Spring 2018 CHE 101C: Mass Transfer Problem Set #10 1.Ammonia, NH3, in air is being absorbed into water within the enclosed tank shown in the figure below. The liquid and gas phases are both well mixed, and mass transfer occurs only at the exposed gas–liquid interface. The diameter of the cylindrical tank is 4 m, total liquid volume inside the tank is constant. The bulk gas pressure of ammonia is maintained at 0.02 atm, and the total gas pressure is constant at 1.0 atm. The system is isothermal at 20 oC. The inlet volumetric flow rate of water is 200 L/h and enters NH3free. You may assume that Henry’s law, pAi= HCAidescribes the equilibrium distribution of NH3between the gas and the liquid phases where H = 0.02 atm/(kgmol/m3). The film mass-transfer coefficients are kG= 1.25 kgmol/m2h atm and kL= 0.05 kgmol/m2h (kg mol/m3). a) Determine the overall mass transfer coefficient, KG. b) Determine the partial pressure of NH3at the gas–liquid interface, pAi. c) Determine WAthe total molar rate of ammonia transfer. d) Develop a material balance for NH3; then determine cALthe concentration of NH3in the outlet liquid stream. 2. A wetted-wall tower is used to ‘‘aerate’’ water using air at 2.0 atm total system pressure and 20oC. The molar composition of air is 21% O2, 78% N2, and 1% other gases. At 20oC, the Henry’s law constant for dissolution of oxygen in water is 40,100 atm, and the mass density of liquid water is 1000 kg=m3 a) What is the maximum mole fraction of oxygen that could be dissolved in the water? b) What is the maximum molar concentration of oxygen that could be dissolved in the water? c) If the total system pressure increases, will the dissolved oxygen concentration in the water (1) increase; (2) decrease; or (3) stay the same? *Note: The axial direction of this cylinder is vertical and radial direction is horizontal.
3. Consider the chemical vapor deposition (CVD) process for the manufacture of solid silicon thin films as illustrated above. A dilute mixture of 0.1 mol% silane, SiH4 (species A), in H2 gas (species B) enters the chamber and flows over a square Si wafer of 15 cm per side at a bulk velocity of 50 cm/s. At the surface of the Si wafer, the following reaction takes place: