Vapor Temperature in Process Vessel
Vapor Temperature in Process Vessel
(OP)
Can one assume that any vapor in a process vessel is the same temperature as the liquid in the process vessel? For example, if I had water at 90C would the vapor in the headspace also be at 90C or would it be at 100C (boiling point of water)?
Thanks.
Thanks.





RE: Vapor Temperature in Process Vessel
RE: Vapor Temperature in Process Vessel
Thanks.
RE: Vapor Temperature in Process Vessel
It is all a matter of semantics. A vapor is a gas (as quark says) that can be condensed. Of course there are vapors in equilibrium with the liquid. At 90oC the vapor pressure of water is 70.1 kPa. Meaning that the gas contains 70.1/101.3 = 69% mol water vapor, under atmospheric pressure (101.3 kPa).
RE: Vapor Temperature in Process Vessel
At the liquid/vapor interface, the temperature will be 90o C due to the overwhelming influence of the liquid inventory.
Near the process vessel wall the temperature will be influenced by many factors, like:
Is the process vessel insulated or bare?
Is that area of the process vessel wall jacket cooled?
Is that area of the process vessel vapor space well mixed or poorly mixed by intentional and unintentional means?
One must think like a molecule to know what really happens in the process!
Good luck,
Latexman
RE: Vapor Temperature in Process Vessel
The vapor pressure of the styrene monomer at 90oC is about 150 mm Hg. Depending on its mol fraction in the liquid, it would -no doubt- appear also in the gas phase.
As Latexman says:
RE: Vapor Temperature in Process Vessel
For all practical purposes, unless your underlying design question is quite unusual, the temperature in the vapor space can be assumed to be the same everywhere in the vessel and is the same as the liquid temperature.
The fact that there is vapor present even at temp < BP explains the fact that a glass of water when left on your bedside table long enough will be empty at some point in time, even if nobody drinks it or knocks it over.
RE: Vapor Temperature in Process Vessel
and the liquid and the vapor are in equilibrium in the first layer between liquid and vapor (energy and mass transfer).
RE: Vapor Temperature in Process Vessel
If non-condensible gases are present (e.g., air), the liquid will exert a partial pressure in the gas phase:
PP = sum(xi*VPi*gammai),
where PP=partial pressure of all condensibles, xi=component i liquid phase mole fraction, VPi=component i vapor pressure at liquid phase temperature, and gammai=component i activity coefficient in the liquid phase. The summation is taken over all components in the liquid phase.
The difference between the vessel pressure and PP will be made up by the non-condensible gas.
RE: Vapor Temperature in Process Vessel
A good example is the modern domestic kettle, which is made of plastic and which switches itself off when the water has boiled. If you half fill it, and then feel the outer walls as it heats up you will feel that the lower portion where the liquid water is gets hot, but the upper section where the water vapor and air are is cold. This is because the plastic shell of the kettle is a poor conductor of heat and little heat is transferred to the gas. But as soon as the water boils the heat transfer into the vapor section is rapid and the walls get hot - and more importantly heat is transferred to the sensor which turns the kettle off.
As always, the devil is in the details.
Katmar Software
Engineering & Risk Analysis Software
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