Introduction and Types of Membrane Separation Processes

Separations by the use of membranes are becoming increasingly important in the process industries. In this relatively new unit operation, the membrane acts as a semipermeable barrier and separation occurs by the membrane controlling the rate of movement of various molecules between two liquid phases, two gas phases, or a liquid and gas phase. The two fluid phases are usually miscible and the membrane barrier prevents actual, ordinary hydrodynamic flow. A classification of the main types of membrane separation is as follows. 

Gas diffusion in porous solid. In this type a gas phase is present on both sides of the membrane, which is a microporous solid. The rates of molecular diffusion of the various gas molecules depend on the pore sizes and the molecular weights. This type of diffusion in the molecular, transition, and Knudsen regions was discussed in detail in Section 7.6. 

Liquid permeation or dialysis. In this case the small solutes in one liquid phase diffuse readily because of concentration differences through a porous membrane to the second liquid phase (or vapor phase). Passage of large molecules through the membrane is more 

Reverse osmosis. A membrane, which impedes the passage of a low-molecular-weight solute, is placed between a solute-solvent solution and a pure solvent. The solvent diffuses into the solution by osmosis. In reverse osmosis a reverse pressure difference is imposed which causes the flow of solvent to reverse as in the desalination of seawater. This process also is used to separate other low-molecular-weight solutes, such as salts, sugars, and simple acids from a solvent (usually water). This process is covered in detail in Sections 13.9 and 13.10. 

Ultrafiltration membrane process. In this process, pressure is used to obtain a separation of molecules by a semipermeable polymeric membrane (M2). The membrane discriminates on the basis of molecular size, shape, or chemical structure and separates relatively high molecular weight solutes such as proteins, polymers, colloidal materials such as minerals, and so on. The osmotic pressure is usually negligible because of the high molecular weights. This is covered in Section 13.11. 

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