Heat transfer & heat exchangers - Lecture 7: Evaporation

Nội dung text: Heat transfer & heat exchangers - Lecture 7: Evaporation

1. Lecture 7 Evaporation
2. Feed characteristic Concentration increases, the viscosity, density and boiling point increase as well, then lower heat coefficient Heat sensitive materials as pharmaceuticals and foods require short residence time Foaming & frothy causes entrainment loss Scale / deposit formation drastically decreases the heat coefficient, then leads to shut down for cleaning Other properties: solubility, specific heat, gas liberation, toxicity
3. Classification Once through / Circulation Natural / Forced / Agitated Falling / Rising Vertical / Horizontal Single / Multiple Internal heater / External heater
4. Classification Natural circulation with low heat transfer coefficient 푣퐿 = 0.3 ÷ 1 Τ푠 , relative cheap, poor circulation, for nonviscous / non deposite scale liquids Forced circulation with high heat transfer coefficient 푣퐿 = 2 ÷ 6 Τ푠 , fouling reduction, high pumping cost Agitated with high heat transfer coefficient, reduce thermal resistance of liquid, high capital cost and low capacity
5. Classification Vertical tube is for foaming liquids Horizontal tube is for low viscosity and non deposit scale liquids
6. Classification Internal heater with short vertical tube = 1 ÷ 2 External heater with long vertical tube = 3 ÷ 10
7. Open kettle & pan evaporator Pan Steam Boiler Jacket Condensate Concentrate
8. Once through evaporator Rising film Falling film External heater External heater
9. Natural circulation evaporator Rising Rising External heater Internal heater
10. Forced circulation evaporator Falling Internal heater Rising Internal heater
11. Forced circulation evaporator Horizontal tube – External heater
12. Agitated evaporator
13. Natural circulation – External heater • Large area for heat transfer, enhanced heat transfer • Short residence time, suitable for heat sensitive Advantages liquids • Rising film is for foaming and frothy liquids • Falling film is for viscous and corrosive liquids • Quite complicated, high capital cost • Cleaning and maintenance is difficult Disadvantages • Space required • Rising is not for viscous, salting and scaling liquids • Falling is not for suspension, salting and scaling liquid
14. Once through evaporator Rising film Internal heater Plate external heater
15. Once through evaporator Heat exchanger Falling film External heater Separation chamber
16. Forced circulation evaporator Rising – External heater
17. Forced circulation evaporator Rising – External heater
18. Single effect with recompression Mechanical recompression
19. Single effect with recompression Mechanical recompression
20. Single effect with recompression Thermal recomp. is better than mechanical recomp. for vacuum operation However, the efficiency of thermal recompression is lower Thermal recompression
