CHE 311 – MASS TRANSFER AND UNIT OPERATIONS TUTORIAL-CHAPTER 2 (DISTILLATION) Question 1 A liquid mixture mixture of 150 150 mol/h propanol-wa propanol-water ter containing containing 30 mol% mol% is fed into batch batch distillation distillation to obtain final propanol content of 10 mol%. The equilibrium data for the mixture is given below: Mol fraction of propanol in liquid, (x A) Mol fraction of propanol in vapour, (y A)
i) ii)
0.00
0.10
0.20
0.30
0.40
0.00
0.22
0.38
0.51
0.63
By using using the Raleig Raleigh h equation equation,, determine determine the the amount amount of feed feed that that has been been distill distilled. ed. Determin Determine e the avera average ge compos compositio ition n of the distil distillate late..
Question 2 A liquid containin containing g 50 mol% n-Hexane n-Hexane and and 50 mol% mol% n-Octane n-Octane is required required to be separated under flash distillation until 65% of the liquid is distilled. The vapor pressure-equilibrium data for the system acetone-methanol at 101.32 kPa is given in the Table 1 below:
Tem, C °
68.7 79.4 93.3 107.2 125.7
Table 1 Vapor-liquid equilibrium data for n-Hexane – n-Octane mixture. Vapor Pressure, Vapor Pressure, Mol fraction of nMol fraction of o o kPa (P A ) kPa (P A ) hexane in liquid n-hexane in phase x A gas phase y A 101.3 16.1 1 1 136.7 23.1 197.3 37.1 284.0 57.9 0.19 0.53 456.0 101.3 0 0
a) By applying applying the Raoult’s Raoult’s Law and Dalton Dalton Law, determine determine the the mol fraction fraction of hexane hexane in o liquid and vapor phase at temperature 79.4 to 93.3 C
b) Determine Determine the composit composition ion of vapor vapor (y A) and liquid (x A) at an equilibrium stage.
c) Further the the vapor stream stream from the the flash distillation distillation is subjected to a condenser condenser before before it fed to a second flash distillation. Again 65 mol% of the liquid is distillated. Determine the composition of the second overhead product (y A). Question 3
A mixture containing 40 mol% of benzene and 60 mol% of chlorobenzene is separated by flash distillation until 70 % of the initial charge is taken off as overhead. Further the vapor stream from the flash distillation is subjected to a condenser before it fed to a fractionation distillation column. A distillated product from the fractionation column containing 95 mol% of benzene and a bottom containing 90 mol% of chlorobenzene. The reflux ratio for this process is 1.3 and the q value used is 0.6. Equilibrium data for benzene-chlorobenzene system is given below. Mol fraction of benzene in liquid, x A Mol fraction of benzene in gas, y A
0
0.1
0.2
0.4
0.6
0.8
0.9
1.0
0
0.37
0.57
0.78
0.89
0.95
0.98
1.0
i) Sketch the schematic diagram of the system. ii) Determine the number of theoretical trays. iii) If the reflux ratio were increased, explain the effect to the number of theoretical stages. Question 4 A saturated liquid feed of 200 mol/h at the boiling point containing 60 mol% n-heptanes and 40 mol% n-octane is to be fractionated at 101.32 kPa abs to give a distillate containing 97 mol% nheptanes and a bottoms containing 10 mol% n-heptanes. The reflux ratio used is 2.5:1. Calculate the mol/h distillate, mol/h bottoms, theoretical number of trays, and the feed tray number. The relative volatility of n-heptane-n-octane is given 2.5. Question 5 A total feed of 100 mol/h having an overall composition of 55 mol% heptane and 45 mol% ethyl benzene is to be fractionated at 101.3 kPa pressure to give a distillate containing 97 mol% heptane and a bottoms containing 10 mol% heptane. The feed enters the tower partially vaporized so that 40 mol% is liquid and 60 mol% vapor. Calculate: a. b. c. d. e.
