Sciencemadness Discussion Board - Vacuum desiccator implosion risk (and how much vacuum is a Buchner funnel designed for?)

Vacuum desiccator implosion risk (and how much vacuum is a Buchner funnel designed for?)

SplendidAcylation - 29-7-2022 at 02:12

Hi,

I have a rather large, glass vacuum desiccator;
I'm not sure the exact size as it is stored away at the moment, but I'd say maybe 40cm diameter, it is surprisingly heavy and the glass is thick.

It is missing the ground-glass adapter that inserts into the top, but otherwise it seems to be in perfect condition, I have never had any need to use it before, although I did evacuate it once (a rubber stopper inserted into the ground-glass socket is sufficient) with no problems.

Anyway, I have finally encountered a situation where I need to use it (the small vacuum-sealer jars that I normally use as vacuum-desiccators are no longer air-tight)

I thought nothing of the implosion risk initially, as these things are so heavy and made of thick-walled glass, but then I looked them up in an old lab equipment catalog, and they are sold with an optional "cage" made of metal mesh to surround them.
The catalog doesn't say what the mesh cage is for (although a Google search revealed that my suspicions were correct; It is supposed to protect you in case of implosion), and it doesn't say the desiccator must be used with the cage, it is sold as an optional extra.

So this is a bit confusing, to be selling a brand-new item with an optional extra that protects you against catastrophic failure, I don't feel too confident in using my old desiccator without some form of protection.

It also occurred to me that perhaps the cage is there to prevent anything hitting the desiccator that may cause it to implode.

Television cathode-ray tubes have a vacuum inside, and they never implode as far as I know, so is it really a possibility that a desiccator, with similarly thick glass, might implode?
Obviously the people selling them think the risk is significant enough to sell a cage for it!

So I guess the point of this thread is:
Why exactly do they sell cages for desiccators, and, knowing that, would it be mad to evacuate one without a cage, especially a rather large one, would that be something you would be nervous about?

I guess I could wrap it up in blankets or something.

Oh, and another related question;
I have a porcelain Buchner funnel (~120mm) that I use for vacuum filtration, quite often if I am filtering something that takes a long time to filter, I will pull a high vacuum and leave it, with the pump off, to filter.
It occurred to me lately that, with a high vacuum, the force pressing down upon the flat perforated porcelain surface is massive, especially since it is a fairly big funnel.
It has withstood it so far, but would you say this is a bad idea?
Are these funnels designed to have 1 ATM between one side and the other?

Thanks in advance for reading this, rather badly constructed, post.

Deathunter88 - 29-7-2022 at 11:10

You're fine and overthinking it, its not going to implode unless you drop something heavy onto it. If it makes you feel better, an effective alternative to a cage is to wrap it with several strips of electrical tape/duct tape in a grid pattern.

SplendidAcylation - 31-7-2022 at 02:04

Quote: Originally posted by Deathunter88

You're fine and overthinking it, its not going to implode unless you drop something heavy onto it. If it makes you feel better, an effective alternative to a cage is to wrap it with several strips of electrical tape/duct tape in a grid pattern.

Thanks for the reply.

I have retrieved the desiccator, and evacuated it to a 20% vacuum, and it is air-tight, however it appears to have some suspicious chips/scratches on the base; There are a few scratches that appear to be superficial but yet they look quite deep due to the reflection/refraction in the glass, so it is hard to tell whether or not they are actually cracks that go right the way through.

Also, how does one tell whether a desiccator is a vacuum one, or an atmospheric pressure one?
They look similar/identical, is it that the bases of both types are identical, and only the lids are different, i.e. the vacuum one has a socket for a vacuum line, or are the bases made differently?

I'm not sure the desiccator I'm using belongs to the lid I'm using, so I was also worried that maybe the base part might not be thick enough for vacuum.

Mateo_swe - 30-10-2022 at 01:29

Desiccators are constructed to withstand vacuum but you maybe shouldnt connect a high vacuum rotary vane pump to it and leave it on.
Why not use an aspirator for the vacuum, i think that will be enough.
Or run the vacuum pump just for a while and then close the valve on the dissicator.
The dissicator should hold the vacuum for at least 24 hours if sealed ok with grease.

For the large buchner funnel i would use aspirator vacuum or one of those small cheap diaphragm vacuum pumps.
I would not use a high vacuum rotary vane pump on a large buchner funnel.
Small cheap diaphragm vacuum pumps are usually enough for filtrations and are cheap to replace if they break down.

