1 / 57

Distillation I

Distillation I. Mass Transfer for 4 th Year Chemical Engineering Department Faculty of Engineering Cairo University. THINGS YOU HAVE TO REMEMBER. Phase Equilibria Dalton’s Law Raoult’s law Antoine Equation. LET’S SAY THAT. For a binary system: A = More Volatile Component

brinda
Download Presentation

Distillation I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Distillation I Mass Transfer for 4th Year Chemical Engineering Department Faculty of Engineering Cairo University

  2. THINGS YOU HAVE TO REMEMBER • Phase Equilibria • Dalton’s Law • Raoult’s law • Antoine Equation

  3. LET’S SAY THAT For a binary system: A = More Volatile Component B = Less Volatile Component PoA>PoB a=Relative volatility (no more recovery)

  4. AND OF COURSE xi= composition of component i in liquid phase yi= composition of component i in vapour phase Pi=yiPT Dalton’s law Pi=xiPoiRaoult’s law Antoine Equation Where T in oK and Po in mmHg Or

  5. Binary System Phase Diagram V T Vapourous Curve L+V Equilibrium Curve y Liquidous Curve 45 line (y=x) L 0% A 100%B 100% A 0%B x,y x Temperature-Composition Diagram x-y Diagram

  6. Binary System Phase Diagram For a pure component evaporation occurs at a constant temperature, so it’s called “BOILING POINT” For a mixture of two or more components evaporation occurs at increasing temperature so it’s called “BOILING RANGE” starting by Bubble point and ending at Dew point T Tdew Tbubble 0% A 100%B 100% A 0%B x,y

  7. WHAT IS THIS?????? It is PTxy diagram for phase equilibrium

  8. Effect of Pressure on Equilibrium

  9. AZEOTROPIC MIXTURE Some systems has what is so called AZEOTROPE like CS2-Acetone and Methanol-Water systems. An azeotropic mixture has a=1 so can’t be separated by distillation. This figure represents a “minimum azeotrope”

  10. AZEOTROPIC MIXTURE This figure represents a “maximum azeotrope” in the system acetone-chloroform

  11. CALCULATING LIQUID VAPOUR EQUILIBRIA FOR BINARY SYSTEMS RELATIONS USED x-y diagram

  12. 1- I f we have TEMPERATURE-COMPOSITION DIAGRAM T 1- Draw tie line 2- Project x and y of that line on the x-y diagram and get a point on the equilibrium curve 3- Repeat the last step several times 4- Connect the points to get the equilibrium curve y x

  13. 2- If vapour pressures are known as a function of temperature:

  14. And by the general equilibrium relation for ideal binary systems: At equilibrium The most used form of equilibrium relation for liquid-vapour systems(Either ideal or non-ideal, binary or multicomponent)

  15. 3- Another form of equilibrium relation ( Only for binary systems if a is known):

  16. A NEW PHASE DIAGRAM ENTHALPY COMPOSITION DIAGRAM H= specific enthalpy of vapour. h= specific enthalpy of liquid. H BTU/Lb H Vapourous Curve Liquidous Curve h 0% A 100%B 100% A 0%B x,y

  17. H Tie line is used for getting points on equilibrium curve as what was done in case of Temperature composition diagram. This diagram is more difficult to draw, however it provides more information about enthalpy of streams. y x

  18. THEORY OF DISTILLATION For any system, once it’s in the wet region, it splits into two phases in equilibrium each of different compositions. The compositions of the two phases can be determined using “TIE LINE” y T x 0% A 100%B 100% A 0%B x,y

  19. Simple Differential Distillation Sometimes called “ASTM distillation”and Used in labs Distillation occurs so that the feed is slowly evaporated and then condensed “DISTILLATE (D)” and remains un-evaporated portion “RESIDUES (W)”. Now to get xwand ῩD By Differential MB: Residues (W) Distillate (D)

  20. Simple Differential Distillation Solved by graphical integration OR Algebraically using the relation: (use when α given) Then calculate ῩD using C.M.B equation

  21. 4- 100 kmoles of a mixture of A and B is fed to a simple still. The feed contains 50 mole % A and a product contains 5 mole % A is required. Calculate the quantity of product obtained. The equilibrium data is presented as follows:

  22. Givens: F=100kmole xf=0.5 xw=0.05 Here we will solve using graphical integration: From xwto xf

  23. 5- A liquid containing 50% n-heptane and 50% n-octane is differentially distilled at 1 atmosphere to vaporize 60 mole% of feed. Find the composition of the distillate and the residue. Find the boiling range during distillation. (The average volatility α=2.15). By trial and error : X w=0.3285 From M.B: ῩD =0.614 Boiling range =109.78-114.586oC

  24. Steam Distillation Experimental method used for thermally sensible materials. For insoluble mixtures (hydrocarbon-water mixtures) each component exerts pressure equals the vapour pressure (x=1). If the summation of the vapour pressures equals the total pressure then the mixture will start boiling at a temperature lower than boiling point of the pure hydrocarbon. Steam is used to perform evaporation at a reduced temperature.

