Distillation

 Chapter -6 

DISTILLATION

 INTRODUCTION 

• Historically distillation was used since 1200 BC in perfumery operations. Early forms of distillation were batch processes using one vaporization and one condensation. Purity was improved by further distillation of the condensate. Greater volumes were processed by simply repeating the distillation. Chemists were reported to carry out as many as 500 to 600 distillations in order to obtain a pure compound. In the early 19th century, the basics of modern techniques including pre-heating and reflux were developed. In 1877 U.S. Patent was granted for a tray column for distillation of ammonia and in the subsequent years for oil and spirits. With the emergence of chemical engineering as a discipline at the end of the 19th century, scientific rather than empirical methods were applied. More accurate designs were used during developments in petroleum industries. Today availability of powerful computers has allowed direct computer simulation of distillation columns.

• Distillation is one of the most important processes for separating the components of a solution. The solution is heated to form a vapour of the more volatile components in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and condensation, individual components in the solution can be recovered in a pure state. Essences and many pure products from the oil refinery industry are processed via distillation

Objective of Distillation: 

• The basic objective of distillation is to separate liquid mixture into two or more components. In a basic distillation column, a feed stream enters in the middle of the column and two streams leave, one at the top and one at the bottom. Components with lower boiling points are concentrated in the stream leaving the top while components with higher boiling points are concentrated in the stream leaving the bottom. Separation is achieved by controlling the column temperature and pressure to take advantage of differences in the relative volatility of the mixture components and therefore tendency to change phase. The lighter, lower boiling point components evaporate and travel up the column to form the top product and the heavier, higher boiling point components condense and travelling down the column to form the bottom product.

BASIC PRINCIPLES  

• Distillation is a process by which a liquid mixture is separated into fractions with higher concentrations of certain components by exploiting differences in relative volatility. Distillation has been used widely to separate volatile components from non-volatile compounds. In industrial settings such as oil refineries and natural gas processing plants this separation process is undertaken using a distillation column. Distillation is a process by which a liquid mixture is separated into fractions with higher concentrations of certain components by exploiting differences in relative volatility. Distillation has been used widely to separate volatile components from non-volatile compounds. In industrial settings such as oil refineries and natural gas processing plants this separation process is undertaken using a distillation column.

• Fractionation: It is another term for distillation also called fractional distillation. 

• Feed: The liquid and/or gas feed into the distillation column. 

• Feed tray: The tray below the inlet nozzle is called the feed tray

• Heavy Component: The component with the lower relative volatility, for example, simple hydrocarbon, this is a component with the higher molecular weight. It is found in higher concentration in the bottom product of the column.

• Light Component: The component with the higher relative volatility, for example, simple hydrocarbon, this is a component with the lower molecular weight. It is found in higher concentration at the top of the column. 

• Stripping section: It is a section that consists of trays between the bottom of the column and the feed tray. In the stripping section the aim is to concentrate the heavier component in the liquid phase.

• Rectifying section: It is a section that consists of trays between the feed tray and the top of the column. In the rectifying section the aim is to concentrate the lighter component in the vapour phase. 

• Top Product: It is a product which leaves from the top of the column, also called distillate. This product is usually passed through a heat exchanger and liquefied. 

• Bottom Product: It is the product which leaves through the bottom of the distillation column. 

• Reflux: A portion of vapors from the top of the column which has been condensed to a liquid and returned to the column as a liquid above the top tray. 

• Reboiler: A heat exchanger at the bottom of the column which boils some of the liquid leaving the column. The vapor generated returns to the column at the bottom of the stripping section. 

• Vapour-Liquid Equilibrium (VLE) Curve: A plot of the actual composition of the lighter component in the vapour phase for a given composition in the liquid phase. Usually it is derived from thermodynamic data. 

• Zeotropic mixture: It is a mixture of liquids with different boiling points. For example, nitrogen, methane, ethane, propane etc.

• Azeotropic mixture: It is a mixture of two or more liquids that has a constant boiling point because vapors have same composition as liquid mixture. 

Principle of Separation: 

• Distillation takes advantage of the difference in relative volatility of the feed mixture components. Generally for two or more compounds at a given pressure and temperature there will be a difference in the vapour and liquid compositions at equilibrium due to component partial pressure. Distillation exploits this by bringing liquid and gas phases into contact at temperatures and pressures that promote the desired separation. During this contact the components with the lower volatility (typically lower boiling point) preferentially moves into the liquid phase while more volatile components move into the vapour phase. 

