5.13b. Steam-stripping of benzene in a sieve-plate column.
A straw oil used to absorb benzene from coke-oven gas is to be steam-stripped in a sieve-plate column at atmospheric pressure to recover the disolved benzene, C6H6. Equilibrium conditions at the operating temperature are approximated by HenryÕs law such that, when the oil phase contains 10 mole % benzene, the equilibrium benzene partial pressure above the oil is 5.07 kPa. The oil may be considered nonvolatile. It enters the stripper containing 8 mole % benzene, 75% of which is to be removed. The steam leaving contains 3 mole % C6H6.
a) How many theorethical stages are required?
b) How many moles of steam are required per 100 moles of the oil-benzene mixture?
c) If 85% of the benzene is to be recovered with the same steam and oil rates, how many theoretical stages are required?
Solution
a)
b)
Kremser equation
c)
(Graphical construction ahead)
Kremser equation
5.15c,d. Absorption of carbon disulfide in a random-packed tower.
Design a tower packed with 50-mm ceramic Hiflow rings for the carbon disulfide scrubber of Problem 5.11. Assume isothermal operation and use a liquid rate of 1.5 times the minimum and a gas-pressure drop not exceeding 175 Pa/m of packing. Calculate the tower diameter, packed height, and total gas-pressure drop. Assume that Ch for the packing is 1.0.

Solution
From Prob. 5.11, the conditions at the bottom of the absorber are:
From Tables 4.1 and 4.2
Run the Packed Tower Design Program of Ap. D
Results:
Estimate the packed height; assume dilute solutions

From Prob 5.11:
Calculate HtOG at the bottom of the tower
Calculate HtOG at the top of the tower
From Prob. 5.11, the conditions at the top of the absorber are:
The mass-tranfer coefficients remain fairly constant along the tower
5.16b,d. Absorption of sulfur dioxide in a random-packed tower.
It is desired to remove 90% of the sulfur dioxide in a flue gas stream at 298 K and 1 atm by countercurrent absorption with pure water at the same temperature, using a packed tower that is 0.7 m in diameter. The tower is packed with 35-mm plastic NORPAC rings. The average gas-pressure drop is 200 Pa/m. Equilibrium is described by HenryÕs law with yi = 8.4xi.. If the liquid flow is adjusted so that the driving force (y Ð y*) is constant, calculate the height of the packed section. Flue gases usually contain less than 1 mole % of SO2, an air pollutant regulated by law. Assume that the properties of the liquid are similar to those of pure water, and that the properties of the flue gases are similar to those of air.

Solution
For the driving force to be constant, the operating line is parallel to the equilibrium line, and the absorption factor A = 1.0
It was found by trial and error with the Packed Column program that, for a pressure drop of 200 Pa/m and D = 0.7 m: