What is Partial Pressure?

14 Sep 2019 at 19:55 // under Simple Saturdays

In the first of my simple Saturday posts, I will cover the term partial pressure. This term is critical to much of chemical engineering and I was very familiar with it during my time at university. But once I graduated and worked in a sugar factory, I didn't use it at all and had just about forgot about it until I was caught by an inspection engineer. I had only been working in the refinery a short period when I came out of a meeting room and the question was sprang on me:

Alistair... What is Partial Pressure?

That was it. Completely out of the blue, no warning or warm up. My brain took a couple of seconds to remember that I was in fact aware of the term, and another few seconds to remember what it meant.

Enough waffling, what is it?

From Wikipedia:

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature.

Make sense? Ok, I will try again.

Pressure is how much air (or anything) pushes on something else. Whenever you are in a normal room, the air will be pushing you on all sides with a pressure of 1 atmosphere. Air is made up of around 80% nitrogen and 20% oxygen¹. 

Air molecules are bouncing around the room. Each time they bounce off of a surface, they push on that surface and that is pressure. 80% of the time, the surface is hit by a nitrogen molecule and 20% of the time it is an oxygen molecule.

So total pressure is 1 atmosphere, 80% of that pressure or 0.8 atmospheres is as a result of the nitrogen. So the nitrogen partial pressure is 0.8 atmospheres.

If I have a reactor sitting with 75% hydrogen and sitting at 20 bara², the hydrogen partial pressure will be 75% of 20 bara or 15 bara. That is, 15 bara is produced by the hydrogen, and 5 bara must be produced by some other gasses.

That’s nice. Why does this matter?

Partial pressure is a bit like a concentration. In liquids, the concentration of a chemical determines the rate of any reactions. The more concentrated the chemical is, the faster the reaction is because there are more molecules available to take part in the reaction.

It is the same when looking at gasses. The more molecules that are available to react, the faster reactions occur. If the partial pressure goes up, either because the fraction of the total gas that is reactive increases or because the total pressure increases, the rate of reaction increases.

This was why the inspection engineer was interested. He knew that there was a problem with the metal used in the pipes and reactors if they see a high hydrogen partial pressure while still at a low temperature.

Final thoughts

Partial pressure is one of those teams that is quite simple but unless you know, it would be difficult to guess (correctly). I have taken some liberties in some of my explanations in the hopes of keeping it simple.

If you have any comments or suggestions for improvement, please let me know by leaving a comment.

1. Ok, I know there are smaller levels of other gasses and I haven’t gotten the proportions quite right, but let's keep things simple ↩
2. Note that this needs to be absolute pressure, not gauge ↩

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