How Do You Calculate Pressure in Supercritical Water Syntheses?

[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.

 

[Chestermiller]

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?

 

[jman1990]

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?

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.

 

[Chestermiller]

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.

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

Chet

 

[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.

 

[Chestermiller]

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

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

 

[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.

 

[Chestermiller]

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.

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

 

[jman1990]

Thanks for the link.

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.

 

[Chestermiller]

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

 

[jman1990]

I see, I get what you're saying.

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