13.1. INTRODUCTION AND TYPES OF MEMBRANE SEPARATION PROCESSES

13.1A. Introduction

Separations by the use of membranes are becoming increasingly important in the process industries. In this relatively new separation process, 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 a gas phase. The two fluid phases are usually miscible and the membrane barrier prevents actual, ordinary hydro-dynamic flow. A classification of the main types of membrane separation follows.

13.1B. Classification of Membrane Processes

1. 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.

2. 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 (or vapor) phase. Passage of large molecules through the membrane is more difficult. This membrane process has been applied in chemical processing separations such as separation of H2SO4 from nickel and copper sulfates in aqueous solutions, food processing, and artificial kidneys, and will be covered in detail in Section 13.2. In electrodialysis, separation of ions occurs by imposing an emf difference across the membrane.

3. Gas permeation in a membrane

The membrane in this process is usually a polymer such as rubber, polyamide, and so on, and is not a porous solid. The solute gas first dissolves in the membrane and then diffuses in the solid to the other gas phase. This was discussed in detail in Section 6.5 for solutes following Fick's law and will be considered again, for the case where resistances are present, in Section 13.3A. In Sections 13.3 to 13.8, different process flow patterns are considered. Examples of membrane separations are helium being separated from natural gas and nitrogen from air. Separation of a gas mixture occurs because each type of molecule diffuses at a different rate through the membrane.

4. 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 is also used to separate other low-molecular-weight solutes, such as salts, sugars, and simple acids from a solvent (usually water). This process will be covered in detail in Sections 13.9 and 13.10.

5. Ultrafiltration membrane process

In this process, pressure is used to obtain a separation of molecules by means of 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 will be covered in Section 13.11.

6. Microfiltration membrane process

In microfiltration, pressure-driven flow through the membrane is used to separate micron-size particles from fluids. The particles are usually larger than those in ultrafiltration. Examples are separation of bacteria, paint pigment, yeast cells, and so on from solutions. This process will be covered in Section 13.12.

7. Gel permeation chromotography

The porous gel retards diffusion of the high-molecular-weight solutes. The driving force is concentration. This process is quite useful in analyzing complex chemical solutions and in the purification of very specialized and/or valuable components.

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