• A distillation column may use either trays or a packed bed to bring the gas and liquid into contact. For a column using trays we can consider the changes to gas and liquid phase compositions as they both enter and exit a single tray. The liquid entering the tray will contact the gas exiting the tray. The hotter vapors phase heats the incoming liquid A distillation column may use either trays or a packed bed to bring the gas and liquid into contact. For a column using trays we can consider the changes to gas and liquid phase compositions as they both enter and exit a single tray. The liquid entering the tray will contact the gas exiting the tray. The hotter vapors phase heats the incoming liquid Where, C is concentration and T is temperature. 

• When a packing is used rather than trays the principle remains the same, in fact packing is often referenced in terms of height equivalent to a theoretical plate (HETP) i.e. what height of packing is equivalent to one theoretical plate. The packing is just an alternative method to bring the liquid and vapors phases into contact with the liquid generally flowing over the surfaces of the packing material, while the vapors passes up through the space between packing elements. 

Typical Operating Parameters:   

The distillation process can be improved by understanding the following operating parameters

• Temperature: The basic temperature profile of a distillation column is hotter at the bottom and cooler at the top. For a simple two component distillation the temperature at the bottom is just lower than the boiling point of the heavier component. The temperature at the top of the column is just above the boiling point of the lighter component. In order to have heavy component to remain as a liquid at the bottom of the column and the lighter component to stay as a gas we set the temperature at the bottom to match this requirement. This temperature is set by adding heat via a heat exchanger. Typically, the heat added to the bottom of the column is easy to control, via steam or hot oil flow rates.

• At the top of the column the situation is reverse. The light component remains as gas while the heavier component is condensed to a liquid and falls back down the column. The top temperature is set just above the boiling point of the lighter component. The temperature control situation is different at the bottom of the column, because the top product to be a liquid when we send it for storage. All of the gas coming out of the top of the column is condensed to liquid. This liquid stream is split with some returning to the column and some going to storage. The top temperature is often controlled by changing the reflux rate, i.e. the flow rate of liquid sent back to the top of the column. A higher reflux rate means cooler liquid falling down the column against the rising warmer gas, and the top temperature is lower. Overall heat is added at the bottom of the column and heat is extracted at the top of the column. Inside the column the temperature balance is created between the hot gas rising up the column and the cooler liquid falling down the column. 

• Pressure: There is typically a pressure gradient across the column with the pressure being higher at the bottom of the column than the top. This pressure gradient occurs as the liquid coming down the column hampers the flow of vapour up the column and imposes a pressure loss on the flow. In steady state distillations the pressure in the column is held constant, and the temperature is varied to control the composition of the product streams.

Applications:

• Distillation is used for many commercial processes, such as production of gasoline, distilled water, xylene, alcohol, paraffin, kerosene, and many other liquids.  
• Distillation is used for purifying solvents and liquid reaction products. 
• It is used in the manufacturing of distilled water, double distilled water used in Water for Injection and other pharmaceutical preparations. 
• Toxic and costly organic solvents are used in extraction, synthesis and analysis of drugs. This solvent can be recovered by distillation for economic as well as the environmental protection benefits.
• Distillation is used in the separation of volatile oils such as clove oil, anise oil, cardamom oil, eucalyptus oil etc. from the plant extracts. 
• It can be used in the separation of volatile components from a mixture of two or more volatile liquids. 
• It can be used as a quality control method for alcohol content in liquid formulations. The alcohol is separated from formulations by distillation and alcohol content is determined. 
• It can be used to liquefy and separate gases from air. For example: nitrogen, oxygen, and argon are distilled from air. 
• Distillation is used in crude fermentation broths to separate alcoholic spirits. 
• It can also be used in the fractionation of crude oil into gasoline and heating oil. 

