API 521 - Supercritical Fluid 1

[Tweec]

API 521 Section 3.15.2.1.2 states that discharge areas for pressure relief on vessels containing "super-critical fluids, gases, or vapors" can be estimated using the equation provided. A) What is meant by super-critical? B) What sizing equation is used when the fluid is NOT super-critical? 3) Why doesn't the equation provided take into account any of the properties of the fluid? 4)The previous section states "the heat absorption equations listed in the following (?) are for..." Since there are no such equations in the following section, what is being referred to here?

 

[Tweec]

I have a 1997 copy of API 521.

 

[dcasto]

Super critle, a fluid whereby this pressure on it is over its criticle pressure. When a fluid is contained at a pressurre above it's criticle pressure, you can drop the temperature to (almost) absolute zero and the fluid wil not be a liquid.

In simple terms, its just a fancy way of saying a gas and keep people from saying the butane in the tank is a liquid be cause evryone knows butane is a liquid.....

 

[sailoday28]

dcasto (Chemical)What if the fluid is above Pc and specific volume,v, is << than v critical?

Regards

 

[dcasto]

doesn't matter its still a very dense gas, it will expand and get less dense with heat input, it has a comprssibility factor, the fluid still follows the gas laws as adjusted by z...

 

[sailoday28]

dcasto (Chemical)
Consider v<<vc and follow a constant spec vol process on a p-v diag as p decreases. Why won't liquid be the end point?
Regards

 

[dcasto]

As you drop the pressure, you drop below the critcle pressure and the fluid will be able to become a liquid or two phase liquid gas.

Here is my simple definition of a super criticle fluid. It has the molecular energy of gas, but it has physical properties of a liquid. So the molecules internal energy is very high and that is the worst case scenerio when trying to control the fluid.

The higher energy in gases is why we shy away from testing vessels with gases and use water instead.

 

[Tweec]

Thanks for the definitions, but I guess I am looking for a quantitative value (or a reference) to determine the critical pressure of a fluid.

Also, does anyone have any insights regarding the other questions?

 

[sailoday28]

Well, if you know Tc, and and equation of state, Pc will occur when for a fixed Tc, the first and second derivatives of pressure with respect to specific volume =0

Otherwise, I would try NIST

regards

 

[dcasto]

http://webbook.nist.gov/chemistry/