Methane selectivity

A7 Ethane/methane selectivity calculated from grand canonical Monte Carlo simulations of mixtures in slit IS at a temperature of 296 K. The selectivity is defined as the ratio of the mole fractions in the pore to the ratio of mole fractions in the bulk. H is the slit width defined in terms of the methane collision diameter (Tch,- (Figure awn from Crackncll R F, D Nicholson and N Quirke 1994. A Grand Canonical Monte Carlo Study ofLennard-s Mixtures in Slit Pores 2 Mixtures of Two-Centre Ethane with Methane. Molecular Simulation 13 161-175.)... 

FoUowiag Monsanto s success, several companies produced membrane systems to treat natural gas streams, particularly the separation of carbon dioxide from methane. The goal is to produce a stream containing less than 2% carbon dioxide to be sent to the national pipeline and a permeate enriched ia carbon dioxide to be flared or reinjected into the ground. CeUulose acetate is the most widely used membrane material for this separation, but because its carbon dioxide—methane selectivity is only 15—20, two-stage systems are often required to achieve a sufficient separation. The membrane process is generally best suited to relatively small streams, but the economics have slowly improved over the years and more than 100 natural gas treatment plants have been installed. 

GP 10] [R 18] The ammonia selectivity of 62% at a p-gauze platinum catalyst is in the range of the technical process (Figure 3.48) the methane selectivity of 30% is only half the industrial performance (70 ml h methane 70 ml h ammonia ... 

Figure 11. The influence of cobalt particle size on the TOF (left) and on methane selectivity (right 220 °C, H2/CO = 2, 1 bar). (Reprinted from Reference [127], 2006, with permission from American Chemical Society).

As usual, employing Co catalysts leads to only little C02 formation. However, the methane selectivity is quite high compared to modern Co catalysts, where a CH4 selectivity of less than 10% is reached at 493 K.23 At this temperature the nanomaterial catalysts, except for the Co/PL catalyst (9% selectivity), exceed this limit already with 11 and 16% for the Co/HB and Co/MW materials, respectively. 

FIGURE 4.9 Activity profile and methane selectivity of an FTS run at 230°C, H2/CO = 2, 20 bar showing drop in activity after stopping H2 flow for 2 h. The H2 flow was reintroduced thereafter.63... 

Along with catalyst activity, product selectivity is a key issue in cobalt-based FTS.1 For GTL processes the preferred product is long-chain waxy hydrocarbons. It is well known that FT reaction conditions have an important effect on product selectivities. High temperatures and H2/CO ratios are associated with higher methane selectivity, lower probability of hydrocarbon chain growth, and lower olefinicity in the products.105... 

FIGURE 9.18 Influence of pressure on reaction rate, olefin content in the C3 fraction, and methane selectivity with cobalt as the catalyst for FT synthesis. Catalyst 100Co-18Th02-100 Si02 (Kieselguhr), H2/CO = 1.8, 175°C. 

Iron-based Fischer-Tropsch synthesis (FTS) catalysts are preferred for synthesis gas with a low H2/CO ratio (e.g., 0.7) because of their excellent activity for the water-gas shift reaction, lower cost, lower methane selectivity, high olefin... 

The data plotted in Figure 16.4a clearly show that CO conversion increases linearly upon increasing the H2/CO inlet ratio. Methane selectivity also increases with the H2/CO ratio, but the effect is less pronounced. On the contrary, the selectivity to both the heaviest hydrocarbons and olefins decreases upon increasing the... 

The results of the experiments, in terms of both CO conversion and light hydrocarbons selectivity, are shown in Figure 16.6a. As expected, CO conversion strongly decreases upon increasing space velocity. Moreover, a slight increase of methane selectivity is also observed, while selectivity to C5+, C25+, and olefinic... 

Kim42,43 found higher CO conversion and higher C5+ and lower methane selectivities when external water was added (20 parts per 100 parts of 2 1 H2 CO feed) during Fischer-Tropsch synthesis. However, when the water concentration in the feed was higher than 28% the CO conversion decreased. 

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

It is interesting to note that the same effect of water, both on activity and C5 + and methane selectivity, is noted for a supported Ru catalyst.52 The same rate equation as was used for Fe and Co-based catalysts, including a term for water, was also valid for the Ru-based catalyst (eqn (4)). 

The initial molar propane/propene-ratio (at 10 % of catalyst life time) is a measure of catalyst activity. It grossly correlates with the catalyst Si/Al-ratio. Fig. 7 concerns further proofs of catalyst life time. No correlation is observed between the amount of coke deposited and the total amount of methanol which is converted during the catalyst life time. However, a correlation appears to exist between (1) coke selectivity and catalyst life time and (2) methane selectivity and coke selectivity. 

Methane-Selective Oxidation of SUica-Supported Molybdenum (VI) Catalysts Structure and Catalytic Performance... 

Initially Mn was discovered to lower the methane selectivity and increase the olefin selectivity. More specifically, it was reported by Kolbel and Tillmetz that more than 50% Mn the remainder being Fe or by Bussemeir et about equal parts of Fe and Mn led to these beneficial F-T effects. In contrast to these results, van Dijk et did not find any changes in the product formation when adding manganese oxide to an Fe F-T catalyst, although this work had been performed at atmospheric pressures, whereas the work of Kolbel and Tillmetz and Bussemeier was done at conditions close to industrial ones. 

The most extensive studies of plasma-polymerized membranes were performed in the 1970s and early 1980s by Yasuda, who tried to develop high-performance reverse osmosis membranes by depositing plasma films onto microporous poly-sulfone films [60,61]. More recently other workers have studied the gas permeability of plasma-polymerized films. For example, Stancell and Spencer [62] were able to obtain a gas separation plasma membrane with a hydrogen/methane selectivity of almost 300, and Kawakami et al. [63] have reported plasma membranes... 

Table 3.3 Effect of fluorination on the carbon dioxide/methane selectivity of various glassy membrane materials...

Base polymer Carbon dioxide/methane selectivity ... 

The benefit obtained from counter-flow depends on the particular separation, but it can often be substantial, particularly in gas separation and per-vaporation processes. A comparison of cross-flow, counter-flow, and counter-flow/sweep for the same membrane module used to dehydrate natural gas is shown in Figure 4.18. Water is a smaller molecule and much more condensable than methane, the main component of natural gas, so membranes with a water/methane selectivity of 400-500 are readily available. In the calculations shown in Figure 4.18, the membrane is assumed to have a pressure-normalized... 

The competitiveness of membrane systems in this application is very sensitive to the selectivity of the membranes for propane, butane and higher hydrocarbons over methane. If the membranes are very selective (propane/methane selectivity of 5-7, butane/methane selectivity of 10-15), the permeate stream from the main set of modules will be small and concentrated, minimizing the cost of the recompressor. Currently, silicone rubber membranes are being considered for this application, but other, more selective materials have been reported [55],... 

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