Definition of a membrane

A simplified working definition of a membrane can be conveniently stated as a semipermeable active or passive barrier which, under a certain driving force, permits preferential passage of one or more selected species or components (molecules, particles or polymers) of a gaseous and/or Uquid mixture or solution (Figure 1.1). 

Although there is no commonly accepted definition of a membrane reactor (MR), the term is usually applied to operations where the unique abilities of membranes to organize, compartmentalize, and/or separate are exploited to perform a (bio)chemical conversion under conditions that are not feasible in the absence of a membrane. In every MR, the membrane separation and the (bio)catalytic conversion are thus combined in such a way that the synergies in the integrated setup entail enhanced processing and improved economics in terms of separation, selectivity, or yield, compared to a traditional configuration with reactor and separation separated in time and space. When the membrane itself carries the catalytic functions, it is mostly referred to as a reactive membrane. ... 

There is no commonly accepted definition of a membrane reactor but the term is applied to membrane (including liquid membrane) processes and devices whose function is to perform chemical conversion, coupling and combining chemical and transport processes, using the unique contacting features of membranes. As a rule, functional definition of this term includes fermentation, catalysis, separation of the products and their enrichment. A few published reviews at this time are available [98-104]. In most of pubhcations the bioreactors, based on enzymes or whole cells, impregnated into the membrane pores (immobihzed or supported hquid membranes) or deposited on the membrane surfaces are discussed. 

What are the three important points with respect to the definition of a membrane ... 

By extending our definition of a membrane, we can include liquids. If we view a membrane as a semlpermeable barrier between two phases, then an immiscible liquid can serve as a membrane between two liquid or gas phases. Different solutes will have different solubilities and diffusion coefficients In a liquid. The product of these two terms is a measure of the permeability. A liquid can yield selective permeabilities and, therefore, a separation. Because the diffusion coefficients in liquids are typically orders of magnitude higher than in polymers, a larger flux can be obtained. 

The exact definition of a membrane is comphcated, but according to Mulder [11], a general definition could be a selective barrier between two phases, the term selective being inherent to a membrane or a membrane process. However, the definition says nothing about membrane structure or membrane function. Membrane science arbitrarily can be divided into seven categories material selection, material characterization and evaluation, membrane preparation, membrane characterization and evaluation, membrane transport phenomena, membrane module design, and process performance [12]. 

A general definition of a membrane is that it is a selective barrier between two phases [ 1 ]. Therefore, using membranes, the feed is separated into two streams, that is, the retentate and permeate streams, as shown in Figure 10.1.1. Either the retentate or the permeate could he product stream, depending upon types of membranes used and the feed stream. The permeate stream is the product stream, if the solvent is purified by removing solutes using a membrane which allows the permeation of solvent and retains the permeation of solutes, such as in the desalination of seawater. If the purpose of the separation process is the concentration of solutes, then the retentate becomes the product. 

It should be recalled that the term surface potential is used quite often in membranology in rather a different sense, i.e. for the potential difference in a diffuse electric layer on the surface of a membrane, see page 443.) It holds that 0 = 0 + X (this equation is the definition of the inner electrical potential 0). Equation (3.1.2) can then be written in the form... 

In mouse liver and kidney and in rat liver, a-D-mannosidase activity appeared to be equally distributed between the two cytoplasmic-granule fractions. With mouse spleen and cancer tissue, a considerable proportion of the enzyme was found free in the cytoplasm. Rat spleen, on the other hand, lacked this cytoplasmic fraction. Inasmuch as the enzyme within the cytoplasmic granules was not fully active in a sucrose homogenate until the membranes had been disintegrated, a-D-mannosidase conforms to the definition of a lysosomal hydrolase. 

The definition of a species permeability in a given membrane is the product of its diffusivity and solubility in tne membrane. It is this product which also results in the membrane s selectivity for one component of a mixture over another(9). However, since the permeability is actually a complicated function of the diffusivity and solubility, it is empirically measured and calculated by Equation 1. 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

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