06/03/2007
KMÜ 346-22 MASS TRANSFER
HOMEWORK I
Instructor : Prof. Dr. Zümriye Aksu
Assistant : Özlem Tunç
Due Date: 12/03/2007
1. Water is held at a constant temperature of 20 oC in the horizontal 15 cm diameter and 3 cm depth tray. The total pressure is 0.9 atm and the medium is dry air. It is assumed that mass transfer coefficient is equal to mass transfer coefficient of molecular diffusion in 15 mm thick gas film above water. Calculate the necessary time for complete evaporation of water inside the tray.
2. Ammonia gas and nitrogen gas are diffusing in counterdiffusion through a straight glass tube 2.0 ft (0.610 m) long with an inside diameter of 0.080 ft (24.4 mm) at 298 K and 101.32 kPa. Both ends of the tube are connected to large mixed chambers at 101.32 kPa. The partial pressure of NH3 in one chamber is constant at 20.0 kPa and 6.666 kPa in the other chamber. The diffusivity at 298 K and 101.32 kPa is 2.30x10-5 m2/s.
a) Calculate the diffusion of NH3 in lb mol/h and kg mol/s.
b) Calculate the diffusion of N2.
c) Calculate the partial pressures at a point 1.0 ft (0.305 m) in the tube and plot pA,pB and P versus distance z. (P.6.2.2)
3. Equimolar counterdiffusion is occurring at steady state in a tube 0.11 m long containing N2 and CO gases at a total pressure of 1.0 atm abs. The partial pressure of N2 is 80 mmHg at one end and 10 mm at the other end. Predict the DAB by the method of Fuller at al.
a) Calculate the flux in kgmol/s.m2 at 298 K for N2.
b) Repeat at 473 K. Does the flux increase?
c) Repeat at 298 K but for a total pressure of 3.0 atm abs. the partial pressure of N2 remains at 80 and 10 mmHg, as in part (a). Does the flux change? (P.6.2.7)
4. Air is flowing inside a naphthalene pipe at a rate of 5 m/s. The pipe diameter is 0.1 meter. Calculate the mass transfer coefficient (ky) of naphthalene to the air.
Good Luck!..