9 replies to this topic

[ramnathsharma]

dear Viewers,

Render your opinion(s) about this:

What is the effect of latent heat of vaporisation under vacuum?
Any thumb rule to calculate Latent heat of vaporisation under vacuum?

Thanks in advance
ramnathsharma

[latexman]

ramnathsharma,

Latent heat of vaporization usually increases as vacuum increases (pressure decreases). The Watson correlation can be used to estimate dHv as a function of temperature. You would need a vapor pressure relation to convert it to a function of pressure.

[engware]

Hi there:

For the saturated steam, it is always:

q = h gas - h liquid

My suggestion is to find any steam tables and look up the values and plug them in the above expression.

Thanks,

Gordan

[Nirav]

dear Viewers,

Render your opinion(s) about this:

What is the effect of latent heat of vaporisation under vacuum?
Any thumb rule to calculate Latent heat of vaporisation under vacuum?

Thanks in advance
ramnathsharma

The difference in enthalpy of vapor and liquid at saturation temperature is defined as LATENT HEAT.
Now, in general, when you decrease the pressure, enthalpy of both vapor and liquid decreases. However, the decrease in enthalpy of liquid is of larger magnitude than that of vapor. Therefore, the difference or the LATENT HEAT will increase as you decrease the pressure.

I have not come across any thumbrule as you required.

What kind of fluid you are talking about? If it is water-steam system, it can be found easily by steam table as already replied in previous post.
However, if you are dealing with mixures of hydrocarbons or other chemical, you need to find out the enthalpy of vapor and liquid at saturation temperature at the specified pressure. You can get LATENT HEAT directly from any of availalbe simulation software. (hysis, pro-II, aspen).

regards,

[ramnathsharma]

Dear viewers ,

Thanks for your replies.

We want specifically the effect of latent heat of vaporisation under vacuum for the product --

"BORON TRIFLUORIDE ETHERATE".

Any idea of how to get the data.

We have searched the internet to some extent, but of no use, probably someone could shed their views and opinions regarding this product.

Thanks in advance

ramnathsharma

dear Viewers,

Render your opinion(s) about this:

What is the effect of latent heat of vaporisation under vacuum?
Any thumb rule to calculate Latent heat of vaporisation under vacuum?

Thanks in advance
ramnathsharma

The difference in enthalpy of vapor and liquid at saturation temperature is defined as LATENT HEAT.
Now, in general, when you decrease the pressure, enthalpy of both vapor and liquid decreases. However, the decrease in enthalpy of liquid is of larger magnitude than that of vapor. Therefore, the difference or the LATENT HEAT will increase as you decrease the pressure.

I have not come across any thumbrule as you required.

What kind of fluid you are talking about? If it is water-steam system, it can be found easily by steam table as already replied in previous post.
However, if you are dealing with mixures of hydrocarbons or other chemical, you need to find out the enthalpy of vapor and liquid at saturation temperature at the specified pressure. You can get LATENT HEAT directly from any of availalbe simulation software. (hysis, pro-II, aspen).

regards,

[Nirav]

Did you try to search for this chemical in any of process simulation software? I could not locate this component on Hysis. If you can find it on any other simulation softwar, it will be easy to find latent heat.

But if you can not find, you can also try to contact the vendor of this chemical. They might have the data you are looking for. The vendor names can be located on internet. Search for MSDS of this chemical. You will find many MSDS based on vendors. So that you can get contact information of those vendors and you can probably get the data of this chemical.

Best of luck !!

[sgkim]

If no actual data available, the Clasius-and-Clapeyron equation would be a last resort.
Refer to the material in the following site to which the temperature dependence of the enthalpy can be referred:

http://www.chemistry...Lab1/intro.html


Stefano

[MrShorty]

I found an MSDS for boron trifluoride etherate (aka boron trifluoride ethyl etherate). From this and catalog entries (TCI sells the stuff, I'm guessing any of the other usual chemical suppliers would supply it as well), it appears that BF3 etherate is officially a complex between BF3 and diethyl ether. With this in mind, I have a few questions that may lead to an answer, but I don't have an answer.

Can we treat this complex as a pseudo-pure compound? I have done a little bit of work with salts. Some salts form complexes (hydrates) with water. These "compounds" have a vapor pressure that can be measured and even correlated, but the pressure is due almost entirely to water that is evolved from the hydrate. In this case, because enthalpy is a state function, the overall enthalpy of vaporization could be modelled as the enthalpy of vaporization for water plus an "enthalpy of association" to account for the heat needed to break up the hydrate.

There's also cases like NO2, where the compound self associates to form N2O4. In this case, you can't separate one species for the other, and so you talk about the heat of vaporization of an equilibrium mixture of the two.

In the case under consideration here, BF3 and diethyl ether certainly exist as pure compounds in their own right. So, I expect we could model the "heat of vaporization" for the complex like we might for a hydrate -- H(vap)tot=H(vap)ether+H(vap)BF3+H(assoc). But we still have the same problem -- what is H(assoc), and of course we would need to know exactly what species exist in the liquid and vapor phases.

With a complex like this, can we determine the overall H(vap) by the vapor pressure curve for the complex? What is the chemistry involved? Namely, does the compound remain associated in the vapor phase, or does it dissociate, or even some equilibrium mixture in the vapor phase? Perhaps the easiest thing to do, if you can't find previous measurements, would be to put the material in a proper calorimeter and measure the heat of vaporization at the conditions you're interested in.

This is an interesting question, I wish I understood it better so I could give you a good answer.

[niallmacdowell]

Hi,

I had a problem similar to this recently, I was predicting the heat of vapourisation at various pressures.

I had data on temperature and pressure values for vapour presssure at room temp, and the boiling point at 1atm. Thus I used the Clapeyron eqn to estimate the values of the constants A & B and then the same values to determine the boiling point at the relevant pressure.

I then used the Watson relation to estimate the boiling point. This method however requires that you know the critical temperature of your material...could be a problem if you are lacking material data....

There are lots of other eqns (see Perrys VI, 3-275) but they require information on the the critical temp & pressure and in some cases the acentric factor

You could also try using the Clausius Clapeyron eqn for the enthalpy of vapourisation, if you have info on how the compressibility factors change with phase change. I understand that these values can be calculated from the redlich-Kwong eqn of state, but again information on critical values is required.

If you have access to HYSYS or some such, you could predict the critical values....

Hope thats of some use..

[hollerg]

I did a quick search on google. This article may have a measured value for the enthalpy of association, if you have access to it.

http://pubs.acs.org/...f?sessid=6006l3

regards