5.17c. Benzene vapor recovery system.
Benzene vapor in the gaseous effluent of an industrial process is scrubbed with a wash oil in a countercurrent packed absorber. The resulting benzene-wash-oil solution is then heated to 398 K and stripped in a tray tower, using steam as the stripping medium. The stripped wash oil is then cooled and recycled to the absorber. Some data relative to the operation follow:

Absorption:
Benzene in entering gas = 1.0 mole %
Operating pressure of absorber = 800 mm Hg
Oil circulation rate = 2 m3/1,000 m3 of gas at STP
Oil specific gravity = 0.88 Molecular weight = 260
HenryÕs law constant = 0.095 at 293 K;
= 0.130 at 300 K
NtOG = 5 transfer units
Stripping:
Pressure = 1 atm Steam at 1 atm, 398 K
HenryÕs law constant = 3.08 at 398 K
Number of equilibrium stages = 5
a) In the winter, it is possible to cool the recycled oil to 293 K, at which temperature the absorber then operates. Under these conditions 72.0 kg of steam is used in the stripper per 1,000 m3 of gas at STP entering the absorber. Calculate the percent benzene recovery in the winter.
Solution
Initial estimates:

b) In the summer it is impossible to cool the recycled wash oil to lower than 300 K with the available cooling water. Assuming that the absorber then operates at 300 K, with the same oil and steam rates, and that NtOG and equilibrium stages remain the same, what summer recovery of benzene can be expected?
Solution
c) If the oil rate cannot be increased, but the steam rate in the summer is increased by 50% over the winter value, what summer recovery of benzene can be expected?
5.19c. Absorption of germanium tetrachloride used for optical fibers.
Germanium tetrachloride (GeCl4) and silicon tetrachloride (SiCl4) are used in the production of optical fibers. Both chlorides are oxidized at high temperature and converted to glasslike particles. However, the GeCl4 oxidation is quite incomplete and it is necessary to scrub the unreacted GeCl4 from its air carrier in a packed column operating at 298 K and 1 atm with a dilute caustic solution. At these conditions, the dissolved GeCl4 has no vapor pressure and mass transfer is controlled by the gas phase. Thus, the equilibrium curve is a straight line of zero slope. The entering gas flows at the rate of 23,850 kg/day of air containing 288 kg/day of GeCl4. The air also contains 540 kg/day of Cl2, which, when dissolved, also will have no vapor pressure.
It is desired to absorb, at least, 99% of both GeCl4 and Cl2 in an existing 0.75-m-diameter column that is packed to a height of 3.0 m with 13-mm ceramic Raschig rings. The liquid rate should be set so that the column operates at 75% of flooding. Because the solutions are very dilute, it can be assumed that both gases are absorbed independenttly.
Gas-phase mass-transfer coefficients for GeCl4 and Cl2 can be estimated from the following empirical equations developed from experimental studies with 13-mm Raschig rings for liquid mass velocities between 0.68Ð2.0 kg/m2-s (Shulman, H. L., et al., AIChE J., 17, 631, 1971):
where:
ds = equivalent packing diameter (0.01774 m for
13-mm ceramic Raschig rings)
r = gas density, kg/m3
Gx,Gy mass velocities, kg/m2-s
For the two diffusing species, take DGeCl4 = 0.06 cm2/s; DCl2 = 013 cm2/s. For the packing, Fp = 580 ftÐ1, e = 0.63.
Determine:
a) Liquid flow rate, in kg/s.

Solution
Initial estimate
b) The percent absorption of GeCl4 and Cl2 based on the available 3.0 m of packing.

Solution (dilute solutions)
5.20c,d. Absorption of carbon disulfide in a structured-packed tower.
Redesign the absorber of Problem 5.15 using metal Montz B1-300 structured packing. The characteristics of this packing are (Seader and Henley, 1998):
Fp = 33 ftÐ1 a = 300 mÐ1 e = 0.93 Ch = 0.482
Cp = 0.295 CL = 1.165 CV = 0.422

Solution
From the Packed Tower Program in Appendix D:
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