5.1a. Overall tray efficiency
Consider a tray absorber with a constant Murphree efficiency EMGE = 0.75, and an average absorption factor A = 1.25.
a) Estimate the overall tray efficiency.
Solution
b) If the absorber requires 5.34 equilibrium stages, calculate the number of real trays.
Solution
Use 8 trays
5.3a. Ammonia stripping from a wastewater in a tray tower.
A tray tower providing six equilibrium stages is used for stripping ammonia from a wastewater stream by means of countercurrent air at 1 atm and 300 K. Calculate the concentration of ammonia in the exit water if the inlet liquid concentration is 0.1 mole % ammonia, the inlet air is free of ammonia, and 1.873 standard cubic meter of air are fed to the tower per kilogram of wastewater. The equilibrium data for this system, in this range of concentrations and 300 K, can be represented by yA,i= 1.414 xA,i (King, C. J., Separation Processes, McGraw-Hill, New York, NY, 1971).
Solution
Initial estimate
5.4a. Ammonia stripping from a wastewater in a tray tower.
The Murphree plate efficiency for the ammonia stripper of Problem 5.3 is constant at 0.581. Estimate the number of real trays required.
Solution
Use 9 trays
5.5b. Ammonia stripping from a wastewater in a tray tower.
If the air flow rate to the absorber of Problems 5.3 and 5.4 is reduced to 1.5 standard m3/kg of water, calculate concentration of ammonia in the exit water if 9 real trays are used and the Murphree efficiency remains constant at 58.1%.
Solution
Initial estimate
5.6b. Absorption of an air pollutant in a tray tower.
A heavy-oil stream at 320 K is used in an absorber to remove dilute quantities of pollutant A from an air stream. The heavy oil is then recycled back to the process where where A is stripped. The process is being run on a pilot plan basis, and information for scale-up is desired. The current absorber is a 16-plate sieve-tray column. Pilot plat data are as follows:
Liquid flow rate = 5.0 moles/hr
Gas flow rate = 2.5 moles/hr
yA,in = 0.04 yA,out = 0.0001 xA,in = 0.0001
Equilibrium for A is given as yA,i = 0.7xA,i. Find the overall column efficiency, and the Murphree plate efficiency. assume that the liquid and gas flow rates are roughly constant.
Solution
5.7b. Absorption of ammonia in a laboratory-scale tray tower.
An absorption column for laboratory use has been carefully constructed so that it has exactly 4 equilibrium stages, and is being used to measure equilibrium data. Water is used as the solvent to absorb ammonia from air. The system operates isothermally at 300 K and 1 atm. The inlet water is pure distilled water. The ratio of L/V = 1.2, inlet gas concentration is 0.01 mole fraction ammonia; the measured outlet gas concentration is 0.0027 mole fraction ammonia. Assuming that HenryÕs law applies, calculate the slope of the equilibrium line.
Solution
Initial estimate
5.8c,d. Absorption of ammonia in a seive-tray tower.
A process for making small amounts of hydrogen by cracking ammonia is being considered and residual, uncracked ammonia is to be removed from the resulting gas. The gas will consist of H2 and N2 in the molar ratio 3:1, containing 3% NH3 by volume, at a pressure of 2 bars and a temperature of 303 K.
There is available a sieve-tray tower, 0.75 m diameter, containing 6 cross-flow trays at 0.5 m tray spacing. The perforations are 4.75 mm in diameter, arranged in triangular pitch on 12.5 mm centers, punched in sheet metal 2 mm thick. The weir height is 40 mm. Assume isothermal scrubbing with pure water at 303 K. The water flow rate to be used should not exceed 50% of the maximum recommended for cross-flow sieve trays which is 0.015 m3/s-m of tower diameter (Treybal, 1980). The gas flow rate should not exceed 80% of the flooding value.
a) Estimate the gas flow rate that can be processed in the column under the circumstances described above.
b) Calculate the concentration of the gas leaving the absorber in part a).
Data:
Liquid density = 996 kg/m3 Surface tension = 0.068 N/m
Foaming factor = 1.0 Liquid diffusivity = 2.42 ´ 10Ð9 m2/s
Gas viscosity = 1.122 ´ 10Ð5 kg/m-s Gas diffusivity = 0.23 cm2/s
Slope of the equilibrium line, m = 0.85
Solution
a)
Run the sieve-tray design program of Appendix E with different values of gas flow rate until D = 0.75 m at f = 0.8. The results are:
mG = 0.657 kg/s Gas pressure drop = 615 Pa per tray
EMGE = 0.841 Fro = 1.042 E = 0.048
b)
Initial estimate