Distillation DEPARTMENT OF CHEMICAL ENGINEERING

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1 Distillation DEPARTMENT OF CHEMICAL ENGINEERING

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4 Weeping in distillation column 4

5 Distillation. Introduction Unit operation Separation process A feed mixture of two or more components is separated into two or more products whose compositions differ from that of the feed. Driving force Relative volatility Principle Phase creation (ESA) Feed liquid or vap-liq mixture Product Bottom product (liquid) + Overhead product (liq or vap-liq ) 5

6 Distillation Examples Separation of binary mixture Separation of Multicomponent mixture 6

7 Vapor Liquid Equilibrium Equilibrium between vapor and liquid is determined experimentally. Temperature Composition diagram Pressure Composition diagram 7

8 T Composition diagram. 8

9 Bubble point and Dew point Bubble point is the point at which first drop of liquid mixture begin to vaporize. Dew point is the point at which first drop of gaseous/vapor mixture begins to condense 9

10 T Composition diagram. 10

11 Partial vaporization and condensation. 11

12 Partial Pressure and Dalton s Law The partial pressure P A of component A in a mixture of vapours is the pressure that would be exerted by component A at the same temperature, if present in the same volumetric concentration as in the mixture. By Dalton s law of partial pressures, the total pressure is equal to the summation of partial pressures. 12

$Raoult s Law In an ideal gas/vapor, the partial pressure of a constituent is proportional to its mole fraction.$

13 Raoult s Law In an ideal gas/vapor, the partial pressure of a constituent is proportional to its mole fraction. For an ideal mixture, the partial pressure of a constituent is related to the liquid phase concentration by Raoult s law. 13

14 Relations from RAOULT S LAW If a mixture follows Raoult s law, then And So But Therefore Hence 14

15 Antoine Equation: Vapor Pressure Relations (1) Riedel Equation: Clapeyron Equation: Where 15

16 Volatility Volatility is defined as the ratio of partial pressure of a component to the mole fraction of that component in liquid phase. For ideal system, volatility of a component is numerically equal to the vapor pressure of the pure component. 16

17 Relative Volatility The ratio of these two volatilities is known as the relative volatility. Replacing partial pressures in terms of total pressure gives a relation for ratio of two components in vapor to the ratio of two components in liquid. 17

18 Relative Volatility For a binary mixture This gives relations for determination of composition of liquid or vapor phases, if the composition of the other phase is known. 18

19 Temperature Dependency of Relative Volatility α varies somewhat with temperature, it remains remarkably steady for many systems. α increases as the temperature falls, so that it is valuable to operate at reduced pressure in order to decrease the boiling point. An average value of α can be used over whole column if the relative volatilities at the top and bottom of column vary by less than 15%. Otherwise the equilibrium curve must be constructed incrementally by calculating the relative volatility at several points along the column. 19

20 K-Value (1) Phase-equilibrium ratio is the ratio of mole fractions of a species in two phases at equilibrium. For vapor liquid systems, the constant is referred to as the K-value or vapor liquid equilibrium ratio. For many systems K is constant over an appreciable temperature range, So it is often much more useful than the simple vapor pressure. Using K-values is particularly useful while dealing with multi component systems. 20

21 K-Value (2) Relative volatility in terms of K-value: 21

22 Methods of Distillation Binary Mixtures Differential Distillation Flash or Equilibrium distillation Rectification 22

23 Differential Distillation (1) A single stage process that starts with a still pot, initially full, and heated at a constant rate. The vapor formed by boiling of liquid is removed at once from the system. Vapor is richer in more volatile component. Liquid becomes steadily weaker in more volatile component. Product quality varies with time. At any instant, liquid is in equilibrium with the vapor formed on that instant. At the end, the remaining liquid is removed as bottom product. A complete separation is possible only at infinite relative volatility. 23

24 Differential Distillation (2) S= number of moles of material in still x= mole fraction of component A in liquid ds= amount of material vaporized from still y= mole fraction of component A in vapor Material balance of component A 24

25 Differential Distillation (3) Integrating Integral can be solved graphically using equilibrium relationship between y and x. Over the range concerned the equilibrium relationship is a straight line of the form y = mx + c 25

26 Differential Distillation (4) or 26

27 Differential Distillation (5) If α remains constant over the range of applicability, then then 27

28 Flash or Equilibrium Distillation (1) Frequently carried out as a continuous process. Feed is usually pumped through a fired heater and enters a still through a valve that causes a reduction in pressure. A part of liquid feed is vaporized in such a way that vapor evolved is in equilibrium with the residual liquid. The still is a separator vessel that provides sufficient times for the vapor and liquid to reach equilibrium. Vapor is removed from top and usually condensed. Liquid is taken from the bottom. 28

