5.9b. Absorption of carbon dioxide in a bubble-cap tray tower.
A plant manufacturing dry ice will burn coke in air to produce a flue gas which, when cleaned and cooled, will contain 15% CO2, 6% O2, and 79% N2. The gas will be blown into a bubble-cap tower scrubber at 1.2 atm and 298 K, to be scrubbed countercurrently with a 30 wt% monoethanolamine (C2H7ON) aqueous solution entering at 298 K. The scrubbing liquid, which is recycled from a stripper, will contain 0.058 mole CO2/mol solution. The gas leaving the scrubber is to contain 2% CO2. A liquid-to-gas ratio of 1.2 times the minimum is specified. Assume isothermal operation. At 298 K and 1.2 atm, the equilibrium mole fraction of carbon dioxide over aqueous solutions of monoethanolamine (30 wt%) is given by
where xA,i is the mole fraction of CO2 in the liquid solution.
a) Calculate the kilograms of solution entering the tower per cubic meter of entering gas.

Solution
For the minimum liquid flow rate, x1max will be in equilibrium with yN1
Initial estimate
Calculate molecular weight of MEA solution
Basis: 100 gm of 30% MEA in water
b) Determine the number of theoretical trays required for part a)

Solution
Generate XY diagram
c) The monoethanolamine solution has a viscosity of 6.0 cP and a density of 1,012 kg/m3. Estimate the overall tray efficiency for the absorber, and the number of real trays required. Seader and Henley (1998) proposed the following empirical correlation to estimate the overall efficiency of absorbers and strippers using bubble-cap trays (it has also been used to obtain rough estimates for sieve-tray towers):
where: EO = overall fractional efficiency
m = slope of equilibrium curve
mL = liquid viscosity, in cP


rL = liquid density, in kg/m3
Hint: In this problem, the equilibrium-distribution curve is not a straight line, therefore m is not constant. Estimate thhe average value of m at liquid concentrations along the operating line and use theaverage in the correlation given.

Solution
5.11c,d. Absorption of carbon disulfide in a sieve-tray tower.
Carbon disulfide, CS2, used as a solvent in a chemical plant, is evaporated from the product in a dryer into an inert gas (essentially N2) in order to avoid an explosion hazard. The CS2-N2 mixture is to be scrubbed with an absorbent hydrocarbon oil. The gas will flow at the rate of 0.4 m3/s at 297 K and 1 atm. The partial pressure of CS2 in the original gas is 50 mm Hg, and the CS2 concentration in the outlet gas is not to exceed 0.5%. The oil enters the absorber essentially pure at a rate 1.5 times the minimum, and solutions of oil and CS2 follow RaoultÕs law. Design a sieve-tray tower for this process. Design for a gas velocity which is 70% of the flooding velocity. Assuming isothermal operation, determine:
a) Liquid flow rate, kg/s



Data (at 297 K):
Oil average molecular weight = 254 Oil viscosity = 4 cP
Oil density = 810 kg/m3 Surface tension = 0.030 N/m
Foaming factor = 0.9 CS2 vapor pressure = 346 mm Hg

Solution
Initial estimates
b) Tower diameter and plate spacing

Solution
Plate design conditions:
f = 0.7 4.5-mm holes in triangular pitch; 12 mm between centers
Weir height = 50 mm; Plate thickness = 2 mm

From the Mathcad program in Appendix E:
D = 0.705 m; t = 0.5 m; Pressure drop = 414 Pa/tray
Fro = 0.712 E = 0.018 EMGE = 0.588
d) Number of real trays required

Solution
Approximation, assuming straight equilibrium and operating lines
Graphical solution: Generate operating line in xy diagram
Graphically, the number of ideal stages is also slightly over 5
Use 10 trays
e) Total gas-pressure drop.

Solution