DISTILLATION

 

DISTILLATION: EXPLANATION OF CONCEPT • Distillation is the unit operation in which the constituents of a liquid mixture (solution) are separated using thermal energy. • Basically the difference of vapour pressure (volatilities) of different constituents at the same temperature is responsible for such a separation. • It is also called as fractionation or fractional distillation. • It is used in chemical and petroleum industries.  TERMINOLOGIES INVOLVED IN DISTILLATION  • Boiling Point : For any given pressure, a pure liquid when heated will boil or vaporise at a certain single temperature. • MVC : More Volatile Component (with Less B.P.) • LVC : Less Volatile Component (with High B.P.) • Driving force : The difference between the existing condition and equilibrium condition. Concentration difference. • Equilibrium stage: It is the one where the vapour leaving the stage is in equilibrium with the liquid leaving the same plate. • Vapour pressure : It is the absolute pressure at which the liquid and it’s vapour are in equilibrium at a given temperature.   RAOULT’S LAW  • It express a quantitative relationship between the concentration and vapour pressure. • It states that partial pressure of each volatile constituent is equal to vapour pressure of the pure constituent multiplied by its mole fraction in the solution at a given temperature.  Suppose Homogeneous mixture of liquid A and B • Partial pressure of component A in Mixture  PA = P°A * xA • Mole fraction of A in solution = xA, Vapour pressure of A in pure state = P°A • Partial pressure of component B in Mixture = PB = P°B * xB • Mole fraction of B in solution = xB Vapour pressure of A in pure state = P°B   DALTON’S LAW • It states that the total pressure exerted by a gas/vapour mixture is equal to the sum of the partial pressures of the components present in the mixture. • Total pressure of Mixture P = PA + PB P = P°A * xA + P°B * xB Where P is the Total Pressure. • For an ideal gas or vapour, the partial pressure is related to its mole fraction in the gas or vapour phase by the relation : PA = yA * P • Where yA is the mole fraction of A in the vapour phase. APPLICATION OF CONCEPT/ EXAMPLES IN REAL LIFE • Production of absolute alcohol from 95% ethanol using benzene, • Separation of crude oil into gasoline, kerosene, fuel oils, production of essential oils, production of alcoholic beverages • Separation of liquid air into N2, O2 and argon etc.   COMPUTATION OF X-Y DATA xA = (P-PB0) / ( PA0 PB0) where, xA=mole fraction of A in liquid phase YA= mole fraction of A in the vapour phase P= total pressure PB0=vapour pressure of pure B PA0=vapour pressure of pure A  RELATIVE VOLATILITY AND X-Y EQUATION  • Volatility - The volatility of any substance in solution may be defined as the equilibrium partial pressure of substance in vapour phase divided by the mole fraction of 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. Volatility of component A, = Partial pressure of A /Mole fraction xA of A in solution   VA = PA/xA Similarly for B, VB = PB/xB • Relative Volatility - Consider a liquid mixture containing two component A and B. In such case, the volatility of one component is expressed in terms of second as below,   • Relative Volatility (αAB) = Volatility of component A (VA) / Volatility of component B (VB)  Y = (α * x) (1+( α-1)x)  RELATIVE VOLATILITY FOR IDEAL SOLUTION Α = PA0 / PB0 VAPOUR LIQUID EQUILIBRIUM (VLE)  • It is a state in which a pure liquid or a liquid mixture exists in vapour and liquid phases in thermodynamic equilibrium with each other. • VLE describes the distribution of components of a liquid mixture between the two coexisting liquid and vapour phases in equilibrium. • The size of distillation column, particularly the height of the distillation column is determined by the VLE data for the liquid mixture under consideration. • It is generally given at constant P or constant T as T-x-y (B.P.Diagram or Temp-composition diagram) data or x-y (Equilibrium curve) data AND P-x-y data at constant T. • VLE is governed by Phase Rule (F=C-P+2). In binary distillation, C=2, P=2, so F=2 (Pressure & Temperature), generally P is constant.   BOILING POINT DIAGRAM   ·             Consider the process of boiling a binary mixture consisting of benzene (mvc) and toluene. ·           ·          The composition of the mixture is plotted on x-axis in terms of mvc and temperature of the mixture is plotted on y-axis. ·         The mixture represented by point A is at a temperature of T1 and contains 50% benzene.          