aonomus
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Scrubbing gasses - solid vs liquid scrubbing effectiveness?
So I've been thinking about how to reduce organic or acidic/basic fumes from experiments running under reflux or distillation, and with the 2 gas
washing bottles I have (bottoms only, will use stopper + glass tube) I could scrub gasses coming out of a setup.
Now the question is, is solid phase or liquid (aqueous) phase more effective at scrubbing gasses?
For organics, as far as I know the best I can do is use activated carbon granules to absorb any solvents, etc. I don't know of any organics that will
absorb most other solvents and not evaporate itself or allow the captured solvents to evaporate off as well.
For acids (HX gasses), I could use powdered Na2CO3/NaHCO3, or, I could use aqueous base to neutralize any acid fumes coming off the reaction at the
added risk of having the apparatus suck liquid backwards during cooling. Additionally one could use a pH indicator to show when the solution had
become discharged (ie: quite acidic).
For bases (ammonia mainly), I could either use some sort of powdered acid (citric, ascorbic, tartaric acid?), or an aqueous solution of HCl or even
just vinegar.
From what I can see, a solid scrubbing material would give more resistance to flow, and have longer contact time, resulting in complete
neutralization. Aqueous solutions would have to be in a very tall tube with a fine dispersion adapter on the end, or the bubbles of gas could float
out before fully reacted.
One problem I can see is the solid scrubbing material could react and block further flow (eg: HCl gas + Na2CO3 --> NaCl crystals form + CO2 gas),
leading to a possibly dangerous pressure buildup.
What is everyones take on this, I do want to try to scrub any unneeded/byproduct gasses if I can, my lungs are valuable to me 
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not_important
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Problem with solids is that they may form a surface film that slows or blocks further reaction, meaning you have to use a much larger amount of them
than you might think.
The ideal is something like a fractionating column, with the absorbent liquid being pumped in at the top and flowing over packing/plates which the gas
stream enters at the bottom and exits from the top. Not much back pressure, suckback is not a problem. For aqueous scrubbers plastics such as PVC,
PE, or PP will work. The main concern is the formation of reaction products with low solubility, potentially meaning the use of dilute solutions and
high liquid flow rates.
A solution of Na2CO3 works well for acidic vapours, as you said, or for fumes of compounds that will form acids on reaction with water. For alkaline
fumes go with H2SO4, H3PO4, oxalic, and maleic acids all are reasonable strong and have low vapour pressures. Most organic acids are weak enough that
ammonia or amine salts tend to disassociate enough that leakage could be a problem; acetic and similar have high enough vapoure pressures at STP that
they would be fume sources in their own right.
High boiling organic liquids such as mineral oil will absorb most solvents, glycerol will absorb the more polar solvents although you want to scrub
acids out first or also include NaOH.
Note that a good cold trap will collect most stuff, dry ice can be OK and LN2 should take care of most items of concern.
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Vogelzang
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How about a thistle tube with a trap like the one shown here?
[Edited on 17-11-2009 by Vogelzang]
[Edited on 17-11-2009 by Vogelzang]
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watson.fawkes
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For an over-full dose of the topic of liquid scrubbing towers, see section 14 of Perry's 8th (P.'s Chemical Engineering Handbook), including the
subsection "Absorption with Chemical Reaction". A lot hinges on the relative speed of reaction vs. diffusion of the reaction products (be it a
chemical or physical process), such as dilutions and pumping speeds for a given input stream. You probably don't need all the information in here, but
a quick read-through (skipping many of the details) might be helpful. In addition, there are other bibliographic references that might be more
specifically useful.
One immediate comment, though, is that there's not a universal method for the problem. Physical methods (solubility, temperature, etc.) are generally
cheaper to run than chemical ones. (The solvent-into-solvent scrubber is of this class.) Scale of operation also matters. Industrially, physical
absorption towers are usually paired with stripping towers for solvent recovery. At the experimental bench scale this is not typically necessary,
unless you're using some expensive solvents.
To understand how solids might work in this application, you can take a look at pressure-swing adsorption systems. The most common example of these
are small medical oxygen concentrators. The problem with surface build-up that not_important noted is addressed in these systems by ping-ponging
between two packed cylinders, alternately pressurizing and depressurizing them. They're not a single-stage solution for waste gas processing, though,
since their result is to shift species between a pair of output streams.
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aonomus
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Indeed overdose, I'm talking about a 125mL gas washing bottle, not building a full sized absorption tower with plates and other features....
Essentially, enough to keep stuff out of my nose and lungs, and from corroding the workspace around me.
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Panache
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Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein
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Forget the solid phase approach in that case, just have an empty safety bottle line-in between your setup and scrubbers, to catch any suckback action.
I find having three on one retort in a circle works well and is easy to get out of the way, that allows for two scrubbers and a safety. Make sure your
safety can hold twice the volume as the total volume of your scrubbing liquids.
Di-octyl phthalate works well for absorbing solvents also and smells nice, pity about the birds eggs. Its good for vacuum.
Vozelgang your picture is too small for me to make out the thistle, nor anything else for that matter lol.
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aonomus
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I went and purchased a few plumbing parts to make a activated charcoal scrubber out of 1.5" ABS pipe, one end with a cap, the other with a screwon
cap, and brass barbs on each end. Granted, this would probably have to be the last scrubber in a series, the brass will corrode upon exposure to HCl,
HNO3, NH3, etc.
One thought I had was with the liquid scrubber, I could use plastic scrubbing pads to increase retention time of gasses, any thoughts?
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DJF90
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From what I can see of Vogelzang's picture, its just a bubbler? I dont see how you could scrub any reasonable quantity of gas using it though. Also
I'd advise a keck clip on the recieving flask 
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Vogelzang
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Quote: Originally posted by DJF90  | | From what I can see of Vogelzang's picture, its just a bubbler? I dont see how you could scrub any reasonable quantity of gas using it though. Also
I'd advise a keck clip on the recieving flask |
The connector between the receiver and condenser is supported by a clamp supported by one of the stands using connectors and a short rod so a third
stand isn't needed. The receiving flask was supported by a jar and rubber stopper.
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entropy51
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| Quote: | | What is everyones take on this, I do want to try to scrub any unneeded/byproduct gasses if I can, my lungs are valuable to me |
My take on protecting your valuable lungs is to build a simple hood. If for no other reason that even if you trap everything with perfect efficiency,
when all is said and done you have to take the apparatus apart and clean the glassware. At that point the traps are useless.
I use traps inside my hood to protect the blower from corrosive fumes, but I would never depend on a trap as the only means to protect my
health. And there's also the little problem of producing a toxic gas that you didn't expect to produce and don't have a trap for. This happens every
now and then. For example, dehydrating oxalic acid by heating sometimes produces measurable amounts of carbon monoxide, which I have never seen a
warning for and wouldn't have expected.
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