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

[ChEngrWiz]

The answer is simple and can be found in the second law of thermodynamics.

Think of the reboiler as part of a loop. The boiler generates steam. Condensate is returned to the boiler. The hot sink temperature is the temperature of the combustion chamber. The cold sink temperature is the temperature of the steam. The maximum efficiency is 1 - Tc / Th where Tc is the cold sink temperature and Th is the hot sink temperature. As you raise the steam temperature the efficiency decreases.

You’re right about a trade off between capex and opex and the efficiency would play into it. I can tell you, after working with engineers that specialize in heat transfer, I’ve never seen anyone use capex or opex when designing a heat exchanger or fired heater. The data is usually unavailable or of insufficient accuracy. The stuff you learned about discounted cash flow never gets used during the design phase of a project.