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Vapour Pressure


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#1 Guest_Guest_*

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Posted 16 March 2005 - 06:18 PM

A liquid will boil if saturated vapour pressure equals atmospheric pressure. However, if the system pressure is not atmospheric would it still boil if vapour pressure = system pressure ?

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Thnx!

#2 mbeychok

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Posted 16 March 2005 - 07:05 PM

Yes, if the liquid vapor pressure at any given temperature equals the system pressure, the liquid will boil.

#3 Art Montemayor

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Posted 16 March 2005 - 07:11 PM

Guest:

Your statement: “A liquid will boil if saturated vapour pressure equals atmospheric pressure” is not a correct statement because it is taken “out of context”. This is a common failing when strict and detailed definitions are not followed precisely as they were intended. This is typically a situation that causes engineering students a lot of grief.

The true statement would be: “A liquid will boil when its vapour pressure is such that it is equal to the system pressure in which it finds itself”. In other words, your definition only works when the system pressure is the atmosphere. My definition is the general definition that works for all cases – where the system pressure is the atmosphere or not.

Boiling involves a change of phase – the liquid molecules are converted to vapor molecules by the addition of the latent heat of vaporization of the parent liquid. If the system that the parent liquid finds itself in is a contained one – i.e., consider a conventional steam boiler – the liquid molecules start to vaporize, exerting their vapor pressure once the liquid achieves its corresponding boiling point. However, if the boiler is blocked-in – i.e., there is no vapor (“steam”) being withdrawn – then the vapor pressure inside the boiler continues to increase if additional heat input is continued. If the heat input is stopped upon reaching a very high pressure level (and the boiler remains blocked-in) the parent liquid water inside the boiler does not “boil”. It remains as water – except that it is categorized as “saturated water”, existing at its corresponding saturated pressure. It is, for practical purposes, in equilibrium with its saturated vapor. This condition is defined very clearly in your Mollier Diagram for steam, on the saturated vapor liquid line. I like the T-S diagram better, because the temperature is a horizontal “tie-line” that communicates the saturated liquid side of the “dome” with the saturated vapor side. If the boiler is maintained in an adiabatic condition (there is no heat loss or gain) while it is blocked-in, there is no “boiling” effect taking place inside. The saturated water level is static and the vapor molecules leaving the surface are equal to the vapor molecules returning to the liquid. This equilibrium can be achieved at any conventional level of pressure (below the Critical Point), and once you open the steam block valve, you will allow the boiler’s vapor pressure to seek a lower level (the effect of a driving force). When this action occurs you will witness “boiling” and, should you continue this while adding the required latent heat of vaporization through the burning of fuel inside the boiler, you will cause continuous boiling and production of steam. In this case the system pressure will be the pressure that you control at by throttling the outlet steam valve. And since the fluid inside the boiler is, for all essential purposes, pure water, then the system pressure is equal to the vapor pressure of water at the saturated temperature.

You can follow the conventional steam cycle through all its circuit on the Mollier Diagram and I highly recommend that you do this with dedicated concentration and study. If you succeed in doing this while identifying all the fluid Thermodynamic paths, you will find a similar, important Thermodynamic process in the mechanical refrigeration cycle using Ammonia – which is also rewarding and a key learning experience for any engineer-to-be.

I hope I explained the answer to your presumed simple question in a manner that allows you not only to understand the “boiling” of water, but the thermodynamic principles and their important applications.

As you can now appreciate, your simple question opens the door to the highly important and rewarding world of Thermodynamics – a subject that is very important to all Chemical Engineers. Welcome to that area of engineering where engineers are made – or broken.

Art Montemayor

#4 virat_chem05

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Posted 24 March 2005 - 01:44 AM

what is red vapour pressure?
what is it's significance?

#5 mbeychok

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Posted 24 March 2005 - 03:25 PM

Virot_chem05:
QUOTE
what is red vapour pressure?

I think you meant Reid Vapor Pressure (RVP), which a method of characterizing the vapor pressure of petroleum products. It is very widely used in the petroleum refining industry because it is a very simple and very quick test method.

