distillation

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DISTILLATION:

DISTILLATION DEEPIKA BAIRAGEE ASSISTANT PROFESSOR Dr. APJ ABDUL KALAM UNIVERSITY, INDORE

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Distillation is an unit operation which involves separation of a vaporizable components from a multi-component system and subsequent condensation of vapours .

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APPLICATIONS

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PRINCIPLE: Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. And also depends on the vapour pressure characteristics of liquid mixtures. For any liquid, the individual molecules within the liquid are continuously in motion A small percentage of these molecules attain enough kinetic energy to leave the liquid phase This exerts an opposing pressure on the atmosphere above the solution known as the vapor pressure, P

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When enough energy, in the form of heat, is imparted to the solution the vapor pressure becomes equal to the atmospheric pressure and the liquid begins to boil.

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Raout’s law Raoult’s law states that, any particular temperature, the partial pressure of one component of a binary mixture is equal to the mole fraction of that component multiplied by its vapor pressure in the pure state at this temperature. Lets us consider two liquids A and B are at constant temperature, where A is more volatile as compared to B According to Raoult’s law pA = Pax pA =partial pressure of component A over a solution X= mole fraction of A PA= Vapor pressure of component A in the pure state at the temperature of the solution

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pB = PB (1-x) pB = Partial pressure of component B over the solution 1-x = Mole fraction of B PB = Vapor pressure of component B in the pure state at the temperature of the solution P = pA + pB = PAx + PB (1- x) Where P represents total pressure Since Y mole fraction of component A in the vapor, is equal to the ratio of the partial pressure of A to the total pressure Y = pA / pA + pB = Pax / PAx + PB (1-x) = PAx /P

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To illustrate Raout ’s law, let us consider the case of benzene and toluene mixture At a temperature of 100˚C toluene has a vapor pressure of 556 mm When partial pressure is plotted against composition, the partial pressures of toluene at various compositions will fall along a straight line from 556mm for pure toluene to zero for pure benzene At this same temperature benzene has vapor pressure of 1350mm, and its vapor pressure will change linearly from zero for 0% benzene to 1350mm for pure benzene The total pressure for any composition will be the sum of the two partial pressures at that composition If the partial pressures are straight lines i.e. Raoult’s law holds then the total pressure will be a straight line between 556mm for pure toluene and 1350 mm for pure benzene

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Roault ’s law applies only to mixtures in which the components are very similar chemically and the molecules of the two substances do not interact in any way Benzene and Toluene – Obey Raoult’s law A mixture of alchol and water, acetic acid and water, methanol and acetone – don’t obey Raoult’s law

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Ideal solution: Ideal solution is defined as a solution that obey’s Raoult’s law. Examples: In these solutions the components have similar structures eg . Benzene and toluene system, n- heptane and n-hexane , ethyl bromide and ethyl iodide etc. In this case total pressure is equal to sum of the partial pressure of the components, i.e. P= ( pA + pB ) The total pressure curve will be a straight line Non-ideal solutions: Solutions those will not obey Raoult’s law are known as non-ideal or real solutions. Most non-ideal solutions shows deviation. The deviations are observed due to uneven solute-solute, solute-solvent and/ or solvent-solvent interactions. Two types of deviations are found: (i) Positive deviation: In these systems the over all vapor pressure is greater than the sum of the partial vaopur pressure of the individual components, i.e. P > ( pA + pB ) When the components differ in their polarity, length of carbon chain or degree of association, the system may show positive deviaiton . Examples: Carbontetrachloride and cyclohexane , benzene and ethanol.

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(ii) Negative deviation: In these systems the over all vapor pressure is lower than the sum of the partial vapor pressure of the individual components, i.e. P< ( pA+pB ) If hydrogen bonding, salt formation and hydration occurs then these systems may show negative deviation. Examples: Chloroform and acetone, pyridine and acetic acid, water and nitric acid.

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Volatility The volatility of any substance in a solution may be defined as the equilibirum partial pressure of the substance in the vapor phase divided by the mole fraction of the substance in the solution. For example, a substance A in a liquid mixture has partial pressure pA and its concentration in the mixture is XA on mole fraction scale. The volatility of A ( vA ) may be mathematically expressed as: vA = partial vapor pressure of A/mole fraction of A in solution = pA /XA

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Relative volatility Relative volatility, α =volatility of component A/ volatility of component B = vA / vB Since, v = p/X, it may be substituted in above equation α = pA /XA/ pB /XB = pAXB / pBXA According to Dalton’s law, the partial vapor pressures of A and B may be expressed as: pA = YA.P pB = YB.P Where YA = mole fraction A in the vapor state YB = mole fraction B in the vapor state P = total pressure of the vapor, kPa Relative volatility may also be expressed by substituting the value of pA and pB in the above equation αAB = YAP.XB/YBP.XA

