While solving mass transfer problems using Fluent, is there a way to replace the scalar diffusivity of a substance with a diffusion tensor at least in 2D? How to go about in solving such a problem? Any suggestions?

Try multicomponent diffusivity option. Its calculation is based upon Stefan-Maxwell formulation.

Swarup

I can invoke the multicomponent diffusion in the material panel for the maxwell-stefan equation. But then, I think the equation uses scalar D as a tensor D. How do i input the coefficients of the tensor? Is it in the form of a UDF?