Sciencemadness Discussion Board

Oil dropping at the vacuum pump outlet

Shikimol69 - 27-6-2013 at 11:12

Hi there!

Today I planned to distill few hundred mls of toluene under reduced pressure, and thus I began by starting my vacuum pump just to preheat it. To recover solvent vapors that have not been condensed by the column, I connected the pump outlet to a trap that consist of a squibb separatory funnel topped by the head of a wash bottle of the Drechsel kind.
After 5 min of preheating, I removed the hose that linked the trap to the pump, and for the 1st time ever I observed the presence of oil at the outlet dropping at a rate of about 1 drop/minute. The gas ballast was initially opened, and it seems that the rate of the dropping slightly increased when I closed the ballast.
Can someone tell me what could be the problem here and what could I do to fix it?

Some informations that may help:
- the pump is a Vacuubrand "chemistry-Hybrid" RC 5 that has been operated for a period of time not exceeding 25 hrs. It consists of an oil sealed sliding vane rotary pump with 2 rotary stages and of a 2 stages diaphragm pump that permanently pumps the outlet of the second stage as well as the oil reservoir. An oil mist filter is integrated to the system ;
- the level of oil is ok (can be directly visualized on the rear side of the pump).

Thank you by advance for your answers ;)

Magpie - 27-6-2013 at 13:22

I'm not an expert on vacuum pumps but IMO what you are seeing is perfectly normal when you are moving a lot of air through the pump. When you have the suction closed I doubt if you will see that oil. Try it.

In normal use, as for vacuum distillations, you are not moving all that air and the pump is virtually "dead-headed" as it is with a closed suction. Any condensables should be dropped out ahead of the pump with a cold-trap.

The reason a pump has a mist eliminator is to minimize the loss of entrained oil by helping it coalesce and fall back into the reservoir.

Shikimol69 - 29-6-2013 at 08:45

Many thanks for giving me some hints Magpie !
Following your advice (and having thoughts about it, it looks just logical^^), I compared the oil flow at the outlet with open vs. closed system: you were right, when the pump is plugged on the distillation system I observed virtually no dripping.
All things considered it was rather expected, but what is the best strategy to isolate the pump from the system being vacuumed? (I mean at the end of the distillation or during the experiment to regulate the blank) :
- Use a simple way valve that when closed will not pass air through the pump ;
- use a valve with a purge system or a 3-way valve which will isolate the vacuum pump on a path ?

Magpie - 29-6-2013 at 09:09

Quote: Originally posted by Shikimol69  
Many thanks for giving me some hints Magpie !


My pleasure.
Quote: Originally posted by Shikimol69  

... but what is the best strategy to isolate the pump from the system being vacuumed? (I mean at the end of the distillation or during the experiment to regulate the blank) :
- Use a simple way valve that when closed will not pass air through the pump ;
- use a valve with a purge system or a 3-way valve which will isolate the vacuum pump on a path ?


My preference is to have an isolation valve between the pump's suction and the distillation apparatus. When distillation is finished, simply close this valve with the pump still running. Then shut off the pump. I have placed a 2nd valve in a tee to atmosphere, upstream of the isolation valve. I can then slowly open it to allow the distillation apparatus to come up to atmospheric pressure. In case this is not clear I will brush up on my paint skills and show a sketch on edit.

Some people have the misunderstanding that you shouldn't run a pump with the suction closed. This does not hurt the pump. A pump is doing no work when it is not moving fluid.

[Edited on 29-6-2013 by Magpie]

vacuum pump sketch.bmp - 703kB

Shikimol69 - 29-6-2013 at 20:31

Nice diagram! You have for sure some paint skills^^
Your picture is very clear, and if my system is a bit different it works exactly the same way: the 2nd valve you placed upstream of the main one is replaced un my system by a single rotary piston valve working in different positions:
- online - the pump and the distillation apparatus communicate
- bleeding hole facing the pump > air flowing to the pump, distillation system isolated (closed)
- bleeding hole facing the distillation apparatus > pump inlet closed, thus allowing the pressure inside the distillation apparatus to equilibrate to atm.
I don't know if you know this kind of valve, it may sound complicated but it's very simple in fact. The valve is made of PE or PP (not sure) and consists of 2 parts: a stationnary one with the notched inlets, and a rotative part with a "bleed" hole and a canal with a bow shape allowing the communic ation at each extremities.

I'm not sure I made myself clear enough with such descriptions, I will post a picture of my faucet in few hours as well of my whole vacuum line & system (I haven't your paint skills, definitely! Lol).
It could be interested to compare our local vacuum system, nop? The choice of the vacuostat, of the regulator, their places, the automatism vs manua... I really think it could be valuable!