6871981

Chemical Engineering Society Council of Student Organizations

De La Salle University –  Manila  Manila

SEPAPRO QUIZ#2 REVIEWER

Questionnaire:................ ...................................... ............................................ ..................... I. Multiple Choice 1. 100 moles of vapor per hour is leaving the top plate of a continuous distillation column and overhead product at the rate of 50 moles/hr is being withdrawn. Then the reflux ratio is a. 0.5 b. 1.0 c. 2.0 d. 3.0 2. Minimum number of ideal stages is required in a fractionating column when the reflux ratio is equal to a. Minimum reflux ratio b. Optimum reflux ratio c. Zero d. Infinity 3. In a binary distillation column if the feed contains 40 mol % vapor, the feed line (or q-line) wikll have a slope of a. -1.5 b. -0.6 c. 0.6 d. 1.5 4. One mole of solute A and B (enthalpy = 900Kcal/Kmol; x=0.4) is mixed with 2 moles of another solution of the same components (enthalpy = 1200Kcal/Kmol; x=0.8). The mole fraction of the mixture is a. 0.475 b. 0.550 c. 0.667 d. 0.750 5. For the rectification of methanol-water, which of the followings will increase the number of ideal stages needed? a. Uses open steam instead of reboiler b. Increases the reflux ratio used c. Increases the operating pressure d. Makes the reflux subcooled

6. Fenske’s Equation is used to calculate number of plates in a distillation column a. A total reflux b. For system having constant relative volatility c. At minimum reflux d. For both (A) and (B) together 7. Which one of the following is not considered an ideal stage in distillation concept? a. Partial condenser b. Total condenser c. Reboiler d. None of the above 8. When open steam is employed for heating in a continuous distillation column for aqueous solution, the stripping section operating line in McCabe-Thiele diagram passes through coordinates (x,y) at the point (xw is the mole fraction of the more volatile component in the bottom product): a. (0,0) b. (xw, xw ) c. (0, xw ) d. (xw,0) 9. For distillation of an equimolar binary mixture of A and B, the equation of the operating lines are: Rectifying section: y = 0.663x + 0.32 Stripping section: y = 1.329x – 0.013 Where x and y are mole fraction of A in the liquid and vapor streams respectively. What is the condition of the feed? a. Saturated liquid b. Saturated vapor c. Subcooled liquid d. Supersaturated vapor 10. A column for distillation of a mixture of A and B operates at total reflux. The mole fraction of liquid leaving the fourth tray from the top is 0.4. If the relative volatility is 1.8, what is the mole fraction of the vapor stream leaving the fifth tray from the top? a. > 0.72 b. 0.72 c. 0.545 d. 0.4

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II. Problem Solving

Answers:

1.

I.

2.

A total feed of 100 lb-mole/hr having an overall composition of 40 mole% A and 60 mol% B is to be fractionated at atmospheric pressure to give a distillate containing 95mol % A and a bottoms containing 7.5 mol% A. The feed enters the column partially vaporized so that 60mol % is liquid and 40mol % is vapor. The column is to operate at a reflux ratio of 3 with a total condenser and a reboiler wherein the relative volatility of A with respect to B can be considered constant at a value of 3. Constant molal overflow may be assumed also. Calculate the following: a. lb-mole/hr distillate produced b. minimum reflux ratio c. theoretical number of plates required d. actual number of stages if the overall column efficiency is 75% e. the reboiler heat load (Btu/hr) if the average molar heat of vaporization of A and B is 10,000 Btu/lb-mole The molar enthalpies of saturated liquid and vapor mixture of A and B at 1 atm total pressure are

 = 20000 + 4000 (saturated vapor enthalpy, KJ/Kmol) ℎ = 9000 + 1000 (saturated liquid enthalpy, KJ/Kmol)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

B D A C C D B D A D

II. 1. a. b. c. d. e.

The relative volatility of A with respect to B is 3. a.

b.

c.

d.

We have 1 Kmol of liquid (x = 0.3, enthalpy = 8500 KJ.Kmol) and 1 Kmol of a vapor ( y = 0.5, enthalpy = 24000 KJ/Kmol). What is the condition of the liquid (subcooled, saturated)? What is the condition of the vapor (supersaturated, saturated)? If a mixture of saturated liquid and saturated vapor in equilibrium has an overall composition of 50 mole% A and an overall enthalpy of 14000 KJ/Kmol, what is its x and y values? When a feed described above (b) is fed to a distillation column at 100Kmol/hr, what would be the number of ideal stages needed to produce an overhead product of 90 mol% A and a bottom of 10 mol% A. The column is equipped with a total condenser and a reboiler operated at 1 atm with a reflux ratio of 2. (A scale of enthalpy from +55000 to -30000KJ/Kmol will suffice to place all relevant points in the h-x diagram). What is the condenser duty (KJ/hr) fpr letter (c).

37.143lnmole/hr Rm = 1.5 5.9 theoretical plates + 1 reboiler 8 plates ...

2. As a.

With x,y the mole fraction of A in liquid and vapor phase respectively.

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b. c. d.

Condition of the liquid: subcooled Condition of the vapor: superheated x = 0.4, y = 0.67 6.8 ideal stages or 5.8 ideal plates + 1 reboiler Q c=2050000 KJ/hr

Goodluck sa exam! Kaya niyo yan :)

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