Calculating Dew Point of a Specific Gas in a Mixture
Calculating Dew Point of a Specific Gas in a Mixture
(OP)
I was trying to get a quick ballpark on what the cooling load and entering water temperature to a condenser, and quickly realized there is a lot of effort that would have to be put forth to calculate this load. 1500 CFM mixture at 95°F, made up of CO2, ethanol, ethyl acetate, air, and acetaldehyde. Acetaldehyde is the target gas that needs to be condensed. It makes up 0.5% of the mixture, and 90% of it needs to be condensed. With it being such a small percentage of the mixture, I would think an extremely low condenser temperature would be required, much lower than the boiling point of 68°F at 1 ATM. The process engineers will be doing these calcs and getting back to me, but I was trying to get a rough idea of cooling load and temp before then, and was curious to know the correct way to come up with these numbers. Thanks.





RE: Calculating Dew Point of a Specific Gas in a Mixture
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Calculating Dew Point of a Specific Gas in a Mixture
My scrap work would be something like this:
Assume: no liquid phase, isothermal, well mixed
*Think about assumptions over coffee*
*Open thermo textbook*
Convert 7.5 CFM (0.5% of 1500 CFM) to mass flow at 95°F
Use heat of vaporization and mass flow to determine the minimum heat duty
*Use heat capacity to determine further cooling requirements*
Find out what your utilities are running at and approximate the heat transfer needs that David mentioned
RE: Calculating Dew Point of a Specific Gas in a Mixture
This is exhaust from a tank being vented to atmosphere, kept around atmospheric pressure. 0.5% of 14.7 PSI gives us a partial pressure of 0.0735 psi for acetaldehyde. The lowest vapor pressure I found on the substance was 2.7 psi at 0°F. If I followed the trend, it looked like I needed to get well below zero to start condensing.
Am I correct in this line of thinking?
RE: Calculating Dew Point of a Specific Gas in a Mixture
Mixtures condense over a range of temperatures, rather than at a single temperature, because the vapor composition progressively changes as the stream passes from the condenser inlet to the condenser outlet. The vapor's dew point at the inlet is relatively high, but it decreases as the mixture passes through the condenser. The extent to which it decreases depends on the mixture composition. The presence of even a very small amount (1%-2%) of "noncondensable gas" (e.g. air, CO2) has a disproportionately high impact. You need to run a process simulation (e.g. Aspen), using a proven physical property set, to estimate the necessary coolant temperature. Without that, you're just shooting in the dark, unless you can reference an existing condenser that is condensing a very similar stream.
RE: Calculating Dew Point of a Specific Gas in a Mixture
RE: Calculating Dew Point of a Specific Gas in a Mixture
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating Dew Point of a Specific Gas in a Mixture
nowadays many different models, say, for example, EOSs with complex mixing rules which surely can do the work, are available in simulators and even in tools as Excel (i.e. prode),
but an experienced engineer is required to tune the model in order to obtain accurate results...
RE: Calculating Dew Point of a Specific Gas in a Mixture
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating Dew Point of a Specific Gas in a Mixture
RE: Calculating Dew Point of a Specific Gas in a Mixture
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating Dew Point of a Specific Gas in a Mixture
anyway we know little about the problem (no detailed information given) and I am afraid there is no way to suggest a "correct way to come up with these numbers" as requested by ME27272727
RE: Calculating Dew Point of a Specific Gas in a Mixture
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculating Dew Point of a Specific Gas in a Mixture
A non ideal mix like this is a hornets nest when it comes to getting reliable thermo physical data - there could be all kinds of wierd azeotropic phases - this is most likely a specialist field many seasoned general skills process engineers will fear to take responsibility for.