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Other Sources of Enthalpy Data

Riedel Equation D H n /RT n = 1.092(In P c - 1,013) 0.930 - ( T n /T c ). Watson Equation D H 2 = (1 - T 2 /T c ) D H 1 (1 - T 1 /T c ). 0.38. Other Sources of Enthalpy Data. Specific heats tabulated (see Appendix of thermodynamic textbook) and graphical data.

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Other Sources of Enthalpy Data

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  1. Riedel Equation DHn/RTn = 1.092(InPc - 1,013) 0.930 - (Tn/Tc) Watson Equation DH2 = (1 - T2/Tc) DH1 (1 - T1/Tc) 0.38 Other Sources of Enthalpy Data Specific heats tabulated (see Appendix of thermodynamic textbook) and graphical data

  2. Example 14b: Calculate QC using Riedel and Watson Equations. What would be the value of QC if the distillate subcools liquid to 30°C?

  3. Specific Heats of Liquids

  4. Specific Heats of Gases

  5. D, xD hD V1 L0 D, xD hD V1 L0 (1) V2 L1 (j) Vj+1 Lj Internal Column Balances Enriching section of the column

  6. D, xD= 0.6 R = 3 V1 L0 (1) V2 L1 Example 14c: What is the composition and enthalpy of vapor entering and leaving the first tray of the distillation column shown in example 14a.

  7. Vk Lk-1 (k) (n) Vn+1 Ln B, xB hB (n) Vn+1 Ln B, xB hB External Column Balances Stripping section of the column

  8. Lewis observed that for a distillation column: enriching section L1 = L2 ….. = Lj = L is constant V1 = V2 ….. = Vj = V is constant stripping section L1 = L2 ….. = Lk = L is constant V1 = V2 ….. = Vk = V is constant but L  L and V  V Lewis Method Important assumptions in distillation column calculation: (1) column is adiabatic (2) specific heat << latent heat (3) latent heat (l) is constant independent of concentration this means one mole of condensed vapor will evaporate 1 mole of liquid (4) saturated liquid and vapor lines in H-x-y diagram are parallel

  9. Lewis Method Vi, yi (i) Equilibrium relation y = Kx Li, xi Vi+1, yi+1 Operating equation rectifying section yj+1 = (L/V)xj + (1-L/V)xD stripping section yk = (L/V)xj - (L/V-1)xB

  10. D, xD = 0.6 hD, R = 3 V1 L0 (1) (2) (5) (3) (4) V6 L5 Example 15: What is the composition and enthalpy of vapor entering and leaving tray 1-5 of the distillation column shown below.

  11. VLE Data

  12. External Column Balances

  13. McCabe-Thiele Method

  14. V4 L3 (4) (9) V10 L9 B, xB = 0.05 hB, boilup ratio = 1 Example 16: What is the composition and enthalpy of vapor entering and leaving tray 4-9 plus the reboiler for the distillation column shown below.

  15. VLE Data

  16. McCabe-Thiele Method

  17. Example 17: A mixture of pentane and toluene was distilled in a distillation column. An analysis of the enriching section is needed to determine whether the column is performing to specification. partial condenser D, yD = 0.9 hD, R = 2 V1 L0 (1) Please determine the composition of liquid and vapor streams leaving each stages for (a) a = 2 and 3.5 with R = 2, (b) a = 3.5 with R = 1 and 4 (2) (3) V4 L3

  18. McCabe-Thiele Method a =2 a =3.5

  19. McCabe-Thiele Method a =3.5 a =3.5

  20. Total and Minimum Reflux Total Reflux D = 0, L0 = V1, R = L0/D =  L/V = L0/V1 = 1 Minimum Reflux D = maximum, L0 = minimum allowable, a =3.5

  21. V6 L5 (6) (9) V10 L9 B, xB = 0.10 hB, boilup ratio = 2 Example 18: Analysis of the stripping section of the distillation column for pentane-toluene separation must be conducted to determine the liquid and vapor compositions leaving each distillation trays. (a) using a total reboiler, (b) using a partial reboiler.

  22. McCabe-Thiele Method a =3.5 a =3.5

  23. L V F V L Internal Column Balances Feed tray Feed Equation: y = -{(L - L)/(V - V)}x + Fzf/(V-V) y = -(Lf/Vf)x + (F/Vf)zf y = {q/(q-1)}x + zf/(1-q) q = (L-L)/F = (H-hf)/(H-h)

  24. Example 19: Find the value of q and draw the feed line for a feed containing 0.4 pentane and 0.6 toluene: (a) the feed is a saturated liquid, (b) the feed contains 0.5 fraction of vapor, (c) the feed was superheated so that each mole of feed vaporizes 10 moles of liquid, (d) the feed was subcooled so that each mole of feed condenses 2 moles of vapor.

  25. McCabe-Thiele Method a =3.5 a =3.5

  26. McCabe-Thiele Method a =3.5 a =3.5

  27. Condenser QC D, xD= 0.9, hD Reflux ratio = L0/D Q=0 F, z, hf (n) = 3 Rmin Reboiler 10 Kmole/min, 0.4 superheated vapor QR B, xB= 0.1, hB Boilup ratio = Vn+1/D Example 20: The distillation column shown in the figure below was used for the separation of 0.4 mole fraction pentane in toluene. The desired distillate and bottom products are 0.1 and 0.9, respectively. The feed enters the column as a superheated vapor that vaporizes 2 moles of liquid per mole of feed. (a) What is q-value of the feed? Plot the feed line. (b) What is the minimum reflux ratio for the separation? (c) If the column reflux was operated at 3 Rmin, where is the optimum feed-plate location? (d) What is the boil-up ratio needed for the separation? (e) How many equilibrium stages is needed to accomplish the desired separation? (f) How much distillate and bottom are produced if the feed rate is 10 kmole/min? (g) What is the minimum number of trays needed for achieve the desired separation?