METHODOLOGY OF SIMPLE DISTILLATION   

Simple distillation is a unit operation in which two liquids with different boiling points are separated

Principle: 

• Simple distillation is a process of heating and cooling liquids in order to separate and purify them. As the liquid being distilled is heated, the vapours that form are richest in the component of the mixture that boils at the lowest temperature. Purified component boils, and thus turns into vapours, over a relatively small temperature range (2 or 3 °C). A careful observation of the temperature in the distillation flask helps to carry out a good separation. As distillation progresses, the concentration of the lowest boiling component steadily decreases. Eventually, the temperatures within the apparatus begin to change and a pure compound is no longer being distilled. As the temperature continues to increase the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate is primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture are separated. In order for simple distillation to perform, the two liquids’ boiling points must have a difference of at least 25 °C (or about 77 °F).

Construction: 

• The set of simple distillation consists of distillation flask with side arm sloping downwards. The mouth of the flask is fitted with cork closure with inserted thermometer. The condenser is attached to sloping arm for cold water circulation with inlet at lower side and outlet at upper side. The cold-water pipe is attached to inlet while outlet discharges water to waste. The condenser outlet delivers liquid product that is collected in a collector or receiver.

Working: 

• Calibration of thermometer: Calibration can be done by placing the thermometer in an ice bath of distilled water. Allow thermometer to reach thermal equilibrium. Now remove from ice water and place it in a beaker of boiling distilled water and again allow it to reach thermal equilibrium. If the temperatures measured does deviate from the expected values by more than two degrees, then use it for recording temperature in distillation process. 

•  Filling the distillation flask: The flask is filled with not more than two third of its volumes to have sufficient space above the liquid surface so that when boiling begins the liquid will not be propelled into the condenser. This is important in viewpoint of purity of the distillate. Porcelain chips should be placed in the distillation flask to prevent superheating of the liquid and to cause a more controlled boiling, eliminating the possibility of liquid to bump into the condenser.

• Heating the distillation flask: The distillation flask is heated slowly until the liquid begins to boil. The vapors' rise up through the neck of the distillation flask and pass through the condenser and condense and drip into the collection receiver. Generally, rate of distillation is approximately 20 drops per minute. Distillation must occur slowly enough that all the vapors condense to liquid in the condenser. Many organic compounds are flammable and if vapors pass through the condenser without condensing, they may ignite as they come in contact with the heat source. 

• Condensation of vapors': As the distillate begins to drop from the condenser, the temperature changes steadily. When it is stable, new receiver is used to collect all the drops that form over a two-to-three-degree range of temperature. As the temperature begins to rise further, a third receiver is used to collect the distillate. This process is repeated; using a new receiver every time the temperature stabilizes or begins changing, until all of the distillate has been collected in discrete fractions. All fractions of the distillate should be saved until it is shown that the desired compound has been effectively separated by distillation. 

Handling Precautions: 

• If direct heating is used stop the heat source from the distillation flask before all of the liquid is vaporized. 

• When all of the liquid is evaporated, the temperature of the glass of the distillation flask rises very rapidly, possibly ignites whatever vapours still is present in the distillation flask. 

• Never distill to dryness. The residue left in the distillation flask may contain peroxides, which could ignite or explode after all the liquid has distilled away. 

• Make sure that all joints are secured very tightly. If any vapour escapes at the connection points, it may come into direct contact with the heat source and ignite. 

• Never heat a closed system, the increasing pressure will cause the glass to explode. 

• If the distillation flask has a tapered neck, the thermometer may be placed in such a way as to block the flow of vapors' up the neck of the flask; in effect creating a closed system; make sure that if using a tapered neck flask, the thermometer is not resting in the lowest portion of the neck.

• If the liquids comprising the mixture that is being distilled have boiling points closer than 25 °C to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for complete separation of the individual compounds. 

Advantages: 

• It is simple, cheap, easy and economic method. (ii) It requires less energy. (iii) This process requires single run and thus is comparatively faster.

Disadvantages: 

• The final product may contain impurities. 

• Azeotropic mixtures cannot be separated by simple distillation. 

• Not suitable for mixtures containing thermolabile components. 

• The volume of mixture should be not more than 2/3rd of the container.

Applications: 

• Simple distillation is primarily used for production of distilled water. 

• Many volatile oils are separated by simple distillation. 

• It is also used in the separation of organic solvents from mixtures. 

• It is used to separate non-volatile components from volatile ones.

• It is used in preparing pharmaceutical spirits.

