Joule-Thomson Effect (Part 2) – my hypothesis

In a previous video (here), I stated my belief that a better understanding of thermodynamics is available by identifying the connections between the micro-world of moving and interacting atoms and the macro-world of classical thermodynamics. My goal is to do just this. My starting point? The Joule-Thomson effect, which is the temperature change that occurs in a gas stream as it is slowly depressurized. In this post I share my hypothesis as to what I believe is happening at the physical level to cause this effect.

Back in the mid-1800s, James Joule discovered that the temperature of a gas changes upon depressurization through a porous plug. At room temperature, most but not all gases cool down. Hydrogen and helium heat up.

Richard Feynman is my guiding light in trying to figure out the physical cause of this effect.

So let’s look at this. What happens as atoms approach each other? Well, at a large distance, nothing. They really don’t “see” each other since the forces of attraction and repulsion are insignificant. The motion is thus “free” and the gas can be modeled as an “ideal gas” with no intermolecular interactions.

As the atoms come closer toward each other, the attractive interaction becomes significant. This interaction happens when the electrons of one atom are attracted to the protons of the other. The atoms accelerate and their speeds increase.

At a certain point, closer still, the electrons of the two atoms repel each other and the interaction switches from attraction to strong repulsion. The atoms decelerate and their speeds decrease.

Since temperature is related to the average speed of atoms and molecules, let’s take a closer look at how these interactions affect the speed of atoms and thus the temperature of the gas as a whole..

Generally speaking, relative to “free motion”, when the attraction interaction is significant, atoms will be moving at higher speeds, and when the repulsion interaction is significant, atoms will moving at slower speeds.

Gas temperature is related to the time-averaged kinetic energy of the atoms and thus depends on the relative amount of time the atoms spend in each of these categories. At low pressure when large distances separate atoms, the interactions are insignificant and “free motion” dominates. At high pressure when small distances separate the atoms, the interactions are significant. So whether heating or cooling occurs during Joule-Thomson expansion from high to low pressure depends on which interaction dominates at high pressure, attraction or repulsion. Per below, attraction dominance leads to cooling, while repulsion dominance leads to heating.

So there’s my hypothesis. Now it’s time to test it. A small group of us is working to employ molecular dynamics simulation to model the above scenarios and, in so doing, uncover why some gases cool while others heat, and also why an inversion point exists. Stay tuned!

Published by Robert T Hanlon

I earned my Sc.D. in chemical engineering from the Massachusetts Institute of Technology and subsequently conducted post-doctoral research at Karlsruhe University in Germany. My professional career took me to Mobil Oil Research & Development Corporation, the Rohm and Haas Company, and then back to MIT where I am currently involved with their School of Chemical Engineering Practice.

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