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Multi Component Flash Distilation


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#1

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Posted 06 October 2008 - 09:12 AM

Hello everyone, long time reader, first time poster. Here i have a problem designing a flash separating tank.

The vessel is part of a gas and condensate plant in the Glycol regeneration phase. The feed consists of hydrocabons, water, nitrogen, CO2 and TEGlycol. The TEG consists of 0.93 of the feeds overall phase mole fraction.
The feed comes in @ T=68 C. P=5750 kPa

However when i did the flash calculation my Bubble point T was higher than the Dew T, which is not reasonable. I dont know where i went wrong for i checked the calculations twice, although i have a concern.

For the component partial pressure calculation i used the antoine constants from Coulson & Richardson's Chemical Engineering design for the Physical Property Data Bank where it has for every component a (maximum & minimum antoine coefficient temperature).

However my question is this, i could not find the Antoine coefficients for every component for the particular temperature, so i used the numbers found even though many were outside the range. Should i find it all, if so could you point me to a reference. Or is there another method to do it, if it turn out to be the case for the mixed up Bubble and dew temperatures.

Your help would be greatly appreciated, thanks.

#2 MrShorty

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Posted 08 October 2008 - 10:09 AM

QUOTE
However my question is this, i could not find the Antoine coefficients for every component for the particular temperature, so i used the numbers found even though many were outside the range.
The Antoine equation is generally not able to represent the nuances of a vapor pressure curve above about 2 atm. For C3, C4, and many of the C5 compounds at 68 C, I would choose a different vapor pressure equation.

Compounds such as N2, CH4, CO2, C2's are above their critical point at 68 C, so any talk about "vapor pressure" is purely hypothetical. For CO2 and C2's, an extrapolation to 68 can probably done reasonably to a hypothetical vapor pressure. For the N2, CH4, though, 68 C is so far removed from the critical point that I expect it's real hard to even begin to talk about a vapor pressure.

I don't know for sure if this fully explains why the bubble/dew point temperatures are off, but it does answer your Antoine query.




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