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I want to build a DIY vacuum filtration apparatus with a bike pump but it's very difficult to find one the could be easily converted into a vacuum pump. This prompted an idea to simply build a reverse version where above atmospheric pressure is forcing the liquid to flow through the filter. I notice pressure filtration is rarely mentioned in write-ups, unlike vacuum filtration. Is there a reason for this lack of popularity or a reason why such a DIY apparatus would likely not work?

Could it be that vacuum is necessary for drying the paste that forms at first because it enhances evaporation?

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    $\begingroup$ I guess it is because people typically have vacuum lines in the labs, but not high pressure lines. $\endgroup$ – Ivan Neretin Jul 9 at 15:00
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    $\begingroup$ @IvanNeretin Every lab I've seen has a 5 or 10 bar pressurised air supply, coming from a big compressor in the basement. No? A vacuum line needs a pump usually in the same room, which is high-maintenance. In my old institute, we had only a few rooms with vacuum lines, mostly for the inorganic first-year lab courses. $\endgroup$ – Karl Jul 9 at 20:09
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    $\begingroup$ @Karl Then I guess we were in different kinds of labs. $\endgroup$ – Ivan Neretin Jul 9 at 21:27
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    $\begingroup$ Not to answer your question, but perhaps to solve your problem: You can buy a plastic Venturi Pump online for less than $20; you attach a hose, screw it into your faucet, let the water flow, and air is sucked through the hose (via the Venturi Effect). I used one myself for at-home vacuum filtration, and it seemed to achieve quite low pressures. Just make sure you don't turn the water on so high that pressure builds up in your water pipes and they leak. $\endgroup$ – electronpusher Jul 9 at 21:28
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    $\begingroup$ @Karl: both vacuum or pressurized air may be used to drive tools. Vacuum in particular is good e.g. for lifting pads and fixation systems. In research labs while many applications may need "vacuum" the particular requirements may differ widely so that locally producing the "right" kind of underpressure is better: e.g. evacuating dewars or IR detectors needs extremely low pressure but almost no flow whereas filtration needs less underpressure but higher flow and the ability to deal with a contaminated gas stream. The mechanical vaccum tools need high flow, no extremely low pressure and ... $\endgroup$ – cbeleites Jul 10 at 9:47
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It's much easier to assemble a system that's safe under external pressure than one that's safe under internal pressure. Convex flasks and tubes resist external pressure better than internal pressure. If something leaks or breaks, you have air rushing into the system, rather than reactants and products blasting out. For that matter, if you've put things together with ground glass joints or rubber stoppers or corks, internal vacuum (external pressure) works to keep them together, while pressurizing the system's interior tends to force them apart.

Also note that a vacuum system will never be under more than one atmosphere of external pressure. With a pressurized system, you need a way to limit the pressure to safe levels, and you need to make sure you know what those levels are.

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    $\begingroup$ Also, it is easier to apply more sample to the filter if it is not under pressure, or wash it with some fresh solution. You do have to turn off the pump when you want to remove the filtrate. Flash chromatography does work with pressure on the column bed rather than vacuum applied to the outlet of the column. $\endgroup$ – Karsten Theis Jul 9 at 22:01
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Pressure filtration is commonly used in industrial applications outside of the lab. In this case vessels and process lines are usually made from metal or plastic and can be easily designed to operate at a given pressure. From an engineering standpoint, pressure filtration offers the ability to have a higher driving force and allows you to limit the required filter area. With vacuum filtration the driving force across the filter is limited to 1 atmosphere. Pressure filters can be safely designed to operate at much higher pressures.

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I'd like to expand on some points that have been mentioned in the comments by @IvanNeretin, @Karl and @electronpusher, but IMHO deserve some more attention:

  • As @electronpusher commented, aspirator (Venturi) pumps are cheap and easily available.

    They are also super robust: each pump is usually made from a single material, and a range of materials (brass, PP, glass) is available. This makes it easy to get one for pretty much any kind of solvent - no worries about pump membranes etc.. Also, no moving parts, no maintenance required other than making sure it doesn't get clogged.
    In addition, with good water supply, you can get quite close to vapor pressure (either of the water driving the pump or of your solvent/eluent/washing medium).
    Yes, the exhaust gas is released and gets mixed with the water. However, you can take care of both locally: collect the water for appropriate treatment and work under a hood that is properly equipped to deal with the substances in question.

  • In contrast, a house-wide vacuum system means that you really have to make sure that no exhaust gas enters that vacuum system where it may condense and/or mix with other exhaust gas or air from other outlets to form corrosive or dangerous (combustion!) mixtures.
    Any mistake here can be very costly and dangerous.
    (Side note: always make sure a vacuum outlet is meant for "chemical" use and not only for workshop/manufacturing/handling purposes such as pneumatic systems with lifting pads or fixation pads)

  • @jeffb and @KarstenTheis have already explained the safety and practical advantages of working with vacuum in lab settings rather than with positive pressure.
    In addition, pressurized air lines often deliver oiled air: it is never safe to assume pressurized air is "just" air.
    This is good for pressure tools (who otherwise need an extra oiler) and also cheaper to produce. There exist oil-free compressors and de-oiling systems that produce oil-free air for the whole house but it is usually more economic to go for oiled pressurized air in general and de-oil only the part of pressurized air that really needs to be oil-free and to the degree needed. Particularly if that pressurized air system is not exclusively for chemical lab use but also used to drive dampers for optical tables, pneumatic system, and all kinds of tools in the mechanical workshop.

    Bonus point: for this reason, be very careful what pressurized air you use for cleaning (not only in terms of chemistry, also optical parts). It's perfectly appropriate to buy expensive bottled "cleaning air" or have an oil-free compressor when there's a pressurized air outlet in the very same lab!

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Positive pressure filtration is extremely common, probably even more common than vacuum filtration.

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source: https://andyjconnelly.wordpress.com/2016/09/28/syringe-filters/

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I worked as a grad student in organic chemistry labs and both vaccuum lines and compressed air are very common.

The reason most filtering setups uses vaccuum (especially for gross extraction after recristallization) is that glassware does not tolerate pressure very well.

We do use air pressure to speed up purification with manually-packed silica gel chromatography columns because installing a vaccuum apparatus for each tube would be a major assle. But even then we use very little pressure as this can break the column. There is specialized glassware for this - it has a crude teflon pressure regulator with a rubber O-Ring (that has to be changed regularly as it dries).

Low-grade vaccuum setups are relatively easy to improvise - with an appropriate adaptor, you can use your home sink tap as a vaccuum source. This setup is often used for rotavapors in organic chemistry labs.

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