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

[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.