2 replies to this topic

[rohubhutia]

1. How does latent heat of vapourisation vary with the pressure of a binary mixture ?

[siretb]

Generally speaking, for one given component, the latent heat of vaporization decreases as temperature increases. Since increasing pressure will increase the boiling point, I'd expect a decrease of the latent heat.
Because it's a binary mixture, if you operate at a vapor fraction less than unity (voporization not complete), the composition will not be the same, because the VLE curve will not be that same at two different pressures.
You have
1) to calculate the VLE equilibrium to get the composition of what you vaporize.
2) fromr this data and the individual component vporization data, calculatae the enthalpy of vaporization of the mix.

Overall enthlpy of vaporization of one mole will decrease when you inccrease pressure.

[Art Montemayor]

rohubhutia:

siretb is correct in his reply and I also agree with his suspicions about the latent heat behavior of a binary mixture.

I always try to visualize the behavior of a fluid by imagining what its Mollier or T-S diagram would look like. Think about the conventional Mollier diagram for steam, for example. You'll recall that the shape of the "dome" is really what determines the latent heat at different saturated conditions. As you go up the ordinate (or vertical axis), you can see that the dome is getting smaller in width - until it reaches the critical point.

In fact, the outline of the dome by definition is precisely the conditions that define the latent heat condition: the curve on the left represents saturated liquid (& solid in some cases) while the section of the dome on the right represents the saturated vapor. And it is the length of the horizontal line (constant pressure) that links both sides of the dome that defines the latent heat of vaporization or sublimation. You will recall that the "lever" rule is used to calculate the amount of flash vapor resulting from a throttling calorimeter in steam thermo lab experiments. The enthalpy value of the saturated vapor less the enthalpy value of the saturated liquid is the Latent Heat.

So if you had a Mollier diagram for your mixture, you would probably have a similar "dome" and the latent heat would be shown as decreasing as the temperature (& pressure) increased -- until you got to the critical point, where there is no latent heat simply because there is no defined liquid or solid phase and this point and beyond. At temperatures and pressures above the critical, you are in the super-critical zone - sometimes described as "mush".

I hope this helps to visualize what is happening in your system.

Art Montemayor
Spring, TX