Hi, I'm trying to model a process by making mass and energy balances using the NDSolve. It reaches the maximum number of steps It is working when I put "-" sign in front of this [Rho]p*(1 - ec)*\[CapitalDelta]H*D[q[z, t], t] (in 3rd equation) in the NDSolve command. But it should be "+" sign. I attached the mathematica file. I would really appreciate some help. This is really important for me.