4 replies to this topic

[arunsneha]

Hello everyone,

What is the difference between VAPOUR and GAS?

 

Regards,

Arun Nair.

[latexman]

Tgas > Tcritical

 

Tcritical > Tvapor > Tboiling

[MrShorty]

The difference between vapor and gas is "fuzzy". Latexman's "definition" using the critical point is one way to draw the line between vapor and gas, and it is the "definition" I usually think of. However, this "definition" will not be consistently applied. For example, we often speak of propane "gas" (Tc ~100 C) and carbon dioxide "gas" (Tc ~30 C) when, if latexman's definition were strictly adhered to, we would normally call these "vapors" at room temperature.

 

Rhetorical thought question: under this definition, what does "liquified gas" mean?

 

Some people might call a gas something that would exist as a vapor/gas at room temperature and pressure (like propane, CO2, butane, nitrogen, etc.) -- normal boiling point < room temperature. Under this thinking, butane is a "gas" but pentane is a "vapor". But what is really different about butane and pentane that merits completely different terms?

 

Ultimately, the answer to your question is somewhat arbitrary. There just is not a property or quantity that delineates vapor from gas.

[NAP]

My way of looking at vapor vs. gas qualitative definition is to see if the component or mixture can exist in liquid phase in the process under consideration and in the range of pressure/temperature existing in the process

 

for ex.

 

LPG.

 

In a sat. gas plant, LPG is obtained from CDU overhead vapors by condensation. So, in the recovery process, LPG gas exists in equilibrium with LPG liquid i.e the gas is saturated at some point in the process...so I would call LPG gaseous state as vapor and not gas (although the definition of LPG and common usage by people is gas)

 

Hydrogen

In a hydrotreater or hydrocracker, hydrogen is used as recycle gas. But throughout the process, and the range of P-T conditions, H2 never comes in liquid state. So I would call hydrogen as gas and not vapor.

 

Classic example is LNG.

 

Natural gas liquefaction or vaporization process uses the term LNG vapor or methane vapor quite frequently (paprticularly when methane is being used as a refrigerant itself in the cascade process). As long as I am looking at the system which in this case is liquefaction process or vapourization process, where I see the gas phase and liquid phase can co-exist, I call that gaseous phase as vapor.

 

One exception to my logic is LNG boil-off gas. When LNG is stored as liquid, it countinuously generates gas which is called as boil-off-gas in engineering parlance. However, I still regard this gas as vapor phase when I am doing simualtions or PFD or H&MB. But thats just me.

 

I am sure there will be lot of other exceptions or deviations or objections to my interpretation of vapor vs gas. But everyone has different way of understanding and interpretation when it comes to qualitative definitions.

[sunny0]

The word vapor in its natural state is a solid or liquid at room temperature. However, a gas in its natural state at room temperature would still be a gas.

Example:
1) steam would be a vapor because at room temperature, it would be water, which is a liquid.
2) Nitrogen (a gas) at room temperature would still be in a gaseous state.

 

Gas is a state of matter while vapor is not.
A gas is a substance above its critical temperature but below its critical pressure, while a vapor is a substance above its boiling point temperature.

 

I hope every thing is clear for you now.

Edited by sunny0, 18 October 2013 - 03:53 AM.