1.1An Overview of Separation Processes
Life on Earth depends on various membrane processes and membranes play an important role in the lives of all of the living beings present on this planet. Examples are the process of respiration through the lungs, which continuously allows the diffusion of O2 and CO2 to and from our bodies to the outside; the purification of groundwater because of the different layers of earth that act as membranes of different porosities; and the skin of living organisms also works as a semipermeable membrane. The skin does not allow microorganisms to enter into our bodies, but allows diffusion of water in the form of sweat and other toxic substances from the body to the outside. Similarly, there are an infinite number of other examples that confirm the presence of different kinds of membranes around all of us.
The word membrane is derived from the Latin word membrana, which means “skin.” Membrane is a sort of barrier that separates things and allows materials to be passed selectively [1]. There are many definitions available for a membrane. A general definition could be: A membrane is a thin barrier, placed between two phases, or mediums, which allow one or more constituents to selectively pass from one medium to the other in the presence of an appropriate driving force while retaining the rest [2]. This definition is based on a macroscopic level, but it should be taken care that the separation is at the microscopic level. Accordingly, it can be said that a membrane process is a combination of both mass and momentum transfer. A membrane can be homogeneous or heterogeneous, symmetric or asymmetric, solid or liquid; it can carry a positive or negative charge or be neutral or bipolar. Transport through a membrane can be affected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure, or temperature gradient. The membrane thickness may vary from as small as 100 microns to several millimeters.
Membrane science is promising and has the upper hand when its advantages are compared to other separation processes like adsorption, distillation, extraction, and crystallization. The important advantages of membranes are their low capital cost, low energy requirement, high separation efficiency, compact design, easy organization with other separation processes, and no requirement of secondary separation processes. The properties of a membrane mainly depend on factors like porosity, pore diameter, pore size distribution, particle size distribution of the solutes, and the affinity between the feed and the membrane material for solubility/diffusivity. There is vastness in the methods available for the preparation of different membranes by using different types of materials. Membrane science, even with these many advantages and options available for preparation and applications, has enough scope for further improvements.
Membranes and membrane processes were first introduced as an analytical tool in chemical and biomedical laboratories. However, their industrial importance increased quite rapidly with significant technical and commercial impact [3]. Membranes now find wide applications in the food/beverage industry; in desalination processes; in the medical industry for the purification of body fluids, antibiotics, and so on; and in separation of industrial effluents, among other processes. This increase in the use of membranes in industry has been attributed to the various advantages associated with membrane processes, such as good product quality, better energy efficiency than conventional separation methods, simple operation, no chemical alteration of constituents, easy scale up, and better environment suitability. Although the rise in membrane usage has been observed only recently, the membrane phenomenon has been long studied.
1.1.1Principle of Membrane Separation
There exists different mechanisms of separation for different membranes; membranes can also be classified based on the mechanisms. Porous membranes separate the feed from permeate on the basis of size and nonporous membranes on th...
