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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?

orthocresol
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Francis L.
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    I guess it is because people typically have vacuum lines in the labs, but not high pressure lines. – Ivan Neretin Jul 09 '19 at 15:00
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    @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. – Karl Jul 09 '19 at 20:09
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    @Karl Then I guess we were in different kinds of labs. – Ivan Neretin Jul 09 '19 at 21:27
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    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. – electronpusher Jul 09 '19 at 21:28
  • @IvanNeretin I think so! ;-) But what are you using them for? Anything except water chemistry ( or other high-boiling solvents) would be a lot of hassle with waste disposal, and slightly dangerous, wouldn't it? – Karl Jul 09 '19 at 21:42
  • Because regular glass parts structurally much stronger when low pressure in - high pressure out due to their cylindric/spherical shape, but much weaker structurally to hold high internal pressure. Pumping any glass cylinder can produce millions of small shrapners, especially if you cannot control the pressure inside. Vacuum pumping can be still dangerous, but the chances that you have more than 1 atm between the two sides of the glass wall is pretty low. – Greg Jul 10 '19 at 07:45
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    @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 ... – cbeleites unhappy with SX Jul 10 '19 at 09:47
  • ... no particular care about contaminated air (the appropriate system for this could directly be used as pre-vacuum for the detector evacuation pump). – cbeleites unhappy with SX Jul 10 '19 at 09:49
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    @electronpusher In some European universities these water venturis are banned already for multiple years, and were collected from both teaching as well as research labs, too. From outside the labs (law) because of their water consumption, as well as for the water pollution; from inside the lab since their power varies too much if multiple room on a floor engage them. – Buttonwood Jul 10 '19 at 18:53
  • @cbeleites Evacuating dewars or IR detectors needs a dedicated turbo pump, costing dozens of k€, that's not what we're talking about here. Agreed about a (20-50 mbar) vacuum line being potentially useful, but I've rarely seen a modern instrument using it. I'd be interested to hear about actual, current uses. The ordinary lab today has a portable membrane pump, which is used a few times a week, for filtration purposes. Schlenk lines, of course, but that's a different story again. – Karl Jul 10 '19 at 19:21
  • @Karl: the vast majority of house-wide pressurized air and vacuum systems I've seen were rather for mechanical (including stabilized tables for analytical instrumentation - but arguably that was optical instrumentation as well, just used to analyze chemical samples) than chemical purposes. And pressurized air orders of magnitude more common than vacuum lines. I've seen vacuum lifting pads in action in the chemical industry for lifting/handling bars of precious metals. My prediction (and dream as spectroscopist working with biological samples) for the not too distant future is, though, that.. – cbeleites unhappy with SX Jul 10 '19 at 19:58
  • ... the automatization technology that is getting smaller now like pick & place machines (they work with vacuum suction, not sure about pressure, though) will become available for us in the form of autosamplers for solid pieces just like pipetting robots are around already. I still guess they'll come with their own vacuum pump. And I totally agree that for most chemical labs, the membrane pumps are the better solution for filtration because you only need to worry what happens in your lab (i.e. what spill/contamination/gas gets into the pump). – cbeleites unhappy with SX Jul 10 '19 at 20:38

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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.

jeffB
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    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. – Karsten Jul 09 '19 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.

ChemEPhD
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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!

Community
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cbeleites unhappy with SX
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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/

DavePhD
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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.

foudian1
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