A special test on the 6-in. distillation tower described in Prob. 8 indicates that the total pressure drop is 2.9 mm Hg when the gas rate is 50 lb/h. For this test, the condenser pressure was maintained at 100 mm Hg, the L/G ratio was the same as shown in Prob. 8, and temperature change was negligible. Operation can be considered to be under preloading conditions. Estimate the total pressure drop at the operating conditions indicated in Prob. 8 for the 12-in. column.

Prob. 8

A random-packed distillation tower with an inside diameter of 6 in. is being operated at a condenser pressure of 100 mm Hg.

The following data are obtained during operation:

Packed height = 3 ft

Gas rate = 100 lb/h

Liquid rate = 90 lb/h

Average molecular weight of gas = 100

Average temperature = 110°F

Total pressure drop = 12 mm

Hg Liquid viscosity = 1.2 centipoises

Liquid density = 55 lb/ft3

A larger distillation tower is needed for the same mixture, and it has been decided to use a 12-in. inside-diameter tower with the same type and size of packing. The packed height will be 3 ft. The condenser pressure will be 50 mm Hg, and the average temperature can be assumed to be 95°F. At this temperature, the liquid viscosity is 1.4 centipoises and the liquid density is 57 lb/ft3. The 12-in. tower will be operated at the same percent of the maximum allowable gas mass velocity as the 6-in. tower, and the same ratio of L/G will be used. The average molecular weight of the gas is unchanged. With the given packing and fluids, the pressure drop at the flooding point can be assumed to be 16 mm Hg/ft of packed height. Neglecting the effect of temperature on fluid densities at the flooding point, estimate the gas rate as pounds per hour at the indicated operating conditions in the 12-in. tower.