21. Multiple effect evaporator Forward: feed flows naturally (without pump), low boiling point for heat sensitive concentrate Backward: pump required, for cold feed and highly viscous concentrate Parallel: for feed is almost saturated, solid crystal Mixed (forward–backward): for very highly viscous concentrate
22. Multiple effect evaporator Backward feed operation
23. Multiple effect evaporator Parallel feed operation
24. Multiple effect evaporator Forward feed operation
25. Multiple effect evaporator
26. Multiple effect evaporator
27. Overall heat transfer coefficient Type Overall coefficient 푼 푾Τ ℃ Vertical tube Natrural circulation 1000 ÷ 2500 Forced circulation 2000 ÷ 5000 Agitated film 1 푃 2000 1푃 1500 100푃 600
28. Raoult’s law ∆ = 𝑖 푛 𝑖: number of ions produced by each molecule of solute : constant by solvent 푛 표푙 푠표푙 푡푒Τ 푠표푙푣푒푛푡 : solute concentration Solvent Boiling point ℃ at 1 푡 풌 풌품℃Τ 풐풍 Water 100 0.512 Ethanol 78.5 1.22 Acetic acid 117.9 3.07 Benzene 80.1 2.53 Diethyl ether 34.5 2.02 Carbon disulfide 46.2 2.34 Carbon tetrachloride 76.5 5.03 Chloroform 61.7 3.63
29. Duhring’s rule ℉ Boiling Boiling of solution point Boiling point of water ℉ Sodium hydroxide – Water at atmospheric pressure
30. Nomograph for boiling point of aqueous solutions
31. Temperature difference 1 Vapor 1 = 푠 푡 + ∆ = 푃, 푤퐹 푃 푠 − 1 (pressure drop, concentration) , 푤 Feed 퐹 퐹 1 Steam 푆 푆 Condensate 1 Concentrate
32. Duhring’s rule 1; 1 2; 2 3; 3 1 2 3 1 2 3 ∆ ∆ 퐹 ∆ + + Feed + 2 3 1 Steam = = 푆 = ′ ′ ′ 2 3 1 concentration ) concentration ) concentration ) ( ( ( ′ ′ ′ 퐿1; 1 퐿2; 2 퐿3; 3 Concentrate Condensate Condensate Condensate ′ ′ ′ 푄1 = 푈1 1 푠 − 1 푄2 = 푈2 2 1 − 2 푄3 = 푈3 3 2 − 3 푄1 = 푈1 1 푠 − 1 푄2 = 푈2 2 1 − 2 푄3 = 푈3 3 2 − 3 Concentration Concentration Pressure drop Pressure drop Concentration Pressure drop Forward feed operation
33. Calculation for single effect evaporator
34. Process variables , , Vapor 퐹 = 퐿 + Mass balances ቊ 퐹푤퐹 = 퐿푤퐿 푃 Energy balances 푄 + 퐹ℎ = 퐿ℎ + ቊ 퐹 퐿 퐹, 푤 , , ℎ 푄 = 푆 푆 − ℎ푆 = 푆∆ 푆 Feed 퐹 퐹 퐹 푆, 푃푆, 푆, 푆 Steam Heat transfer rate 푄 = 푈 푆 − 푄, 푈, 푆, 푃 , , ℎ 푆 푆 푆 Condensate 퐿, 푤 ; , ℎ 퐿 퐿 Concentrate
35. Calculation for multiple effect evaporator
36. Process variables 1; 1 2; 2 3; 3 푃1 푃2 푃3 퐹, Feed 퐹 1 2 3 Steam 푆 퐿1; 1 퐿2; 2 퐿3; 3 Concentrate Condensate Condensate Condensate Concentration, hydrostatic head & friction loss are negligible: 푄푖 = 푈푖 푖∆ 푖 푄1 = 푈1 1 푠 − 1 푃1 푄2 = 푈2 2 1 푃1 − 2 푃2 푄3 = 푈3 3 2 푃2 − 3 푃3 Forward feed operation
37. Process variables 푄1 푄2 푄3 3 푃1 푃2 푃3 2 3 푄1 퐿3; 3 Concentrate 1 2 If 퐹 = 1 and no heat loss 푄1 = 푄2 = 푄3 = ⋯ = 푄푖 and 푈1 = 푈2 = 푈3 = ⋯ = 푈푖 Then 푖∆ 푖= 표푛푠푡 Forward feed operation
38. Process variables 푄1 + 푄2 + 푄3 1 + 2 + 3 푃1 푃2 푃3 푄 = 푈푆 푆∆ 푆 푈 = 푈 푄 = 푄1 + 푄2 푆 푖 +푄3 ∆ 푆= ෍2 ∆ 푖 = 푛∆ 푖 3 퐿3; 3 Concentrate If 퐹 = 1 and no heat loss 푄1 = 푄2 = 푄3 = ⋯ = 푄푖 푄 = ෍ 푄푖 = ෍ 푈푖 푖∆ 푖 and 푈 = 푈 = 푈 = ⋯ = 푈 Then ∆ = 표푛푠푡 1 2 3 푖 푖 푖 푄 = 푛푈푖 푖∆ 푖 If 1 = 2 = 3 = ⋯ = 푖 Then ∆ 1= ∆ 2= ∆ 3= ⋯ = ∆ 푖 푆 = 푖 Forward feed operation