Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a physical separation process, and not a chemical reaction.
Water is distilled to remove impurities
Air is distilled to separate its components
Distillation of fermented solutions to produce distilled beverages with a higher alcohol content.
The premises where distillation is carried out, especially distillation of alcohol, are known as a distillery .
VARIOUS TYPES OF DISTILLATION
Single Vaporization/Condensation cycle of a mixture that produces a distillate that is always impure
Therefore, it is impossible to completely separate the components in a mixture with Simple Distillation
Relatively pure substances can be obtained from a mixture with Simple Distillation if the boiling points of the components differ by a large amount (>100 o C)
If a small increment of the initial distillate is separated and redistilled and this process is repeated many times, effectively producing multiple sequential Vaporization/ Condensation Cycles, an increasingly pure solution can be attained.
Accomplishes the same thing as Multiple Simple Sequential Vaporization / Condensation Cycles, by inserting a Fractionating Column (a Vigreux Column) between the Distillation Flask and the Distillation Head.
The Fractionating Column, of which there are many types containing a variety of packing materials, subjects the mixture to many Vaporization/Condensation Cycles as the material moves up the column toward the Distillation Head, which is attached to the Condenser.
With each cycle within the column, the composition of the vapor is progressively enriched in the lower boiling liquid.
This process continues until most of the lower boiling compound is removed from the original mixture and condensed in the receiving flask
Schematic of Distillation
Fractional Distillation (Cont.)
When the lower boiling liquid is effectively removed from the original mixture, the temperature rises and a second fraction containing some of both compounds is produced.
As the temperature approaches the boiling point of the higher boiling point compound, the distillate condensing into the third receiving flask is increasingly pure in the higher boiling point compound.
Column Efficiency:- A common measure of the efficiency of a Fractionation Column is given by its number of Theoretical Plates
One Theoretical Plate is equivalent to a Simple Distillation, i.e., one Vaporization / Condensation Cycle. The smaller the boiling point difference, the greater the number of theoretical plates a fractionating column must have to achieve separation of mixtures
Simple & Fractional Distillation
An azeotrope is a mixture of two or more liquids (chemicals) in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture.
Because their composition is unchanged by distillation, azeotropes are also called constant boiling mixtures .
positive azeotrope is 95.63% ethanol and 4.37% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.2°C.
negative azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water (by weight). Hydrogen chloride boils at −84°C and water at 100°C, but the azeotrope boils at 110°C.
Azeotropic distillation usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two, immiscible liquid phases), such as the example below with the addition of benzene to water and ethanol. In actual fact, this practice of adding an entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof. In some senses, adding an entrainer is similar to extractive distillation.
Example - distillation of ethanol/water, using normal distillation techniques, ethanol can only be purified to approximately 96% . Some uses require a higher percentage of alcohol, eg. when used as a gasoline additive.
Material separation agent- The addition of a Material Separation Agent, such as benzene.
Molecular sieves- For the distillation of ethanol for gasoline addition, the most common means of breaking the azeotrope is the use of molecular sieves.
Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually less than atmospheric pressure) causing evaporation of the most volatile liquid(s) (those with the lowest boiling points). This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure. Vacuum distillation is used with or without heating the solution.
Temperature sensitive materials (such as beta carotene) require vacuum distillation to remove solvents from the mixture without damaging the product. Vacuum distillation is sometimes referred to as low temperature distillation. This type of distillation is in use in the oil industry.
Advantages of Vacuum Distillation
Industrial-scale vacuum distillation has several advantages.
One tool to reduce the number of stages needed is to utilize vacuum distillation. Vacuum distillation columns typically used in oil refineries have diameters ranging up to about 14 meters (46 feet), heights ranging up to about 50 meters (164 feet), and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).
Vacuum distillation increases the relative volatility of the key components in many applications. Lower pressures increase relative volatilities in most systems.
A second advantage of vacuum distillation is the reduced temperature requirement at lower pressures. For many systems, the products degrade or polymerize at elevated temperatures.
Vacuum distillation can improve a separation by:
Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures.
Increasing capacity, yield, and purity.
Another advantage of vacuum distillation is the reduced capital cost, at the expense of slightly more operating cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a distillation column.
A special application of the simple distillation is molecular distillation, known also as evaporative distillation or short path distillation.
Theory of molecular distillation:-
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. For material that are regarded as non volatile under ordinary conditions of temperature and pressure are generally removed by this by increasing the mean free path.
Characteristics of the molecular distillation process:-
Very high vacuum
Evaporating surface must be close to the condensing surface
The liquid area is large to avoid boiling and evolution of the vapors is from surface only.
Molecular Distillation (cont.)
Application of molecular distillation:-
Purification of oils
Separating of vitamins
Batch distillation refers to the use of distillation in batches, meaning that a mixture is distilled to separate it into its component fractions before the distillation still is again charged with more mixture and the process is repeated. This is in contrast with continuous distillation where the feedstock is added and the distillate drawn off without interruption.
Batch distillation has always been an important part of the production of seasonal, or low capacity and high-purity chemicals. It is a very frequent separation process in the pharmaceutical industry and in wastewater treatment units.
Because each of the distillate fractions are taken out at different times, only one distillate exit point (location) is needed for a batch distillation and the distillate can just be switched to a different receiver, a fraction-collecting container. Batch distillation is often used when smaller quantities are distilled.