FLASH DISTILLATION    

• Flash distillation, also called "equilibrium distillation", is a single stage separation technique. Simple flash separations are very common in industry, particularly petroleum refining. Even when some other method of separation is to be used, it is not uncommon to use a "pre-flash" to reduce the load on the separation itself.

Principle: 

• The separation using flash distillation is based upon the flash vaporization. In flash vaporization hot liquid mixture when passes from high pressure zone to low pressure zone suddenly get vaporized. Reduced pressure reduces the boiling point which leads to the vaporization of the liquid. The energy for vaporization is taken-up from the liquid itself, which causes decrease in temperature. The molecules in vapors phase with low boiling point get condensed while high boiling point molecules remain as vapors. The vapors and condensed liquid fraction remain in contact till saturation. The liquid falls at the bottom and is collected whereas vapors' are further allowed to condense. 

Construction:     

• Flash distillation unit consists of pump attached to feed tank from where it pumps feed at high pressure. This unit has a heating chamber through which the liquid mixture carrying pipe passes. The chamber is insulated to avoid heat loss during operation and maintain desired temperature. There is a pressure control valve fitted between heating chamber and the flash drum. The other end of the pipe directly opens into the flash drum. In single stage flash distillation unit liquid outlet is provided at the bottom. In case of multistage distillation, the liquid and vapors are taken to next unit for further distillation. When designing a flash system, it is important to provide enough disengaging space in the flash drum. Flash drum can also be designed as cyclone separators. 

Working:

• A liquid mixture feed is pumped through a heater to raise the temperature and enthalpy of the mixture. It then flows through a valve and the pressure is reduced, causing the liquid to partially vaporize. Once the mixture enters in a big enough volume, the "flash drum", liquid and vapoors get separated. Because the vapoor and liquid are in such a close contact the "flash" occurs and the product liquid and vapoor phases approach equilibrium. Flash distillation can be operated in continuous mode as well as at multi-level. The operational setting is synchronized in such a way that input equals the output of the process and thus the vapors and liquid proportions at any instance remain constant in the flash drum.

Application: 

• Flash distillation is used in petroleum industry for refining crude oil. 
• It is used in the desalination of ocean water by multi-stage flash distillation. 
• It can also be used for separation of heptane from octane.

Advantages: 

• Flash distillation is a continuous process. 
• The equipment is smaller than the multi-stage flash distillation. 

• The operating costs are low compared to multi-stage flash distillation.

Disadvantages:

•  Flash distillation is not effective in separating components of comparable volatility. 

• It is not suitable for two component systems. 

• It is not an efficient distillation when nearly pure components are required, because the condensed vapour and residual liquid contain both components to some extent.     

FRACTIONAL DISTILLATION

• The basic idea behind fractional distillation is the same as simple distillation. The difference between simple and fractional distillation is the number of times that the liquid is vaporized and condensed. Simple distillation condenses the liquid once, so the boiling points of the two liquids must be far apart to make it efficient. Simple distillation is performed on a mixture of liquids with similar volatilities and the resulting distillate is in more concentrated form in the more volatile compound than the original mixture and it may contain a significant amount of the higher boiling compound. If the distillate of simple distillation is distilled again, the resulting distillate is again of furthermore concentrated form of the lower boiling compound, but still a portion of the distillate contain the higher boiling compound. The number of simple distillations in a fractional distillation depends on the length and efficiency of the fractionating column.  

• 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 vapour 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. Fractional distillation is different from simple distillation. In simple distillation, vapour is directly passed through the condenser. In fractional distillation the vapour must pass through a fractionating column in which partial condensation of vapour is allowed to occur. In simple distillation, condensate is collected directly into the receiver, while in fractional distillation condensation takes place in the fractionating column, so that a part of the condensing vapour returns to the still.

Principle:

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

Construction:

•  The equipment used for fractional distillation consists of special type of still-heads known as fractionating columns. In still-heads condensation and revaporisation are affected continuously. Fractionating column is an essentially a long vertical tube in which the vapour moves upward and partially gets condensed. The condensate flows down the column and is returned to the flask. The columns are constructed to provide a large cooling surface for the vapour to condense and obstruct the ascending vapour to allow easy condensation. The obstruction also retards the downward flow of liquid, which is a high boiling component. Fractionating columns used are packed columns and plate columns.

