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Course Name: Mass Transfer Operations - II (MTO-II) Course Code: 20CHC14 Instruction: 3L + 1T



(L- Lecture; T- Tutorial)



Instructor: Dr. Prasanna R. Redapangu Credits: 4



Duration of SEE: 3 h SEE: 60 Marks CIE: 40 Marks



UNIT-I Distillation: VLE phase diagrams, Tie lines and mixture rule; Raoult’s law, Relative Volatility Flash vaporization and differential distillation for binary mixtures Steam distillation. Batch distillation with reflux for binary mixtures.



Objectives: This unit helps the students to understand the: • Vapor- Liquid Equilibrium and Phase Diagrams



• Different types of distillation



Introduction • Most industrial chemical plants involve processes comprised of multiple components in multiple phases. The components can be of constant composition or varying composition. • Processes such as distillation, absorption, and extraction bring phases of different composition into contact, and when the phases are not in equilibrium, mass transfer between the phases alters their compositions. • Both the extent of change and the rate of transfer depend on the departure of the system from equilibrium. Thus, for quantitative treatment of mass transfer, the equilibrium T, P, and phase compositions must be known.



6



Nature of Equilibrium Equilibrium : A static condition in which no changes occur in the macroscopic properties of a system with time. Transfer of material or energy across phase boundaries occurs till equilibrium is established between the phases. Equilibrium: • It is a static condition in which the net transfer of material between the phases ceases for a given set of operating conditions that exists for all combinations of phases. • Equilibrium refers to the absence of any tendency for a change to take place and thus represent an end point of any naturally occurring process.



Driving force: When two phases, which are not at equilibrium are brought into intimate contact, the phases will tend to approach equilibrium due to a tendency for change to take place. The difference between the existing condition and equilibrium condition is the driving force which causes a change. A concentration difference is the driving force for mass transfer analogous to a temperature difference for heat transfer. Equilibrium stage: It is one in which the two phases not at equilibrium are brought into contact, time is provided to attain equilibrium, the phases are separated and streams leave the stage in equilibrium. For a given set of operating conditions, an equilibrium stage gives the maximum possible composition change, so it is also known as the ideal or theoretical stage. In actual practice, equilibrium is not achieved and hence stage efficiencies are always less than 100 percent.



Vapor Liquid Equilibrium (VLE) Vapor/Liquid Equilibrium (VLE) is a state of co-existence of vapor and liquid phases. The basic data for distillation are the equilibria existing between the vapour and liquid phases of a system under consideration. The equilibrium in vapour-liquid systems is governed by Phase rule. For a two phase binary system, C = 2, P = 2. Therefore, F = C – P + 2 = 2-2+2 = 2. Degrees of freedom (F) or the number of intensive variables that must be fixed to define the equilibrium state of the system is F = 2. Thus, for VLE calculations, at least two variables must be fixed so that other variables can be estimated. In distillation, there are four variables: temperature, pressure, vapour phase composition and liquid phase composition. i.e., The variables in VLE are. T, P, x, y 9



The qualitative behavior of VLE can be understood using 3 phase diagrams i. Constant-pressure diagram (T-xy), ii. Constant-temperature diagram (P-xy) iii. Equilibrium diagram (x-y)



Boiling Point: For any given pressure, a pure liquid when heated will boil or vaporize at a certain single temperature known as the boiling point of the liquid. It is the temperature of a liquid at which the vapor pressure of a liquid equals the prevailing pressure. The boiling point of a liquid increases with increase in pressure and vice versa. The normal boiling of a liquid is the temperature at which its boiling takes place under a pressure of 1 atm. Boiling points at a given pressure vary greatly for different liquids. For example, the boiling point of water is 373 K (100oC), that of toluene is 383.6 K (110.6oC) and that of methanol is 337.7 K (64.7oC) at one atmosphere (101.325 kPa).



Vapor Pressure: The vapor pressure of a liquid is the pressure exerted by the vapor at equilibrium condition – the conditions at which the rate of vaporization equals the rate of condensation.



