Distillation

1. Distillation Prepared by Dr.Nagwa El Mansy Chemical Engineering Department Cairo University Fourth year

2. References:- 1-Coluson and Richerdson, Chemical Engineering vol , vol II , vol III. 2- Geancoplis, Principles of Unit Operation. 3- Mc-Cabe and Smith, Unit operations for Chemical Engineering. 4- Traybal, Mass Transfer Operations. 5- Sherwood, Mass Transfer. 6-Perry’s , Chemical Engineering. 7- “Separation Process Principles”, 2nd ed, Seaderet’al . 8- Site on Google search, Separation Processes.

3. Distillation Distillation:- Is a method of separating homogenous mixture based on differences in boiling points. Distillation is done by:- a-partial vaporization ( liq → vap) b-partial condensation (vap → liq) c- changing in pressure ( gas → liq)

4. Distillation has a number of applications:- 1- It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. 2-Water is distilled to remove impurities, such as salt from seawater. 3- Air is distilled to separate its components oxygen, nitrogen, and argon for industrial use.

5. Separation in distillation depend on:- Relative volatility ( α ) :-

6. Vapor-liquid Equilibrium:- [ When the liquid phase behaves as an ideal solution all molecules have the same size; all intermolecular forces are equal; the properties of the mixture depend only on the properties of the pure components of the mixture]. Phase Diagram (Binary Mixture) In our analysis of phase diagram, we shall consider only Two component mixture, e.g. A (more volatile) and B (less volatile). There are 2 types of phase diagram: constant pressure or constant temperature. Constant Pressure Phase Diagram Figure(1) shows a constant pressure phase diagram for an ideal solution (one that obey ) Raoult's Law

8. A typical equilibrium curve for a binary mixture on x-y diagram. It contains less information than the phase diagram (i.e. temperature is not included), but it is most commonly used. It is useful for graphical design in determining the number of theoretical stages required for a distillation column.

9. Constant Temperature (Isothermal) Phase Diagram:- Figure(2)shows the constant temperature phase diagram. The constant pressure phase diagram is more commonly used in the analysis of VLE, but the constant temperature Phase diagram is also useful in the analysis of solution behavior that deviates from Raoult's Law. [ From this Figure (constant temperature phase diagram) we see that the more volatile liquid will have a higher vapor pressure (i.e. pA at xA = 1.0). Note also the regions of vapor-only, liquid-only and vapor-liquid mixture. ]

11. Effects of Increased Pressures:- Although most distillations are carried out at atmospheric or near atmospheric pressure, it is not uncommon to distill at other pressures. High pressure distillation (typically 3 - 20 atm). At elevated pressures, the vapor phase deviates from ideal gas behavior, and modifications to the VLE data is required. After pressure P3, the critical pressure of the more volatile component is exceeded, and there is no longer a distinction between vapor and liquid. Distillation is no longer possible beyond this point. The majority of distillations are carried out at pressures below 70% of the critical pressure.

13. Abnormal mixtures:- 1-Azeotropic Mixture:- Very large deviations from ideality lead to a special class of mixtures known as azeotropes , azeotropic mixtures, or constant-boiling mixtures. Azeotrope is a special class of liquid mixture that boils at a constant temperature at a certain composition. At this condition, it behaves as if it was one component with one constant boiling point. A boiling liquid mixture at the azeotropic composition produces a vapor of exactly the same composition, and the Liquid does not change its composition as it evaporates.

14. Two types of azeotropes are known:- minimum boiling and Maximum boiling a- Minimum-boiling azeotropes: a- One of the best known minimum-boiling azeotrope is the ethanol-water system which at 1 atm occurs at 89.4 mole% ethanol and 78.2 oC. b- carbon-disulfide - acetone (61.0 mole% CS2, 39.25 oC, 1 atm) c- benzene - water (29.6 mole% H2O, 69.25 oC, 1 atm) b-Maximum-boiling azeotropes:- a-Hydrochloric acid - water (11.1 mole% HCl, 110 oC, 1 atm) b-Acetone - chloroform (65.5 mole% chloroform, 64.5 oC, 1 atm)

15. a-Minimum Azeotrope:- Figure(5) show the constant pressure phase diagram plus equilibrium curve for a minimum-boiling azeotropic mixture of carbon disulfide (CS2) and acetone. At point L, the concentration in the vapor phase is the same as the concentration in the liquid phase ( y = x ), and a = 1.0. This concentration is known as the azeotropic composition (0.61 mole fraction CS2). At this point , the mixture boils at a constant temperature (39.25 oC under 1 atm) and without change in composition. On the equilibrium diagram, it can be seen that at this point, the equilibrium curve crossed the diagonal

16. b-Maximum boiling Azeotrope:- The Figure(6) shows the constant pressure phase diagram plus equilibrium curve for a maximum boiling azeotrope mixture of acetone and chloroform. The Azeotropic composition is 0.345 mole fraction acetone. Point L in the Figures is now a minimum on the constant temperature phase diagram, and a maximum (64.5 oC, under 1 atm) on the constant pressure phase diagram