• Packed columns: In this column some form of packing is used to affect the necessary liquid/vapour contact. The packing consists of single turn helices (spirals) of wire or glass, glass rings, cylindrical glass beads, stainless steel rings etc. The 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. The same type of fractionating columns can be obtained in various lengths. 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. Packing must be uniform so as to obtain proper channels. If packing is irregular, mass transfer becomes less effective. Example is Widmer column.

• Plate columns: Many forms of plates are used in the distillation columns. These can be divided into Bubble cap plates and Turbo grid plates. The bubble cap column is used in large distillation plants. The column consists of a number of plates mounted one above the other. Caps are present on each plate, which allow the vapour to escape by bubbling through the liquid. Ascending vapour from the still passes through the bubble-caps on plate A and the rising vapour will be richer in the more volatile component. This vapour passes through the liquid on plate B and partially condensed. The heat of condensation partially vaporizes the liquid. The process of condensation and vaporization is repeated at plate C and so on all the way up the column. Each bubble-cap plate has the same effect as a separate still. The bubble cap plate is effective over a wide range of vapour-liquid proportions. There is an excellent contact as the vapour bubbles through the liquid. The major limitation is a layer of liquid on each plate that results in considerable hold-up of liquid over the entire column. There is a need to force the vapour out of the caps, through the liquid that leads to a large pressure drop through the column. In addition, column does not drain when it is not in use. The structure of bubble plate is so complicated that makes construction and maintenance expensive.  

Working: 

•  In fractional distillation, the vapors formed from the boiling mixture rise into the fractionating column where they condense on the column's packing. This condensation is similar to a single run of simple distillation; the condensate is more concentrated in the lower boiling compound than the mixture in the distillation flask. As vapors continue to rise through the column, the condensed liquid revaporizes. Each time this occurs the resulting vapours are more and more concentrated in the more volatile substances. The length of the fractionating column and the material it is packed with impact the number of times the vapors will recondense before passing into the condenser. The number of times the column will support this is referred to as the number of theoretical plates of the column. The procedures of simple distillation are so similar to those involved in fractional distillation; the apparatus that are used in the procedures are also very similar. In fractional distillation, a packed fractionating column is attached to the top of the distillation flask and beneath the condenser. This provides the surface area on which rising vapors condense, and subsequently revaporize.

• The fractionating column is used to supply a temperature gradient over which the distillation can occur. In an ideal situation, the temperature in the distillation flask would be equal to the boiling point of the mixture of liquids and the temperature at the top of the fractionating column would be equal to the boiling point of the lower boiling compound; all of the lower boiling compound would be distilled away before any of the higher boiling compound. In reality, fractions of the distillate must be collected because as the distillation proceeds, the concentration of the higher boiling compound in the distillate being collected steadily increases. Fractions of the distillate, which are collected over a small temperature range, will be essentially purified; several fractions should be collected as the temperature changes and these portions of the distillate should be distilled again to amplify the purification that has already occurred.  

• Efficiency of Fractional distillation: The efficiency of separation by fractional distillation of a mixture may depend upon various factors that include fractionating column, reflux ratio, heat input and column temperature etc. Reflux ratio is the quotient of the amount of liquid returning through the column to the amount collected into the receiver during the same interval of time. A column that operates under total reflux will not yield distillate and thus it should be high. It is controlled by selecting proper still. Other experimental conditions necessary for good separation are comparatively large amount of liquid continuously returning through the column, thorough mixing of liquid and vapour and a large active surface of contact between liquid and vapour. The number of vaporization-condensation cycles that can occur within a fractionation column determines the purity which can be attained. The efficiency of a column depends upon column length and composition. Generally column is packed with copper sponge. This increases the surface area that the ascending vapour encounters and results in more vaporization-condensation cycles compared to an empty column.

• Theoretical Plates: A measure of efficiency of a column is known as the number of theoretical plates of that column. One theoretical plate is equivalent to one vaporizationcondensation cycle which is equivalent to the one simple distillation. Thus a fractionation column that can attain the equivalent of three simple distillations would be said to have three theoretical plates

Applications:

• Fractional distillation is used for the separation of miscible liquids such as acetone and water, chloroform and benzene. 

• 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 composition curves (zeotropic mixtures). Examples include benzene and toluene, carbon tetrachloride and cyclohexane, and water and methanol. 

