Carbon dioxide methane selectivity

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. 

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

Base polymer Carbon dioxide/methane selectivity ... 

Improved hydrogen sulfide, carbon dioxide/methane-selective membranes make membranes the low-cost method of separating acid gases from natural gas. 

Despite these failures, microporous carbon membranes continue to be a subject of research by a number of groups [67-70], The selectivities obtained are often very good, even for simple gas mixtures such as oxygen/nitrogen or carbon dioxide/methane. However long-term, it is difficult to imagine carbon membranes... 

Figure 8.6 The difference between selectivities calculated from pure gas measurements and selectivities measured with gas mixtures can be large. Data of Lee et al. [13] for carbon dioxide/methane with cellulose acetate films. Reprinted from S.Y. Lee, B.S. Minhas and M.D. Donohue, Effect of Gas Composition and Pressure on Permeation through Cellulose Acetate Membranes, in New Membrane Materials and Processes for Separation, K.K. Sirkar and D.R. Lloyd (eds), AIChE Symposium Series Number 261, Vol. 84, p. 93 (1988). Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1988 AIChE. All rights reserved...

For the substrates of interest, ammonia and methane, the most stable products of oxidation are nitrogen and carbon dioxide respectively. Selectivity, accordingly, in the oxidation of ammonia refers to the production of NO or N2O for methane oxidation selective reaction products may be formaldehyde or carbon monoxide. Oxidation of methane alone over platinum is usually of low selectivity. In the case of selective oxidation of CH4/NH3 mixtures, HCN is the product of interest. 

The expected return of oil price increases over the next decade will again spur investigation into conversion processes for alternative fuel sources such as coal. Coal gasification will produce substantial amounts of carbon dioxide as a by-product. If this carbon dioxide could be converted economically to methanol or methane, established zeolite catalytic processes could convert these intermediates to gasoline. Furthermore, concern about providing a substitute for natural gas to the established gas pipelines has led the Gas Research Institute to sponsor investigation of carbon dioxide conversion selectively to methane (2). 

Here we present evidence that this can be achieved in three different cases ethylene epoxidation, carbon dioxide methanation and Fischer-Tropsch synthesis [6], and demonstrate the advantages for ethylene epoxidation with a 2-D computer model of the process. Table 2 summarizes the rationale for selecting these processes as examples. Since the purpose was to demonstrate feasibility and advantage for foam-supported catalysts, no attempt was made to incorporate promoters to improve activity or selectivity. 

Because oxygen, carbon dioxide, methane, and other alkanes are completely miscible with dense supercritical water, combustion can occur in this fluid phase. Both flameless oxidation and flaming combustion can take place. This leads to an important application in the treatment of organic hazardous wastes. Nonpolar organic wastes such as polychlorinated biphenyls (PCBs) are miscible in all proportions in supercritical water and, in the presence of an oxidizer, react to produce primarily carbon dioxide, water, chloride salts, and other small molecules. The products can be selectively removed from solution by dropping the pressure or by cooling. Oxidation in supercritical water can transform more than 99.9 percent of hazardous organic materials into environmentally acceptable forms in just a few minutes. A supercritical water reactor is a closed system that has no emissions into the atmosphere, which is different from an incinerator. 

Table 4.8 Permeation Coefficients and Selectivity in Carbon Dioxide Methane Mixtures ...

A potential large scale application for gas separation emerges in natural gas purification. For this reason, the carbon dioxide/methane system is subject to extensive research. Sulfonated PPE and a blend of PPE with heteropolyacids (HPA)s has been compared with respect to their separation efficiency for mixtures of carbon dioxide and methane. The permeation coefficients and the selectivity are shown in Table 4.8. 

Nickel and ruthenium [342,343] catalysts are those most frequently under investigation for the selective methanation of carbon monoxide. The main issue of concern is that the concentration of carbon dioxide is much higher in the reformate compared with carbon monoxide and thus the catalyst used for carbon monoxide methanation has to be very selective. Normally, the operating window of methanation catalysts is relatively small, around 250 °C, because a trade-off is required between sufficient activity and selectivity. Well above 250 °C all methanation catalysts tend to be selective for carbon dioxide methanation. 

Wang Y, Takahashi Y, Ohtsuka Y (1998) Carbon dioxide-induced selective craiversion of methane to C2 hydrocarbons on Ce02 modified with CaO. Appl Catal Got 172 L203-L206... 

When methane and oxygen are converted in the gas phase at temperatures between 850 and 1150 K, C2 hydrocarbons are formed besides carbon monoxide along with minor amounts of C3 j hydrocarbons, formaldehyde, methanol and carbon dioxide. The selectivities depend on the conditions applied i.e., partial pressures of methane and oxygen respectively, as well as temperature. 

A novel nanocomposite membrane, poly dimethyl siloxane (PDMS)/Au was prepared for carbon dioxide/methane separation. Synthesis of stabilized nano particles is also reported. The nanoparticles were characterized by UV-visible spectroscopy and transmission electron microscopy (TEM). The hybrid membrane was characterized morphologically by scanning electron microscope (SEM) and the change in inter-segmental distance due to filler loading by wide angle X-ray diffraction patterns (WAXD). The gas transport properties were measured at different pressures and temperatures. The effects of filler loading on permselectivity, diffiisivity selectivity and solubility selectivity are reported for CO2/CH4 separation. Reverse selective phenomena of PDMS/Au nanocomposite membrane over the conventional PDMS membrane is explained based on sorption kinetics of CO2. 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

The composition of the gas mixture, which is introduced into the tube bundle reactor (tubes of 6-12 m length and 20-50 mm diameter, filled with the Ag catalyst) consists of 15-50 vol % ethylene, 5-9% oxygen, as much as 60% methane as dilution gas, and 10-15% carbon dioxide. The reaction therefore proceeds above the upper explosion limit. The ethylene conversion runs up to 10% per cycle through the reactor. The ethylene oxide selectivity amounts to 75-83 % maximum. The formed ethylene oxide is recovered by scrubbing with water and the newly formed carbon dioxide is separated from the cycle gas, e.g., by hot potash washing process. 

The selectivity is 100% in this simple example, but do not believe it. Many things happen at 625°C, and the actual effluent contains substantial amounts of carbon dioxide, benzene, toluene, methane, and ethylene in addition to styrene, ethylbenzene, and hydrogen. It contains small but troublesome amounts of diethyl benzene, divinyl benzene, and phenyl acetylene. The actual selectivity is about 90%. A good kinetic model would account for aU the important by-products and would even reflect the age of the catalyst. A good reactor model would, at a minimum, include the temperature change due to reaction. 

Selective Conversion of Methane to Ci Hydrocarbons using Carbon Dioxide as an Oxidant over CaO-MnO/CeOi Catalyst... 

GP 2[ [R 3a[ Catalysts need to be initially activated on-stream with a mixture of 20% ethylene and 20% oxygen in methane as balance [44], The temperature was raised until first formation of carbon dioxide became notable. The initial selectivity is close to 70% and after time-in-stream for 1 day at 250 °C decreases to 62% at 1.3% conversion. This loss in selectivity at the expense of conversion is a general phenomenon during all investigations conducted in [44], Non-promo ted catalysts show a certain decrease in selectivity within a few days, particularly at high temperature and conversion. 

Of great interest and importance are studies on carbon dioxide reduction on copper electrodes, performed primarily by Japanese scientists. Under certain conditions, formation of methane and ethylene with high faradaic yields (up to 90%) was observed. The efficiency and selectivity of this reaction depends very much on the purity and the state of the surface of the copper electrode. For this reason, many of the published results are contradictory. 

---