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For example, a mixture of methyl alcohol and water is having a total vapor pressure of 101.31 kPa . This liquid contains 0.40 mole fraction of methyl alcohol and the equilibrium vapor contains 0.729 mole fraction. XA = 0.4 ; YA = 0.729 ; XB = 0.6; YB = 0.271 Relative volatility = 0.729 X 0.60/ 0.271 X 0.40 = 4.035 Sometime s, relative volatility may change with concentration especially if the binary solution do not obey Raoult’s law. However, mixtures that obey Raoult’s law show only a slight change in relative volatility with concentration variation.

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HENRY’S LAW The partial pressure of a component over a solution is proportional to its mole fraction in the liquid. This can be expressed as pA = Cx Where pA = partial pressure of component A X = mole fraction of A in liquid phase C = Henry’s law constant; C is constant only at constant temperature Raoult’s law is essentially a special case of Henry’s law where the constant C in equation is the vapor pressure of the pure component

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GENERAL EQUIPMENT FOR DISTILLATION STILL: It is a vaporizing chamber and used to place the material to be distilled. The still is heated by a suitable means for vaporization of the volatile constituents On laboratory scale round bottom flasks made of glass are used so that the progress of the distillation can be noticed. A condenser is attached to the still using appropriate joints. A trap is inserted between distillation flask and condenser.

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CONDENSER Used to condense the vapor It is kept cold by circulating water/air through jacket. TYPES Single-surface condensers Straight Tube Bulb Tube Spiral Coiled Type Double-surface condensers Multi-tubular condensers The condenser is connected to a receiver through a suitable adapter.

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RECEIVER It is used to collect the distillate It may be a simple flask It immersed in ice-bath to minimize loss of volatile matter Florentine receivers are used for the separation of oil and water Types of Florentine receivers Type I : For separation of oil heavier than water Type II: For separation of oil lighter than water

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Classification of Distillation methods

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SIMPLE DISTILLATION Simple distillation is a process of converting a single constituent from a liquid (or mixture ) into its vapor, transferring the vapor to another place and recovering the liquid by condensing the vapor, usually by allowing it to come in contact with a cold surface. This process is known differential distillation, as distillation is based on the differences in volatilities and vapor pressure of the component in the mixture.

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PRINCIPLE Liquid boils when its vapor pressure is equal to atmospheric pressure. Simple distillation is conducted at its boiling point. The higher the relative volatility of a liquid, the better is the separation by simple distillation. Heat is supplied to the liquid so that it boils. The resulting vapor is transferred to a different place and condensed.

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APLLICATIONS For the preparation of distilled water and water for injection Volatile and aromatic waters are prepared Organic solvents are purified A few official compounds are prepared by distillation. Examples are spirit of nitrous ether and aromatic spirit of ammonia Non-volatile solids are separated from volatile liquids.

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ASSEMBLING OF APPARATUS It consists of a distillation flask with a side arm sloping downwards. Condenser is fitted into the side arm by means of a cork. The condenser is usually water condenser, i.e., jacketed for circulation of water. The condenser is connected to a receiver flask using an adapter with ground glass joints. On a laboratory scale, the whole apparatus is made of glass.

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PROCEDURE The liquid to be distilled is filled into the flask to one-half to two-third of its volume. Bumping is avoided by adding small pieces of porcelain before distillation A thermometer is inserted into the cork and fixed to the flask. The thermometer bulb must be just below the level of the side arm. Water is circulated through the jacket of the condenser. The content s are heated gradually. The liquid begins to boil after some time. The vapor begins to rise up and passes down the side arm into the condenser. The temperature rises rapidly and reaches a constant value. The temperature of the distillate is noted down, which equal to the boiling point of the liquid. The vapor is condensed and collected into the receiver. The flame is adjusted so that the distillate is collected at the rate of one to two drops per second. Distillation should be continued until a small volume of liquid remains in the flask.

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FLASH DISTILLTION

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Flash distillation is defined as a process in which the entire liquid mixture is suddenly vaporized (flash) by passing the feed from a high pressure zone to a low pressure zone. Flash distillation is also known as equilibrium distillation, i.e., separation is attempted when the liquid and vapor phases are in equilibrium. This method is frequently carried out as a continuous process and does not involve rectification.

