Membrane processes types

There are five types of membrane processes, which are commonly used in water and wastewater treatment ... 

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... 

Another factor is the ease with which various membrane materials can be fabricated into a particular module design.1618 Almost all membranes can be formed into plate-and-frame, spiral-wound and tubular modules, but many membrane materials cannot be fabricated into hollow fine fibres or capillary fibres. Finally, the suitability of the module design for high-pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can be important factors.4-11 The types of module generally used in some of the major membrane processes are listed in Table 16.2. 

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

A number of transport mediators are transport proteins in the absence of an external energy supply, thermal motion leads to their conformational change or rotation so that the transported substance, bound at one side of the membrane, is transferred to the other side of the membrane. This type of mediator has a limited number of sites for binding the transported substance, so that an increase in the concentration of the latter leads to saturation. Here, the transport process is characterized by specificity for a given substance and inhibition by other transportable substances competing for binding sites and also by various inhibitors. When the concentrations of the transported substance are identical on both sides of the membrane,... 

There are three major types of membrane processes, each with different physical means of operation reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF). In addition, electrodialysis (ED) is also considered to be a membrane process. 

Tab. 13.1 Types and characteristics of widely practiced membrane processes.

A simple system in which transport processes occur that are also characteristic of membrane processes is the Hquid junction formed between two electrolyte solutions in the same solvent. The region in which one electrolyte passes into the other is frequently a porous diaphragm of various construction (fig. 2.2). A second type of Hquid junction is the free diffusion region (fig. 2.3). 

In the supported liquid membrane process, the liquid membrane phase impregnates a microporous solid support placed between the two bulk phases (Figure 15.1c). The liquid membrane is stabilized by capillary forces making unnecessary the addition of stabilizers to the membrane phase. Two types of support configurations are used hollow fiber or flat sheet membrane modules. These two types of liquid membrane configuration will be discussed in the following sections. 

Film coefficients of mass transfer inside or outside tubes are important in membrane processes using tube-type or the so-called hollow fiber membranes. In the case where flow inside the tubes is turbulent, the dimensionless Equation 6.25a, b (analogous to Equation 5.8a, b for heat transfer) provide the film coefficients of mass transfer k [7]... 

Although many types of membrane modules are used for various membrane processes, they can be categorized as follows. (It should be mentioned here that such membrane modules are occasionally used for gas-liquid systems.)... 

One explanation might be found in the differences in topography of "antigenic receptors" 1n different synaptic contacts, differences both 1n the number of receptors and in their distribution. Another explanation might be found 1n differences among synaptic contacts with respect to the type of membrane process that 1s altered by the binding of antibody molecules. The number of such processes 1s substantial and continues to grow. 

The types of modules generally used in some of the major membrane processes are listed in Table 3.8. 

Natural gas is usually produced from the well and transported to the gas processing plant at high pressure, in the range 500-1500 psi. To minimize recompression costs, the membrane process must remove impurities from the gas into the permeate stream, leaving the methane, ethane, and other hydrocarbons in the high-pressure residue gas. This requirement determines the type of membranes that can be used for this separation. Figure 8.30 is a graphical representation of the factors of molecular size and condensability that affect selection of membranes for natural gas separations. 

Membrane processes for the separation of gaseous and liquid mixtures are important examples. In these cases there are already large numbers of applicable materials and processes. Further improvements (mostly concerning better selectiv-ities at acceptably high permeabilities), often needing real jumps in performance, are, however, still needed in many cases. This applies, although in the opposite sense, also to barrier materials where permeations at least of certain types of molecules will be extremely small. Other areas concern biomaterials or material systems for the controlled release of drugs. 

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