Continuous distillation ,
a form of distillation, is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation.
A distillation produces at least two output fractions. These fractions include at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid, and practically always a bottoms (or residuum) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor.
In a continuous distillation, each of the fraction streams is taken simultaneously throughout operation; therefore, a separate exit point is needed for each fraction. In practice when there are multiple distillate fractions, each of the distillate exit points are located at different heights on a fractionating column.
The bottoms fraction can be taken from the bottom of the distillation column or unit, but is often taken from a reboiler connected to the bottom of the column.
Continuous Distillation (cont.)
In a continuous distillation, the system is kept in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include feed input rate, output stream rates, heating and cooling rates, reflux ratio, and temperatures, pressures, and compositions at every point (location). Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, steady state is normally maintained.
Since a continuous distillation unit is fed constantly with a feed mixture and not filled all at once like a batch distillation, a continuous distillation unit does not need a sizable distillation pot, vessel, or reservoir for a batch fill. Instead, the mixture can be fed directly into the column, where the actual separation occurs. The height of the feed point along the column can vary on the situation and is designed so as to provide optimal result.
Flash evaporation is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a valve or other device. If the valve or device is located at the entry into a pressure vessel so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a flash drum.
If the saturated liquid is a single-component liquid (for example, liquid propane or liquid ammonia), a part of the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional vapor compression refrigeration systems.
If the saturated liquid is a multi-component liquid (for example, a mixture of propane, isobutane and normal butane), the flashed vapor is richer in the more volatile components than is the remaining liquid.
Flash Distillation (cont.)
Flash distillation (sometimes called "equilibrium distillation") is a single stage separation technique. 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 a big enough volume (the "flash drum"), the liquid and vapor separate. Because the vapor and liquid are in such close contact up until the "flash" occurs, the product liquid and vapor phases approach equilibrium.
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.
Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.
The method of extractive distillation uses a separation solvent, which is generally nonvolatile, has a high boiling point and is miscible with the mixture, but doesn't form an azeotropic mixture. The solvent interacts differently with the components of the mixture thereby causing their relative volatilities to change.
The solvent must alter the relative volatility by a wide enough margin for a successful result. The quantity, cost and availability of the solvent should be considered. The solvent should be easily separable from the bottom product, and should not react chemically with the components or the mixture, or cause corrosion in the equipment. A classic example is of aniline as suitable solvent.
Typical Distillation Setup
In a solution of two miscible liquids (A & B) the partial pressure of component “A” (P A ) in the solution equals the partial pressure of pure “A” (P A o ) times its mole fraction (N A )
Partial Pressure of A in solution = P A = (P A o ) x (N A )
Partial Pressure of B in solution = P B = (P B o ) x (N B )
When the total pressure (sum of the partial pressures) is equal to or greater than the applied pressure, normally Atmospheric Pressure (760 mm Hg), the solution boils
P total = P A + P B = P A o N A + P B o N B
If the sum of the two partial pressures of the two compounds in a mixture is less than the applied pressure, the mixture will not boil. The solution must be heated until the combined vapor pressure equals the applied pressure
Vapor Pressure / Boiling Point
According to Kinetic Theory, the molecules in a liquid are in a constant state of Thermal Motion and some of these molecules are moving fast enough to escape from the liquid forming a vapor above the liquid. This vapor exerts a pressure on the surface of the liquid, i.e., Vapor Pressure
Vapor Pressure – The pressure of the vapor coexisting with a confined liquid or solid, i.e., the pressure in an evacuated container containing a liquid at constant temperature after the liquid and escaping molecules near the surface of the liquid – the vapor - reach equilibrium
The Vapor Pressure of a liquid increases, generally exponentially, with temperature
Boiling Point – As a liquid is heated, the vapor pressure of the liquid increases to the point at which it just equals the applied pressure - usually atmospheric pressure. The liquid now begins to bubble (boil)
Vapor Pressure / Boiling Point (Con’t)
Different liquid compounds or mixtures of liquids have different vapor pressures at a given temperature.
Liquids with high vapor pressures (Volatile compounds) require relatively little energy (heat) to increase the vapor pressure to match the applied (atmospheric) pressure, and thus, boil, i.e. they have low boiling points.
Liquids with low vapor pressures require considerably more energy to increase the vapor pressure to the point where it matches the applied pressure, thus, they have relatively high boiling points.
The individual compounds in a mixture each exert its own pressure – partial pressure.
The sum of the partial pressures equals to the total vapor pressure of the solution
As the distillation proceeds, the composition of the liquid and the vapor are continuously changing
The Horizontal and Vertical Lines represent the processes that occur during a fractional distillation.
Each Horizontal Line (L 3 V 3 , L 2 ,V 2 ), etc., represents both the vaporization step of a given vaporization/condensation step and the composition of the vapor in equilibrium with the liquid at a given temperature.
At 53 o C with a liquid composition of 80% A and 20% B (L 4 V 4 on the diagram), the vapor would have 95% A and 5% B when equilibrium has been established between the liquid and the vapor.
At 63 o C with a 50/50 liquid mixture of A&B (L 3 V 3 on the diagram), the vapor would have a composition of 80% A & 20% B at equilibrium.