17. 2- Partial liquid miscbility:- Some substances exhibit such large deviations from ideality that they do not dissolve completely in liquid state, e.g., Isobutanol-water

18. 3-Complete immisciblity Mixtures:- The mutual solubility of some Liquids is so small that they can be Considered substantially insoluble (Figure(8)) e.g., hydrocarbon and water. The vapor pressure of either component cannot be influenced by the other and each one exerts its vapor pressure at the prevailing temperature. When the sum of the separate vapor pressures equals the total pressure, the mixture boils at a temperature lower than the boiling point of each component:- PA + PB = PT , or P◦A + P◦B = PT

19. Calculation of Vapor/Liquid Equilibrium:- 1- By experimental methods:- (under certain P&T to get y = f(x)). 2-Published data. 3-Prediction of K values. For binary mixture:-

23. One Stage Distillation 1-Simple (differential) distillation (ASTM):- We will consider a binary mixture of A (more volatile) and B (less volatile). The set-up is as shown in the Figure(9) . The system consist of a batch of liquid feed (F) (fixed quantity) with a composition( xF) inside a kettle (or still) fitted with heating element or steam jacket, and a condenser to condense the vapor produced. The condensed Vapor is known as the distillate (D) with a composition (yD). The distillate is collected In a condensate receiver. The liquid remaining in the still is known as the residual (W) With a composition ( xW).

25. At any time t, the amount of liquid in the still is L , with mole fraction of A in the liquid being (x). After a small differential time (t + dt) , a small amount of vapor dL is produced, and the composition of A in the vapor is (y) (mole fraction). The vapor is assumed to be in equilibrium with the residue liquid . The amount of liquid in the still is thus reduced from L to (L - dL), while the liquid composition changed from x to (x - dx). See the followingFigure (10):-

28. After performing the integration assume xw, (usually xw unknown) . If left hand side=right hand side your assumption is correct if not repeat your assumption.

30. Steps for calculation the correct xw:- 1-plot x-y diagram. 2- assume xw. 3-costruct the previous table between x and 1/(y*- x). 4 - plot x vs 1/(y*- x) as shown in the last figure. 5- calculate the area under the curve = Ln F/W. 6- if area = Ln F/W , your assumption of xw is correct if not reassume xw. 7- finally calculate the correct xw.

31. Equation(1) can be used to determine (xw) if (F, xF , D) are known. Also the average distillate composition can be Calculated ( yD ) by simple material balance:- F = D + W --------(2) F xF = D yD + W xW -------(3) By solving the three equations all the quantities and compositions can be calculated.

32. 2-Flash (equilibrium) distillation:- A single-stage continuous operation where a liquid mixture is partially vaporized,then flows through a pressure reducing valve to the separator. In the separator, separation between the vapor

33. and liquid takes place. How much of A is produced in the vapor (and remained in the liquid) depends on the condition of the Feed, (see Figure(13)). Vapor amount (V) with composition (y), and a liquid amount (L) with composition (x) are produced. The two streams leaving the flash drum (y and x )are in equilibrium with each others. Flash distillation is done by:- 1-Flash vaporization (liquid →heating→throttling valve → Separation ). 2-Flash condensation ( vapor →cooling →separation ). 3- Feed at high pressure → change to low pressure. Some times flash distillation is used as a method for changing conditions of a mixture.

34. A single-stage flash operation can rarely produce the required purity or fractional recoveries. An obvious but inefficient approach is to apply a series of flash separators condensing part of the vapor and boiling part of the liquid products from successive stages. Its preferred to use systems with high relative volatility in flash distillation to obtain considerable separation .

35. Operating line equation for flash distillation:- For binary mixture(A &B)

37. Multi-flash distillation:-

40. Temperature calculations inside flash drum:- Figure(16) shows an enthalpy balance for continuous operation. The material and enthalpy balance are:-

42. Steps for calculation T :- 1- assume T. 2- calculate Ki. 3- assume L/V. 4- calculate yi . 5- check ∑ yi =1. 6- calculate xi . 7- check the enthalpy balance, if (LHS = RHS )→ your assumption is correct, if not reassume T . Usually, T is assumed between the bubble and the dew Points of the mixture.

43. Calculations of dew point and bubble point:- A-Bubble point:-

44. B- Dew point :-

45. Trials to calculate bubble point Trials to calculate dew point

46. 3- Steam distillation:- It is a method for distilling organic compounds which are heat sensitive materials (contaminated by traces of non volatile impurities).This process involves using bubbling steam (which is completely insoluble with the organic compound) through a heated mixture of the raw material to give maximum contact between steam and the compound.

47. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure)leaving Impurities and condensed steam in the still. The vapor mixture (carried by steam) is cooled and condensed, usually yielding a layer of organic compound(c) and a layer of water(w). The vapor pressure of each component (c&w) cannot be influenced by the presence of the other and each one exerts its vapor pressure at the Prevailing temperature. When the sum of the separate vapor pressures equal the total pressure, the mixture boils at a lower temperature (lower than boiling points of c & w). The use of steam reduces the partial pressure of feed components and thus permits there vaporization at temperature below there normal boiling point.

50. At boiling:-