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Active Contributor

Join Date: Nov 2010
Posts: 10

Calculating Temperature at which 100ppm HF Gas Condenses

11/24/2010 8:11 PM

I am working on an application that requires measuring around 100 ppm HF gas and I need to be sure that the HF gas will not condense in the pipeline or inside the measuring instrument. Therefore I need to know how to calculate the temperature at which a 100ppm HF gas will condense. First if the balance gas is a reasonably dry SF6 (less than -30 oC dew point) and also if the balance is dry N2.

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Join Date: Apr 2010
Location: Western Australia
Posts: 92
#1

Re: Calculating Temperature at which 100ppm HF Gas Condenses

11/26/2010 11:44 AM

Ok, lets work this through from first principles.

The dew point is where the vapour pressure of the liquid HF equals the partial pressure in the gas mixture.

Say we have 100 ppm by mass of HF in dry N2. That gives us the mass fraction HF as .0001. By adjusting for the molecular weights of each gas, 20 and 28 respectively, we have a mole fraction HF of .00014. So the partial pressure is .00014 atmospheres, providing your stream is at atmospheric pressure.

Now we need to know the vapour pressure of HF. This can be found on the following graph:

Looking at that graph, our value of .00014 is off scale. We could extend the graph by various means, but we can take a quick estimate and say that the temperature will be around -100 C. I suspect that will be sufficiently accurate for your purposes?

In the case of SF6, the mole fraction will be higher, but the SF6 would begin to condense before the HF did, I think. I'll leave that one to you.

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Join Date: Nov 2010
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#2
In reply to #1

Re: Calculating Temperature at which 100ppm HF Gas Condenses

11/26/2010 12:08 PM

Many Thanks Morrie, That's what I need to assure me that it should never condense in the tubing or inside the instrument as these are in ambient air. I have also learned how to calculate it for other gases! Gharieb