Question

A species A diffuses radially outwards from a sphere of radius ro. The following assumptions can be made. The mole fraction of species A at the surface of the sphere is XAO. Species A undergoes equimolar counter-diffusion with another species B. The diffusivity of A in B is denoted DAB. The total molar concentration of the system is c. The mole fraction of A at a radial distance of 10ro from the centre of the sphere is effectively zero. (a) Determine an expression for the molar flux of A at the surface of the sphere under these circumstances. Likewise determine an expression for the molar flow rate of A at the surface of the sphere. [12 marks] (b) Would one expect to see a large change in the molar flux of A if the distance at which the mole fraction had been considered to be effectively zero were located at 100ro from the centre of the sphere instead of 10ro from the centre? Explain your reasoning. [4 marks] (c) The situation described in (b) corresponds to a roughly tenfold increase in the length of the diffusion path. If one were to consider the case of 1-dimensional diffusion across a film rather than the case of radial diffusion from a sphere, how would a tenfold increase in the length of the diffusion path impact on the molar flux obtained in the 1-dimensional system? Hence comment on the differences between spherical radial diffusion and 1-dimensional diffusion in terms of the relative change in molar flux produced by a tenfold increase in the diffusion path.

Answer 1

(a) Molar flux of A at the surface of the sphere:Molar flux (NA) is defined as the number of moles of A that passes through a unit area per unit time. In radial flow, the molar flux of A is:NA = -DAB(∂CA/∂r) = -DAB(CA/rt)Where, rt = radius of the sphere and CA = concentration of A.Since the mole fraction of A at the surface of the sphere is XAO, then we can express the molar flow rate of A at the surface of the sphere as:NA0 = NA|rt=ro = -DAB(CAO/ro)(XAO/1 - XAO)(b) If the distance at which the mole fraction was considered to be effectively zero were located at 100ro from the centre of the sphere instead of 10ro from the centre, then there would be a large change in the molar flux of A.This is because the concentration gradient between the centre of the sphere and 100ro from the centre of the sphere would be much steeper than between the centre of the sphere and 10ro from the centre. Therefore, there would be a larger concentration gradient driving the diffusion of A, which would result in a larger molar flux of A.(c) If one considers the case of 1-dimensional diffusion across a film rather than the case of radial diffusion from a sphere, then a tenfold increase in the length of the diffusion path would result in a roughly tenfold decrease in the molar flux obtained in the 1-dimensional system. This is because the molar flux is directly proportional to the concentration gradient, and a tenfold increase in the length of the diffusion path would result in a tenfold decrease in the concentration gradient.In terms of the relative change in molar flux produced by a tenfold increase in the diffusion path, there is a greater relative change in molar flux produced by a tenfold increase in the diffusion path in the case of 1-dimensional diffusion across a film than in the case of radial diffusion from a sphere. This is because the concentration gradient is much steeper in the case of radial diffusion from a sphere, which means that the molar flux is less affected by a change in the length of the diffusion path.