[krukrupol]

Hello fellow chemical engineering students!

My question involves the evaporation of Heptane, which has a vapor pressure of 40 torr.

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Let's say that we have a bubble chamber of heptane with an inlet and an outlet.

Nitrogen enters the bottom of the heptane chamber and bubbles up to the top of the liquid.

At the top of the chamber is an outlet which connects to the atmosphere at 760torr (1 atm).

Let's assume the nitrogen is entering very slowly, such that the heptane vapor is at equilibrium with the heptane liquid as it leaves the chamber.

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My confusion arises when thinking about the partial pressures.

If I assume that everything is at atmospheric pressure, then that would mean that the chamber has 40torr of heptane and 720torr of nitrogen.

Before the nitrogen enters the heptane chamber though, I was also under the assumption that the pressure there is 760torr just from nitrogen.

Is 40torr of nitrogen gone when it enters the chamber, or am I thinking about this wrong?

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If the nitrogen is being forced at a constant volume flow rate (say 10 cm^3 / min), then maybe I am wrong to assume that this part of the line is at 760torr, since the nitrogen would have to be at slightly higher pressures to work its way out of the bottom of the heptane chamber.

Even so, if 10 (cm^3 / min) is the volume flow rate in, that has to equal the volume flow rate out, right? If 10^cm3 / min is the flow rate out, what is the composition of that vapor, is it going to be (40/760) mol % heptane?


Sorry for the long question, I hope someone can give me some insight or further reading on this. It seems like an straight forward problem 

[fledarmus]

It seems to me that if the nitrogen is entering the tank slowly enough that the heptane vapor is always in equilibrium with the heptane liquid, then you can safely assume that the vapor leaving the tank has the composition you stated - 720 torr nitrogen, 40 torr heptane, for a total of 760 torr. The nitrogen entering the tank is entering at 760 torr as well. You actually have slightly higher volume leaving the tank than entering. The difference in volume should be the difference between 40 torr of heptane as a liquid, and 40 torr of heptane as a gas, since the liquid is evaporating and the nitrogen is filling the space that it left.