When we heat the mixture it will boil at a temperature T2, vapours will contain more of mvc. ·         The vapour sat C is in equilibrium with liquid at B and thus BC is known as the tie line. ·         If we reheat the condensate obtained at this stage, it will boil at T3 and the vapours issuing will contain more of mvc,thus enrichment of benzene takes place . ·         In the process of boiling, the mixture boils over a temperature range, so the term used is bubble point. ·         The liquid represented by any point on the lower curve is at its bubble point and the lower curve is called bubble point temperature curve. ·              When a mixture of vapours is cooled, at a point condensation starts. The first drop of liquid will have composition represented by point K. While cooling the vapour becomes richer in mvc than liquid. The condensation starts at any point on the upper curve. The upper curve is the dew point temperature curve. EQUILIBRIUM DIAGRAM (X-Y DIAGRAM)   EFFECT OF INCREASED PRESSURE ON VLE   EXPLANATION OF CONCEPT  Principle methods of distillation: 1. Simple or differential distillation 2. Flash or equilibrium distillation 3. Fractionation or rectification  Fractional distillation is better because reflux is there in fractionation and hence purity of the product is higher.   SIMPLE OR DIFFERENTIAL DISTILLATION  Since the composition of the vapour changes with time, the mathematical approach should be differential.         ·         Simple distillation is Batch and single stage distillation. ·         Simple distillation is process in which liquid mixture is heated to convert volatile component from the liquid into vapour which is then condensed and collected as a product. ·         A known quantity of mixture is charged into a jacketed kettle or still. ·         The jacket is provided for heating the liquid mixture in the steel with help of heating medium such as steam. ·         The vapour formed is withdrawn and fed to condenser where it is liquefied and collected as distillate. ·         The product distillate can be collected in several receivers. ·         The distillation is continued till boiling point of the liquid  reaches a predetermined valve and the content of the still is finally removed as residual liquid containing majority of less volatile component. RAYLEIGH EQUATION: Let F be moles of liquid mixture containing xF mol fraction of A, D kmoles of distillate and W kmoles of residual liquid in still Let yD and xW be the mol fr of A in distillate and bottom residual liquid. Let L be kmoles of liquid in the still let x be mol fr of A in liquid. Let very small amount dD kmol of distillate of composition y in equilibrium with the liquid is vaporized. Then composition and quantity of liquid decreases to (x-dx) and L to (L-dL) respectively.   Overall material balance is L=L-dL+dD, Or dL= dD Material balance for component A is Lx=(L-dL)(x-dx)+ydD   Lx= Lx-Ldx-xdL+dLdx+ydD , dLdx=0   0= -Ldx-xdL+ydL   But dD=dL,   i.e. 0=-Ldx-xdL+ydL, Ldx=(y-x)dL, dL/L=dx/(y-x)   Integrating the equation between the limits L=F, x=xF, L=W x=xW   F xF ʃdL/L= ʃdx/(y-x) W xW FLASH OR EQUILIBRIUM DISTILLATION     In this method, a definite fraction of the liquid feed is vapourised in such a manner that vapour produced is in equilibrium with the residual liquid. Therefore flash distillation is also called as Equilibrium distillation.   Flash distillation is carried out in a continuous manner. In this method, a liquid mixture is partially vaporized the vapor and liquid are allowed to attained equilibrium and finally withdrawn separately     Consider one mole of liquid mixture having xf mole fraction , f moles of feed that is vapourized and of composition y. Then ( 1-f) will be the moles of residual liquid obtained. Let x be the mole fraction of more volatile component in liquid. Material balance for more volatile component is                                                                      xF = fy+(1-f)x  OR y= -(1-f)x/f+(xf/f)  The above equation is operating line for flash distillation with slope = -(1-f)/f and y- intercept = xF/f  The point of intersection of operating line and diagonal (x=y) is (xF ,xF ) FRACTIONATION OR RECTIFICATION:     Fractioning column consist of 1)      A cylindrical shell divided into sections by series of Trays 2)      A reboiler 3)      A condenser •      The liquid mixture to be speared is introduced in the cylinder column more or less centrally. •      