Separation of gas mixtures

We have seen several examples of a technique for separation of gas mixtures which, in contrast with most commercial processes, requires no physical transfer of solvent, handling of solids, or cycling of temperature or pressure. The energy requirements can also be far lower The thermodynamic minimum work of separation is, under isothermal conditions, the free energy difference between the process stream and byproduct, or permeate, stream. When this difference is due only to the partial pressure difference of component 1, it becomes ... 

Rao, M.B. and S. Sircar, Nanoporous carbon membranes for separation of gas mixtures by selective surface flow, /. Membr. Sci., 85(3), 253-264, 1993a. 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

The chemical modification of poly(arylene ether sulfone)s has already been described in numerous papers. They relate to sulfonation, fluorination, and halomethylation. These derivatives are particularly suitable for the preparation of hydrolysis- and temperature-resistant separation membranes. They are used already for sea water desalination, and also for the separation of gas mixtures. 

The importance of pressure ratio in the separation of gas mixtures can be illustrated by considering the separation of a gas mixture with component concentrations of iiio and iijo at a feed pressure p0. A flow of component i across the membrane can only occur if the partial pressure of i on the feed side of the membrane (riiop0) is greater than the partial pressure of i on the permeate side of the membrane (niepe), that is,... 

S.A. Stern, Industrial Applications of Membrane Processes The Separation of Gas Mixtures, in Membrane Processes for Industry, Proceedings of the Symposium, Southern Research Institute, Birmingham, AL, pp. 196-217 (1966). 

Stern, S. A., and Koros, W. J. (2000). Separation of gas mixtures with polymer membranes A brief overview, Chimie Nouvelle 18(72), 3201-3215. 

Specific properties of molecular sieves are the reason for which these materials found application as adsorbents for separation of gas mixtures, especially air [2]. In fact, one can say that carbon-based molecular sieves played fundamental role in commercialisation of the pressure swing adsorption process (PSA) used for separation of nitrogen from air [3]. 

Carbon molecular sieve (CMS) is useful in air separation processes because of its ability to selectively discriminate on the basis of molecular size and hence adsorb the smaller oxygen molecule over nitrogen. The difference in the adsorption kinetics of various gases allows the separation of gas mixtures into pure components using pressure swing adsorption (PSA). 

Zhu, H.Y., W.H. Gao and E.F. Vansant 1994, The pillared clay-carbon composite membrane the preparation, vapour permeation and separation of gas mixtures, presented at 3rd Int Conf. Inorg. Membr., Worcester, MA, USA. 

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. 

M. Hassan, J.D. Way, P.M. Thoen and A.C. Dillon, Separation of gas mixtures using hollow fiber silica membranes, in Y.H. Ma (Ed.), Proceedings of 3rd International Conference on Inorganic Membranes (ICIM3), July 10-14,1994, Worcester, MA, USA. Distributed by Worcester Polytechnic Institute, 100 Institute Rd. Worcester, MA 01609, USA. pp. 325-335. 

Lundy, K. "Separation of Gas Mixtures via Multi-Layered Composite Membrane" MS Thesis, Chem. Dept., State Unlv. of N.Y. ESF Syracuse, N.Y. (1984). 

For the separation of gas mixtures (permanent gases and/or condensable vapors) where the feed and permeate streams are both gas phase, the driving force across the membrane is the partial pressure difference. The membrane is typically a dense film and the transport mechanism is sorption-diffusion. The dual-mode transport model is typically used with polymer materials that are below their glass transition temperature. 

Pressure-swing adsorption. Although adsorption is most often used as a purification process to remove small amounts of material, a number of applications involve separations of gas mixtures with moderate to high concentration of adsorbates. These are called bulk separations, and they often use different operating procedures than for gas purification. Pressure-swing adsorption (PSA) is a bulk separation process that is used for small-scale air separation plants and for concentration of hydrogen in process streams. 

Membranes for the separation of gas mixtures are of two very different kinds one a microporous membrane, the other nonporous. Microporous membranes were the first to be studied and the basic law governing their selectivity was discovered by Graham.1 When pore size of a microporous membrane is small compared to the mean-free-path of the gas molecules, permeate will be enriched in the gas of the lower molecular weight. Since molecular weight ratios of most gases are not very large and since the selectivity is proportional to the square... 

N. I. Timofeev, F. N. Berseneva, V. M. Makarov, New palladium-based membrane alloys for separation of gas mixtures to generate ultrapure hydrogen, Int.]. Hydrogen Energy 1994, 39(11), 895-898. 

In chemical, petrochemical and related industries, separation processes are usually responsible for a major part of the production costs. In the separation of gas mixtures, adsorptive processes like Pressure Swing Adsorption (PSA) are being used by small and medium-sized industries, mainly because they have been found more efficient and economical than traditional separation methods. As part of her research project for a... 

Since IBM s offer greater stability than ILM s and greater selectivity and permeability than PM s, it would be useful to be able to model transport processes in these materials and to predict the effectiveness of facilitated transport based on relevant physical properties (RPP). Although it may be necessary to modify the model developed for ILM s in order to completely describe transport processes in IBM s, it is likely that moat of the same RPP s of the system will be Important. The purpose of this section is to point out that measurement of RPP s in IBM s, especially permselective IBM s, may be difficult. Although problems with model development and property measurement exist, carrier Impregnated IBM s can produce rapid and selective separations of gas mixtures. Way and co-workers have incorporated the monoprotonated ethylenediamlne cation into Nafion membranes to achieve the separation of carbon dioxide from methane (25). 

Vorotyntsev V M, Malyshev V M Mochalov G M, et al. 2001. Separation of gas mixtures by the gas hydrate crystallization method. Theoretical Foundations of Chemical Engineering, 35(2) 119. 

Sivakumar, S.V. and Rao, D.P. (2012) Adsorptive separation of gas mixtures Mechanistic view, sharp separation and process intensification. Chem. Eng. Process., 53, 31-52. 

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