# How Do You Calculate Pressure in Supercritical Water Syntheses?

• jman1990
In summary, the pressure in a partially filled closed vessel at a supercritical temperature can be calculated using the table in Chet's link.
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.

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?

Chestermiller said:
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.

jman1990 said:
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

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

jman1990 said:
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

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.

jman1990 said:
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

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.

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.

## 1. What is supercritical water pressure?

Supercritical water pressure refers to the state of water at a pressure and temperature above its critical point, where it exists as a single phase with properties of both a liquid and a gas.

## 2. What is the critical point of water?

The critical point of water is the temperature and pressure at which the liquid and gas phases of water become indistinguishable, resulting in a single supercritical phase.

## 3. What are the properties of supercritical water?

Supercritical water has unique properties, including its ability to dissolve both polar and non-polar substances, high density and low viscosity, and increased reactivity compared to regular water.

## 4. How is supercritical water pressure used in scientific research?

Supercritical water pressure is used in a variety of scientific research, including in chemical reactions, material synthesis, and waste treatment. It also has potential applications in renewable energy production.

## 5. What are the challenges of working with supercritical water pressure?

Working with supercritical water pressure can be challenging due to its high temperature and pressure, which requires specialized equipment and safety precautions. It also has a high reactivity, which can make it difficult to control in experiments.

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