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Topic: Calculate the pressure of a closed system  (Read 4756 times)

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Offline divomaster

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Calculate the pressure of a closed system
« on: November 05, 2016, 10:39:26 AM »
Hi guys I'm new on this forum, I'm a polytechnic student studying chemical engineering.
Please help me on this question.

I have a 10ml vial filled with 3ml of ethyl acetate, density=0.897kg/m^3 bp=77°C.
This vial is heated up in a microwave reactor at 300W. The vial is sealed so its a closed system.
The temperature of ethyl acetate is kept at 100°C, how do we determine the system pressure in the vial?

It seems that if I increase the volume of ethyl acetate used therefore decreasing the head space volume above the liquid surface, the pressure seems to drop. Therefore i suspected that the vapor pressure kept the liquid from vaporizing any further.

I tried with Antoine, Clausius Clapeyron and the ideal gas equation. However they don't seem to apply when i change the volume of ethyl acetate used in the microwave reactor.
I tried with the ideal gas equation, gave me 94 bars when i calculated it, so I assumed that the liquid has not vaporized fully yet.
I used Antoine and Clausius Clapeyron equation but they do not include the volume of liquid used in the vial.
The following are the data values of the pressure exerted shown on the data logger when I changed the volumes used to heat up ethyl acetate in the vial:

2ml used, I get 2.4 bars shown on screen.
3ml used, I get 0.8 bars shown on screen.
5ml used, I get 0.3 bars shown on screen.

All of these pressure values reached a plateau in the data logger so its stabilized at 100°C. This pressure is measured at the top of the vial when the seal starts expanding due to the increase in pressure. This question is to test whether the measure of pressure in the system is correct or not. I know there is no such thing as a perfect system due to energy losses and whatsoever in the environment but calculations should be able to yield a value close to the ones that i see on the data logger, otherwise if a correct calculation is stated and it deviates massively from the one shown on the data logger, I need to change the pressure reading device as soon as possible.

Thank You. Hope its an interesting question to solve. ;D ;D ;D
« Last Edit: November 05, 2016, 10:53:58 AM by divomaster »

Offline Borek

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Re: Calculate the pressure of a closed system
« Reply #1 on: November 05, 2016, 11:22:07 AM »
As long as there is liquid present (and from the looks of it that should be the situation in all three cases) the final pressure at the top of the vial should be identical.

It should be also different at the bottom, but I doubt you will be able to measure the differences, as they are quite minute.

The only thing that matters is the temperature inside of the vial.
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Offline silverz89

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Re: Calculate the pressure of a closed system
« Reply #2 on: January 10, 2017, 11:50:17 AM »
double check your dimensional analysis. From my math using ideal gas law it appears you are missing one of the divisions to convert from mL to moles of ethyl acetate by 1000..

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