Ex. 36: In a lab experiment, the solute A is being absorbed from a
mixture with an insoluble gas in a falling film of water at 30°C and a
total pressure of 1.45 bar. The gas phase mtc at the given gas velocity
is estimated to be k.=90.3 m/sec. It is known that 13.6% of the total
mass transfer resistance lies in the gas phase. At a particular section
inside the tower, the mole fraction of the solute in the bulk gas is
0.065 and the interfacial concentration of the solute in the liquid is
x=0.00201. The equilibrium solubility of the gas in water at the given
temperature is p=3.318x10* x' (p is in mm Hg). Calculate:
The
(a) The absorption flux of the gas at the given section of the
apparatus (b) the bulk liquid concentration at that section of the
driving
apparatus (c) K (d) The individual and overall gas phase driving
forces in terms of Ap and Ay.
Answers
Answer:
Abstract
Falling films have broad applications in chemical engineering. We present a numerical study to investigate the effects of surface waves of falling films on the mass transfer from flowing gas phase into the liquid film. The model employs the volume of fluid method to explicitly track the gas-liquid phase interface and a one-fluid formulation to model the multiphase flow coupled with mass transfer between the two phases. The surface waves are generated by the time-varying injection rate of the falling film. The effects of surface wave frequencies and amplitudes on the overall mass transfer also are investigated for a counter-current gas-liquid flow of oxygen and water. Because of the solubility difference in the two phases, the model can capture the concentration discontinuity across the liquid-gas interface. Good agreement is obtained between the numerical results and experimental data reported in the literature. By systematically varying the frequencies and amplitudes of flow rate for the falling film, the surface waves generated on the film surface enhance mass transfer in general. The model prediction shows a proportional increase in mass transfer for small frequencies of surface waves and saturation at higher frequencies.