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PRINCIPLE When a hot liquid mixture is allowed to enter from a high-pressure zone into a low-pressure zone, the entire liquid mixture is suddenly vaporized. This process is known as flash vaporization. During this process the chamber gets cooled. The individual vapor phase molecules of high boiling fraction get condensed, while low boiling fraction remains as vapor. USES Flash distillation is used for separating components, which boil at widely different temperatures. It is widely used in petroleum industry for refining crude oil. ADVANTAGES Flash distillation is a continuous process. DISADVANTAGES It is not effective in separating components of comparable volatility. It is not an efficient distillation when nearly pure components are required, because the condensed vapor and residual liquid are far from pure.

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WORKING The feed is pumped through a heater at a certain pressure. The liquid gets heated, which enters the vapor-liquid separator through a pressure –reducing value. Due to the drop in pressure, the hot liquid flashes, which further enhances the vaporization process. The sudden vaporization induces cooling. The individual vapor phase molecules of high boiling faction get condensed , while low boiling fraction remains as vapor. The mixture is allowed for a sufficient time, so that vapor and liquid portions separate and achieve equilibrium. The vapor is separated through a pipe form above and liquid is collected from the bottom of the separator. By continuously feeding into the still, it is possible to obtain continuous flash distillation. The operating conditions can be adjusted in such a way that the amount of feed exactly equals the amount of material removed. Therefore, vapor and liquid concentrations at any point remain constant in the unit.

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VACUUM DISTILLATION (DISTILLATION UNDER REDUCED PRESSURE)

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The distillation process in which the liquid is distilled at a temperature lower than its boiling point by the application of vacuum. Vacuum pumps, suction pumps, etc are used to reduce the pressure on the liquid surface. Distillation under the reduced pressure is based on the principle of the simple distillation with some modification.

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PRINCIPLE Liquid boils when vapor pressure is equal to the atmospheric pressure, i.e., pressure on its surface. If the external pressure is reduced by applying vacuum, the boiling point of liquid is lowered. Therefore, the liquid boils at a lower temperature. This principle is illustrated using an example of water. Water boils at an 100˚C at an atmospheric pressure is 101.31kPa. At 40˚C , the vapor pressure of water is approximately 9.33kPa. Hence, the external pressure is reduced to 9.33pKa where water boils at 40˚C . The net result is the increase in rate of mass transfer into vapour .

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ASSEMBLING OF APPRATUS It consists of a double-neck distillation flask known as Claisen flask. Thick walled glass apparatus with interchangeable standard glass joints are used for vacuum distillation. In one of the necks of the Claisen flask, a thermometer is fitted. The second neck prevents splashing of the violently agitated liquid. Bumping occurs readily during vacuum distillation. Placing a fine capillary tube in the second neck of the flask can prevent bumping. The capillary tube is dipped in the boiling liquid, so that a stream of air bubbles is drawn out. Water bath or oil bath is used for heating The Claisen flask is connected to a receiver through a condenser. Vaccum pump is attached through an adapter to the receiver. A small pressure gauge should be inserter between the pump and the receiver.

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APPLICATION Preventing degradation of active constituents Enzymes – malt extract, pancreatin Vitamins- thiamine, ascorbic acid Glycosides – anthraquinones Alkaloids – hyocyamine to atropine DISADVANTAGES In vacuum distillation, persistent foaming occurs. This may be overcome by adding carpryl alcohol to the liquid or by inserting a fine air capillary tube in the second neck of the Claisen flask. The stream of air is drawn in and breaks the rising foam. The above method is not suitable for the preparation of semisolids extracts by distillation under vacuum.

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MOLECULAR DISTILLATION It is defined as a distillation process in which each molecule in the vapor phase travels mean free path and gets condensed individually without intermolecular collisions on application of vacuum. Molecular distillation is based on the principle of the simple distillation with some modifications. This is also called Evaporation distillation or short path distillation.

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PRINCIPLE The substances to be distilled have very low vapor pressure. Example are viscous liquids, oils, greases, waxy materials and high molecular weight substances. These boil at very high temperature. In order to decrease the boiling point of the liquids, high vacuum must be applied. The pressure exerted by vapors above the liquid is much lower. At very low pressure, the distance between the evaporating surface and the condenser is approximately equal to the mean free path of the vapor molecules. Molecules leaving the surface of the liquid are more likely hit the condenser surface nearby, each molecule is condensed individually, the distillate is subsequently collected.

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THEORY The mean free path of a molecule is defined as the average distance through which a molecule can move without coming into collision with another. Λ = ŋ√3/p ρ Where, p = vapour pressure, kPa ρ = density, kg/m3 ŋ=viscosity, Pa.s Λ = mean path length, m

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APPLICATION S Purification of chemicals such as thricresyl phosphate, dibutyl phthalate and dimethyl phthalate. More frequently used in the refining of fixed oils. Vitamin A is separated from fish liver oil. Vitamin E is concentrated by this method from fish liver oils and other vegetable oils. Free fatty acids are distilled at 100˚C. Steroids can be obtained between 100˚C and 200˚C, while triglycerides can be obtained from 200˚C onwards. Proteins and gums will remains as non-volatile residues. Thus, the above mixture can be separated by molecular distillation.

