Enthalpy of components of Natural Gas

[Gerard Lardner]

I am trying to calculate the enthalpy of natural gas at a number of temperature/pressure states. I am working in Excel. I have put together a string of the components and their molar concentrations, but I am having problems.

Four components are not behaving as expected, even when I try to get their enthalpy as a sole component: 

• isoButane
• neopentane
• isopentane
• n-Pentane

The first three give a #VALUE error; the last, n-Pentane, gives a negative enthalpy. I have been careful to ensure the fluid names are as listed in the help file for List of fluids. For example, n-Pentane at 0.5 barg and 8°C:

=PropsSI("H", "P", 0.5*100000+101325, "T", 8+273.15, "n-Pentane")/1000 returns -64.59 kJ/kg, and

=PropsSI("H", "P", 0.5*100000+101325, "T", 8+273.15, "neoPentane")/1000 returns #VALUE.

The other dozen or so components are all returning sensible values, as expected.

What am I doing wrong?

Kind regards,

Gerard

[Ian Bell]

Do you also get sensible densities? I don't see an obvious reason why you would get invalid enthalpies if the densities are ok.

[Gerard Lardner]

Hi Ian,

Thank you; and good thought: I hadn’t tried for any other outputs. In changing the output to Density rather than Enthalpy, I noticed also that the capitalisation of some of the names in the List of Fluids was not exactly as I had used them; changing that detail fixed some of the issues – they now all calculate an output, both density and enthalpy. However, I am puzzled by negative enthalpies calculated for some fluids at 1 bara/25 °C and again at 1.5 bara/8 °C. The fluids that now display negative enthalpies in my list are Isopentane and n-Pentane. Do I need to set a reference state? For all the components of natural gas? (I am more familiar with using the IF97 back-end of CoolProp.) How do I set a reference state, say ASHRAE, in Excel? I can do it in SMath Studio, though it seems a change of reference state has to be declared separately for each component fluid.

One other question: I notice that, for hydrogen sulphide at 1 bara/25 °C, Coolprop gives a density of 1.404 kg/m^3; but it is given in Engineering Toolbox as 1.505 kg/m^3. Is there that much variation in different calculations?

 Many thanks for the help, and for getting me to look closer at what I am doing.

 Gerard

[Ian Bell]

Enthalpies are always relative to some reference state. I recommend not worrying about exact values of enthalpy. I never change reference states and recommend you do the same. The problem is especially messy for mixtures, and CoolProp does not allow reference states to be set for mixtures.

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