# Computation of maximum pressure in heated closed vessel

• Mattoo
In summary, Nidum is trying to create hydrocarbons by heating up rocks with organic matter in the presence of water. He wants to do this at low pressures in order to limit costs. He has looked into using a vacuum oven but it would be impractical and would not produce good results. He has found a pressure vessel that is already certified to meet Australian standards and he is looking for comments and suggestions on how to proceed.
Mattoo
I have a relatively simple design problem but my memories of thermodynamics are very rusty and I can't figure it out on my own.

To make it short, I want to put a mix of solid, water and air in a 500mL pressure vessel and heat it all up to 250'C for several weeks. T and P are at room conditions initially and the amount of water, air and solid is known. The solid part has a compressibility that is significant. Both its density and compressibility are known.
I need to know what will be the final pressure or at least a rough estimate with a safety margin in order to choose the right pressure vessel to use.
The pressure vessel will of course be equipped with a pressure relief valve but I need to keep the water inside the vessel the whole time.

I only find cases of heated vessels entirely filled with liquid. I guess the presence of air that is much more compressible than water will act as a buffer and I will get lower pressures than equivalent cases of water filled vessels (is that right?). The volume ratio water/air is not imposed and I can play with it to match a target pressure.

I've looked at steam tables, water phase diagrams, thermodynamics basics but I'm still stuck. Anyone can help or direct me to the right resources?

If the solid material is inert then all you really have is a pressure cooker .

With some water and some steam space in the vessel then pressure only depends on temperature .

Small amounts of air initially in the steam space generally don't make that much difference though you could estimate the effect if you wanted to .

So 250 C means a pressure of about 40 bar .

Even though your vessel is very small that pressure takes you well into the area of pressure vessels , dangerous equipment , design codes , regulations and inspection .

Not a project for someone with no technical knowledge . If you have good reasons to pursue this work then I suggest that you seek help from a suitably qualified professional engineer .

If you tell us what you actually want to do then we may be able to suggest safer and less problematic ways of doing it .

Last edited:
Hello Nidum,

I work in facilities with high pressure equipments, hence there is already an HSE framework with safety procedures, insurances, etc... I cannot use the pressure vessels we have here because it would exceed their temperature range.
It is very true I have no technical knowledge. I intend to get a pressure vessel that is already certified to meet the Australian standard for pressure vessels. The vessel will be in an oven that has a fume evacuation system in case the pressure relief valve opens.

As for my project, I want to reproduce the concept of hydrous pyrolysis but with pressures as low as possible to limit the cost (some hypy experiments go above 1000bar). the concept is to heat up a rock sample with organic matter in the presence of water to reproduce the maturation process that yields hydrocarbons in natural rock formations. The process will produce gas in small amounts (negligible for my samples compared to the water vapor). Having the sample under pressure isn't of paramount importance. What matters most is to keep the water in the reaction chamber and limit the exposure to oxygen from the air. (The initial oxygen in the vessel is not ideal but fine if air isn't regularly renewed).

I thought of using a vacuum oven but this would force me to open the oven regularly and add more water every now and then. That would give very poor results. Hence I can't see any other solution than a pressure vessel. Moreover I already have the right oven to do the job.

Comments and suggestions are very welcome of course.

## 1. How is maximum pressure in a heated closed vessel calculated?

The maximum pressure in a heated closed vessel is calculated using the ideal gas law, which states that pressure is directly proportional to temperature and number of moles of gas, and inversely proportional to the volume of the vessel. The equation for this is P = (nRT)/V, where P is pressure, n is the number of moles, R is the gas constant, T is temperature, and V is volume.

## 2. What factors affect the computation of maximum pressure in a heated closed vessel?

Several factors can affect the computation of maximum pressure in a heated closed vessel. These include the amount and type of gas present in the vessel, the temperature of the gas, the volume of the vessel, and the presence of any other substances that may react with the gas.

## 3. How does the type of gas affect the computation of maximum pressure in a heated closed vessel?

The type of gas present in a heated closed vessel can affect the computation of maximum pressure in several ways. Different gases have different molecular weights, which can affect the number of moles present in the vessel. They also have different properties, such as specific heat capacity and thermal conductivity, which can impact their behavior when heated.

## 4. Can the computation of maximum pressure in a heated closed vessel be affected by external factors?

Yes, external factors such as changes in temperature or volume of the environment surrounding the vessel can impact the computation of maximum pressure. This is because these factors can affect the temperature and volume of the gas within the vessel, which are key components in the ideal gas law equation.

## 5. How is the computation of maximum pressure in a heated closed vessel used in practical applications?

The computation of maximum pressure in a heated closed vessel is important in various industries, such as chemical engineering and material science, where it can be used to determine the safety and stability of pressure vessels. It is also used in the design and development of engines, boilers, and other equipment that operate under high pressure and temperature conditions.

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