5.21c,d. Absorption of ammonia in a random-packed tower.
It is desired to reduce the ammonia content of 0.05 m3/s of an ammonia-air mixture (300 K and 1 atm) from 5.0 to 0.04% by volume by water scrubbing. There is available a 0.3-m-diameter tower packed with 25-mm ceramic Raschig rings to a depth of 3.5 m. Is the tower satisfactory, and if so, what water rate should be used? At 300 K, ammonia-water solutions follow HenryÕs law up to 5 mole % ammonia in the liquid, with m = 1.414.
Solution
Calculate minimum water flow rate
Try
From Appendix D
Try
From Appendix D
Try
From Appendix D
Try
From Appendix D
Cubic spline interpolation
5.22b,d. Absorption of sulfur dioxide in a structured-packed tower.
A tower packed with metal Montz B1-300 structured packing is to be designed to absorb SO2 from air by scrubbing with water. The entering gas, at an SO2-free flow rate of 37.44 moles/m2-s of bed cross-sectional area, contains 20 mole % of SO2. Pure water enters at a flow rate of 1976 moles/m2-s of bed cross-sectional area. The exiting gas is to contain only 0.5 mole % SO2. Assume that neither air nor water will transfer between the phases and that the tower operates isothermally at 2 atm and 303 K. Equilibrium data for solubility of SO2 in water at 303 k and 1 atm have been fitted by least-squares to the equation (Seader and Henley, 1998):
a) Derive the following operating line equation for the absorber:
Solution
b) If the absorber is to process 1.0 m3/s (at 2 atm and 303 K) of the entering gas, calculate the water flow rate, the tower diameter, and the gas-pressure drop per unit of packing height at the bottom of the absorber.
Solution
From Ap. D, by trial-and-error:
c)
At the top of the column:
From Ap. D, by trial-and-error:
Average mass-transfer coefficients
Average height
Calculate NtG
Initial estimate