Che425: Problem set #1



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Solution
(a) From form of Eq. (3-18),

(b)


Distance from Bulb 1, cm

yA

yX

cm/s

cm/s

0 (Bulb 1)

0.7500

0.2500

0.0066

0.7500

5

0.6125

0.3875

0.0081

0.6111

10

0.4750

0.5250

0.0104

0.4760

15

0.3375

0.6625

0.0147

0.3367

20 (Bulb 2)

0.2000

0.8000

0.0248

0.1996















(c) Because we have equimolar, countercurrent diffusion, the molar average velocity of the mixture is zero.


6.2 HCl gas diffuses through a film of air 0.1 in. thick at 20oC. The partial pressure of HCl on one side of the film is 0.08 atm and zero on the other. Estimate the rate of diffusion in mol HCl/scm2, if the total pressure is (a) 10 atm, (b) 1 atm, (c) 0.1 atm. The diffusivity of HCl in air at 20oC and 1 atm is 0.145 cm2/s.

(b) NH = 1.98 x 10-6 mol/s-cm2
(c) NH =3.82 x 10-5 mol/s-cm2

7. Water in an open disk exposed to dry air at 25oC vaporizes at a constant rate of 0.04 g/hcm2. It the water surface is at the wet-bulb temperature of 11.0oC, calculate the effective gas-film thickness (i.e., the thickness of a stagnant air film that would offer the same resistance to vapor diffusion as is actually encountered). Diffusivity of water vapor in air at 1 atm and 291 K is 0.24 cm2/s.

Note that the bulk flow has little effect here.


8. A polyisoprene membrane of 0.8 m thickness is used to separate methane from H2. Estimate the mass transfer fluxes using the following data:





Partial pressure, MPa

Solubility

Diffusivity




Membrane side 1

Membrane side 2

S, mol/m3-Pa

D, m2/s

Methane

2.5

0.05

1.14 x 10-4

8.0 x 10-11

Hydrogen

2.0

0.20

0.17 x 10-4

109 x 10-11



Solution ----------------------------------------------------------------------------------------------

Species

p, Pa

H, mol/m3-Pa

D, m2/s

Flux, N, mol/m2-s

Methane

2.45 x 106

1.14 x 10-4

8.0 x 10-11

0.028

Hydrogen

1.80 x 106

0.17 x 10-4

109 x 10-11

0.042



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