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# Energy Balance - Enthalpy Change of a Fluid at Changing Pressure

## Energy Balance - Enthalpy Change of a Fluid at Changing Pressure

(OP)
Hey guys I hope things are going well. I am a process engineer at a agricultural processing plant and have been doing some Mass and Energy Balance work around our soybean oil extraction plant and have some questions around some energy balance/enthalpy change calculations:

For example, I am trying to calculate the BTU/lb that is required to take liquid hexane from an initial state of 135 deg F and a pressure -0.3 IWC pressure to the final state of vapor at 142 deg F and a pressure of -20 inHG pressure. I have laid out my process paths as the following:

Hexane (liquid, -.5” H2O @ 135 °F) to Hexane (liquid, 1 atm, 69 °C) to Hexane (gas, 1 atm, 69 °C) to Hexane (gas, -20” Hg @ 99 °F) to Hexane (gas, -20” Hg @ 142 °F)

We know the latent heat of vaporization of Hexane at 1 atm and 69 deg C. And we know the saturation temperature of hexane at -20" HG using boiling point calculation from the antoine equation.

I am having trouble determining the right way to calculate the enthalpy change going from Hexane (gas, 1 atm, 69 °C) to Hexane (gas, 20” Hg @ 99 °F) because it is a changing pressure and temperature process. How would solve across this process path? Is there a way I can directly estimate the enthalpy change from the initial state to final state without the process paths? Thanks.

### RE: Energy Balance - Enthalpy Change of a Fluid at Changing Pressure

Hexane is one of the fluids in the Thermophysical Properties of Fluid Systems in the NIST Chemistry WebBook. See if that will help you.

dH/dP is usually very small. It is frequently neglected.

Good Luck,
Latexman

### RE: Energy Balance - Enthalpy Change of a Fluid at Changing Pressure

There is some tabular data for n-C6H12 on page 2-276, table 2-221 in the 8th edn of Perry Chem Engg Handbook also

### RE: Energy Balance - Enthalpy Change of a Fluid at Changing Pressure

H,
Agree with all above but you can use an EOS to calculate:
dh =cp*dT+[v-T*∂V/∂T)p]*dp
then integrate the equation above.

Pierre

edit: Using PR EOS:
1.01325 bar @69C ...z=0.9576, Ro= 3.2 kg/m3, v=2.689 e-02m3/mol, dv/dt = 8.853e-05 SI units
0.335974 bar @ 37.22C ... z= 0.9821, Ro= 1.14 kg/m3, v=7.5431e-02 m3/mol, dv/dt = 2.55 e-04 SI units
the integral part related to P change should be 280 J/mol and the integral part related to T should be (5100-9810) Joule/mol
[/b]

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