The column itself is divided into two sections- Rectifying and stripping Sections. •      The section above the feed plate or tray is called the Rectifying section •      The portion below the feed plate including feed plate is called stripping section. •      Trays are gas-liquid contacting devices on which gas/vaopur and liquid are brought into intimate contact for mass transfer to occur between two phases. •      Vapors are generated in are boiler and are fed to bottom of column. •      The liquid from the fractionators rich is less volatile component is called the bottoms or bottom product. •      The vapour issuing from the top of column is fed to a condenser . •      A part of condensed liquid is returned to column (reflux) and remaining part is withdrawn as top product or distillate which is more volatile component. •      As we move up the column, the vapour becomes richer and richer in more  volatile component and as we move down the column, the liquid becomes richer and richer in less validate component. •      As liquid is at its bubble point and vapour is at its dew point •      Temperature is maximum at the bottom and minimum at the top. •      The part of condensed liquid produced from a column that is returned to top of column is called reflux.   ·         The plate are numbered serially from  the top down. ·         Consider the nth plate from the top in the cascade. ·         The plate n-1 is immediately above the n-1 plate and the plate n+1 is immediately below the n plate. ·         On every plate two different fluid (vapour  and liquid) stream no at equilibrium are brought into intimate contact. the mass transfer between the phases takes place, the phases are separated, and finally two fluid streams i.e the two phases, leave the plate in equilibrium with each other. ·         The plate under consideration will receive the liquid stream of Ln-1 moles/h of composition Xn-1 and a vapour stream of Vn+1 moles/h of composition yn+1 from plate n-1 and n+1 respectively. ·         A liquid stream Ln of composition Xn and vapour stream vn of composition yn leaves this plate n+1 and n-1 respectively in equilibrium with each other.   AZEOTROPIC DISTILLATION ·         Azeotrope is a liquid mixture with an equilibrium vapour of the same composition as the liquid. ·         The dew point and bubble point are identical at azeotropic composition and the mixture vaporizes at a single temperature, so azeotropes are called constant boiling mixtures. ·         when an azeotrope is boiled, the vapour produced will have the same composition as the liquid from which it is produced. boiling of an ordinary solution takes place from the bubble point to the dew point, whereas the boiling point of an azeotropes remains constant till the entire liquid is vaporized. ·         The temperature composition diagrams and equilibrium diagrams for liquid mixtures forming azeotopes at constant pressure are shown in fig. here we will see that the equilibrium curves cross the diagonal –which is an indication of the existence of an azeotrope. ·         For all mixtures of composition less than P ,the equilibrium vapour is  richer in more volatile component than the liquid, while for all mixtures of compositions greater than P , the equilibrium vapour is less rich in more volatile component than the liquid . the mixture composition P gives the vapour composition identical with the liquid.   STEAM DISTILLATION: ·         Steam distillation is adopted in cases where substance involved cannot withstand temp of distillation and decompose. Substance of this kind can be separated by reducing the partial pressure of the volatile component. ·         This can be done by making use of inert vapour that decreases the temperature of distillation. The inert vapour used should be practically immiscible with components to be distilled. Steam is used for this purpose. ·         In steam distillation, steam is directly admitted into the liquid in the still. The mixed vapour containing desired component is taken as overhead, condensed and desired component is separated from water phase by gravity while non volatile material remains behind in the still. APPLICATION OF STEAM DISTILLATION:  1. For separating high boiling component from non volatile impurities. 2. For separating high boiling mixture into different fractions wherein the decomposition of material might occur if direct distillation were employed 3. Where vaporization temperature cannot be reached by heat.