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Falling film Molecular still or Wiped Film Molecular still Vaporization occurs from a film of liquid flowing down a heated surface under high vacuum. The vapour travels a short distance and strikes the condenser nearby. Each molecule is condensed individually. The distillate is subsequently collected.

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CONSTRUCTION The vessel has a diameter of one meter. The walls of the vessel are provided with suitable means of heating (jacket). Wipers are provided adjacent to the wall. Wipers are connected to a rotating head through a rotor. The condensers are arranged very close to the wall. Vacuum pump is connected to a large diameter pipe at the center of the vessel. Provisions are made for collecting the distillate and the undistilled liquid residue at the bottom.

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WORKING The vessel is heated by suitable means. Vacuum is applied at the centre of the vessel and wipers are allowed to rotate. The feed is entered through the inlet of the vessel. As the liquid flows down the walls, it is spread to form a film by PTFE wipers, which are moving at a rate of 3m per second. The velocity of the film is 1.5 m per second. Since the surface is already heated, the liquid film evaporated directly. The vapour travels its mean free path and strikes the condenser. The condensate is collected into the vessel. The residue is collected form the bottom of the vessel and re-circulated through the feed port for further distillation. (1000L/h)

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Centrifugal Molecular Still Liquid feed is introduced into a vessel, which is rotated at very high speed. On account of heating, vaporization occurs form a film of liquid on the sides of the vessel. The vapour travels a short distance and gets condensed on the adjacent condenser. Each molecules is condensed individually. The distillate is subsequently collected.

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CONSTRAUCITON It consists of a bucket-shaped vessel having a diameter of about 1 to 1.5m. It is rotated at high speed using a motor. Radiant heaters are provided externally to heat the fluid in the bucket. Condensers are arranged very close to the evaporating surface. Vacuum pump is connected to the entire vessel at the top. Provisions are made for introducing the feed into the center of the bucket, for receiving the product and residue for re-circulation.

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WORKING Vacuum is applied at the centre of the vessel. The bucket shaped vessel is allowed to rotate at high speed. The feed is introduced form the centre of the vessel. Due to centrifugal action of the rotating bucket, liquid moves outward over the surface of the vessel and forms a film. Since, the radiant heaters heat the surface, the liquid evaporates directly form the film. The vapor travels its mean tree path and strikes the condenser. The condensate is collected into another vessel. The residue is collected form the bottom of the vessel and is re circulated through the feed port for further distillation.

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STEAM DISTILLATION Steam distillation is method of distillation carried out with aid of steam. It is used to separate High boiling substance from non-volatile impurities Separate immiscible liquids

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PRINCIPLE A mixture of immiscible liquids begins to boil when sum of their vapors pressure is equal to atmospheric pressure. In case of mixture of water and turpentine, mixture boils below the boiling point of pre water, though the turpentine boils at a much higher temperature than that of water. Example Boiling point of Turpentine = 160˚C Boiling point of water + Turpentine mixture = 95.6˚C At this temperature Vapor pressure of Water = 86.245kPa (647mmHg) Turpentine = 15.06kPa (113mmHg) Sum of the vapor pressure = 101.31kPa (760mmHg) Which is normal atmospheric pressure and thus high boiling liquid may be distilled with water at a temperature much below its boiling point.

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APPLICATION It is used to separate immiscible liquids like water and toluene. It is useful in purification of liquid with high boiling point, like essential oil of almond. Camphor is distilled by this method. Aromatic waters are prepared by this method. It is used to extract volatile oils like clove, anise and eucalyptus oils. ADVANTAGES Volatile oils can be separated at a lower temperature in steam distillation, without any decomposition and loss of aroma. DISADVANTAGES Steam distillation is not suitable when immiscible liquid and water react with each other.

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CONSTRUCTION Metallic steam can fitted with cork having two holes. Safety tube inserted up to bottom through one hole to maintain pressure inside steam can, more over when steam comes out from safety tube indicates that can is empty. Though other hole band tube is passed and other end of this tube is connected to flask containing non-aqueous liquid in which tube is dipped. Flask and condenser is connected with delivery tube. Condenser is connected to receiver with help of adopter. Provision are made to heat both steam can and flask separately.

