Volume diffusion in purification by sublimation

The amount sublimated of large organic molecules varies with the square of the diameter of the tube in which sublimation occurs. This implies that the velocity profile in the tube is nearly flat, consistent with volume diffusion, but not with laminar flow, Knudsen diffusion, or slip flow. However, molecular simulation calculations show that under the conditions used, the velocity profile is near parabolic when there is no deposition on the tube wall, inconsistent with volume diffusion, but in agreement with laminar flow. These calculations also show that deposition on the wall does result in velocity which is almost constant with radial position. The result is a laminar flow profile which is nearly flat, and hence a total flux proportional to the square of the tube diameter, which is observed both in these simulations and in physical experiments.