What Is the Joule Thompson Effect and How Is It Calculated for Different Gases?

[Snoop06]

Hey all, I'm not sure this is a homework problem, more a problem I'm having with equations and this effect.

Experimentally we measured dT and dp, I plotted them, then determined the Joule Thompson coefficients for 3 gases (He, CO2, N2). These values came out to be

μJT(CO2) = .815 bar/K
μJT(N2) = .1319 bar/K
μJT(He) = -.0949 bar/K

which seemed reasonable enough to me.

Now, I need to calculate theoretical values using various equations of state. I began with van der Waals, and got

μJT = 1/Cp((2a/RT)-b)

I thought this would be simple, just looking up the Cp, and using known van der Waals coefficients, but my calculation for CO2 comes out to be something like -4 or so, so something is wrong, but I can't figure out what it is. Also, I could not find anywhere theoretical μJT values, which I am supposed to put in my lab report.

Thanks for any tips!

 

[Nate810]

It is possible that the equation you are using to calculate the Joule-Thomson coefficient is not applicable for the particular conditions of your experiment. The ideal gas law equation is used to calculate the Joule-Thomson coefficient for an ideal gas, but in real gases the coefficients can be different. You should look for literature on the Joule-Thomson coefficient specific to the type of gas you are working with and its conditions.

 

[baba89]

Hello,

The Joule Thompson effect is a thermodynamic phenomenon in which a gas experiences a change in temperature and pressure when it undergoes a throttling process. This effect is commonly observed in the expansion of gases through a small orifice, such as in a gas turbine or a refrigerator.

Based on your experimental results, it seems like you have successfully measured the Joule Thompson coefficients for three different gases. These values indicate the change in temperature per unit change in pressure for each gas. It is important to note that these coefficients can vary depending on the specific conditions of the experiment, such as the initial temperature and pressure of the gas.

Now, when it comes to calculating theoretical values using different equations of state, it is important to consider the assumptions and limitations of each equation. For the van der Waals equation, it assumes that gases behave as ideal gases, which may not always be the case. Additionally, the van der Waals coefficients (a and b) are specific to each gas and can vary depending on the temperature and pressure range.

To calculate the theoretical Joule Thompson coefficient using the van der Waals equation, you will need to use the values of a and b specific to each gas at the temperature and pressure of your experiment. These values can be found in tables or calculated using the van der Waals constants for each gas.

I am not sure where you are supposed to find the theoretical values for your lab report, but you can try looking for them in textbooks or scientific journals. You can also try calculating them using different equations of state, such as the Redlich-Kwong or the Peng-Robinson equations.

I hope this helps and good luck with your lab report!