Membrane Separation

1. Membrane Separation
 Membrane SeparationProcesses.
Membrane separations represent a new type of unit operation. In membrane
separation process, a feed consisting of a mixture of two or more components is
partially separated by means of a semi-permeable barrier which separates
components of a mixture or solution by selectively controlling their movement
from one side of the membrane to the other. In some cases, particles or molecules
of sizes larger than those of the pores of the membrane are excluded right at the
upstream side while in others, they move through the membranes at differential
rates depending upon their molecular weights or volumes so that separation
becomes possible.
 Classification of membrane processes.
In their early stage of development, membrane processeswere classified as
follows:
A. Gaseous diffusion or effusion and permeation in separation of gases
B. Dialysis and permeation in separation of liquids.
 According to present practice, membrane processes are classified in terms of
different types of pressure driven membrane filtration as:
 Microfiltration (MF) for separation of colloidal to micron particles and
microorganisms,
 Ultrafiltration (UF) for separation of organics with molecular weight of above
5000,
 Nanofiltration (NF) for separation of divalent salts and organics while
permeating monovalent salts such as NaCl,
 Reverse osmosis (RO) for very high degree of salt separation including NaCl
and soluble organics.

5.  GeneralMembrane Equation.
It is not possible at present to provide an equation, or set of equations, that
allows the prediction from first principles of the membrane permeation rate and
solute rejection for a given real separation.

6. The general membrane equation is an attempt to state the factors which may be
important in determining the membrane permeation rate for pressure driven
processes.
Ex1. A brackish water is to be desalinized using a thin film compositemembrane
having a flux rate coefficient (kw) of1.5 x10-6
s/m.Thenetoperating pressure
will be 2500 kpa and permeate flow rate is 0.010 m3
/s.
Estimate membrane area.