SEPARATION TECHNIQUES IN CHEMICAL INDUSTRY

Introduction:- In chemistry and chemical engineering, a separation process, or a separation

technique, or simply a separation, is a method to achieve any mass transfer phenomenon that converts a mixture of substances into two or more distinct product mixtures (which may be referred to as fractions), at least one of which is enriched in one or more of the mixture's constituents.

In some cases, a separation may fully divide the mixture into its pure constituents. Separations are carried out based on differences in chemical properties, or physical properties such as size, shape, mass, density, or chemical affinity, between the constituents of a mixture, and are often classified according to the particular differences they use to achieve separation. Usually there is only physical movement, no substantial chemical modification. In the case that no single difference can be used to accomplish a desired separation, multiple operations will often be performed in combination to achieve the desired end.

Need for Separation:-The purpose of a separation may be analytical, i.e. to help analyse components in the

original mixture without any attempt to save the fractions, or may be preparative, i.e. to "prepare" fractions or samples of the components that can be saved. The separation can be done on a small scale, effectively a laboratory scale for analytical or preparative purposes, or on a large scale, effectively an industrial scale for preparative purposes, or on some intermediate scale.

Many industries now find separations indispensable: the petroleum industry separates crude oil into products used as fuels, lubricants, and chemical raw materials; the pharmaceutical industry separates and purifies natural and synthetic drugs to meet health needs; and the mining industry is based on the separation and purification of metals.Reasons for Separations:-

There are two general reasons for performing separations on mixtures. First, the mixture may contain some substance that should be isolated from the rest of the mixture: this process of isolating and thus removing substances considered to be contaminants is called purification. For example, in the manufacture of synthetic drugs, mixtures containing variable proportions of several compounds usually arise. The removal of the desired drug from the rest of the mixture is important if the product is to have uniform potency and is to be free of other components that may be dangerous to the body.

The second reason for performing separations is to alter the composition of a sample so that one or more of the components can be analysed. For example, the analysis of air pollutants to assess the quality of the air is of great interest, yet many of the pollutants are at a concentration too low for direct analysis, even with the most sensitive devices. Pollutants can be collected by passing samples of air through a tube containing an adsorbent material. By this process the pollutants are concentrated to a level such that straightforward analysis and monitoring can take place.

In a second example, several impurities in a sample may interfere with the analysis of the substance of primary interest. Thus, in the analysis of trace concentrations of metals in rivers, organic substances can cause erroneous results. These interferences must be removed prior to the analysis.

Classification of Separation:-Classification is based on the physical or chemical phenomena utilized to effect the

separation. These phenomena can be divided into two broad categories:i) Equilibrium and rate (kinetic) processes. Table 1 lists some separation methods based on equilibria, and Table 2 indicates those methods based on rate phenomena.

Table 1:

Separations based on Phase Equilibria

All equilibrium methods considered in this section involve the distribution of substances between two phases that are insoluble in one another. As an example, consider the two immiscible liquids benzene and water. If a coloured compound is placed in the water and the two phases are mixed, colour appears in the benzene phase, and the intensity of the colour in the water phase decreases. These colour changes continue to occur for a certain time, beyond which no macroscopic changes take place, no matter how long or vigorously the two phases are mixed. Because the dye is soluble in the benzene as well as in the water, the dye is extracted into the benzene at the start of the mixing. But, just as the dye tends to move into the benzene phase, so it also tends to be dissolved in the aqueous phase.

Thus dye molecules move back and forth across the liquid-liquid interface. Eventually, a condition is reached such that the tendencies of the dye to pass from benzene to water and from water to benzene are equal, and the concentration of the dye (as measured by the intensity of its colour) is constant in the two phases. This is the condition of equilibrium. Note that this is static from a macroscopic point of view. On a molecular level it is a dynamic process, however, for many molecules continue to pass through the liquid-liquid interface (although of equal number in both directions).

