I need to do an energy balance on a heat exchanger where an aziotropic mixture of 90% methanol and 10% water at 21°C is being heated to 261°C. The heat source is high pressure saturated steam at 31 bar and it is assumed that the steam leaves as condensate at the same pressure (31bar).(adsbygoogle = window.adsbygoogle || []).push({});

Calculating the steam required at the end is relatively straightforward using m = Q/ΔHv, as the steam merely goes from saturated vapour to saturated liquid.

However, calculating the actual heat required to raise the temperature of the methanol feed solution from 21°C to 261°C at 2.2 bar is where I get lost.

In this calculation we have to take into account the amount of energy gainedm*Cp*dTand the amount of energy required to vaporize the water m*ΔHv. Which isn't too hard, except I don't know which value of Cp to use. Normally I would use Cp = 4.18kJ/kg K and calculateQ = m*(4.18)*(261-21) + m*ΔHvbut this Cp value would only apply for water at 21°C, not steam at 261°C, which would have a different Cp.

The other option would be to useQ = m*∫CpdT + m*ΔHv, except I don't have an equation for Cp in terms of T.

I guess my question is:

What value(s) of Cp do I use when calculating the the total energy required to heat a mixture of 90% Methanol/10%water solution from 21°C to 261°C?

There is probably a very straightforward explanation for this and I would very much appreciate it if someone could please explain it to me.

Thank you for your time.

TheLorax

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# Energy Balance on heat exchanger

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