APPLICATIONS

It is often necessary to separate liquid and gas phases in a certain stage of an operation or process. Since both the condition of the wet gas stream or more generally the gas-liquid stream and the required efficiency may vary widely, care shall be taken in selecting a separator in order to match the specific duty. For instance, a gas-liquid separator upstream of a gas compressor would need to be very efficient, whereas in other cases a simple knock-out vessel may be sufficient if only bulk separation of the gas and liquid phases id required. There are many chemical process materials and biological substances occur as mixtures of different components in the gas, liquid, or solid phase. The application of separation process is to separate or remove one or more of the components from its original mixture. In order to separate or remove one or more of the components from its original mixture, it must be contacted with another phase. The two phases are brought into more or less intimate contact with each other so that a solute or solutes can diffuse from one to the other. The two bulk phases usually somewhat miscible in each other. The two-phase pair can be gas-liquid, gas-solid, liquid-liquid or liquid-solid. In this case, we will look more into gas-liquid separation which involving gas and liquid phase only. The phases are then separated by simple physical methods. Below, there are some types of separation process which shows the applications of gas-liquid separation process.

Absorption

When the two contacting phases are gas and liquid, the operation is called absorption. A solute or several solutes are absorbed from the gas phase into the liquid phase in absorption. This process involves molecular and turbulent diffusion or mass transfer of solute A through a stagnant, on diffusing gas B into a stagnant liquid C. the reverse of absorption is called stripping or desorption, and the same theories and basic principles hold. When the gas is pure air and the piqued s pure water, the process is called humidification. Dehumidification involves removal of water vapor from air.

Gas absorption unit does one of the popular absorption processes. Gas absorption/desorption is a process in which a gaseous mixture is brought into contact with a liquid and during this contact a component is transferred between the gas stream and the liquid stream. The gas may be bubbled through the liquid, or it may pass over streams of the liquid, arranged to provide a large surface through which the mass transfer can occur. The liquid film in this latter case can flow down the sides of columns or over packing, or it can cascade from one tray to another with the liquid falling and the gas rising in the counter flow.  The gas, or components of it, either dissolves in the liquid (absorption) or extracts a volatile component from the liquid (desorption). 

Gas absorption equipment is designed to achieve the greatest practicable interfacial area between the gas and the liquid streams, so that liquid sprays and gas-bubbling devices are often employed. In many cases, a vertical array of trays is so arranged that the liquid descends over a series of perforated trays, or flows down over ceramic packing that fills a tower.

For the hydrogenation of oils, absorption is followed by reaction of the hydrogen with the oil, and a nickel catalyst is used to speed up the reactions. Also, pressure is applied to increase gas concentrations and therefore speed up the reaction rates. Practical problems are concerned with arranging distribution of the catalyst, as well as of oil and hydrogen. Some designs spray oil and catalyst into hydrogen, others bubble hydrogen through a continuous oil phase in which catalyst particles are suspended.For the stripping of volatile flavours and taints in deodorizing equipment, the steam phase is in general the continuous one and the liquid is sprayed into this and then separated. In one design of cream deodorizing plant, cream is sprayed into an atmosphere of steam and the two streams then pass on to the next stages, or the steam may be condensed and fresh steam used in the next stage.

Other separation process applications

Ion exchange

Ion exchange process is removal of certain ions by an ion-exchange solid. This separation process closely resembles adsorption because of the ion-exchange solid. It is widely used in in the food & beverage, hydrometallurgy, metals finishing, chemical & petrochemical, pharmaceutical, sugar & sweeteners, ground & potable water, nuclear, softening & industrial water, semiconductor, power, and a host of other industries.

In preparation of high purity water:Most typical example of application is preparation of high purity water for power engineering, electronic and nuclear industries for examples polymeric or mineralic insoluble

In treatment of water:Widely used for water softening, water purification, water decontamination, etc.

In household:Widely used in household (laundry detergents and water filters) to produce soft water. This is accomplished by exchanging calcium Ca2+and magnesium Mg2+ cations against Na+ or H+ cations (see water softening). Another application for ion exchange in domestic water treatment is the removal ofnitrate and natural organic matter.

In industry:Industrial and analytical ion exchange chromatography is another area to be mentioned. Ion exchange chromatography is a chromatographical method that is widely used for chemical analysis and separation of ions. For example, inbiochemistry it is widely used to separate charged molecules such as proteins. An important area of the application is extraction and purification of biologically produced substances such as proteins (amino acids) and DNA/RNA.

Adsorption

Adsorption process, one or more components of a liquid or gas stream are adsorbed on the surface or in the pores of a solid adsorbent and a separation is obtain.

In preserving vacuum: In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enters into the annular space either due to glass imperfection or diffusion though glass is adsorbed.

In glass masks:All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the atmosphere are preferentially adsorbed and the air for breathing is purified.

In clarification of sugar:Sugar is decolorized by treating sugar solution with charcoal powder. The latter adsorbs the undesirable colors present.

In paint industry:The paint should not contain dissolved gases as otherwise the paint does not adhere well to the surface to be painted and thus will have a poor covering power. The dissolved gases are therefore, removed by suitable adsorbents during manufacture. Further, all surfaces are covered with layers of gaseous, liquid or solid films. These have to be removed before the paint is applied. This is done by suitable liquids which adsorbs these films. Such liquids are called wetting agents. The use of spirit as wetting agent in furniture painting is well known.

