Introduction to Membrane Science and Technology
A membrane is a selective barrier that permits the separation of certain species in a fluid by combination of sieving and sorption diffusion mechanism. Separation is achieved by selectively passing (permeating) one or more components of a stream through the membrane while retarding the passage of one or more other components. See the Figure. Membranes can selectively separate components over a wide range of particle sizes and molecular weights, from macromolecular materials such as starch and protein to monovalent ions. Membranes have gained an important place in chemical technology and are used in a broad range of applications.
The key properties determining membrane performance are high selectivity and fluxes, good mechanical, chemical and thermal stability under operating conditions, low fouling tendencies and good compatibility with the operating environment; and cost effective and defect-free production.
Although the major uses of membranes are in the production of potable water and separation of industrial gases, they can be used for many other important applications such as filtration of particulate matter from liquid suspensions, air or industrial flue gas and the dehydration of ethanol azeotropes.
More specialised applications include ion separation in electrochemical processes, dialysis of blood and urine, artificial lungs, controlled release of therapeutic drugs, membrane-based sensors, etc.
Membrane processes are characterized by the fact that a feed stream is divided into 2 streams: retentate and permeate. The most general process can be depicted by the following Figure:
The retentate is that part of the feed that does not pass through the membrane, while the permeate is that part of the feed that does pass through the membrane. The optional "sweep" is a gas or liquid that is used to help remove the permeate. The component(s) of interest in membrane separation is known as the solute. The solute can be retained on the membrane membrane and removed in the retentate or passed through the membrane in the permeate.
It is important to note that there are 3 different mechanisms by which membrane can perform separations:
By
having holes or pores which are of such a size that certain species can pass
through and others cannot. This mechanism is called size exclusion.
By
selective retardation by the pores when the pore diameters are close to molecular
sizes. This mechanism is called pore flow.
By
dissolution into the membrane, migration by molecular diffusion across the membrane,
and re-emergence from the other side. This is called solution diffusion.
Click here for examples of industrial membrane processes.
Membrane processes have a number of advantages and disadvantages compared to alternative means of performing separations. Click here for more information.