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Author: Subject: Rotary cane vacuum basics: use CaCl2 drying tube instead of cold trap
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[*] posted on 9-8-2020 at 00:19
Rotary cane vacuum basics: use CaCl2 drying tube instead of cold trap

Hey all
I’ve recently purchased a rotary vane pump for vacuum distillation of high boiling liquids and in addition to my pump I purchased a cold trap. Now this cold trap looks more like a gas bubbler with a test tube like jacket and a long stem that fits inside.

Now my question is this: For distilling polar liquids that’s main impurity will be water, can I fill this “Cold trap” up with calcium chloride instead of using dry ice and acetone to cool it and collect vapors before my pump?

I have yet to use my pump but the first few liquids I plan to distill are DMSO and Sulfuric acid. Now, thinking about going out and buying dry ice every time I plan to perform a vacuum distillation makes my wallet hurt. Plus it just seems like a hassle and I’d rather reserve the errand for more sensitive distillations. I’m sure and a cooled CaCl2 trap would suffice in keeping water out of my vacuum, no?

[Edited on 9-8-2020 by Opylation]

[Edited on 9-8-2020 by Opylation]
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[*] posted on 9-8-2020 at 05:57

In refrigeration, for water contaminated systems it is recommended to preheat the pump and to use the gas ballast till worst is out then close ballast and the heat drive most the moisture from the oil.

then post purge. All so changing oil frequently is best for performance, but this is the HVAC realm. I occasionally use my pump for distillations, following much the same procedures used on water contaminated systems.
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[*] posted on 9-8-2020 at 07:13

I'm not sure if CaCl2 would work as intended with respect to water, but you still want a cold trap to capture the high boiling point solvent as not all of it will be condensed by your water condenser. Unless this pump is very cheap, I wouldn't vacuum distill sulfuric acid. Most rotary vane pumps are machined out of cast iron which corrodes in the presence of H2SO4. The rotor and vanes have fairly tight tolerances and the latter are forced against the stator by a spring. It doesn't take much corrosion to get the vanes to stick in the rotor and ruin the pump.

(I've considered vacuum distilling H2SO4 on my PTFE diaphragm pump but it's too scary for me. Boiling H2SO4 at 337 C atmospheric pressure seems safer than boiling H2SO4 at 100-200 C under reduced pressure with the added risk of glassware implosion. :o)

[Edited on 8-9-2020 by monolithic]
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[*] posted on 9-8-2020 at 07:52

I don't think CaCl2 keeps hold of water under vacuum. You can even dry CaCl2 by leaving it in very dry air. I used to have a beaker covered with filter paper with some hygroscopic compound next to a hygrometer. Whenever the relative humidity would get lower than 15%, the compound would crystallize, and dissolve in water again when humidity went up (it only gets that dry here when it freezes outside).
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