Middle East Technical University
Department of Chemical Engineering
ChE 327 – Heat and Mass Transfer Operations
April 3, 2013
HOMEWORK VI
Due April 12, 2013
Question 1. Air at 27C and 1 atm pressure flows over a porous plate that is soaked in
ammonia. The molar concentration of ammonia in air, cA, is given by
c A 3 e  1.2 z
where cA is in kmol/m3 and z is the distance measured from the surface of the plate in meters.
Calculate the molar flux of ammonia from the surface.
Question 2. Component A diffuses through a composite wall of two different materials B &
C. The steady state concentration profile along the mass transfer direction is measured as
shown in the figure
B
C
CA1
CA2
J*A
a) Qualitatively draw flux of A versus position.
b) Comment on the interface discontinuity in the concentration. What else can you deduce
from the concentration profile ?
Question 3. Show that the following two forms of Fick’s law are equivalent:
n A   A (n A  n B )  D AB   A   A v  D AB   A
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N A  x A ( N A  N B )  cD AB  x A  c A v *  cD AB  x A
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Question 4. Predict the diffusivity of gas pairs CO2-N2 and N2-O2 at 1 Atm. and 273.2 K, and
compare with the experimental data at pp517 of Transport Phenomena by Bird, Stewart,
Lightfoot, 2002, John Wiley&Sons.
Question 5. 0.5 L of ethanol is poured into a cylindrical tank of 2 L capacity and the top is
quickly sealed. The total height of the cylinder is 1 m. Calculate the mass transfer coefficient
if the ethanol concentration in the air reaches 2% of the saturation value in 5 minutes. The
cylinder temperature is kept constant at 20C. Ethanol properties: PAsat = 43.6 mm Hg;
MA= 46 g/gmol; ρA = 789 kg/m3.
Question 6. One way of measuring the diffusion coefficients of vapors is to place a small
amount of liquid in a vertical capillary, generally known as Stefan diffusion tube, and blow a
gas stream of known composition across the top as shown in the figure. Show how one can
estimate the diffusion coefficient by observing the decrease in the gas-liquid interphase as a
function of time.
Cömertlikte ve yardım etmede akarsu gibi ol
Şevkat ve merhamette güneş gibi ol
Başkalarının kusurunu örtmede gece gibi ol
Hiddet ve asabiyette ölü gibi ol
Tevazu ve alçak gönüllülükte toprak gibi ol
Hoşgörürlülükte deniz gibi ol
YA OLDUĞUN GİBİ GÖRÜN YA GÖRÜNDÜĞÜN GİBİ OL
Mevlana Celaleddin Rumi