# Partial Pressure

**URL:** <https://boards.straightdope.com/t/partial-pressure/440>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [January 17, 2000, 12:59am UTC](https://boards.straightdope.com/t/partial-pressure/440 "2000-01-17T00:59:00Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![perkitl](https://avatars.discourse-cdn.com/v4/letter/p/9de053/32.png) [@perkitl](https://boards.straightdope.com/u/perkitl)\
**Post date:** [January 17, 2000, 12:59am UTC](https://boards.straightdope.com/t/partial-pressure/440/1 "2000-01-17T00:59:00Z")

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Can anyone explain in laymans terms what partial pressure is in relation to oil refining?

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**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\
**Post date:** [January 17, 2000, 1:37am UTC](https://boards.straightdope.com/t/partial-pressure/440/2 "2000-01-17T01:37:00Z")

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I don’t know about “in relation to oild refining”, but I’ll take a shot at it anyway.

Any gas exerts a pressure (on its container or the things immersed in it). For some calculations, it is convenient to pretend that the overall pressure is the sum of the “partial pressures” of the individual constituents of the gas. The partial pressure of each constituent is the total pressure times the fraction of the gas’s mass that is contributed by each constituent. If a gas is 75% nitrogen (by weight) and the pressure is 100 psi, then the partial pressure of teh nitorgen is 75 psi.

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jrf

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**Author:** ![Boris\_B](https://avatars.discourse-cdn.com/v4/letter/b/fbc32d/32.png) [@Boris\_B](https://boards.straightdope.com/u/Boris_B)\
**Post date:** [January 17, 2000, 5:34am UTC](https://boards.straightdope.com/t/partial-pressure/440/3 "2000-01-17T05:34:00Z")

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I don’t know exactly how partial pressure works with oil refining, but I’ll take a guess:  
In order to separate crude oil into its constituents, the stuff is heated up. Since all the constituents have different boiling points, and evaporate at different temperatures, you can assume various things about the pressures in your gas mixture. So at one temperature, most of the total pressure will be provided by pentane and hexane, and at a different temperature the pressures of those constituents will be much lower. So when you suck off gas from crude at a certain temperature, you’ll know pretty much what you’re getting.

That’s just a guess though

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**Author:** ![JillGat](https://avatars.discourse-cdn.com/v4/letter/j/9fc29f/32.png) [@JillGat](https://boards.straightdope.com/u/JillGat)\
**Post date:** [January 17, 2000, 7:09am UTC](https://boards.straightdope.com/t/partial-pressure/440/4 "2000-01-17T07:09:00Z")

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This appears to be another question not related to Cecil’s columns. Topics like this are best posted in the “General Questions” forum.  
Jill