Advantages:  

• Frictional distillation gives good solvent recovery 

• Fractional distillation is easy to use and operate. 

• Fractional distillation is also highly efficient, especially for systems that use stacked distillation columns, which produce more output at lower costs. 

• It helps to produce much-needed fuel.

Disadvantage:

• It is distilled disturbs ecology. For example, refining crude oil.  
• It is expensive because it requires large structures, heavy-duty materials, and specialized machinery
• It also requires staff to be fully trained in the operation of systems to ensure they know how to use the distillation equipment and won’t make mistakes.

DISTILLATION UNDER REDUCED PRESSURE 

• The boiling point of water increases when the external pressure is increased whereas decreased external pressure decreases the boiling point. This principle is used in the process of freeze drying. Many compounds cannot be distilled at atmospheric pressure because their boiling points are so high. At their normal boiling points, the compounds decompose. Thus, some of these materials can be distilled under reduced pressure because the required temperature to boil the liquid can be lowered significantly. If the boiling point is lowered by 10 °C each time the external pressure is halved. To vaporize a liquid, its temperature can be raised or its pressure can be decreased. Distillation under reduced pressure can also be called as vacuum distillation. During vacuum distillation, the pressure inside the distillation column is maintained at a vacuum to lower the temperature needed to vaporize the liquid. This method of distillation is used for heat sensitive products, liquids with low viscosities, and liquids that tend to foul or foam. In vacuum distillation, vacuum pumps are added to the distillation system to decrease the column pressure below atmospheric pressure. Careful pressure control is important because the separation is dependent on the differences in relative volatility at a given temperature and pressure. Changes in relative volatilities could adversely affect the separation.

Principle:

• This distillation method works on the principle that boiling occurs when the vapour pressure of a liquid exceeds the ambient pressure. Vacuum distillation is used with or without heating the mixture.

Construction: 

• The vacuum distillation unit consists of a distillation column, condensing distillate, and reboiler. Vacuum pumps and vacuum regulators are added to distillation columns to maintain the column at a vacuum. Many mixtures can be distilled at much more economical temperatures with the use of these vacuum distillation columns. 

Working:

• A vacuum distillation is also called as low temperature distillation. The target product in this distillation could either be the remaining product, the distilled product or a purified product. The vacuum pressure associated with a distillation depends on the product to be distilled. For example, volatile substances like those used in oil refineries are likely to undergo vacuum distillations at above 1 Torr, perhaps 20-50 mmHg

Applications: 

• The products of normal distillation are further distilled using vacuum distillation. The high boiling point hydrocarbons, such as lubricants and waxes, are separated at economical temperatures.

• Vacuum distillation is also used in the separation of sensitive organic chemicals and recovery of organic solvents. 

Advantages: 

Vacuum distillation reduces the number of stages needed in distillation. 

• The product output per day is very high. 

• It increases the relative volatility of the key components in many applications. Lower pressures increase relative volatilities in most systems. 

• It requires lower temperatures at lower pressures. 

• Vacuum distillation can improve a separation by prevention of product degradation because of reduced pressure leading to lower tower bottoms temperatures. 

• It has very high capacity to handle liquid mixtures giving high yield and highest purity.

• It requires low capital cost, at the expense of slightly more operating cost. Using vacuum distillation one can reduce the height and diameter, and thus the capital cost of a distillation column. 

• Columns can be operated at lower temperatures.

Disadvantages:

• High energy costs of vacuum pumps. 

• Pressure and energy losses due to any leaks or cracks. 

• Large column diameters needed for the process to be efficient. 

STEAM DISTILLATION

• The steam distillation is a process in which concentration and isolation of an essential oil from reliable and often cheaper natural sources is carried out. This is an important technique and has significant commercial applications. Many compounds, both solids and liquids, are separated from complex mixtures by taking advantage of their volatility in steam. A compound must satisfy three conditions to be successfully separated by steam distillation. It must be stable and relatively insoluble in boiling water and must have a vapor pressure in boiling water that is of the order of 1 kPa (0.01) atmosphere. If two or more compounds satisfy these three conditions, they will generally not separate from each other but will be separated from everything else

Principle: 

• The principle behind steam distillation is a way of separating miscible liquid based on their volatilities. The boiling point of the products is so minimized that it permits the constituents to get vaporized. The vapour pressure exerted by the liquids differs in strength which is a function of temperature. The boiling of the liquids takes place and at a certain instance of time the boiling point of the natural products in the liquid form surpass the atmospheric pressure. The result is that the vapor pressure of the whole system increases. 