In chromatographic analysis:The selective adsorbent of certain substances from a solution by a particular solid adsorbent has helped to develop technique for the separation of the components of the mixture. This technique is called chromatographic analysis. For example: in column chromatography a long and wide vertical tube is filled with a suitable adsorbent and the solution of the mixture poured from the top and then collected one by one from the bottom.

In catalysis:The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the solid catalyst. As a result, the concentration of the reactants increases on the surface and hence the rate of reaction increases. The theory is also able to explain the greater efficiency of the catalyst in the finely divided state, the action of catalyst promoters and poisons.

In adsorption indicators:Various dyes which owe their use to adsorption have been introduced as indicators particularly in precipitation titrations. For example: KBr is easily titrated with AgNO 3 using eosin as an indicator.

In softening of hard water:The use of ion exchangers for softening of hard water is based upon the principle of competing adsorption just as in chromatography. In removing moisture from air in the storage of delicate instruments: Such instruments which may be harmed by contact with the moist air are kept out of contact with moisture using silica gel.

Crystallization

Crystallization is a natural occurring process but also a process abundantly used in the industry. Crystallization is used at some stage in nearly all process industries as a method of production, purification or recovery of solid materials. Crystallization is an example of a separation process in which mass is transferred from a liquid solution, whose composition is generally mixed, to a pure solid crystal. Soluble components are removed from solution by adjusting the conditions so that the solution becomes supersaturated and excess solute crystallizes out in a pure form. This is generally accomplished by lowering the temperature, or by concentration of the solution, in each case to form a supersaturated solution from which crystallization can occur. The equilibrium is established between the crystals and the surrounding solution, the mother liquor. The manufacture of sucrose, from sugar cane or sugar beet, is an important example of crystallization in food technology. Crystallization is also used in the manufacture of other sugars, such as glucose and lactose, in the manufacture of food additives, such as salt, and in the processing of foodstuffs, such as ice cream. In the manufacture of sucrose from cane, water is added and the sugar is pressed out from the residual cane as a solution. This solution is purified and then concentrated to allow the sucrose to crystallize out from the solution.

Membrane processing

Of the several separation technologies available, membrane separation technology brought a significant change in dairy food processing. There are several advantages of membrane separation technology when compared to other processes. These include, separation of components at a lower temperature, separating the component in its native state, less energy use etc. Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF) and Microfiltration (MF) are four commonly used membrane separation process in the dairy food processing. These processes have evolved from consistent research and development in the area of new membrane material development as well as applications. Membrane separations have been extensively used in dairy process industry and are used for selective separation of different species. Commonly used separation processes are Microfiltration, Ultrafiltration, Nanofiltration and Reverse osmosis. These processes differ in membrane characteristics, their pore size and operating pressures to which they are exposed to. Reverse osmosis is mainly

used to concentrate all the solutes present in a mixture, while removing water in the process. Nanofiltrationis used to concentrate the solutes while partially allowing the passage of some lactose and monovalent salts, thereby minimizing the effect of osmotic pressure. Conventional ultrafiltration is used to remove lactose and soluble salts from dairy mixtures. Microfiltrationis widely used to remove bacteria, somatic cells, fat and lately micellar casein from skim milk.

Leaching

The process is called leaching if a fluid is being used to extract a solute from a solid. Sometimes, it is also called extraction.

Leaching processes for biological substances:Many biological organic and inorganic substances occur in a mixture of different components in a solid. In order to separate the desired solute constituent or remove an undesirable solute component from the solid phase, the solid is brought into contact with a liquid. The solid and liquid are in contact and the solute or solutes can diffuse from the solid into the solvent, resulting in separation of the components originally in the solid. This separation process is called liquid-solid leaching or simply leaching. Because in leaching the solute is being extracted from the solid this is also called extraction. In leaching, when an undesirable component is removed from a solid with water, the process is called washing.

In the biological and food processing industries, many products are separated from their original natural structure by liquid-solid leaching. An important process for example is the leaching of sugar from sugar beets with hot water. In the production of vegetable oils, organic solvents such as hexane, acetone, and/or ether are used to extract oil from nuts, beans and seeds.

In the pharmaceutical industry, many different pharmaceutical products are obtained by leaching plant roots, leaves, and stems.

Leaching processes for inorganic and organic materials:Leaching is extensively used in metal processing industries. The useful metal may occur in mixtures with very large amounts of undesirable constituents, and leaching is used to remove the metals as soluble salts. The use of acids is prevalent in the metal processing industry, Sulphates are normally used to remove metals from the solid phase, these produce harmful environmental byproducts such as sulphates.

Liquid-liquid extraction

Liquid-liquid extraction is when the two phases are liquid, where a solute or solutes are removed from one liquid phase to another liquid phase. Biofuels and chemicals produced by biological processes such as fermentation and algae often require liquid-liquid extraction (LLE) as the first step in recovery and purification. Many of these chemicals are higher boiling than water or from azeotropes, resulting in high-energy requirement for distillation.

LLE often offers a process with significantly reduced energy requirement, and as such can provide a cost effective process that has minimal energy utilization, a key factor for effective biofuel technology.

Distillation

A volatile vapor phase and a liquid phase that vaporizes are involved. A simple example is distillation of an ethanol-water solution, where the vapor contains a concentration of ethanol greater than in the liquid. Another example is distillation of an ammonia-water solution to produce a vapor rich in ammonia. In the distillation of crude petroleum, various fractions, such as gasoline, kerosene, and heating oils, are distilled off.