The condition of equilibrium in this example can be described in terms of the distribution coefficient, K, by the equation

in which the concentrations in the equilibrium state are considered. For K = 1, there are equal concentrations of the dye in the two phases; for K > 1, more dye would be found in the benzene phase at equilibrium.

In Table 1 most of the important chemical equilibrium separation methods are subdivided in terms of the two insoluble phases (gas, liquid, or solid). A supercritical fluid is a phase that occurs for a gas at a specific temperature and pressure such that the gas will no longer condense to a liquid regardless of how high the pressure is raised. It is a state intermediate between a gas and a liquid.

Table 2:

Separation based on Rate Phenomena

Rate separation processes are based on differences in the kinetic properties of the components of a mixture, such as the velocity of migration in a medium or of diffusion through semipermeable barriers.

The separation of mixtures of proteins is often difficult because of the similarity of the properties of such molecules. When proteins are dissolved in water, they ionize (form electrically charged particles). Both positive and negative electrical charges can occur on various parts of the complex molecule, and, depending on the pH of the solution, a protein molecule as a whole will be either net positively or negatively charged. For a given set of solution conditions, the net charges on different proteins usually are unequal.

Electrophoresis takes advantage of these charge differences to effect a separation. In this method, two electrodes are positioned at opposite ends of a paper, starch gel, column, or other appropriate supporting medium. A salt solution is used to moisten the medium and to connect the electrodes electrically. The mixture to be separated is placed in the centre of the supporting medium, and an electrical potential is applied. The positively charged proteins move toward the negatively charged electrode (cathode), while the negatively charged proteins migrate toward the positively charged electrode (anode). The migration velocity in each direction depends not only on the charge on the proteins but also on their size: thus proteins with the same charge can be separated.

This example demonstrates the separation of charged species on the basis of differences in migration velocity in an electric field. The extent of such a separation (based on the rate of a process) is time-dependent, a feature that distinguishes such separations from those based upon equilibria.

The velocity can be either positive or negative, depending on direction. It depends not only on the size and electrical charge of the molecule but also on the conditions of the experiment (e.g., voltage between the two electrodes). In analogy to equilibrium methods, the separation factor can be defined as the ratio of migration velocities for two proteins:

The extent of separation (i.e., how far one protein is removed from another) depends on the different distances traversed by the two proteins:

where t is the time allowed for migration. Thus the extent of separation is directly proportional to the time of migration in the electric field.

Another major category of rate separation methods is based on the diffusion of molecules through semipermeable barriers. Besides differing in charge, proteins also differ in size, and this latter property can be used as the basis of separation. If a vessel is divided in half by a porous membrane, and a solution of different proteins is placed in one section and pure water in the other, some of the proteins will be able to diffuse freely through the membrane, while others will be too large to fit through the holes or pores. Still others will be able to just squeeze through the pores and so will diffuse more slowly through the membrane. The extent of separation will thus be dependent on the time allowed for diffusion to take place.

Table 2 lists the various barrier separation methods discussed in this article. The differences in the methods involve the type of substances diffusing through the semipermeable barrier and whether an external field or pressure is applied across the membrane.

PARTICLE SEPARATION:-Up to this point, only separations at the molecular level have been discussed.

Separations of particles are also important in both industry and research. Particle separations are performed for one of two purposes:

(1) to remove particles from gases or liquids, or

(2) to separate particles of different sizes or properties.

The first reason is widely used in the electronics industry requires dust-free “clean rooms” for assembly of very small components. The second purpose deals with the classification of particles from samples containing particles of many different sizes. Many technical processes using finely divided materials require that the particle size be as uniform

as possible. In addition, the separation of cells is important in the biotechnology industry. The more important particle separation methods are filtration, sedimentation, elutriation, centrifugation, particle electrophoresis, electrostatic precipitation, flotation, and screening

BY

PRAMODKUMAR

MBA Tech (CHEMICAL) 4th YR