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Old Aug 16th 2019, 11:38 AM   #1
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Unhappy BWRS with negative accentric factor?

I have inherited our thermodynamic programs from a predecessor who has passed away. We have not had issues with them before but now we are.

One of our simulations has introduced a component with a negative accentric factor. Some components do have this, like H, He, Ne. We are now getting errors with our simulations because of this.

The BWRS equation of state uses the Pitzer accentric factor. Co takes the square root of this value. Obviously the program cannot take a square root of a negative number, so it throws an error.
I am trying to add a safeguard to the program that will continue the calculation even if the omega is negative. But i don't know what to do instead. I can't set it to 0 or 1, i don't think I can just change the negative value to positive.

Does anyone have any hints? I have searched the internet and cannot find any guidance about this.

Thank you,
SC KBR
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Old Aug 18th 2019, 12:38 PM   #2
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I have not heard of the BWRS equation of state before
so I had a quick look on <wikipedia>

I found that there are actually many alternative <equations of state>
Could you perhaps adopt one of the alternatives when the BWRS fails?

Also a negative accentric factor indicates something about the gas,
perhaps one of the simpler equations (<Van der Waals> or <Redlich-Kwong> equations) which do not include an accentric factor, can be used when the accentric factor falls below zero.
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Last edited by Woody; Aug 18th 2019 at 12:40 PM.
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Old Aug 19th 2019, 05:18 AM   #3
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Thanks, but...

Thank you for your reply. Unfortunately we are stuck with the BWRS equation of state for a variety of reasons. It is best suited for the cryogenic temperatures that the process is operating in, and we have over 20 years of using it for our process with successful results.

I just don't think that B, W, R, or S (ha ha) thought about the possibility of negative acentric factors, or like you, would just say "use something else."

To clarify one point, after further investigation, i found that *some* negative acentric factors can be used. Small negatives like that for H2 or He work in the equation. We have those components in our gas mixture. But if the omega gets below -0.25 or so, there is when it fails. Neon and Arsenic are so far the only two elements I can find that have omegas lower than that.

Arsenic is rare to see in a gas mixture. This is the first time we've come across this issue that I know of.

I'll have to keep looking....

Thank you again.
Regards,
SC
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