BelCha - 28-10-2019 at 03:58
Hi all,
Long-time reader, first-time poster - be gentle. I am looking to purchase a vacuum pump for solvent extraction using my DIY rotavap. My solvent is
ethanol and I would like extract between 50mL-500mL of ethanol using a water bath set at 40°C and hence require a vacuum pressure that can reach at
least 27 torr (35mbar). Ideally, the vacuum would reach lower as to compensate for small leaks in my rotavap.
After reading reviews in this forum, it seems like water aspirators can pull a decent vacuum without the troubles and maintenance that comes with
rotary vane pumps. However, I don't want to go through the trouble of making a home water aspirator and I found a circulating water vacuum pump within
my price range on eBay. Only problem, I could not find individual user reviews on whether they pull an the effective vacuum (with or without ice
water). Does anybody here use them or have experience with them? Will really appreciate your help.
Here is the link
https://www.ebay.com.au/itm/180W-Lab-Circulating-Water-Vacuu...
Another problem is that in the description it says the maximum vacuum is 0.098Mpa. Megapascal? Isn't that effectively atmospheric pressure? There
also seems to many identical listings with significantly differing prices. Couldn't easily determine what was different - they all pulled the same
vacuum and had the same flow rate.
As a side note, I have link some photos of my DIY rotavap for those interested or so can provide some feedback to improve the design.
https://imgur.com/Ejmg5mQ
https://imgur.com/L9ixjoR
https://imgur.com/mWgYdnk
Thanks in advance
[Edited on 28-10-2019 by BelCha]
Sulaiman - 28-10-2019 at 05:00
This eBay listing for a similar looking item shows the gauges clearly;
https://www.ebay.co.uk/itm/AM5-220V-180W-Circulating-Water-V...
from that I infer that the minimum pressure is -0.098 MPa relative to atmospheric pressure.
monolithic - 28-10-2019 at 10:34
They might not be the right tool for the job if 27 torr is your minimum requirement. Assuming the venturi works perfectly, you need water at 27 C to
get a vacuum of 27 torr, since you're limited by the vapor pressure of water. If the venturi or pump are poorly made (poor flow characteristics of the
venturi, insufficient or inconsistent/surging pump flow rate or pressure, etc.) then your maximum vacuum will degrade. Likewise, your maximum vacuum
will degrade as the pump dumps heat into the water with prolonged use.
Something like this might be better. https://www.ebay.com.au/itm/KNF-Neuberger-UN726-1-2TTP-Vacuu... I own one and it works great. Highly resistant to almost every acidic or organic
vapor as they have PTFE heads and diaphragms. The heads can be plumbed in series or parallel for high vacuum (10 torr) or high flow (50 torr) simply
by changing around the tubing -- just Google the instruction manual.
[Edited on 10-28-2019 by monolithic]
Sulaiman - 28-10-2019 at 17:22
Plus, there will be 50ml to 500ml ethanol dissolved in the recirculating water,
no way you will maintain 27 torr 
I would definitely go with an electro-mechanical vacuum pump.
A ptfe diaphragm pump seems suitable.
You could buy a 3cfm rotary pump
https://www.ebay.com.au/itm/Vacuum-Pump-Single-Air-Condition...
pus accesories plus lots of oil for less cost.
You would need to bleed air into the system to control the pressure,
the air bleed will cause the pump to heat up somewhat,
which causes the ethanol to boil off out of the oil.
There will be a significant loss of oil as mist = lots of oil mist to be vented away.
SWIM - 28-10-2019 at 19:41
How did you arrive at 27 Torr as the needed temperature?
BP of ethanol at that pressure is about 8 degrees C.
A 32 degree difference between bath temp and BP sounds like more than you'd need unless you're in a mighty big hurry.
AvBaeyer - 28-10-2019 at 19:42
I say go ahead and buy a good recirculating water pump. Where I live using a running water aspirator would be crazy expensive. I have a recirculating
water pump I bought used several years ago made by Boekel and I use it constantly for vacuum filtration and distillation. It has been the best $200
lab purchase I have made. It will easily pull 20 - 25 mm for distillations or product drying on any given day (I use an Hg manometer to measure
vacuum.) With fresh cooled water I can easily get to 15 mm constant vacuum over a period of 1 -2 hours. I use it for solvent removal with an ice trap
between the receiver and the pump which allows a constant vacuum. The water temperature in the pump typically is ambient and is usually around 17-20
C. Do not put ice in the water tank as it will make one heck of a noise.
I have a good (but cheap) rotary vane vac pump which easily pulls <1 mm Hg. I would never consider using it for solvent removal under ordinary
circumstances. Anyway, bleeding these kinds of pumps much above 3- 4 mm Hg makes way too much noise and spits out too much oil vapor.
Overall, you cannot go wrong with a good recirculating pump for everyday routine lab operations requiring modest vacuum.
AvB
BelCha - 28-10-2019 at 20:06
Thanks for the responses – there have been very helpful. I think I will still go with the water aspirator as a don't like the idea of moving my
whole apparatus outdoors due to oil vapour spewing out.
I have relied on the attached Table for the extent of vacuum I need to pull ethanol.
Unfortunately a PFTE diaphragm pump is out of my price range at this point, though I agree this is the simplest approach.
