Question

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?

Answer 1

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

Step-by-step explanation:

KNOWN:

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

FIND:

Molar diffusion flux.

ASSUMPTIONS:

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

ANALYSIS: 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

COMMENTS: The mass flux is:

n_A,x=M_a*N_A,x

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