However, RVP differs somewhat from the True Vapor Pressure (TVP). You can find methods of converting RVP to TVP in the USA's Environmental Protection Agency (EPA) publication "AP-42, Compilation of Air Pollutant Emissions". Chapter 7, Section 1 of that publication deals with vapor emissions from storage tanks and is available online at:

www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf

Scroll down to pages 43 and 44 of that section and you will find a nomograph for converting RVP to TVP. You will also find equations for doing the conversion (obtained by linear regression of the nomograph).

By the way, you should have started a new topic thread with your question rather than appending it to an existing topic.

#6 virat_chem05

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Posted 04 April 2005 - 06:24 PM

QUOTE (mbeychok @ Mar 24 2005, 03:25 PM)
Virot_chem05:
QUOTE
what is red vapour pressure?

I think you meant Reid Vapor Pressure (RVP), which a method of characterizing the vapor pressure of petroleum products. It is very widely used in the petroleum refining industry because it is a very simple and very quick test method.

However, RVP differs somewhat from the True Vapor Pressure (TVP). You can find methods of converting RVP to TVP in the USA's Environmental Protection Agency (EPA) publication "AP-42, Compilation of Air Pollutant Emissions". Chapter 7, Section 1 of that publication deals with vapor emissions from storage tanks and is available online at:

<a href=http://www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf>www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf</a>

Scroll down to pages 43 and 44 of that section and you will find a nomograph for converting RVP to TVP. You will also find equations for doing the conversion (obtained by linear regression of the nomograph).

By the way, you should have started a new topic thread with your question rather than appending it to an existing topic.

thanks, but i want to know exact understanding of reid vapour pressure and it's significance, unsure.gif

#7 gvdlans

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Posted 04 April 2005 - 08:24 PM

A simple Google search would do the job:

QUOTE
Vapour Pressure - a measure of liquid’s volatility. (The vapour pressure of water at 212°F is one atmosphere.) Under the ASTM Method D 323 (Reid vapour pressure), it is the absolute vapour pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will evaporate. Unlike distillation data, vapour pressure provides a single value that reflects the combined effect of the individual vapour pressure of the different petroleum fractions in accordance with their mole ratios. It is thus possible for two wholly different products to exhibit the same vapour pressure at the same temperature - provided the cumulative pressures exerted by the fractions are the same. A narrow-cut distillate, for example, may exhibit the same vapour pressure as that of a dumbbell blend, where the effect of heavy fractions is counterbalanced by that of the lighter ones. In conjunction with other volatility data. Reid vapour pressure plays a role in the prediction of gasoline performance.


(quote from http://www.cpieng.com/Reference.htm)

#8 mbeychok

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Posted 05 April 2005 - 06:28 AM

gvdlans:

The definition you found may be slightly misleading. I think it should be pointed out that the Reid Vapor Pressure (RVP) is not "the absolute vapour pressure exerted by a liquid at 100°F". It is only a measure of the absolute vapor pressure exerted by a liquid at 100 °F.

The RVP differs from the True Vapor Pressure of the sample due to some small amount of sample vaporization as well as the presence of water vapor and air in the confined space of the test chamber containing the sample liquid being tested.

#9 BehnamPro

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Posted 29 April 2005 - 07:22 PM

Hi,
liquids boils when they are in equilibriume with their vapor ,so you can use steam table particularly for water to find out the equilibruime pressure and temp.

#10 mbeychok

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Posted 30 April 2005 - 11:12 AM

BehnamPro:

QUOTE
liquids boils when they are in equilibriume with their vapor

Did you take the time to read all of the preceding messages in this thread before you decided to add your comment? You should have ... because your comment (quoted above) is completely wrong!!

A liquid boils when the vapor pressure of the liquid is equal to the system pressure. A liquid can be in equilibrium with its vapor without the liquid boiling. In other words, if the vapor pressure of a liquid is lower than the system pressure, the liquid will not boil even though the liquid is in equilibrium with its vapor ... at such a condition, the liquid is called a "sub-cooled liquid".

For example, water in a vessel at say 2 barg and at say 20 °C has a vapor pressure that is much lower than the system pressure of 2 barg. Therefore, it is a subcooled liquid and it will not boil ... even though the water is in equilibrium with its vapor. For that water to boil, the water temperature would have to be increased until the vapor pressure of the water became equal to 2 barg.




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