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Vacuum Operation

edited January 2021 in Usage

Hello Everyone,

I am new to this forum. I have a couple of questions:

  1. Does anyone operate distillation @ vacuum condition?
  2. If yes, the agitator shaft provide enough sealing?

Thank you for any comment and shared experience!


  • You may want to start with this tag search, though I don't recall that the sealing of the agitator shaft ever came up in that context. Someone else may know the answer to that.

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  • edited January 2021

    It is highly unlikely a shaft vapor seal designed for atmospheric distillation will survive vacuum. Vacuum rated shaft seals are either incredibly expensive, or they use packing which requires regular replacement.

    Plus this all depends on the level of vacuum.

    Most vacuum distillation is done using magnetically coupled agitation. Also keep in mind that scorching is less likely under vacuum, so agitation may not always be required.

    The other issue is the vacuum rating of the kettle. Tanks must be designed for vacuum service generally.

  • Our mechanical seals are rated for 15 psi.

    Notwithstanding the occasional oversight when we are doing the mad scramble to get stuff out the door to the customer, we pump the tanks up to 1bar to pressure test before they leave the floor. They hold fine at 1bar.

    StillDragon North America - Your StillDragon® Distributor for North America

  • pez
    edited January 2021

    When you pull a vacuum the forces on the vessels change from a tensile hoop stress to a buckling load. If you have any creases that will concentrate the stress and initiate an early collapse the vessel. The damage is normally catastrophic.

    I would make sure to reinforce before pulling down.

  • edited January 2021

    Thank you all for answers. My project is from natural flavours extraction. I want to evaporate alcohol to concentrate flavours, hence I need to lower the temperature to 50°C. I would need approximately 4.7 psi (330 mbar) inside the vessel.

    @Smaug said: Our mechanical seals are rated for 15 psi.

    Notwithstanding the occasional oversight when we are doing the mad scramble to get stuff out the door to the customer, we pump the tanks up to 1bar to pressure test before they leave the floor. They hold fine at 1bar.

    Dear @Smaug,

    Do you think the boiler can withstand internal pressure of 4.7 psi (330 mbar)?

    Thank you

  • It will fail at that level of vacuum, and for straight evap you don’t need a still.

    Buy a commercial vacuum chamber and use a ventilated cap over the product vessel.

    You will concentrate the flavors, but lose overall alcohol and volume. Any water, etc will remain.

    Can you provide more info on your project?

    Have a look at AS1210 or the ASME standards for vacuum vessels.

  • edited January 2021

    Thank you @pez for your valuable comment.

    The project aims to extract flavours from some Amazon's pecial fruits and plants. Extraction is made by maceration in ethanol as solvent. Then we need to recover the solvent and concentrate the flavors, water, etc..

    Therefore, besides vacuum chamber, I also need a solvent collector and some cooling system. This is why I had initially thought about a still.

  • As you normalize pressure the vapor will recondense. Just put your sample in a container on a trivet, cover but allow for venting out the sides and pull the vacuum. Once it cools you’ll have ethanol in the bottom and solution in the container.

    Note that any compounds with a lower boiling temp than the ethanol will also be removed.

    A nice packed column could make you plenty of ethanol

  • @pez, Do you mean to recover the ethanol at the outlet of the vacuum pump?

  • edited January 2021

    You can build an extract rig out of tri-clamp pipe however, most tri-clamp spool is heavy enough, especially with heavy duty clamps, to be able to support the level of vacuum you are looking for (gasket leaks are actually the biggest problem).

    Keep in mind, if you are using ethanol to first extract, you can then remove the solvent (ethanol) in a second step, with a vacuum still that is a simpler design. A common design for this is a 12" x 12" tri-clamp spool as the kettle. This would give you about 4 gallons usable. Small external heating pad is all that's required under vac. An interesting design would be to use 1 or 2 12" sight-glass tees (column sections). The heavy flange and clamp would serve as a reinforcing ring around the body. Someone would obviously need to volunteer to test the glass (which should be fine).

    StillDragon condensers hold up fine in vacuum situations, I've regularly pulled StillDragon piping and condensers to 100 mbar very confidently. Pure ethanol is boiling around 80f at that level of vacuum. Keep in mind you need active condenser cooling to pull this low. We use low temperature chillers. I've gone lower but it becomes impossible to control as boiling point approaches ambient temp (bumping and foaming).

    Straight/pure ethanol extractions can be run in a semi-continuous fashion, which completely eliminates the need for huge boilers, and a huge rig. But depends on what you are extracting, and if your product is extracted with the solvent, or left behind in the boiler (recovery chamber).

  • edited January 2021

    thank you @grim, indeed I can use pipes to make the boiler and the whole rig. But I think StillDragon doesn't have 12" diameter components.

    I just need to extract ethanol, and everything else is left behind. This is why also a commercial vacuum chamber might be fine for me. I was wondering if by using a diaphragm vacuum pump (oil free) I can recover the ethanol at the outlet of the pump itself (at ambient temperature) thus eliminating the low temperature condenser between the boiler and the pump. Clearly in this case the vacuum pump must be able to have ethanol vapors passing through it.

    What do you think?

  • We do, you know.

    StillDragon Australia & New Zealand - Your StillDragon® Distributor for Australia & New Zealand

  • Dear All, I succesfully managed to extract ethanol under vacuum using a vacuum chamber, a vacuum pump and a condenser. Thank you all for the comments and advice provided!

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