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30 Flash or Equilibrium Distillation (2) F= molar flow rate of feed V= molar flow rate of vapor S= molar flow rate of liquid x= mole fraction of A in liquid product y= mole fraction of A in vapor product x f = mole fraction of A in feed Total material balance Component A balance 30

31 Flash or Equilibrium Distillation (3) Rearranging Equation of straight line that passes through point (x f,y f ) and has a slope 31

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33 Example 11.6 An equimolar mixture of benzene and toluene is subjected to flash distillation at 100 kn/m 2 in the separator. Using the equilibrium data given in Figure 11.9, determine the composition of the liquid and vapour leaving the separator when the feed is 25 per cent vaporised. For this condition, the boiling point diagram in Figure may be used to determine the temperature of the exit liquid stream. 33

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35 Rectification The term rectification is derived from the Latin words rectefacere, meaning to improve. Distillation process that enables successive vaporization and condensation to be accomplished in one unit. The essential merit of rectification is that it enables a vapour to be obtained that is substantially richer in the more volatile component than is the liquid left in the still. Such a unit is known as a fractionating column. 35

36 Fractionating Column: Schematic 36

37 Fractionating Column: Major Parts Rectifying section Stripping section Reboiler Condenser Reflux drum / Accumulator 37

38 Fractionating Column: Streams Feed stream Overhead product Bottom product Reflux stream Boil-up stream Utilities Hot Cold 38

39 Fractionating Column: Operation Fractionating column is a tall cylindrical vessel that is divided into several sections by means of perforated trays. The trays allow the vapor to pass through their perforations in the upward direction. The liquid flows across each tray, then over a weir, and then through a down comer to the next tray in the downward direction. The vapor from top of the column is condensed and then passed to a reflux drum (or accumulator or reflux divider). A fraction of the condensed vapor is drawn as top product (distillate). The remainder is returned to the top tray as reflux. 39

40 Fractionating Column: Operation The liquid from the bottom of the column is heated by means of hot oil or steam in a reboiler. The bottom product is drawn as liquid that flows over the weir in the reboiler. Since the bottom product is hot, it may be used top reheat the incoming feed. The vapor generated in the reboiler is returned to the bottom tray of the column. 40

41 Fractionating Column: Operation At each tray, vapor is partially condensed to give a vapor that is richer in more volatile component. Liquid is partially vaporized and becomes weaker in more volatile component. This is because of vaporization of some part of more volatile component from the liquid, and condensation of some part of less volatile component from the vapor. The partial condensation of rising vapor and partial vaporization of reflux liquid occurs at each stage. Top vapor, distillate product and reflux liquid have same composition. 41

42 Fractionating Column: Operation Feed is introduced on some intermediate tray where the liquid has approximately same composition as the feed. The part of the column above the feed tray is known as rectifying section. The lower portion below the feed tray is known as stripping section. Vapor and liquid leaving from an ideal tray are in equilibrium with each other. Vapor and liquid entering a tray are not in equilibrium. Assumptions of equimolar counter diffusion and constant molar overflow are often valid in the system. 42

43 Equimolar Counter diffusion When the mass transfer rates of the two components are equal and opposite, the process is said to be one of equimolar counter diffusion. It occurs in the distillation column when the molar latent heats of the two components are the same. The more volatile component is transferred from liquid to vapor, and the less volatile components is transferred from vapor to liquid. If the molar latent heats of the two components are equal, the condensation of a given amount of less volatile component releases exactly the amount of latent heat required to volatilize the same molar quantity of the more volatile component. 43

44 Constant Molar Overflow (CMO) Heat losses from a distillation column are usually small and may be neglected. For an ideal system, heat of mixing is zero. With these assumptions, molar heat of vaporization may be taken as constant and independent of composition. For such systems, if one mole of vapor condenses, exactly one mole of vapor is liberated. With CMO, molar flowrates of liquid and vapor remain constant in the rectifying section (and in stripping section) unless material enters (or is withdrawn from) the column. 44

Introduction to Distillation. Binous - Introd. to Distillation

Introduction to Distillation. Binous - Introd. to Distillation Introduction to Distillation 1 Exploits differences in boiling point, or volatility Requires the input of energy Handles a wide range of feed flow rates Separates a wide range of feed concentrations Produce