General Membrane Separation Process

Fig. 19.2 The operation principle of the most common membrane separation processes, with membranes separating the feed (left) from the permeate phase (right). Circles and stars indicate volatile and non-volatile compounds, respectively. Driving forces acting upon solutes are indicated by arrows as gradients of pressure (P), activity (a) and electrostatic potential (V )- It shoidd be noted that all these driving forces are eventiudly based on a gradient of the chemical potential in its most general form...

Pervaporation is a membrane separation process in which a dense, non-porous membrane separates a liquid feed solution from a vapour permeate (Fig. 19.2c). The transport across the membrane barrier is therefore based, generally, on a solution-difliision mechanism with an intense solute-membrane interaction. It... 

This book provides a general introduction to membrane science and technology. Chapters 2 to 4 cover membrane science, that is, topics that are basic to all membrane processes, such as transport mechanisms, membrane preparation, and boundary layer effects. The next six chapters cover the industrial membrane separation processes, which represent the heart of current membrane technology. Carrier facilitated transport is covered next, followed by a chapter reviewing the medical applications of membranes. The book closes with a chapter that describes various minor or yet-to-be-developed membrane processes, including membrane reactors, membrane contactors and piezodialysis. 

The locations of the singular points in reactive membrane separation processes depend on both the separation effects (distillation/membrane separation) and the reaction effect. Singular points can be generally obtained as steady-state solutions of Eq. (88) ... 

For a more generalized analysis of the qualitative influence of membranes on the singular points, the reactive membrane separation process is now considered with a nondiagonal [/c]-matrix. The condition for a kinetic arheotropes is given by... 

The clogging effect can be considered as a reduction in the value of the surface filtration constant for practical purposes. Indeed, when clogging takes place, the surface filtration constant can be given by its initial value ko multiplied by a decreasing time function. This assumption is frequently used when the function is obtained from experiments [3.19, 3.20]. In our example, if we do not consider the friction (and heat transfer) we can note that only a concrete mass transfer problem can be associated with the membrane separation process. The first step before starting to build the general mathematical model, concerns the division of the system into different elementary sections. Indeed, we have a model for the filtration device (i.e. the membrane and its envelope), for the pump (P) and for the reservoir of concentrated suspension (RZ) (Fig. 3.7). 

For general reading on membranes see a. T. Matsuiua, Synthetic Membranes and Membrane Separation Processes, CRC Press, Boca Raton, 1994 b. RW. Baker, in Kirk-Othmo" Encyclopedia of Chemical Technology, Vol. 16, (1995), 13... 

The general subject has been explored in a number of reviews, as annotated in Hoffman (2003), and has been a concern in such serial publications as the Journal of Membrane Science and Membrane Separation Processes. The subject has also been of interest to the Gas Research Institute (now the Institute of Gas Technology) as per workshops that have been held on the subject (Gas Research Institute, 1981, 1982). In fact, the Gas Research Institute jointly sponsored a project with the Dow Corning Corporation and others aimed at correlating and predicting the permeability behavior of membranes from the chemical structure, as noted in Hoffman (1987) and Hoffman et al. (1988). 

Membrane-based separation processes to capture either H2 or CO2 from the gasifier are new and less studied methods of CO2 separation and capture. Membranes separate the desired gas component without requiring phase changes or chemical or physical sorption. The cost of membrane separation is generally dictated by the overall pressure drop. Membranes made of various types of materials such as polymers, metals, and rubber composites have been investigated. Palladium and molecular sieves are currently under study. ° ... 

Maxwell-Stefan (dusty gas) approach by taking the membrane to be the additional component in the mixture. When the model is extended to account for thermodynamic nonidealities (what may be considered to be a dusty fluid model) almost all membrane separation processes can be modeled systematically. Put another way, the Maxwell-Stefan approach is the most promising candidate for developing a generalized theory of separation processes (Lee et al., 1977 Krishna, 1987). 

Ultrafiltration, like reverse osmosis, is a pressure-driven membrane separation process. The applied pressures usually range from about 7 X 10" to 7 X 10 Pa, and the solvent, most often water, passes through the membrane. Material that does not pass through the membrane includes particulate matter, colloids, suspensions, and dissolved macromolecules of molecular weight generally greater than 10,000 and often greater than 2000. Rejection is usually close to complete. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Today, membrane separation processes are finding an extensive and ever-increasing use in the pharmaceutical, food, and petrochemical industries, as well as in environmental applications. Membrane separation processes present some advantages over more classical separation processes since they are less energy-consuming and more compact, and generally require less initial capital investment... 

Figure 9.1 General schematic of the membrane separation process, (a) Details the solution-diffusion mechanism used in gas separations.

For membrane separation processes, two types of membrane modular units are generally employed hollow fiber and spiral wound. The hollow fiber-based modules comprise bundles of hollow fibers encapsulated at hoth ends with the feed stream separated from the permeate stream hy the hollow fibers. For spiral wound units, alternate layers of membrane and spacers (to allow gas flow across the membrane on both the feed side and the permeate side of the membrane) are employed. Gas separation processes generally use hollow fibers, whereas desalination and water filtration processes generally employ spiral wound constructions. For membrane bioreactors for sewage water treatment, hollow fiber constructions are common. 

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