In a binary mixture (a two component system), the component with a lower boiling point, i.e., the component with a higher vapor pressure at a given temperature is termed as the more volatile or lighter, while the component with a higher boiling point or with a lower vapour pressure at a given temperature is termed as the less volatile or heavier. Thus, in case of a binary mixture of methanol and water, methanol (B.P. = 64.7°C) is a more volatile component and water (B.P. = 100°C) is a less volatile component, since the vapor pressure of methanol is higher than that of water at any given temperature. The whole mixture of methanol and water will boil somewhere between 337.7 K (64.7°C) and 373 K (100°C) at atmospheric pressure (101.325 kPa).



Constant-Pressure Vapor-Liquid Equilibria : The compositions of the vapor and liquid phases, that are in equilibrium, are usually expressed in terms of mole fractions of the more volatile component in the respective phases (y and x) - we use mole fractions as a measure of concentrations. Equilibrium data for a binary mixture at constant total pressure are represented in graphical forms by means of (i) the temperature-composition diagram (T-xy) or (ii) the x-y diagram where the vapor phase composition (y) is plotted against the liquid phase composition (x).



Constant-pressure or Boiling point diagram –T-xy • Plots of temperature (T) vs mole fractions of vapor (y) and liquid (x) of more volatile component (component with low boiling point) Consider a liquid mixture (Point A) heated slowly at constant pressure. The temperature of the mixture increases and reaches a temperature at which the liquid starts to vaporize.



T2



B



Tie line C T-y



The point at which the first drop of a liquid mixture begins to vaporize is called bubble point and the corresponding temperature is bubble point temperature (at B).



A



T1 T-x



14



3 regions: • Sub-cooled liquid: region below T-x curve (Bubble curve) • Superheated vapor: region above T-y curve (Dew curve) • Vapor-Liquid Mixture: Region between the curves T-x curves are below the T-y curves • Point B gives the liquid mole fraction (x) and the vapor mole fraction (y) in equilibrium is given by point C. • Line BC is called the tie-line. A tie-line is a line that connects the equilibrium liquid and vapor mole fractions. Many tie-lines will be there in a phase diagram. • Here y is always more than x. • Temperature at x = 0 is T2 (boiling point of component 2) and at x = 1 is T1 (boiling point of component 1). The dew point is the point at which the first drop of a gaseous mixture begins to condense. 15



Mixture Rule • Infinite number of tie lines can be drawn in a phase diagram. • For example, for the tie line DF, the mixture on the lower curve at point D is a saturated liquid., a mixture on the upper curve at point F is a superheated vapor. • Any point between DF is a two-phase mixture, consisting of liquid phase composition at D and a vapor phase composition at F, in such proportions that the average composition of the entire mixture is represented by E. • The relative amounts of the equilibrium phases are related to the segments of tie line.



Moles of D = Moles of F



line EF line DE



Tie line E D



F



This is called mixture rule. It gives the average composition of the entire mixture. 16



Constant-Temperature diagram –P-xy Plots of pressure vs mole fractions of vapor and liquid of more volatile component (component with higher vapor pressure) P1sat



Consider a vapor-liquid mixture of two components (1-2) where component 1 is more volatile than component 2.



A Tie line B



Pressure at x = 1 is the vapor pressure of component 1 (P1sat) and pressure at x = 0 is P2sat.