Fery - 30-10-2022 at 02:09

If a water aspirator pump gives vacuum -0,90 atm and a rotary vane oil vacuum pump almost -1,00 atm then the difference of forces on glass is only 10%. Water aspirator vacuum pump is usually enough for desiccator as well the best choice for vacuum filtering through Buchner funnel. Rotary vane oil pump (especially 2 stage) is necessary when you distill something with boiling point above 220 C or vacuum sublimation of substances with very high b.p. I distilled 2-nitrotoluene at 70 C (b.p. 220 C at normal atm. pressure) using cheap chinese rotary vane oil 2-stage pump and also dinitrotoluenes distilled at IIRC 150 C but already solidified in the condenser (some amount of dinitrotoluenes was present in mononitration of toluene).
Also remember that some solvents may already start to evaporate at room temperature during vacuum filtration (acetone, methanol, etc) even at the strength of water aspirator pump. But water pump binds the vapor into water which is some advantage. That happens when crystals reduce the air flow through the filter paper too much. Boiling may happen also when vacuum filtering hot water solution.
So start with water aspirator pump.

Sulaiman - 30-10-2022 at 04:14

I suspect that a grain of sand in the greased joint would be enough to crack the glass and cause an implosion,
so protection is a very good idea.

I too think that the difference in pressure between a water and rotary pump is small compared to atmospheric pressure.

I don't see the value in a rotary vane pump for filtration, so many solvents boil.
A water pump should be more than adequate.
I use a small -0. 7 to -0.8 bar pump and so far never felt the need to use my rotary vane pump.
BUT I'm not very experienced - others may have different opinions.

macckone - 31-10-2022 at 11:51

I think the biggest reason for a cage is to protect the desiccator.
If they implode you usually get a few large pieces and a bunch of very small particles.
Having had a glass cylinder break on my birkeland-eyed reactor, I can testify that heat is a bigger danger than pressure or vacuum.
Vacuum desiccators are usually way overdesigned so they can withstand some abuse.

Assuming the substance you are desiccating is not too hygroscopic, the vacuum doesn't need to be very high.

Mateo_swe - 1-11-2022 at 01:03

Quote: Originally posted by Fery

Rotary vane oil pump (especially 2 stage) is necessary when you distill something with boiling point above 220 C or vacuum sublimation of substances with very high b.p. I distilled 2-nitrotoluene at 70 C (b.p. 220 C at normal atm. pressure) using cheap chinese rotary vane oil 2-stage pump and also dinitrotoluenes distilled at IIRC 150 C but already solidified in the condenser (some amount of dinitrotoluenes was present in mononitration of toluene).

So my Vacuubrand diaphragm vacuum pump isnt usable for fractional distillation of essential oils that have boiling points above 200°C?
Do i need a 2-stage rotary vane vacuum pump for similar applications?
I thought my diaphragm vacuum pump could do most things but maybe it isnt so then.

Organikum - 4-11-2022 at 19:30

Quote: Originally posted by Mateo_swe

Quote: Originally posted by Fery

But you can use your Vacuubrand as backing pump for a two stage rotary vane and achieve real good vacuum with the setup (put the diaphragm pump only online after the rotary vane has reachedabout 30 mbar or less or the diaphragm pump gets damaged).
Also you can pump all kinds of vapors then, water too, goes straight through the rotary then.

Naturally thats hardcore but effective.
It depends on the sensibility of the substrate if your Vacuubrandt suffices or not. A 4-head, 3-stage does most, the 2-head 2-stage ones are hard on the limit, sometimes yes, sometimes no. Rotary vane pumps are asscheap used, at least here where I live, get one with a (separate so possible) 3-phase motor and a cheap chinese inverter (240V in 3x 240V out) and you are in the top range of pumping glory!

Sulaiman - 4-11-2022 at 21:49

AFAIK Buchner funnels are commonly designed to use a water aspirator as a vacuum pump,
typically 10 to 15 torr pressure.
A perfect vacuum is 0 torr
The difference in force is therefore (760 - 0)/(760 - 15) =
a 2% increase in force = negligible.

The differemce is that a rotary vane pump is more likely to cause boiling/frothing of the liquid filtrate.

One significant difference with a rotary vane pump is the cost of pressure sensors/gauges.

wg48temp9 - 5-11-2022 at 01:41

Quote: Originally posted by Organikum

[
But you can use your Vacuubrand as backing pump for a two stage rotary vane and achieve real good vacuum with the setup (put the diaphragm pump only online after the rotary vane has reached about 30 mbar or less or the diaphragm pump gets damaged).
Also you can pump all kinds of vapors then, water too, goes straight through the rotary then.

Not all rotary vane pumps like having vacuum on their outlet port. The oil seal on the drive shaft may leak and there may be a valve on the inlet port that is operated by the differential pressure between the in and out ports.
The vacuum may also reduces the oil flow thru the pump chambers.
The vacuum can cause the oil to foam up and get sucked into the outlet port.

The following double lip double spring loaded oil seal is needed.