• Author / Uploaded
• Carlos Miguel Dacaimat

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• Ingeniería de procesos químicos
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REVIEW QUESTIONS Gas Absorption 1. Acetone is to be recovered from an acetone-air mixture by counter-current scrubbing with water in a packed tower. The inlet gas mixture has 5 mole% acetone. The gas flow rate is 0.5 kg/m 2s (MW = 29) and the liquid flow rate is 0.85 kg/m2s (MW = 18). The overall mass transfer coefficient K ya may be taken as 0.0152 kg-mole/(m 3.s.mole fraction). The system may be considered as dilute. a) What should be the height of the tower to remove 98% of the entering acetone? b) Repeat the calculations for the tower height if the liquid rate used is 1.20 kg/m 2.s The equilibrium data for the mixture are: mole fraction acetone in liquid (xA) mole fraction acetone in gas (yA)

0.0076 0.0099

0.0156 0.0196

0.0306 0.0361

0.0333 0.0400

2. Acetone is being absorbed by water in a packed tower having a cross-sectional area of 0.186 m 2 operating at 293 K and 101.32 kPa. The inlet air contains 2.6 mole% acetone and the outlet gas contains 0.5 mole% acetone. The total inlet gas flow is 13.65 kg-mole/hr (MW = 29). The pure water inlet flow is 45.36 kg-mole water/hr (MW = 18). The individual mass transfer coefficients are k xa = 0.06098 kg-mole/(m3.s.mole fraction) and kya = 0.03795 kg-mole/ (m3.s.mole fraction) respectively. Calculate Kya and the tower height required. mole fraction acetone in liquid (xA)

0.00 0 0.0

partial pressure of acetone in vapor (mm Hg)

0.03 3 30.0

0.07 2 62.8

0.117

0.171

85.4

103.0

3. Sulfur dioxide is to be recovered from an air-SO 2 mixture by contacting the gas mixture with pure water in a packed tower. The tower will be operated at 30 oC and 1 atm (abs). The entering gas contains 5% mole SO 2 and the exiting gas is to contain no more than 0.5% mole SO 2. Since the concentration of SO 2 in water is small, the system can be considered as dilute system. The total gas flow rate entering the tower is 100 kg-mole/m 2.hr. The molecular weights of SO2 and H2O are 64 and 18 respectively. If the overall mass transfer coefficient for the vapor phase is K ya = 60 kg-mole/(m3.hr. mole fraction), determine: a) The minimum pure water rate (in kg-mole/m2.hr) needed for this separation, b) The height of the tower (in m) required if the actual water rate entering the tower is 1.2 times the minimum value. The equilibrium data for SO2-water system at 30oC and 1 atm (abs) are given below: Mass SO2 per 100 mass H2O Partial Pressure of SO2 in Vapor pA (mm Hg)

7.5

5.0

2.5

1.5

1.0

0.7

0.5

0.3

0.1

0.05

688.0

452.0

216.0

125.0

79.0

52.0

36.0

19.7

4.71

0.70

4. A tray tower is to be used to remove ammonia from an entering air stream containing 6 mole% ammonia at 293 K and 1.013 x 105 Pa. The entering pure water flow rate is 144 kg/m 2.hr (MW of H2O = 18) and the total inlet gas flow rate is 116 kg/m2.hr (MW of gas = 29). The ammonia concentration in the outlet gas is to be not more than 0.4 mole %.The system can be assumed to be dilute. a) Calculate the number of theoretical trays needed. b) If the overall efficiency is 25%, determine the actual number of trays needed for the separation. 5. It is desired to absorb 90% of the acetone in a gas containing 1.0 mole% acetone in air in a counter-current tray column. The total inlet gas flow to the column is 30.0 kg-mole/hr, and pure water is to be used to absorb the acetone. The process is to operate isothermally at 300 K and a total pressure of 101.32 kPa. The equilibrium solubility relationship for acetone in water is y = 2.53 x. Calculate: a) The minimum pure water flow in kg-mole/hr. b) The number of theoretical trays required for the separation if the pure water flow is 1.2 times the minimum. c) Compare your answers for part (b) using the Kremser Equation. d) Using the Kremser equation, find the number of theoretical trays required for the separation if the pure water flow is 1.4 times and 2.0 times the minimum.