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3 years ago

5

A thin plastic membrane is used to separate helium from a gas stream. Under steady-state conditions the concentration of helium

in the membrane is known to be 0.02 and 0.005 kmol/m^3 at the inner and outer surfaces, respectively. If the member is 1mm thick and the binary diffusion coefficient of helium with respect to the plastic is 10^-9 m^2/s, what is the diffusive flux?

1 answer:

Juli2301 [7.4K]3 years ago

5 0

Answer:

N_A=1.5*10^-8 kmol/s.m^2

Explanation:

<u>KNOWN: </u>

Molar concentration of helium at the inner and outer surfaces of a plastic membrane. Diffusion coefficient and membrane thickness.

<u>FIND:</u>

Molar diffusion flux.

<u>ASSUMPTIONS:</u>

(1) Steady-state conditions, (2) One-dimensional diffusion in a plane wall, (3) Stationary medium, (4) Uniform C = C_A + C_B.

<u>ANALYSIS:</u> The molar flux may be obtained from

N_A=D_AB/L(C_A,1-C_A,2)

=10^-9 m^2/s/0.001 m(0.02-0.005)kmol/m^3

N_A=1.5*10^-8 kmol/s.m^2

<u>COMMENTS:</u> The mass flux is:

n_A,x=M_a*N_A,x

n_A,x=6*10^-8 kg/s m^2

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