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Back Chemical Chemical Engineering Dictionary Thermodynamics Fixed Bed Separation and Adsorbent & Adsorption Process

Fixed Bed Separation and Adsorbent & Adsorption Process

For separating gaseous and liquid mixtures, the fluid is brought into contact with some small porous solid. The solid then selectively adsorb certain component or make complexes with certain component in the feed. This process is called adsorption.

In adsorption the solid is keep fixed on a fixed bed and a fluid is continuously passed over a solid until solid is completely saturated the flow is then switched to a next bed.

When the bed is completely saturated it is taken out for regeneration and sometimes a new bed is placed. Water to be soften or deionized is passed over bed of ion exchange resin until the resin becomes nearly saturated.

Continuous counter current operation can be achieve by moving the solids over the bed and continuously replacing spent particle with fresh ones. It is not always easy to get uniform flow of solids that is why this operation is not often used.

Another process which is similar to Adsorption is called Chromatography. In this process liquid or gas mixtures are passed over a bed of porous of particle. Feed is not introduced continuously like in Adsorption process. The feed is introduced in small pulses. The individual component that moves over the bed at different rates is collected at the exit. The bed is regenerated whenever require by passing carrier gas or liquid and can be operated for long period. In this process only small amount of feed mixtures are separated.

Adsorbent and Adsorption Process:

Most of the adsorbent are very porous material. The adsorption process takes place at the wall of the pores or at particular site inside the particle. The internal surface area is greater in magnitude than the external area and is often 500 to 1000 m2/g because the pores are generally very small. Separation takes place on the surface of porous material because of the following factors.

  1. Molecular weight
  2. Shape
  3. Polarity

Some molecule gets attracted to the surface of the porous material because of polarity or the pores are very small to hold the larger molecule.
In many cases the adsorbate does not let any other component to be sticked on the adsorbents and in this way after Regeneration the collected adsorbate is highly concentrated or in nearly pure form.

Applications of Adsorption

There are numerous application of Adsorption which includes the recovery of organic solvents used in paints, printing inks and solutions for fabric coating or film casting. For the collection of some of the solvent laden air is sent to the condenser. This step is generally impractical to cool the gas below ambient temperature to eliminate solvent losses. The air containing small amount of solvent is then pass through abed of carbon adsorbent particles, it reduces the concentration of solvent less than 1 ppm.. carbon is used to remove several pollutant like hydrogen sulphide and carbon sulphide and other odorous organic compounds.

The gases are usually dried by passing the gas over silica gel, alumina or other inorganic porous solids. The gases with low dew point (-75oc) are produced by passing the gases over zeolites or molecular sieves. These are natural or synthetic alumino-silicates. They possess fine porous structure adsorption is carried out on molecular sieve to separate normal paraffin from branched paraffin and aromatics.

Adsorption is carried out in liquid phase too. The organic compounds are removed from drinking water. Colored impurities are removed sugar solutions and vegetable oils. Water is removed from organic liquids. When the products of the reaction are not separated by crystallization and distillation the adsorption method is utilized. In vapor phase and liquid phase adsorption dome of same kind of solid is used, in case of liquids larger pores adsorbent are mostly preferred.

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