Moles per hour distillate and bottoms Minimum reflux ratio Rm. Minimum steps and theoretical trays at total reflux. Theoretical number of trays and feed tray number required for an operating reflux ratio is 1.6Rm. Number of actual trays if the overall tray efficiency is 80%. x A y A
Question 6
0 0
0.080 0.230
0.250 0.514
0.485 0.730
0.790 0.904
1.000 1.000
A mixture containing 70 mol% of benzene and 30 mol% of toluene enter a fractioning tower at 101.32 kPa. The distillate is to contain 95 mol% of benzene while the bottoms is 92 mol% toluene. The feed is entering the tower at its dew point. The relative volatility of benzene and toluene is 1.5. Determine the number of theoretical stages and location of feed stage when the reflux ratio is 4:1. Question 7 A saturated liquid feed of 200 mol/h at the boiling point containing 42 mol% heptanes and 58 mol% ethyl benzene is to be fractionated at 101.32 kPa abs to give a distillate containing 97 mol % heptanes and a bottoms containing 1.1 mol% heptanes. The equilibrium data are given below at 101.32 kPa abs pressure for the mole fraction n-heptane. Mol fraction of n-heptane in liquid phase (x A) Mol fraction of n-heptane in vapor phase (y A)
0 0
0.08 0.23
0.25 0.51
0.49 0.73
0.79 0.90
1 1
a) Determine the minimum reflux ratio, R m. b) Determine the minimum number of theoretical plates at total reflux. c) The reflux ratio used is 2.27R m, determine: a. The distillate and bottoms flow rate in mol/h. b. The theoretical number of trays. c. The feed tray number. Question 8 An outlet stream 50 mol/h of reactor consists of 40 mol% heptane and 60 mol% of ethyl benzene is to be further fractionated at 101.32 kPa. A distillate contains 96 mol% heptane and a bottoms of 94 mol% ethyl benzene are to be obtained. The equilibrium data for heptane-ethyl benzene system is given in Table 1. Mol fraction of heptane in liquid phase (x A) 0 0.08 0.25 0.485 0.79 1 Mol fraction of heptane in vapor phase (y A) 0 0.23 0.514 0.73 0.904 1 a) Determine the distilate and bottom flowrate in mol/h. b) Determine the value of q when: i) This stream enters the column as an equilibrium mixture at 20 mol/h of liquid and 30 mol/h of vapor. ii) This stream enters as saturated liquid. iii) This stream enters as saturated vapor. iv) This stream enters as saturated mixture and the enthalpy values are given: Enthalpy of feed at entrance condition: 150000 kJ/kgmol Enthalpy of feed at boiling point: 8800 kJ/kgmol Enthalpy of feed at dew point: 144500 kJ/kgmol c) d)
Determine the minimum number of stages of the column. Use the q value when the stream enters the column as an equilibrium mixture at 20 mol/h of liquid and 30 mol/h of vapor. Determine: i) The minimum reflux ration, R min. ii) The number of the theoretical stages if the actual reflux ratio is 1.72R min. iii) The actual number of the stages if the overall efficiency is 80%. Question 9
A feed of 100 mol/h of mixture containing 60 mol% carbon tetrachloride and 40 mol% carbon disulfide is fed to an intermediate point of a continuous distillation column. The column operates at atmospheric pressure to give a distillate containing 91 mol% purity and contain 85% of the more volatile component in the feed. The value of L/V in the stripping section is 1.6. The feed enters the column as 40 mol% vapor. The equilibrium data for the solution is given in the table below: Mol fraction carbon tetrachloride in a liquid phase, x A Mol fraction carbon tetrachloride in a vapor phase, y A a) b) c) d) e) f)
0 0
0.08 0.23
0.25 0.51
0.49 0.73
0.79 0.90
1 1
Calculate the molar flow rate at distillate and bottom. Calculate the mole fraction of more volatile component in the bottom. Calculate the q value. Find the reflux ratio value. Determine the number of theoretical stages and the location of feed stage. Calculate the actual number of stage if the overall efficiency is 85%
Question 10 a.
In continuous distillation, one of the parameter which used in McCabe-Thiele method is the parameter feed line (q). By sketching a relevant x-y diagram, sketch and label the qline for five (5) different types of pahse in feed, namely, subcooled liquid, saturated liquid, saturated mixture, saturated vapor and superheated vapor. State as well the velue or range of feed line (q) for each feed condition.
b.
A continuous distillation column handles a mixture consisting of 35 mol% of benzene and 65 mol% of toluene at the rate 160 kmol/h and separates it into a product containing 97 mol% of benzene in overhead and a liquid containing 98 mol% toluene. The feed enters the column as 35 mol% vapor. Table Vapor-liquid equilibrium data for benzene-toluene mixture Mole fraction of benzene in liquid, x Mole fraction of benzene in vapour, y
i. ii. iii. iv.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
0.22
0.38
0.51
0.63
0.70
0.85
0.85
0.91
0.96
1.00
Calculate the molar flow rate of distillate in kmol/h. Calculate the molar flow rate of bottom in kmol/h. Calculate number of theoretical plate required if reflux ratio of 2.7 is employed. Calculate the actual number of plate if the column efficiency is 85%.
Question 11
An equimolar mixture of acetone and water from the outlet stream of flash column is to be further fractionated at 101.3 kPa, while entering the tower at its boiling point. The distillate is to contain 95 mol% acetone and the bottoms 5 mol% acetone. During the process, 3 moles of distillate is removed for every 4 moles returned to the unit as reflux. The equilibrium data for this system can be represented by the following equation: y = 1.9(x). Determine: i)
The schematic diagram of the system.
ii) The molar flowrates of the distillate and bottoms products.
iii) The q value. iv) The number of theoretical stages and the position of the feed stage. v) The actual number of stage if the efficiency is 65%.
Question 12 As an assistant engineer, you have been assigned to analyze the problem that occurred in the distillation column which cannot produce the desired product purification. Suggest four (4) possible solutions to overcome this problem so that the distillation column achieved the desired product purification. Question 13 a.
Describe the following terminologies in the distillation column process: i)
Volatility
ii)
Reflux ratio.
iii)
Total reflux ratio.
iv)
Minimum reflux ratio.
v)
Enriching column
vi)
Stripping column
b.
Explain the principle of distillation process with appropriate diagram.
c.
Derive the equation for the simple batch distillation by Raleigh equation.
