Solving diffusion type equation using Poisson Solver

Hi,

I am trying to solve a 3D diffusion type equation with periodic in X and Y and Neumann boundary condition  in Z direction using the MKL Poisson Solver and facing couple of problems.

First of all the BCTYPE if i use 'PPPPNN' and put  

bd_az[i + j * (nx+1)]= 0.0
bd_bz[i + j * (nx+1)]= 0.0

in the Z boundary, is it considering the Zero Neumann condition acurately at the boundaries?

and the other question is if i write the diffusion equation in terms of poisson equation then my RHS or the 'f' will be time dependent as du/dt. So, in that case will there be any conflict between the time scheme (u_old , u_new)?

Or is there any other way to use the MKL Poisson solver for the time dependent equations with the above mentioned boundary conditions ?

Please explain.

Thanks,

Swagnik