Construction:

• A large scale stainless steel steam distillation unit usually has capacity from 0.5 to 15 thousand liters. Its usual diameter varies from 1.5 to 5 meter. The still contains jacket through which steam can be passed for heating the content, Fig. 6.8. Steam vessel is hydrostatically tested at 125 psi to serve as the distillation tank. Low pressure or high-pressure steam is supplied by a boiler. The steam vessel can hold extract approximately 3 to 5 liters of essential oil per distillation. The size of the tub is designed to provide oil in sufficient quantity for industrial evacuation or for analysis. The vapors of water and volatile oil are condensed in condenser attached. The distillate is collected as two layers which is separated by Florentine receiver. Following the distillation, the vessel can be disconnected from the cold-water condenser and rotated on swivels to a horizontal position, permitting easy removal and refilling of plant material. The entire extraction unit (vessel, condenser, boiler and oil collector) is suitable for mounting and transportation. It is built to extract volatile essential oils from aromatic plants. 

Working: 

• Steam distillation is a process employed to extract essential oils from organic plant matter by passing steam generated through the plant material. Usually a chamber is filled with holes (perforations) in the bottom for steam to come through with either fresh or dried herbs.

• Temperature sensitive compounds which would normally decompose through simple distillation vaporize at lower temperatures when subjected to steam in the distillation column. This allows for the separation of essential oils, which tend to be less soluble in boiling water, from chemically complex materials. When the steam is passed through the organic material, tiny pockets holding the essential oils open to release the essential oil molecules without doing any damage to these delicate components. A lid keeps the oil from diffusing into the air when the steam is applied to it. The distillate obtained contain a mixture of water vapour and essential oils which returns to their liquid form in the condensing apparatus. The condensed water and oil droplets are collected and channeled them through a filter, which separates the water from the oil. They are separated using a Florentine separator. In case of essential oils, using steam allows the distillate to retain the more delicate flavours and aromas which would otherwise breakdown if high temperatures are applied. 

Advantages:

• Steam distillation is useful for extracting most fats, oils and waxes. This process works well for types of substances that do not mix with water. 

• It can be a cost-effective method to invest in to extract a diverse array of immiscible substances. 

•  Since steam temperature can remain at the boiling point of water; this process also has a cost benefit of requiring less fuel for the steam boiler. 

• The amount of steam and the quality of the steam can be controlled. 

• Lower risk of thermal degradation as temperature generally do not rise above 100 °C. (vi) Most widely used process for the extraction of essential oils on a large scale.

• It is the standard method of extracting flavour and fragrance.

Disadvantages: 

• Need trained operator in order to operate the equipment. 

• The process has a hidden cost of maintaining and repairing equipment. 

• There is a much higher capital requirement and with low-priced oils the payback period can be over 10 years. 

• Requires higher level of technical skill and fabrication and repairs and maintenance require a higher level of skill. 

Applications: 

• Steam distillation is used to extract essential oils from aromatic plants to flavour liqueurs.

• It is used at wide scale for the manufacturing of essential oils like perfumes.

• It is used in synthesis of complex organic compounds. 

• Orange oil and eucalyptus oil are obtained at industrial scale using this method. 

• It is also used in petroleum industries and in the production of consumer food products. 

• It is used for extraction of peppermint and spearmint oils. 

MOLECULAR DISTILLATION    

• Molecular distillation is a type of short-path vacuum distillation, characterized by an extremely low vacuum pressure (≈ 0.01 torr). It is a process of separation, purification and concentration of natural products, complex and thermally sensitive molecules. This process is characterized by short term exposure of the distillate liquid to high temperatures in high vacuum in the distillation column and a small distance between the evaporator and the condenser around 2 cm. In molecular distillation, fluids are in the free molecular flow regime. The mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. The motion of molecules is in the line of sight, because they do not form a continuous gas anymore. Thus, a short path between the hot surface and the cold surface is necessary.

