Gas mixtures, separation

The energy required to reversibly separate gas mixtures is the same as that necessary to isothermally compress each component in the mixture from the partial pressure of the gas in the mixture to the final pressure of the mixture. This reversible isothermal work is given by the familiar relation... 

Temperature Two modes of temperature parametric-pumping cycles have been defined—direct and recuperative. In direct mode, an adsorbent column is heated and cooled while the fluid feed is pumped forward and backward through the bed from reservoirs at each end. When the feed is a binary fluid, one component will concentrate in one reservoir and one in the other. In recuperative mode, the heating and cooling takes place outside the adsorbent column. Parametric pumping, thermal and pH modes, have been widely studied for separation of liquid mixtures. However, the primary success for separating gas mixtures in thermal mode has been the separation of propane/ethane on activated carbon [Jencziewski and Myers, Ind. Eng. Chem. Fundam., 9, 216-221 (1970)] and of air/S02 on silica gel... 

Rodicker, H. 1974. Separating gas mixtures using metal coated membranes. East German Patent 107,859. 

The importance of the pressure ratio in separating gas mixtures can be illustrated by considering the separation of a gas mixture with component concentrations (mol%) nio and njo at a feed pressure of p0. A flow of component across the membrane can only occur if the partial pressure of component i on the feed side of the membrane, is greater than the partial pressure of component i on the permeate side of the membrane, nkpi. That is... 

These polychelates are used as stationary phase in molecular sieve GC to separate gas mixtures of rare gases. 

Immersion calorimetry can be apply successfully to the characterisation of CMS to evaluate their pore size distribution and, in this way, their ability to separate gas mixtures as a function of their molecular size. On the other hand, carbon molecular sieves can be prepared from coconut shells by activation with C02. These materials can be used for the separation of gas mixtures such as O2/N2, CO2/CH4 and n-C4Hio/i-C4Hio. 

Lemcoff, N.O. and Gmelin, R.C. (1993). Pressure swing adsorption method for separating gas mixtures. US Patent 5,176,722. 

The specific applications section is classified and grouped according to the type of solutes separated gas mixtures, conversion, degradation, separation, and purification of biochemical products (membrane bioreactors) at wastewater treatment, types of module tested and types of carrier used. 

The study of gas transport in membranes has been actively pursued for over 100 years. This extensive research resulted in the development of good theories on single gas transport in polymers and other membranes. The practical use of membranes to separate gas mixtures is, however, much more recent. One well-known application has been the separation of uranium isotopes for nuclear weapon production. With few exceptions, no new, large scale applications were introduced until the late 1970 s when polymer membranes were developed of sufficient permeability and selectivity to enable their economical industrial use. Since this development is so recent, gas separations by membranes are still less well-known and their use less widespread than other membrane applications such as reverse osmosis, ultrafiltration and microfiltration. In excellent reviews on gas transport in polymers as recent as 1983, no mention was made of the important developments of the last few years. For this reason, this chapter will concentrate on the more recent aspects of gas separation by membranes. Naturally, many of the examples cited will be from our own experience, but the general underlying principles are applicable to many membrane based gas separating systems. 

Gas permeation is used to separate gas mixtures, for example, hydrogen fixjm methane. High pressures on the order of 500 psia are used to force the molecules through a dense polymer membrane, which is packaged in pressure-vessel modules, each containing up to 4,000 ft of membrane surface area. Membrane modules cost approximately 35/ft of membrane surface area. Multiple modules are arranged in parallel to achieve the desired total membrane area. 

Absorption processes are mainly used to separate gas mixtures and to purify gases. With a gas separation, the dissolved com-... 

Table 9. The Clusius-Dickel method of separating gas mixtures...

Activated carbons are widely used as adsorbents in either the gas or the liquid phase, and also as catalysts and catalyst support. In some cases their adsorption behaviour depends basically on their textural characteristics, i.e., porous structure and pore volume. As an example carbon molecular sieves (CMS) are a kind of activated carbons that make use of a narrow pore size distribution, of a few angstroms in diameter, to selectively separate gas mixtures [1], such as N2/O2, CO2/CH4, C3H6/C3H8 and some others. But also the surface chemistry can condition in many cases [2,3] the adsorption behaviour, as well as the activity as catalyst and catalyst support [4, 5]. This means that the adsorption properties of the activated carbons cannot be easily explained only on the basis of textural characteristics (surface area and pore size distribution) the nature of the chemical surface must also be taken into account. Therefore, for a complete characterization of the activated carbon surface, textural and chemical characteristics must be assessed. 

It is clear from the above discussion, carbon membrane still requires much improvement and a long journey to go through before it will become a dominant commercialized inorganic membrane in this century. However, caibon membrane has a great potential to replace other inoiganic membranes in the market because it has a number of unique characteristics and is able to separate gas mixtures, which have similar size of gas molecules, efficiently. 

Carbon membranes are effective for separating gas mixtures with similar... 

This is beeause they have many useful characteristics and are able to efficiently separate gas mixtures that have molecules of similar sizes. A number of recommendations are made for future investigative work. 

Polymers separate gas mixtures because they are more permeable to some components in the mixture than to others. The ability of a polymer to be more permeable to component A in a mixture of A and B is characterized by the ideal selectivity, CCa/B which is the ratio of permeabilities of the two components (2) ... 

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