  25. Steam Distillation Steam functions: 1- Heating the batch (material to be distilled) to the bubble point. 2- Giving the batch the latent heat needed to vaporize. 3- Carrying the vapours.

  26. Steam Distillation To calculate the necessary amount of steam needed we will need to know: 1- Q1=heat required to heat the batch to Tb 2- Q2=Latent heat gained by the batch 3- amount of steam carrying the vapours

  27. Steam Distillation Q1=mbatch*CP ) batch*(Tb-TF) Q2=mbatch*lbatch Amount of steam carrying the vapours: Vapours leaving=water vapour+hydrocarbon vapour PT=Pwater+PHC

  28. Steam Distillation Q1=mw1*lw mw1=Q1/lw Q2=mw2*lw mw2=Q2/lw

  29. Problem 8: 10 Kg batch of ethylaniline is to be steam distilled from small amount of non-volatile impurity. Saturated steam at 25 Psia is used. Initial temperature of ethylaniline is 40 oC and the distillation takes place at atmospheric pressure. a- At what temperature will the distillation proceeds? b- Determine the composition of the vapour phase. c- How much steam is used? Data: • Heat capacity of ethylaniline is 0.4 KCal/Kg.C • Heat capacity of steam is 0.35 KCal/Kg.C • Latent heat of vaporization of ethylaniline is 72 Kcal/Kg • Vapour pressures of water and ethylanilineare given in the table below:

  30. Operation occurs at 1 atm=760 mmHg SO Temperature at which Ptotal=760 mmHg is 99.15oC

  31. Composition of vapour phase: At 99.15oC: Pw=737.2 mmHg PEA=22.8 mmHg yw=737.2/760=0.97 yEA=22.8/760=0.03

  32. Amount of steam used: CP)EA= 0.4 KCal/Kg.C CP)Steam= 0.35 KCal/Kg.C lEA= 72 Kcal/Kg lSteam= 540 Kcal/Kg Q1=10*0.4*(99.15-40)=236.5 Kcal m1=Q1/lSteam= 236.5/540=0.438 Kg Q2=10*72=720 Kcal m2=Q2/lSteam= 720/540=1.333 Kg mw=0.438+1.333+48.1=49.87 Kg

  33. Flash Vaporization Distillation Flash vaporization or equilibrium distillation is a single stage operation where a liquid mixture is partially vaporized, the vapour allowed to come to equilibrium with the residual liquid and the resulting vapour and liquid phases are separated and removed. It’s easy but not efficient, and no packing or trays are needed. Generally used for easy separation (very high relative volatility) or as a primary separation step.

  34. How to Reach Flashing Conditions Changing Temperature Changing Pressure T T 0% A 100%B 100% A 0%B 0% A 100%B 100% A 0%B x,y x,y

  35. How to Reach Flashing Conditions Changing Temperature Changing Pressure

  36. Calculating Tbubble and Tdew Flashing occurs at a temperature between bubble and dew points. So calculating bubble and dew points is necessary to specify suitable flashing conditions. T bubble >Tflashing > T dew

  37. Calculating Tbubble Bubble point is the temperature at which JUST ONE bubble of gas evaporates and is in equilibrium with the liquid whose composition will not be affected. T y Tbubble x 0% A 100%B xf 100% A 0%B

  38. Calculating Tbubble Then xi=xFi And yi=KixFi Where Ki=Poi/PT TO GET Tbubble: • Assume Tbubble • Calculate yi’s • Calculate Syi. if equals 1 OK, else reassume Tbubble • Interpolate T y Tbubble x 0% A 100%B xf 100% A 0%B

  39. (Tbubble)1Sy1 (Tbubble)TRUESy=1 (Tbubble)2Sy2 Sy OR 1 (Tbubble)TRUE Tbubble

  40. Calculating Tdew Dew point is the temperature at which JUST ONE point of liquid condenses and is in equilibrium with the vapour whose composition will not be affected. T y Tdew x 0% A 100%B yf 100% A 0%B

  41. Calculating Tdew Then yi=xFi And xi=yFi/Ki Where Ki=Poi/PT TO GET Tdew: • Assume Tdew • Calculate xi’s • Calculate Sxi. if equals 1 OK, else reassume Tdew • Interpolate T y Tdew x 0% A 100%B xf 100% A 0%B

  42. (Tdew)1Sx1 (Tdew)TRUESx=1 (Tdew)2Sx2 Sx OR 1 (Tdew)TRUE Tdew

  43. Flash Distillation Calculations Overall Material Balance: F=L+V Component Material Balance: F xFi=L xi+Vyi Equilibrium Relation: yi=ki xi

  44. Flash Distillation Calculations (L+V) xFi=L xi+Vyi (L+V) xFi=L xi+Vki xi (L+V) xFi=(L+V ki) xi It’s also solved by trial and error on liquid and vapour flow rates.

  45. Flash Distillation Calculations For the easiness of trials: (÷V)

  46. Flash Distillation Calculations Finally: Now trials are done by changing ONLY L/V till Sx and Sy are BOTH equal one

  47. Flash Distillation Calculations Once L/V is calculated, all needed variables can be calculated: F=L+V

More Related