Selectively permeable membrane

Boundary layer effects Membranes (selective permeability for ions, gases etc.), ion exchangers, controlled release of pharmaceuticals. 

To improve process economics, an integrated process shown conceptually in Fig. 27 has been proposed. A per-vaporation subsystem is equipped with a membrane selectively permeable to water and ammonia, but rejects ethanol and ethyl lactate. The retentate stream carrying these reactants may be returned to the reactor to help drive the reaction toward completion. 

Carbon dioxide sensor — An electrochemical device for measuring the -+ partial pressure of carbon dioxide, originally developed for measurements with blood by Sever-inghaus and Bradley [i]. It consists of a glass electrode as used for pH measurements covered with a membrane selectively permeable for CO2, e.g., silicone. Between membrane and glass electrode an electrolyte solution containing NaCl and Na2C03 is entrapped. 

Ilias and Govind(lO) have reviewed the development of high temperature membranes lor membrane reactor application. Hsieh(4) has summarized the technology in the area of important inorganic membranes, the thermal and mechanical stabilities of these membranes, selective permeabilities, catalyst impregnation, membrane/reaction considerations, reactor configuration, and reaction coupling. 

Catalyst-membrane systems are promising structured catalysts. The perspectives for control of heterogeneous catalytic reactions by the combination of a catalyst and a membrane selectively permeable for one of the reactants have been discussed [1]. Catalyst-membrane systems enhance reaction rate and selectivity due to the directed transfer of reactants and energy. 

In all cells, the plasma membrane acts as a permeability barrier that prevents the entry of unwanted materials from the extracellular milieu and the exit of needed metabolites. Specific membrane transport proteins In the plasma mem brane permit the passage of nutrients Into the cell and metabolic wastes out of It others function to maintain the proper Ionic composition and pH ( 7.2) of the c3d osol. The structure and function of proteins that make the plasma membrane selectively permeable to different molecules are discussed In Chapter 7. 

The magnitude of the membrane electric potential is the same (59 mV for a tenfold difference in ion concentrations), except that the right side is now positive with respect to the left (Figure 7-13c), opposite to the polarity obtained across a membrane selectively permeable to Na ions. 

Although membranes could be used for the entire alcohol purification process, a great many stages would be needed, and it is cheaper to use distillation for most of the separation. If a membrane selectively permeable to alcohol could be developed, it might be feasible to separate alcohol from dilute solutions entirely by membranes. 

The electric field applied across a membrane would determine the direction of ion movement. Cations move toward the cathode and anions move toward the anode. With the membrane selectively permeable for only cations or only anions, a separation process would take place. 

A technical way to fulfill this plan is to surround the reacting mixture by a membrane selectively permeable to one of the products at least (Integrated Membrane Reactor). As a result, the reaction rate of the reverse reaction is lower than that of the forward reaction and the conversion increases theoretically to the value corresponding to the complete depletion of the limiting reactant... 

Consequently, membrane bioreactors are an example of the combination of two unit operations in one step for example, membrane filtration with the chemical reaction. In a typical membrane bioreactor, as weU as acting as a support for the biocatalyst, the membrane can be a very effective separation system for undesirable reactions or products. The removal of a reaction product from the reaction environment can be easily achieved thanks to the membrane selective permeability, and this is of great advantage in thermodynamically unfavourable conditions, such as reversible reactions or product-inhibited enzyme reactions. A very interesting example of a membrane bioreactor is the combination of a membrane process, such as microfiltration or ultrafiltration (UF), with a suspended growth bioreactor. Such a set up is now widely used for municipal and industrial wastewater treatment, with some plants capable of treating waste from populations of up to 80 000 people (Judd, 2006). 

FIGU RE 5.6 A reverse osmosis system for the removal of ions and other impurities from water. Highly pressurized water is forced through a membrane selectively permeable to water so that desalinated purified water penetrates the membrane and a waste brine is rejected. Membrane filtration processes that employ filters with... 

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