Consider the hydrogen-removal process described in Problem 14.46, but under conditions for which the mass diffusivity of the hydrogen gas (A) in the sheet material (B) is D_AB = 1.8 x 10^-11 m² /s (instead of 2.6 x 10^-8 m²/s). With the smaller value of D_AB' a uniform concentration may no longer be assumed to exist in the material during the removal process.

(a) If the sheet material is left exposed to the air stream for a very long time, what is the final content of hydrogen in the material (kg/m³)?

(b) Identify and evaluate the parameters that describe the transient mass diffusion process in the sheet.

(c) Determine the time required to reduce the hydrogen mass density at the center of the sheet to twice the limiting value calculated in part (a).

(d) Assuming a uniform concentration at any time during the removal process, calculate the time required to reach twice the limiting density calculated in part (a). Compare the result with that obtained from part (c), and explain the differences.