D



P-x C



P2sat



P-y



17



• Consider a liquid at Point A. Reduction in pressure at constant temperature causes the liquid to vaporize, the first bubble of vapor forms at point B. The vaporization goes to completion at point C and further reduction in pressure leads to the production of superheated vapor. 3 regions: • Liquid: region above P-x curve (Bubble curve) • Vapor: region below P-y curve (Dew curve) • Vapor-Liquid Mixture: Region between the curves P-x curves are above the P-y curves • Point B gives the liquid mole fraction (x) and the vapor mole fraction (y) in equilibrium is given by point D. • Line BD is called the tie-line. • Pressure at x = 0 is P2sat (vapor pressure of component 2) and at x = 1 is P1sat (vapor pressure of component 1) 18



Equilibrium diagram –x-y • Plots of mole fractions of vapor and liquid in equilibrium • Also called distribution diagram The tie-line in T-xy and P-xy diagrams becomes a point here. Each of this point on the curve gives the values of x and y. The diagonal line is the x=y line x-y line



The xy diagram is also prepared at constant total pressure data. It can be constructed from the boiling point diagram by drawing horizontal tie-lines. The intersections of these curves with bubble point curve gives x and that with dew point curve will give y. 19



Effect of temperature and pressure on VLE



20



Raoult's law : It is commonly used for predicting the vapor-liquid equilibrium for an ideal solution in equilibrium with an ideal gas mixture from the pure component vapor pressure data. It states that the equilibrium partial pressure of a constituent/component in a solution at a given temperature is equal to the product of its vapor pressure in the pure state and its mole fraction in the liquid phase. Thus, for a binary (two component) system, if pA is the equilibrium partial pressure of A, pAo is the vapour pressure of 'A' in the pure state and xA is the mole fraction of 'A' in the liquid phase, then



(i)



Dalton's Law: It states that the total pressure exerted by a gas/vapor mixture is equal to the sum of the partial pressures of the components present in it. Mathematically, for a binary system: P = pA + pB where P is the total pressure. For an ideal gas or vapor, the partial pressure is related to mole fraction of the component in gas or vapor phase by the relation: Partial pressure = Mole fraction × Total pressure Thus, for component ‘A’ pA = y A · P (ii) where yA is the mole fraction of 'A' in the vapor phase.



Equilibrium Relations:



Relations between x, y, P, T or xA, yA, PA0, PB0, P



From (i) and (ii), (1)



Other relations: y P x A  A0 PA



(a)



PA0 x A yA  P



This is the VLE equation. If total pressure and vapor pressure are known at constant temperature, xA and yA can be found, which are the Equilibrium compositions.



(b)



PA0 x A  PB0 xB  P (c)



1 P y A / PA0  y B / PB0



(e) (d)



Relative volatility(α) Volatility (of Component A): It is defined as the ratio of the partial pressure of 'A' to the mole fraction of 'A' in the liquid phase.



The relative volatility of a component A with respect to a component B is the ratio of the volatility of 'A' (the more volatile component) to the volatility of 'B'. It is also known as the volatility of 'A' with respect to 'B' and is denoted by the symbol αAB.



 AB 



( pA / x A) ( pB / x B ) Thus, the relative volatility is the ratio of the concentration ratio of A to B in the vapor phase to that in the liquid phase.



Construction of Equilibrium Curve (x-y diagram) using Relative volatility(α) The equilibrium curve can also be constructed using a method based on relative volatility (α).



Self-Assignment: Derive this equation using the equilibrium relations. From the above equation, knowing 'α' for a given binary system x – y data (equilibrium data) can be generated by taking x = 0, 0.1 …… to 1 and evaluating the corresponding values of 'y' (equilibrium vapor phase composition). For an ideal system, volatility is equal to the vapor pressure of the pure component. The relative volatility of A with respect to B is also defined as the ratio of the vapor pressure of A to the vapor pressure of B at the same temperature. Therefore,



Summary of α Relations:



y A / yB   x A / xB



  PA / PB 0



0



x A yA  1  (  1) x A



26



Significance: Relative volatility is a measure of the separability by distillation (i.e., it is a measure of the ease with which the components are separated).  When α = 1, separation by distillation is not possible.  The separation by distillation is possible for relative volatility values greater than one.  Larger the value of the relative volatility, the easier is the separation by distillation.



Example: A mixture of benzene and toluene boils at 368 K (95°C) under a pressure of 101.325 kPa. Determine the composition of the boiling liquid assuming that mixture obeys Raoult's law. At 368 K (95°C), the vapor pressure of benzene is 155.56 kPa and that of toluene is 63.98 kPa.



