Vapor Temperature in Process Vessel 2

[RJB32482]

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.

 

[quark]

At 90C water there should not be any vapor (but there will be gases) if the pressure is not below 1 atmosphere. Generally, both liquid and gas should be at saturation temperature with respect to the pressure in the system.

 

[RJB32482]

OK so you are saying only air will be the only gas in the system? Not even some water vapor? Why do some plants need to capture emissions from reactors even though the reaction temperature is less than the boiling point of the reactant. For example, a plant I studied needed styrene condensors when they venting a reactor a 90C during reaction (polystyrene production). But the boiling point of styrene is above 90C. Isn't there still some styrene in vapor form that needs to be condensed so it doesn't leave the reactor?

Thanks.

 

[25362]

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 90^oC 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).

 

[Latexman]

RJB32482,

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

 

[25362]

The vapor pressure of the styrene monomer at 90^oC 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:

One must think like a molecule...

 

[epoisses]

I'd rather think like an engineer...

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.

 

[Yuman]

I think, the temperature at the top of the vessel is lower with just a few degrees because the steam (he has taken an example of water) is not superheated by it stills humid and this humidity has an effect on the temperature comparing to temperature liquid which is more than that of the steam or gas.
and the liquid and the vapor are in equilibrium in the first layer between liquid and vapor (energy and mass transfer).

 

[UmeshMathur]

If there are no non-condensable gases, the vapor space will be at the temperature of the liquid and its pressure will correspond to vapor pressure of the liquid. It's composition can be found from a bubble point calculation of the liquid.

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.

 

[katmar]

There is no definitive yes/no answer to this question. Latexman listed the major specifics that have to be looked at in solving this heat transfer problem. Every tank will be a new problem to solve.

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

http://katmarsoftware.com