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Topic: Pressure Calculation - closed system  (Read 13075 times)

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Offline Berkeley04

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Pressure Calculation - closed system
« on: February 25, 2009, 07:30:30 PM »
Hi all,

it's been a while since I've dug into my thermo book. Please help.

I'm heating a closed vessel (500mL) containing Ethanol to 120 - 180C.  The system is then purged with Nitrogen at atmospheric pressure.
I want to calculate the resulting pressure as a function of temperature.  Also, does this value change depending on head space.  For instance, if I fill the container with 450 mL or 250mL, is there a difference in resulting pressure?  Please help with this calculation. 

I have access to Perry's HB and Chem CAD. 

Thanks in advance.

Offline Borek

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Re: Pressure Calculation - closed system
« Reply #1 on: February 26, 2009, 02:39:01 AM »
The system is then purged with Nitrogen at atmospheric pressure.

How are you planing to introduce atmospheric pressure nitrogen to a closed vessel with increased pressure? And how are you going to purge closed vessel? Perhaps my English fails me, but it doesn't make sense to me.
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Offline Berkeley04

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Re: Pressure Calculation - closed system
« Reply #2 on: February 26, 2009, 12:46:32 PM »
Sorry for the confusion... I mixed up the nitrogen addition order.  Thats the first step. 

I add nitrogen to the system while leaving a bleeder valve open to remove any O2.  Therefore, the added nitrogen does not increase system pressure.  It remains 1 atm prior to heating.   

Then I close all valves and heat the steel container.   The container will also contain 10% water.  Instead of Ethanol, I'm going to be using Hexane. 

Thanks again.

Offline Borek

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Re: Pressure Calculation - closed system
« Reply #3 on: February 26, 2009, 06:25:38 PM »
Two possible cases.

One is that pressure is low enough so that container is heated above boiling point. If so, PV=nRT will do the trick as a first approximation, perhaps some more advanced equation (http://en.wikipedia.org/wiki/Van_der_Waals_equation) will be necessary as pressure will be probably high enough so that the gas can't be treated as ideal.

Second case is when the pressure gets so high, that boiling point is above container temperature. Situation gets more complicated then. It should be possible to solve using some combination of ideal gas equation, mass balance (liquid/vapor) and Clausius-Clapeyron equation, but that's only intuition, I can be missing something.
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