Example: Calculate the equilibrium compositions of the liquid and the vapor phases for a mixture of methyl alcohol and water at a temperature of 323 K (50°C) and under a pressure of 40 kPa. Assume that both the liquid and vapor behave ideally. Data : Vapor Pressure of methanol at 323 K = 53.32 kPa Vapor Pressure of water at 323 K = 12.33 kPa.



Tutorial: The vapor pressures of n-heptane (A) and n-octane (B) are given in the following table. Assume that Raoult's and Dalton's laws apply,



i. ii.



Calculate the value of average relative volatility and Obtain the Equilibrium relation.



Compute vapour-liquid equilibria data (x-y) using the equation obtained in (i) at constant pressure of 101.325 kPa. iii. Draw T-(x,y) and (x-y) diagrams. Data: Boiling point of n-heptane (A) = 371.4 K (98.4°C) Boiling point of n-octane (B) = 398.6 K (125.6°C)



Solution : The more volatile component is n-heptane (A). x = xA = mole fraction of n-heptane in the liquid y = yA = mole fraction of n-heptane in the vapour Here we have to compute the vapour-liquid equilibrium compositions at different temperatures by using the following relationships : where P is pressure of the system (101.325 kPa), pAo and pBo are the vapour pressures of n-hetane and n-octane, respectively.



Example: Binary system acetonitrile(1)/nitromethane(2) conforms closely to Raoult’s law. Vapor pressure for the pure species are given by the following Antoine equations, where T is in K and pressure in kPa: sat 1



ln P



ln P2sat a) b) c) d)



2,945.47  14.2724  T  49.15 2,972.64  14.2043  T  64.15



Prepare a graph showing P vs. x1 and P vs. y1 at temperature 750C Mark the vapor pressures of both components on the plot. Draw tie lines and generate x-y data graphically. Draw the distribution diagram using the data generated in (d).



BUBL P calculations are required.











P  P2sat  P1sat  P2sat x1



y1 P  x P



sat 1 1



At 750C (348.15K) , the vapor pressures are calculated from Antoine equation; sat sat



P1



• • •



 83.21 P2  41.98



Now generate the data P-xy by taking each x value (from 0 to 1), and calculate corresponding P and y1 values. Tabulate the values of x, y ,P Plot P vs xy. Sample calculation: for x= 0.6 P  41.98  83.21  41.980.6 P  66.72kPa



x1P1sat 0.683.21 y1    0.7483 66.72 P 35



Calculation Table:



Module - 2



Distillation :



Distillation is a unit operation in which the constituents of a liquid mixture (solution) are separated using thermal energy. Basically, the difference in vapour pressures (volatilities) of different constituents at the same temperature is responsible for such a separation. This unit operation is also termed as fractional distillation or fractionation. With this technique it is possible to separate the liquid mixture into its components in almost pure form and due to this, distillation is the most important of all the mass transfer operations.



 In distillation, the phases involved are: liquid and vapour (the vapour phase is created by supplying heat to the liquid) and mass is transferred from both the phases to one another, by vaporisation from the liquid phase and by condensation from the vapour phase.  The net effect is an increase in composition of the more volatile component in the vapour (phase) and that of the less volatile component in the liquid.  The basic requirement for a separation of components by distillation is that the composition of the vapour be different from the composition of the liquid with which it is in equilibrium - the vapour is always richer in the more volatile component than the liquid from which it is formed.  If the vapour composition is the same as the liquid composition, distillation technique will not effect a separation.



Distillation is commonly used in chemical and petroleum industries as a means of separating the liquid mixture into its component parts. Separation of ethanol and water mixture, production of absolute alcohol from 95% ethanol using benzene, separation of petroleum crude into gasoline, kerosene, fuel oils, etc. are the typical examples of distillation.



