Methanation of carbon oxides

In Situ Methanation of Carbon Oxides. During the bench-scale test program, we noticed that carbon oxides were apparently being metha-nated in the reactor because more hydrocarbons and less carbon oxides were present in the products than would be indicated by carbon and carbon oxides balances. We then carried out two tests with no oil shale... 

We also made thermodynamic equilibrium calculations to see how much methane could be formed by methanation of carbon oxides. These calculations show that, for all conditions within the reactor tube, it is possible (thermodynamically) to form methane in quantities even greater than those observed. 

Additional data obtained in the countercurrent tests verified the ability to suppress carbonate decomposition by adding carbon dioxide to the feed hydrogen. We also discovered that substantial methanation of carbon oxides occurs in the shale bed. 

D. Summary of Mechanism Studies.—The recent research does not solve old problems. There are areas of disagreement in the findings, no doubt due to different catalysts and experimental conditions. No single primary product can be identified. Carbon oxides are often the major products leaving the catalyst surface, in relative concentrations dependent upon the efficiency of the catalyst for the CO shift reaction (4). On Rh the CO content tends to be high and the CO 2 low, probably indicative of lower activity for the shift reactions compared with Ni, where CO tends to be lower. Methane however can be formed directly from the higher hydrocarbons at 600 °C and above, as well as by methanation of carbon oxides. 

Men, Y, Kolh, G, Zapf, R, Hessel, V, Lowe, H. Selective methanation of carbon oxides in a microchannel reactor—Primary screening and impact of additives. Catal. Today 2007 125 81-87. 

Kowalczyk, Z. Stolecki, K. Raro g-Pilecka, W. Mis kiewicz, E. Wilczkowska.E. Karpin ski, Z. Supported Ruthenium Catalysts for Selective Methanation of Carbon Oxides at Very Low CO /H Ratios. Appl. Catal. A Gen. 2008, 342, 35-39. 

Synthesis Gas Preparation Processes. Synthesis gas for ammonia production consists of hydrogen and nitrogen in about a three to one mole ratio, residual methane, argon introduced with the process air, and traces of carbon oxides. There are several processes available for synthesis gas generation and each is characterized by the specific feedstock used. A typical synthesis gas composition by volume is hydrogen, 73.65% nitrogen, 24.55% methane, <1 ppm-0.8% argon, 100 ppm—0.34% carbon oxides, 2—10 ppm and water vapor, 0.1 ppm. 

Steam Reforming Processes. In the steam reforming process, light hydrocarbon feedstocks (qv), such as natural gas, Hquefied petroleum gas, and naphtha, or in some cases heavier distillate oils are purified of sulfur compounds (see Sulfurremoval and recovery). These then react with steam in the presence of a nickel-containing catalyst to produce a mixture of hydrogen, methane, and carbon oxides. Essentially total decomposition of compounds containing more than one carbon atom per molecule is obtained (see Ammonia Hydrogen Petroleum). 

The rich gas from the absorption operation is usually stripped of the desirable components and recycled back to the absorber (Figure 8-57). The stripping medium may be steam or a dry or inert gas (methane, nitrogen, carbon oxides—hydrogen, etc.). This depends upon the process application of the various components. 

In this work, we will show that the addition of TCM to the feedstream in the methane conversion process results in the enhancement of the conversion of methane and the selectivity to C2 hydrocarbons on praseodymium oxide primarily as a result of the formation of praseodymium oxychloride, in contrast with the production of carbon oxides on praseodymium oxide in the absence of TCM (8-10). The surface properties of these catalysts are characterized by application of adsorption experiments and X-ray photoelectron spectroscopy (XPS). 

Although other experimental evidence also exists for a formate to methoxy mechanism, Baiker argues that formate intermediates are not involved in the reduction of carbon oxides to methanol on Zr02-based catalysts.8,35,624 Rather, formates are intermediates in methanation (see Section 3.2.1). 

The main problems associated with the direct oxidation of methane are the higher reactivity of the products (methanol and formaldehyde) compared to methane, and the thermodynamically more favorable complete combustion of methane to carbon oxides and water ... 

Purification of Synthesis Gas. This involves the removal of carbon oxides to prevent poisoning of the NIT3 catalyst. An absorption process is used to remove the bulk of the C02, followed by methanation of the residual carbon oxides in the methanator, Modern ammonia plants use a variety of C02-removal processes with effective absorbent solutions. The principal absorbent solutions currently in use are hot carbonates and cthanolamincs. Other solutions used include methanol, acetone, liquid nitrogen, glycols, and other organic solvents. 

The methanator effluent is cooled by heat exchange with boiler feedwater and cooling water. Tile synthesis gas leaves the methanator containing less than 10 parts per million (ppm) of carbon oxides. 

Compared with the common high-temperature conversion of natural gas and further carbon oxide conversion on a catalyst [131], the current process promotes process simplification the reaction is implemented at relatively low temperature (860-900 °C instead of 1400-1600 °C for existing non-catalytic processes of methane conversion) and an additional unit for catalytic conversion of carbon oxide is excluded (in NH3 production). 

In carbonisation systems, coke forming in the absence of oxygen restricts the formation of carbon oxides to what can be formed from water and oxygen present in the coal. Some hydrogen is formed by the water gas shift reaction but most (and the methane formed) is a consequence of decomposition of the large coal hydrocarbons into the elements. The typical gas composition is shown in Table 11.1. 

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