4.11b. Pressure drop and approach to flooding in structured packing.
Repeat Example 4.3, but using Montz metal B1-200 structured packing (very similar to the one shown in Figure 4.3). For this packing, Fp = 22 ft2/ft3, a = 200 mÐ1, e = 0.979, Ch = 0.547, Cp = 0.355 (Seader and Henley, 1998).

Solution
Packed Column Design Program
This program calculates the diameter of a packed
column to satisfy a given pressure drop criterium,
and estimates the volumetric mass-transfer coefficients.
Enter data related to the gas and liquid streams
Enter liquid flow rate, mL, in kg/s
Enter gas flow rate, mG, in kg/s
Enter liquid density, in kg/m3
Enter gas density, kg/m3
Enter liquid viscosity, Pa-s
Enter gas viscosity, Pa-s
Enter temperature, T, in K
Enter total pressure, P, in Pa
Enter data related to the packing
Enter packing factor, Fp, in ft2/ft3
Enter specific area, a, m2/m3
Introduce a units conversion factor in Fp
Enter porosity, fraction
Enter loading constant, Ch
Enter pressure drop constant, Cp
Enter allowed pressure drop, in Pa/m
Calculate flow parameter, X
Calculate Y at flooding conditions
Calculate gas velocity at flooding, vGf
As a first estimate of the column diameter, D, design for 70% of flooding
Calculate gas volume flow rate, QG, in m3/s
Calculate liquid volume flow rate, QL, in m3/s
Calculate effective particle size, dp, in m
Iterate to find the tower diameter for the given pressure drop
Column diameter, in meters
Fractional approach to flooding
4.12b, d. Pressure drop in beds packed with second-generation random packings.
A packed tower is to be designed for the countercurrent contact of a benzene-nitrogen gas mixture with kerosene to wash out the benzene from the gas. The gas enters the tower at the rate of 1.5 m3/s, measured at 110 kPa and 298 K, containing 5 mole % benzene. Esentially, all the benzene is absorbed by the kerosene. The liquid enters the tower at the rate of 4.0 kg/s; the liquid density is 800 kg/m3, viscosity is 2.3 cP. The packing will be 50-mm metal Pall rings, and the tower diameter will be chosen to produce a gas-pressure drop of 400 Pa/m of irrigated packing.
(a) Calculate the tower diameter to be used, and the resulting fractional approach to flooding.
(b) Assume that, for the diameter chosen, the irrigated packed height will be 5 m and that 1 m of unirrigated packing will be placed over the liquid inlet to act as entrainment separator. The blower-motor combination to be used at the gas inlet will have an overall mechanical efficiency of 60%. Calculate the power required to blow the gas through the packing.

Solution
(a) Design for conditions at the bottom of the tower where the maximum flow of gas and liquid occur
Benzene entering with the gas:
Assuming that all of the benzene is absorbed:
From the Lucas method for mixtures of gases:
Using the Packed Column Design Program:

D = 0.913 m f = 0.825
(b) Calculate the pressure drop through the dry packing on top
From the Lucas method for mixtures of gases:
From equation (4-11):
(c) Estimate the volumetric mass-transfer coefficients for the gas and liquid phases. Assume that DL = 5.0 ´ 10Ð10 m2/s.
From the wilke-Lee equation, DG = 0.0885 cm2/s
From Table 4.2, CL = 1.192, CV = 0.410
Using the Packed Column Design Program:
kLah = 0.00675 sÐ1; kyah = 0.26 kmol/m3-s
4.13b, d. Pressure drop in beds packed with structured packings.
Redesign the packed bed of Problem 4.12, but using Montz metal B1-200 structured packing (very similar to the one shown in Figure 4.3). For this packing, Fp = 22 ft2/ft3, a = 200 mÐ1, e = 0.979, Ch = 0.547, Cp = 0.355, CL = 0.971, CV = 0.390 (Seader and Henley, 1998).

Solution
Using the Packed Column Design Program:
D = 0.85 m f = 0.859
kLah = 0.00861 sÐ1; kyah = 0.376 kmol/m3-s
4.14c, d. Air stripping of wastewater in a packed column.
A wastewater stream of 0.038 m3/s, containing 10 ppm (by weight) of benzene, is to be stripped with air in a packed column operating at 298 K and 2 atm to reduce the benzene concentration to 0.005 ppm. The packing specified is 50-mm plastic Pall rings. The air flow rate to be used is 5 times the minimum. Henry's law constant for benzene in water at this temperature is 0.6 kPa-m3/mole (Davis and Cornwell, 1998). Calculate the tower diameter if the gas-pressure drop is not to exceed 500 Pa/m of packed height. Estimate the corresponding mass-transfer coefficients. The diffusivity of benzene vapor in air at 298 K and 1 atm is 0.096 cm2/s; the diffusivity of liquid benzene in water at infinite dilution at 298 K is 1.02 ´ 10Ð5 cm2/s (Cussler, 1997).
Solution
Calculate m, the slope of the equilibrium curve:
For water at 298 K,
Calculate the minimum air flow rate:
Convert liquid concentrations from ppm to mole fractions
At these low concentrations the equilibrium and operating lines are straight, and
y2 (max) = y2* = mx2
Using the Packed Column Design Program:
D = 1.145 m f = 0.766
kLah = 0.032 sÐ1; kyah = 0.335 kmol/m3-s