Evaporation is concerned with the separation of a solution containing a non-volatile solute and a volatile solvent, whereas distillation is concerned with the separation of solution where all the components are appreciably volatile. Thus, the separation of a brine into salt and water is a evaporation, whereas the separation of a mixture of alcohol and water into its components is a distillation.



METHODS OF DISTILLATION - BINARY SYSTEMS : Basically, distillation is carried out in two ways : 1. The liquid mixture to be separated is heated to create a vapour. The vapour formed is condensed in a condenser and withdrawn as product. As there is no reflux, products of relatively low purities are obtained. …….. Without Reflux 2. The liquid mixture to be separated is heated to create a vapour, the vapour formed is condensed in a condenser. A part of the condensed liquid is returned to the distillation still (as reflux) and the remaining part is withdrawn as product. In this method, the liquid and vapour are brought into intimate contact for a number of times and almost pure product can be achieved. The part of the condensed liquid returned to the distillation unit is called reflux and the operation is called rectification or fractionation. The term rectification originated in the alcohol industry, whereas the term fractionation is popular in the petroleum industry. …….. With Reflux



Reflux is a part of the condensed liquid that is returned back to the distillation still.



Common methods used in distillation practice are : 1. Differential or Simple Distillation, 2. Flash or Equilibrium Distillation, 3. Rectification or Fractionation. Out of these three methods, distillation with rectification or simply called rectification is the most important. The first two methods are carried out without reflux and the third one is carried out with reflux (which is nothing but returning a part of the condensed liquid back to the distillation still).



Differential or Simple Distillation : In this distillation technique, a known quantity of a liquid mixture is charged into a jacketed kettle or still. The jacket is provided for heating the liquid mass in the still with the help of a heating medium such as steam. The charge is boiled slowly, the vapors formed is withdrawn and fed to a condenser where it is liquefied and collected in a receiver as a distillate. In the early stage of distillation, the vapor, so the distillate, leaving the still is rich in the more volatile component and as the distillation proceeds the liquid in the still becomes lean with respect to the more volatile component. The composition of the less volatile component thereby increases and hence the boiling point increases. The product (distillate) from such units can be collected in several receivers, called cuts, to give the products of various purities over the length of distillation period. The distillation is continued till the boiling point of the liquid reaches a predetermined value and the content of the still is finally removed as residual liquid containing majority of the less volatile component.



Differential or Simple Distillation :



Material balance - binary mixtures – Derivation of Rayleigh’s Equation: As the composition of the vapor issuing from the distillation still and that of the liquid remaining in it changes during the course of operation, the mathematical approach should be differential. Let 'F' be the kmol of a liquid mixture (A + B) containing xF mole fraction of A which is charged to a distillation still. D Let 'D' be kmol of distillate and 'W' be kmol of residual liquid yD in the still which are obtained at the end of operation. Let yD and xW be the mole fraction of 'A' in the distillate and dD, y the bottom residual liquid. F xF L, x Let 'L' be kmol of liquid left in the still at any time during the course of distillation and let 'x' be the mole fraction of 'A' in the liquid. W xW Let a very small amount 'dD' kmol of distillate of composition 'y' in equilibrium with the liquid is vaporized.



Then, the composition and the quantity of liquid decreases from x to x – dx and L to L–dL, respectively.



(i) (ii) (iii)



From Eq.(ii), dD = dL



(iv)



(v) (vi)



Eq.(v) in (iv)



(vii) This is known as the Rayleigh equation. It is used to determine F, W, xF and xW when three of these are known.



where F = initial moles in the still/kettle, W = moles left in the still. xF and xW are the initial and final composition of the liquid, respectively.



The R.H.S. of Eq. (vii) is evaluated graphically by plotting 1/(y – x) against x and determining the area under the curve between x = xF and x = xW. The required data for the above procedure are taken from the vapour-liquid equilibrium relationship.



If yD is the composited distillate composition, then it is obtained by taking a material balance of A.



