Gas separation by membrane permeation

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

Gas separation by membranes will always have to compete with other separation processes such as cryogenics, absorption and adsorption systems. Membranes usually are less competitive in very large scale operations where the fast gas is less than about 20% of the feed gas, unless the slow gas is the desired product. Membranes also are not usually the method of choice when extremely pure product gas is required. Membranes do, however, have distinct advantages in small to medium scale operations, in situations where gas is available at pressure, in situations where high recovery is paramount, and in applications where simplicity and minimal maintenance are of prime importance (such as in remote locations). Membranes are very well suited for applications in which the non-permeate is the product of interest, since it is obtained at pressure. Examples are acid gas removal from natural gas and gas dehydration. 

Assumptions used and ideal flow patterns. In deriving theoretical models for gas separation by membranes, isothermal conditions and negligible pressure drop in the feed stream and permeate stream are generally assumed. It is also assumed that the effects of total pressure and/or composition of the gas are negligible and that the permeability of each component is constant (i.e., no interactions between different components). 

The cross-flow model for reverse osmosis is similar to that for gas separation by membranes discussed in Section 13.6. Because of the small solute concentration, the permeate side acts as if completely mixed. Hence, even if the module is designed for countercurrent or cocurrent flow, the cross-flow model is valid. This is discussed in detail elsewhere (HI). 

First we will illustrate the minimum energy required to separate a small amount of mixture for the following processes evaporation of water from a saline solution recovery of water by reverse osmosis separation of an ideal binary gas mixture by membrane permeation. Then we will consider the definition of net work consumption for thermally driven processes. Next we will consider a variety of separation processes vis-k-vis their minimum energy requirement for separation. 

Gas purification and separation by membrane permeation has many advantages, including... 

Gas separations by distillation are energy-consuming processes. The driving force for the gas permeation is only the pressure difference between two compartments separated by the membrane. The permeation is governed by two parameters—diffusion and solubility ... 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

Figure 19.6. Gas permeation equipment and performance, (a) Cutaway of a Monsanto Prism hollow fiber module for gas separation by permeation, (b) Flowsketch of a continuous column membrane gas separator, (c) Composition profiles of a mixture of C02 and Oz in a column 5 m long operated at total reflux [Thorman and Hwang in ( Turbak, Ed.), Synthetic Membranes II, American Chemical Society, Washington DC, 1981, pp. 259-279],...

Teramoto M, Takeuchi N, Maki T, and Matsuyama H. Gas separation by liquid membrane accompanied by permeation of membrane liquid through membrane physical transport. Sep Purif Technol, 2001 24(1-2) 101-112. 

Guha AK, Majumdar S, and Sirkar KK. A larger-scale study of gas separation by hollow-fiber-contained liquid membrane permeator. JMem Sci, 1991 62(3) 293-307. 

In membrane systems, which require segregative properties e.g. in gas separation, usually large permeation in combination with a good separation factor (selectivity) is required. This is obtained by applying an external pressure gradient and a low partial pressure at the permeate (low pressure) side of the membrane (see also Section 9.3). A frequently used membrane system is schematically given in Fig. 9.11 as an example. 

In membrane technology, processes consisting of more than one stage are known in gas permeation and for the production of boiler feed water from seawater. Furthermore, cascades will be necessary if organic mixtures are to be separated by membrane processes only. 

The first field for which concentration polarization was deeply investigated (since the 1960s ) was liquid separation by membrane processes, such as ultrafiltration and reverse osmosis. On the contrary, for a long time it had been generally accepted that concentration polarization had only a negligible effect on membrane performance in gas separation. This was justified by the fact that membranes were quite thick and permeating flux very low, and, moreover, that... 

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