Carbon dioxide conventional separation processe

Other gas permeation applications include separation of hydrogen from methane, hydrogen from carbon monoxide, and removal of components such as carbon dioxide, helium, moisture, and organic solvents from gas streams. Gas permeation for such operations may provide a more economical and more practical alternative than conventional separation processes such as cryogenic distillation, absorption, or adsorption. 

The environmentally benign, nontoxic, and nonflammable fluids water and carbon dioxide (C02) are the two most abundant and inexpensive solvents on Earth. Water-in-C02 (w/c) or C02-in-water (c/w) dispersions in the form of microemulsions and emulsions offer new possibilities in waste minimization for the replacement of organic solvents in separations, reactions, and materials formation processes. Whereas the solvent strength of C02 is limited, these dispersions have the ability to function as a universal solvent medium by solubilizing high concentrations of polar, ionic, and nonpolar molecules within their dispersed and continuous phases. These emulsions may be phase-separated easily for product recovery (unlike the case for conventional emulsions) simply by depressurization. 

Emissions of carbon dioxide are comparable with emissions from a conventional coal plant. However, should future environmental regulations require the removal of CO2, an IGCC plant can separate and sequester CO2 from the process at a significantly lower cost than conventional technologies. 

Supercritical fluid extraction (SFE) is a method that circumvents some problems associated with conventional separation techniques. Carbon dioxide, as an inert, inexpensive, nonflammable, and environmentally acceptable gas is the solvent of choice because of its moderate critical temperature and pressure (76). SFE has been used effectively to refine marine oils and remove cholesterol, polychlorinated biphenyls (PCB), Vitamin E, and other components (77). The disadvantages of this process include the use of extremely high pressures and the high capital cost. 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

The oxidative coupling of isobutene can be performed in two separate steps, coimected with reduction of catalyst and reoxidation of the reduced catalyst afterwards. The two step process leads to an improvement of DMH selectivity as compared to the conventional process. The formation of carbon dioxide requires surface lattice oxygen from tbe catalyst, while formation of DMH occurs by abstraction of protons and electrons at the catalyst surface. They are absorbed on the catalyst bulk and, finally, react to water there. Thus, the rate of carbon dioxide formation is more affected by catalyst reduction than the rate of DMH formation. 

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