  28. McCabe-Thiele Method a =3.5 a =3.5

  29. Condenser QC D, yD= 0.95, hD Reflux ratio = L0/D Q=0 F, z, hf (n) Reboiler 1 Kmole/min, 0.25 saturated liquid QR B, xB= 0.1, hB Boilup ratio = Vn+1/D Example 21: The distillation column shown in the figure below was used for the separation of 0.25 mole pentane from heptane. The desired distillate and bottom products are 0.05 and 0.95, respectively. The vapor flowrate in the enriching and stripping sections of the column are 2 D and 3B, respectively. (a) What are the flowrates of distillate and bottom? (b) What is the (L/V)enriching and plot the top operating line? (c) Express the the operation reflux ratio, R as n Rmin (d) What is the boil-up ratio? Plot the bottom operating line. (e) Is the feed subcooled, saturated liquid, mixture, saturated vapor or superheated vapor? (f) How many equilibrium stages is needed to accomplish the desired separation? (g) Where is the optimum location of the feed plate?

  30. McCabe-Thiele Method a =2 a =2

  31. D, yD= 0.6, hD (a) Derive the top and bottom operating equation for the stripping column. (b) Plot the top and bottom operating line (c) Plot the feed line and determine the q-value of the feed. Q=0 F, z, hf 15 Kmole/min, 0.10 (n) Reboiler QR B, xB= 0.1, hB Boilup ratio = Vn+1/D = 4 (d) What are the allowable feed in a stripping section i.e., subcooled, saturated liquid, mixture, saturated vapor and superheated vapor? and why? (e) Determine the number of stages needed for the separation. (f) What is the minimum reflux ratio for this separation column? Example 22: A stripping column shown in the figure below was used to remove oil from contaminated water. The water leaving the bottom must be at least 99.7 % pure. The VLE data is plotted in the figure below.

  32. McCabe-Thiele Method a =3.5 a =3.5

  33. McCabe-Thiele Method a =3.5 a =3.5

  34. C, xc, hc F, z, hf 75 Kmole/min, 0.20, 25 % vapor Example 23: Crude oil could be extracted from sand found in Canadian province of Saskatchewan. Steam is used in the extraction process and the oil-water mixture is send through a series of distillation column. The final column known as the dehydrating column is employed for removing the final traces of water from the crude to meet the industrial maximum tolerance level of 0.01 mole fraction water. Instead of a condenser saturated liquid water was used directly as coolant. This arrangement has the added benefit of diluting the oil that remains in the water recovered at the distillate. The water from the distillate is then sent to settling tank to remove the final traces of oil before discharge. D, yD= 0.8, hD (a) Derive the top operating equation for dehydration column. (b) Derive the bottom operating equation (c) Derive the feed equation (d) Plot the respective top and bottom operating line as well as the feedline Q=0 (n) Reboiler QR B, xB= 0.01, hB Boilup ratio = Vn+1/D = 3 (e) Determine the number of stages needed for the separation and the optimum feed plate location if the total tray efficiency is 0.25.

  35. McCabe-Thiele Method a =3.5 a =3.5

  36. Condenser QC D, xD= 0.9, hD Reflux ratio = L0/D Q=0 F, z, hf (n) = 2 Rmin Reboiler 10 Kmole/min, 0.4 superheated vapor QR B, xB= 0.1, hB Boilup ratio = Vn+1/D Example 24: The distillation column shown in the figure below was used for the separation of 0.5 mole fraction methanol-water solution. The desired distillate and bottom products are 0.10 and 0.95, respectively. The feed enters the column as a superheated vapor that vaporizes 2 moles of liquid per mole of feed. (a) What is q-value of the feed? Plot the feed line. (b) What will happen if the feed condition changes from superheated to sat. vapor to sat. liquid and subcooled liquid. (c) What is the minimum reflux ratio? (d) What is the minimum number of plates?

  37. McCabe-Thiele Method a =3.5 a =3.5

  38. Column Efficiency Overall Column Efficiency Eo = Nequil/Nactual Murphree Efficiency EMV= actual change in vapor change in vapor for equilibrium stage = yj - yj+1 yj* - yj+1

  39. Condenser QC D, xD= 0.9, hD Reflux ratio = L0/D Q=0 F, z, hf (n) = 2 Rmin Reboiler 10 Kmole/min, 0.5 Subcooled liquid QR B, xB= 0.2, hB Boilup ratio = Vn+1/B Example 25: The distillation column shown in the figure below was used for the separation of 0.5 mole fraction methanol-water solution. The desired distillate and bottom products are 0.20 and 0.9, respectively. The feed enters the column as a subcooled liquid that condenses 2 moles of vapor per mole of feed. (a) What is q-value of the feed? Plot the feed line. (b) What is the number of equilibrium stages? (c) What is the actual number of stages if the EMV = 0.5? (d) Solve the problem using Fenske, Gilliland and Underwood methods.

  40. McCabe-Thiele Method a =3.5 a =3.5

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