Supercritical Water Pressure

1. Oct 28, 2015

jman1990

Hello All,

I am doing supercritical water syntheses in a sealed reaction container. I am looking for a way to calculate the exact pressure inside the reaction vessel as a function of the fill level and the temperature. This is to make sure I stay within the safe limits of my reaction vessel.

2. Oct 28, 2015

Staff: Mentor

You are starting with a partially filled closed vessel at some temperature, and you are trying to find the pressure at a supercritical temperature, correct?

3. Oct 28, 2015

jman1990

That's correct. I have a 75 mL vessel that is partially filled with water (the actual fill level varies between 50%-70% depending on the reaction). I am bringing the vessel to 400 degrees C, which is just over the critical point.

4. Oct 28, 2015

Staff: Mentor

Is there air initially in the head space or just water vapor?

Chet

5. Oct 28, 2015

jman1990

Yes, the remaining head space is filled with air, the only water vapor present would be due to the vapor pressure at ambient conditions.

6. Oct 28, 2015

Staff: Mentor

At these conditions, I don't think that the air would add much to the final pressure, and we can estimate that using the ideal gas law. Are you familiar with the use of the Steam Tables?

Chet

7. Oct 28, 2015

jman1990

I am not.

I am just beginning to work under these conditions and I am the first in our lab to do so, so this is pretty new to all of us.

8. Oct 28, 2015

Staff: Mentor

Check this out, and then please get back with me:
che.sut.ac.ir/People%5CCourses%5C76%5CSteam%20Table.pdf

I'll tell you how to use the table to get what you want.

Chet

9. Oct 28, 2015

jman1990

Okay, I'm looking at the tables. I'm guessing I want the table B.4 (superheated water vapor)?

How can I use this information to back out the pressure of my system?

Thanks for the help.

10. Oct 28, 2015

Staff: Mentor

If you have 50% fill to start with, the specific volume of the gas at the final temperature will be 0.002 m3/kg, right? So, look up in the table 0.002 m3/kg in the superheated water vapor tables at 400 C? That's your pressure due to the water. (You may have to interpolate in the table).

Chet

11. Oct 28, 2015

jman1990

I see, I get what you're saying.

Thanks a lot for the helps, that's exactly what I need.