Principle:

• Molecular distillation is considered as the safest mode of separation and to purify the thermally unstable molecules and related compounds with low volatility and elevated boiling points. The process distinguishes the short residence time in the zone of the molecular evaporator exposed to heat and low operating temperature due to vacuum in the space of distillation. The separation principle of molecular distillation is based on the difference of molecular mean free path. The passage of free path for molecules should be collision free.

Construction:

• A simple molecular distillation has a unit which is placed on a hot surface. The distillate moves a very short distance before it gets condensed. If the substance is not too viscous, it will drip from the point on the glass condensing surface and run down to the receiving point. The sophisticated apparatus with a different design will have the liquid distilled down on a heated surface close to the condenser. The movement of a film prevents a build-up of non-volatile materials on the surface of the material to be distilled as this might cause the distillation to stop. 

Centrifugal Molecular Distillation: 

• This is a technique of purification applied under molecular distillation utilized worldwide for food processing, pharmaceutical applications, petroleum industries, and chemical industries. The main principle behind the unit is low pressure and very short residence time. Degassing of feed material leads to the next stage where the material is flown into a spinning disc which is pre-heated. The entire process of distillation gets over in less than fraction of a second because the material that is feed into expands on the spinning pre-heated disc. The distillate finally condenses on the outer extremes of the shell and then slowly flow into the collecting vessel due to gravity. The residual matter is collected in the gutter around the spinning disc and finally into the collecting vessel below Fig. 6.10. The molecular distillation process occurs at a very low temperature and hence can avoid thermal decomposition. The high vacuum applied helps in eliminating the oxidation due to exposure to atmospheric air. The free path distillation is carried out at a very low pressure of 10−2 Torr while in molecular distillation the pressure is kept at 10−3 Torr Q

Working: 

• The molecular distillation process is carried out at a very low pressure so that the distance between hot and condensing surface is less than the mean free path of the molecules. Each of the unit is a single stage but has several units in series. Molecular distillation is applied to thermally sensitive high molecular weight materials. The contact times in commercial units may be low as 0.001 seconds. The film thickness is of the order of 0.05 – 0.1 mm. In vacuum operations, the air ingress is very much possible, whereas in pressure operations the vapour emissions are likely to occur. The distillation process is inherently hazardous with flammables and the presence of huge volume of flammables in reboilers, in column internals and adjacent piping pose huge explosion hazards in these distillation units. Concentration gradient between top and bottom of the column has a bearing on safety. Concentration of impurities in the column can lead to hazards.

Advantages:

• Toxicity: Avoids the problem of toxicity of solvents used as the separating agent. 

• Thermal stability: Minimizes losses due to thermal decomposition. 

• Continuous process: It can be used in a continuous feed process to harvest distillate without having to break vacuum. 

• Stability: The vacuum allows oils to be processed at minimal temperatures, reducing the risk of oxidative damage. 

• Purity: Separating the oil’s components by weight allows contaminants to be reduced far below industry standards. 

• Concentration: Weight grouping allows the processor to concentrate fatty acids. 

• Short residence time: This process has short residence time of the feed liquid. 

• It works at a significantly lowered temperature due to high vacuum capability

• It has optimal efficiency in mass and heat transfer.

• It is suitable for processing high value products. 

Disadvantages:

•  Cost: The cost for this complicated technology is relatively high.

• Natural form: The starting natural triglyceride form is lost in the distillation process.

Applications:

• It is used for separation of vitamins and polyunsaturated fatty acids. 

• Molecular distillation is used industrially for purification of oils. 

• It is also used to enrich borage oil in γ-linolenic acid (GLA) and recover tocopherols from deodorizer distillate of soybean oil. 

• It can be used for the production of synthetic and natural vitamin E.

• Capsicum red pigment containing 1% to 2% of the solvent can be separated after two stage molecular distillation. 

• It is used for separation of strong spices like volatile substances. 

• It is used for highly heat sensitive materials. 

• It is a common process in deodorization, decolonization and purification. 

• It is used for deoxidation, level off odour or purification and for bleaching or purification. 

• It is used in fractionation of dimers of fatty acids, separation of radioactive nuclides from melts of irradiated media, lanolin purification, preparation of high concentrated monoglycerides, recovery of carotenoids from palm oil, 

• It is used in synthesis of pure diglicyde ether of bisphenol-A.