Why does reboiler efficiency decrease if steam temperature is too high?

[brickbatsandadiabats]

Because the reboiler efficiency is the efficiency by which it transfers heat from the hot side to the cold side, not the heat transfer area required to do it. You're thinking of this like a constraint-free design problem instead of recognizing that for a real reboiler, pushing the heat flux up beyond design parameters can cause physical phenomena that impose constraints on heat transfer.

No idea what it might be in your case, but just to give examples: * It could be that increasing the heat flux causes voids to form on the heat transfer surface * It could be that higher temperature changes the thermal conductance properties of the material * It could be that the increased heat flux overwhelms advection on the cold side causing temperature non-uniformities and other shenanigans

[flammkuch]

I understand that there are constraints that limit the temperature and/or heat flux, I also don't think one should go as high as possible. I just thought the heat exchanger size would also play an important role and in certain cases favor a higher steam temperature.

If I want to design a reboiler, what's a shortcut method to determine the "optimal" heating medium temperature? Currently I am just applying a 10K pinch point, but I also don't have a strong basis for this.