice), a violent, highly exothermic reaction best carried out at low temperatures in an inert solvent (71, 72). Instead of water, hydrolysis of A12(S04)3- 16 H20 can be used. In both cases the reaction is evidenced by the evolution of methane during hydrolysis ... 

A tube-wall reactor, in which the catalyst is coated on the tube wall, is conceptually ideally suited for highly exothermic and equilibrium-limited reactions because the heat generated at the wall can be rapidly taken away by the coolant. Previous work (1) has numerically demonstrated that for highly exothermic selectivity reactions, the optimized tube-wall reactor is superior from both steady state production and dynamic points of view to the fixed-bed reactor. Also, the tube-wall reactor is being advanced as a possible reactor for carrying out methanation in coal gasification plants (2). From a reaction engineering point of view, it therefore seems appropriate to analyze the reactor for the analytically resolvable case of complex first-order isothermal reactions. 

The value of AH° for the chlorination of methane is about —105.1 kJ/mol (—25.0 kcal/mol). This is a highly exothermic reaction, with the decrease in enthalpy serving as the primary driving force. 

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