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Computation of maximum pressure in heated closed vessel

  1. Jun 1, 2017 #1
    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?
  2. jcsd
  3. Jun 2, 2017 #2


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    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: Jun 2, 2017
  4. Jun 5, 2017 #3
    Hello Nidum,

    Thank you for your answer.

    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 vaccuum 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.
  5. Jun 5, 2017 #4
    Comments and suggestions are very welcome of course.
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