Membranes - Principles of Chemical Separations with Environmental Applications - page 233

9

Membranes

A wide screen just makes a bad film twice as bad.

- SAMUEL GOLDWYN

9.1 Objectives

1 Identify the three major driving forces for membrane separations.
2 Define permeability, permeance, selectivity, and rejection.
3 List the transport mechanisms for membrane separations.
4 List some environmental applications of each type of membrane separation.
5 Describe the advantages and disadvantages of membrane technology.

9.2 Membrane definition

A membrane can be defined as [1]:

... a semi-permeable barrier between two phases. This barrier can restrict the movement of

molecules across it in a very specific manner. The membrane must act as a barrier between
phases to prevent intimate contact. This barrier can be solid, liquid, or even a gas. The semi-
permeable nature is essential to insuring that a separation takes place. If all species present could
move through the membrane at the same rate, no separation would occur. The manner in which
the membrane restricts molecular motion can take many forms. Size exclusion, differences
in diffusion coefficients, electrical charge, and differences in solubility are some examples. A
membrane separation is a rate process. The separation is accomplished by a driving force, not
by equilibrium between phases.

There are three important points to note with respect to this definition. First, a mem-
brane is defined by what it does (function), not by what it is. So, a wide range of materials