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WORKING The non-aqueous liquid is placed in the flask. A small quantity of water is added to it. Steam can is filled with water. The steam generator and the flask are heated simultaneously, so that a uniform flow of steam passes through the boiling mixture. The mixture gets heated. The steam carries the volatile oil and passes into the condenser, which is cooled by cold water. The condensed immiscible liquid is collected into the receiver. Distillation is continued until all the non-aqueous liquid has been distilled.

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FRACTIONAL DISTILLATION Fractional distillation is a process in which vaporization of liquid mixture gives rise to a mixture of constituents from which the desired one is separated in pure form. This method is also known as rectification, because a part of the vapor is condensed and returned as a liquid. This method is used to separate miscible volatile liquids, whose boiling points are close, by means of a fractionating column.

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PRINCIPLE When a liquid mixture is distilled, the partial condensation of the vapor is allowed to occur in a fractionating column. In the column, ascending vapor from the still is allowed to come in contact with the condensing vapor returning to the still. This results is enrichment of the vapor with the more volatile component. By condensing the vapor and reheating the liquid repeatedly, equilibrium between liquid and vapor is set up at each stage, which ultimately results in the separation of more volatile component.

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APPLICATION Fractional distillation is used for the separation of volatile miscible liquids with near boiling point such as acetone and water, chloroform and benzene. DISADVANTAGES Fractional distillation cannot be used to separate miscible liquids, which form PURE azeotropic mixtures.

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FRACTIONATING COLUMNS In fractional distillation, special type of still-heads are required so that condensation and re- vaporisation are affected continuously. These are known as fractionating columns. A fractionating column is essentially a long vertical tube in which the vapor passes upward and partially condensed. The condensate flows down the column and is returned eventually to the flask. The columns are constructed so as to offer the following advantages simultaneously. It offers a large cooling surface for the vapour to condense. An obstruction to the ascending vapour allows easy condensation.

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Fractionating columns Types Packed columns and Plate columns

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Packed columns Some form of packing is used in the column to affect the necessary liquid/ vapour contact. The packing may consist of single turn helices of wire or glass, glass rings, cylindrical glass beads, stainless steel rings etc. Construction: Packed column consists of a tower containing a packing that becomes wetted with a film of liquid, which is brought into contact with the vapour in the intervening spaces. A long fractionating column is necessary when the boiling points of the constituents are lying fairly close together. A short fractionating column is necessary when the boiling point of the constituents differ considerably. APPLICATIONS Packing must be uniform so as to obtain proper channels. If packing is irregular, mass transfer becomes less effective.

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B. Plate columns Many forms of plate are used in the distillation using different columns. It can be divided into two types, which are commonly used in pharmacy. Bubble cap plates Turbo grid plates Construction: The column consists of a number of plates mounted one above the other. Caps are present on each plate, which allow the vapor to escape by bubbling through the liquid. Working: Ascending vapor from the still passes through the bubble-caps on plate A and the rising vapour will be richer in the more volatile component. This vapor passes through the liquid on plate B and partially condensed. The heat of condensation partially vaporize the liquid. The process of condensation and vaporization will be repeated at plate C and so all the way up the column. Each bubble-cap plate has the same effect as a separate still.

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ADVANTAGES The bubble cap plate is effective over a wide range of vapour -liquid proportions. There is excellent contact as the vapour bubbles through the liquid. DISADVNATAGES A layer a liquid on each plate results in considerable hold-up o f liquid over the entire column. The need to force the vapor out of the caps, through the liquid, led to a large pressure drop through the column. The column does not drain when it is not in use. The structure is complicated making construction and maintenance expensive.

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THEORY Fractional distillation is suitable for a system when the boiling point of the mixture is always intermediate between those of pure components. There is neither a maximum nor a minimum in the compostion curves. These systems are known as azeotrophic mixtures. Examples Benzene and toluene Carbon tetrachloride and cyclohexane

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AZEOTROPIC AND EXTRACTIVE DISTILLATION In which azeotorpic mixture is broken by the addition of third substance, which forms a new azeotrpe with one of the components. Extractive distillation The third substance added to the azeotorpic mixture is relatively nonvolatile liquid compared to the components to be separated.

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APPLICATION The liquor from fermentation process is a common source of ethanol and contains approximately 8 to 10%. Absolute alcohol can be prepared by azeotrophic distillation.

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DESTRUCTIVE DISTILLATION It is a distillation method in which the distillate is decomposition products of the constituents of the organic matter burnt in the absence of air. This process is also known as dry distillation.

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COMPRESSION DISTILLATION Compression distillation method was developed to meet the needs of Navy and Army for fresh water, which is obtained from sea-water. The product obtained is quite pure and pyrogen -free.

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