I am not 100% sure that I understand you, because you have two different definitions of Fr, namely
Fr = m*Cp*(1-exp((-F*U*A)/(m*Cp)));
In any case, it seems that your problem is straightforward.
I suppose that you are trying to maximize eta. In that case, you should maximize Q and A and minimize I_t.
So minimizing I_t means taking I_t = 400.
Maximizing A means taking d = 15.
Q = A*Fr*(S - U (Ti - To));
Maybe I misunderstand something, but it doesn't seem to me that there are any non-monotone things here.
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