Though the simple or differential distillation as a method of separation is not effective, many such units are used, especially where, (i) the components to be separated have widely different boiling points and (ii) methods giving sharp separations are not necessary.



Flash or Equilibrium Distillation : Flash distillation is normally carried out in a continuous manner. In this method, a liquid mixture is partially vaporised, the vapour and liquid are allowed to attain equilibrium by providing a sufficient contact time and finally withdrawn separately.



Consider one mole of a liquid mixture having xF mole fraction of the more volatile component, is fed to a flash distillation unit. Let 'f’ be the fraction of the feed that is vaporized and is of composition 'y'. Then, (1 – f) will be the moles of the residual liquid obtained. Let 'x' be the mole fraction of the more volatile component in the liquid.



Above Eq. is the material balance/operating line for flash distillation with a slope equal to – (1 – f)/f and an intercept equal to xF/f.



Cases:



Method of obtaining the equilibrium compositions of vapour (y1) and liquid (x1) for a given f.



Basis : Feed containing 40 mole % benzene xF = mole fraction of benzene in the feed = 0.4 Given : 50 mole % of the feed is vaporized.



f = mole fraction of feed that is vaporized = 0.5



Slope of the operating line for flash distillation = –(1 – f)/f Slope = – (1 – 0.5)/0.5 = – 1.0



Draw the equilibrium curve with the help of data given.



The point of intersection of the operating line and the diagonal is (xF, xF), i.e., (0.4, 0.4). Mark the point (0.4, 0.4) on the diagonal. Draw the operating line through it with a slope equal to – 1.0 (θ = – 45o) which will cut the equilibrium curve at point say P. Through 'P' read the equilibrium liquid phase and vapor phase compositions from the x-axis and y-axis, respectively.



X=0.3, y=0.5



Example 8.11 : A liquid mixture containing 40 mole % methanol and 60 mole % water is fed to a differential distillation at atmospheric pressure, with 60 mole % of the liquid is distilled. Find the composition of the composited distillate and the residue.



Solution : Basis : 100 kmol of feed Let F, D, W be the kmol of the feed, distillate and residue, respectively. Given : 60 mole % of the feed is distilled



R.H.S. of this equation is evaluated graphically by plotting 1/y–x as ordinate v/s x as abscissa and measuring the area under the curve between the limits x = xF and x = xW. L.H.S. of the Rayleigh equation = Plot 1/(y – x) v/s x and measure the area under the curve from xF = 0.40 till the area equals 0.916 and then read the value of x representing xW (composition of the residue).



Practice Problems:



Steam Distillation : Steam distillation (It is a distillation process with open steam) is used : (i) for separating a high boiling component from the non-volatile impurities, (ii) for separating a high boiling mixture into different fractions wherein the decomposition of material might occur if direct distillation were employed, (iii) in cases where vaporization temperature cannot be reached by steam heat.



Steam distillation is especially adopted in cases where the substances involved cannot withstand temperature of distillation and decompose (i.e., for heat sensitive materials).



At atmospheric pressure high boiling liquids can be boiled at lower temperature by adding an immiscible phase to the liquid mixture. Substances of this kind can be separated by reducing the partial pressure of the volatile component. This can be done by making use of an inert vapor that decreases the temperature of distillation. The inert vapor used should be practically immiscible with the components to be distilled. Steam is usually used for this purpose and the operation is called steam distillation. Steam is widely used since : it is immiscible with many organic compounds, it provides required heat of vaporization and it is easily available at low cost. By reducing the pressure (vacuum pressure distillation), the relative volatility between components increases and thereby separation can happen. So, vacuum pressure distillation is mainly preferred when relative volatility between the components is less. Under vacuum the boiling will happen at lower temperature.



Steam distillation is a differential distillation in which live steam is passed in to the distilling system to boil the high boiling liquid at low temperature.



Conditions for steam distillation:



Steam distillation is possible only when the liquid mixture to be distilled does not react with water and immiscible with the volatile liquid component. Application: This method is often used to separate high boiling component from small amounts of non-volatile or less volatile components, and particularly where decomposition might occur if direct distillation is employed.



Theory for steam distillation:



If there are two immiscible liquid phases, each will exert its own vapor pressure at a given temperature and can not be influenced by the presence of other. When the sum of the vapor pressures equal to total pressure, the liquid boils. If pAv is the vapor pressure of water, and pBv is the vapor pressure of the high boiling component. The total vapor pressure is pAv+ pBv. When P equals to pAv+ pBv. The mixture boils. P= pAv+ pBv yA= pAv/P yB= pBv/P As long as liquid water is present, the high boiling component boils at low temperatures well below normal boiling point without using vacuum. The vapours of water and high boiling component are condensed in a condenser and the resulting two immiscible phases separated. The disadvantage of this method is lot of energy is used to vaporize both water and high boiling liquid.



Turpentine boils at 160°C. Mixture of water + Turpentine boils at 95°C Total vapor pressure = Vapor pressure of water + vapor pressure of Turpentine at 95°C = 647 mm Hg + 113 mm Hg = 760 mm Hg The moles of B to moles of A,



nB/nA= pBv/pAv



Degrees of freedom: By phase rule, In this case, C = 2, P = 3 and F = C-P+2 = 2-3+2 = 1 The number of variables that can be varied independently is one. In steam distillation, if pressure is fixed then it boils at one temperature.



Example: A mixture containing 50g of water and 50 g of ethyl aniline, which can be assumed to be essentially insoluble, is boiled at standard atmospheric pressure. Find out at what temperature it boils and composition of the vapour.



Since the liquids are insoluble, each exerts its own vapor pressure, and when the sum of these equals 760 mmHg the mixture boils.



The mixture boils at 99.15°C



yA*= pAv/P = 737.2/760 = 0.97 yB*= pBv/P = 22.8/760 = 0.03



Temperature C



VP of water, VP of ethyl mmHg aniline, mmHg



38.5



51.1



1



52.1



64.4



199.7



5



205



80.6



363.9



10



374



96



657.6



20



678



99.15



737.2



22.8



760



113.2



1225



40



1265



P= pAv+ pBv



Batch distillation with reflux Batch distillation is used extensively in small scale production units where the same piece of equipment is to be used for many different mixtures and where small quantities of the liquid mixtures are to be handled. It is useful when more than one product is to be obtained (i.e., different quality products) and when the liquid mixture to be separated are high in solid content, for example, tar etc. as it keeps the solids separated (in the reboiler/still) which are removed at the termination of the process.



In batch distillation, the specific quantity of a liquid mixture is charged to a reboiler/still, heating is applied, vapour generated flow upward through a fractionating column and a part of the liquid from a condenser runs down the column as reflux. The entire fractionating column acts as a enriching section. As the distillate will be rich in the more volatile component, the liquid in the reboiler becomes steadily weaker in the more volatile component as the operation proceeds and hence the purity of the product will steadily fall.



Batch distillation operation may be carried by varying the reflux ratio so as to get a constant overhead composition. In this case, initially the column is operated under total reflux and then some value of the reflux ratio is adjusted. But as the distillation proceeds, the top product quality may fall. Thus, to keep the top product quality to be constant, the reflux ratio is increased. Reflux ratio is continually increased till it reaches a maximum value and then it is reduced and the cut is taken into a separate receiver. It may be charged in a next batch. The reflux ratio is defined as the ratio of the liquid returned to the column divided by the liquid removed as product, i.e., R = L/D.



Total reflux is a condition where all the vapor condensed is returned to the column.



Another method of operating batch distillation unit is to operate the column under a constant reflux ratio. Column is operated under total reflux initially and then the reflux is set to a predetermined value. As the distillation proceeds, top products quality will steadily fall but distillate is collected in the same receiver until the average distillate composition is reached at the desired value and then overhead product is collected in a second receiver till the termination of operation and the same may be charged in the next batch.