Pumpin problems...

[raffike]

Raf acquired a pump lately,it seems to be only slightly used and 2 staged,550 watts,gas ballast and all.Raf was testing his pump,distilling pure safrole to be more precise,safrole started boiling at 60 C and temp constantly rose and finally stabilized at 108-110 which translates roughly to 10 torr vacuum.For some reason vacuum was falling throghout distillation process.Raf was wondering what would happen if he connected two of these pump in a row for more effiency?Sounds like a good idea,doesn't it?

[Rhodium]

A dry ice trap to protect the pump you already have would be a better investment, a likely reason for the falling vacuum was that as you distilled, volatile low-boiling terpenes from the oil dissolved in the vacuum pump oil, reducing the maximum attainable vacuum as more and more volatiles dissolved.

Osmium has posted a lot on this subject, go back and search for vacuum (pumps) and his name.

[ragnaroekk]

And also already by Osmium advised but seldom followed:
The use of metal tubing with at least double the diameter as the plastic tubing seen on the picture gives impressive better results. (the last meter to the flask is always flexible of course - use metal as far as possible) A aspirator as forepump and the ballast valve open max. helps to keep oil clean and is good enough to achieve a very good vacuum.

[raffike]

Oil wasn't probably the problem,he distilled safrole 5 times with it and every time boiling started at 60-70 and ended at 110 C.He should try to distill some isosafrole to get more precise results as distilling iso isn't as tricky as distilling plain safrole,will search for that Os post...

[hypo]

> The use of metal tubing with at least double the diameter as the plastic
> tubing seen on the picture gives impressive better results.

really? shouldn't this only become an issue, when the mean free path
length is approaching the diameter of the tubing? i.e. below 1mbar?

[ragnaroekk]

And below a certain vacuum it becomes essential. Gladly you added the <1mbar at the end - otherwise it would have been complete impossible to decrypt this piece of art. Oh? This is a sentence? Sorry....

In the range of vacuum raffike works  the metal tubing is for sure measurable. The manufacturer of the pump didn´t attach this first class flange on his pump for to attach a gardenhose - a nibble would have sufficiced if this had been intended. Also in laboratories fixed installations are only used because they are more expensive, look better and make a better sound when you bang with the head against such a metal tube. The named "mean free path length" is probably measured in "meanometer" or "french" as unit? Just a guess....

[raffike]

Let me tell you what raf will do:he'll throw away that "garden hose" and replaces it with PP black plastic tubing,.5" in dia,keeps lenght minimal and tries how pump pulls,then he also tries connecting two identical pumps in row.Results to come...

[ragnaroekk]

5" (about 12cm, arn´t you european?) is to much volume added! will take to long to empty it and makes no sense you don´t have a turbomolecular highspeed pump on work   . Please behold! Use 2" (5cm) tube, PP is ok - the heavier the better, there is watertubing available (PP-R reinforced PP) with 1cm walls - cheap and easy to work with. Just a example of course.
An aspirator would be better as a second pump - you have the dander to reach the vapour pressure of the oil of the first pump if two staged up. Might ruin it. A cooled trap with ATF (automatic transmission fluid) or the same oil as in the pump is easy and sucks exactly such vapours which would contamine the pumpoil. Clean it by boiling/heating.

I reread your first post and I believe it possible you have a nasty leak somewhere probably on some elastic part which blows open dureing use but kind of selfseals when vacuum is gone. Just a guess.

[Rhodium]

What's wrong with my table?

[Prince_Charles]

Pressures above about 10^-2 mbar have viscous flow conditions and the conductance of tubing varies with the fourth power of its diameter: doubling tube diameter increases the conductance 16 times.

Conductance varies inversely to tube length: doubling the tube length halves the conductance

This means that tube diameter is much more important than length, though length should be reduced as far as possible.

(below 10^-2 mbar, in molecular flow conditions, the conductance is proportional to the third power of the diameter and inversely proportional to length.)

Keep the tubing short and wide so that the performance bottle neck is with the conductance of the glassware and the pumping speed of the pump, which are fixed.

Typically tube conductance should be greater than 5 times pumping speed. I'd go for at least 20 mm internal diameter tubing.

[hypo]

> What's wrong with my table?

the first row has 3 cells, the second 5 cells and the rest
has 4 cells. they must all have the same # of cells.

(i think i just invented a new profession: table corrector   )

[xboXer]

If nature abohrs a vacuum, why then does the universe employ one as the medium for the area between the stars and planets?

Many have stated: Working with a vacuum sucks, although, working with pressure blows. Working tables is what one does after getting a degree in fundemental geography, and not suprisingly, this is a prime example of a vacuum constant.

Use joint lube on glass glass joints,
Teflon tape on threaded connectors,
And when working with a vacuum, be aware that some labware is not neg.pressure friendly, and thus will implode at the worst possible moment.
Try to eliminate as many connections as is feasible, and also test the vac. pump to spec.s to assure that it is a sound unit at all levels of vacuum inducement.  

[raffike]

Isomerized some safrole(230 C and 3 hours,only 55% yield,will switch to vac isomerizing soon) and tried my pump again this time with 20 mm hose which was very short,same results,120 C was the bp of isosafrole which means it pulls as much as last time with safrole(safrole came over at (108-110).Will try 2 pumps in a row soon.

[Prince_Charles]

Raffike, what condition is your pump oil? Did it come with the pump or is it likely to have been changed? An oil change is the first thing I would do with a used pump.

[sYnThOmAtIc]

Raff, brand new yellow jackets cost $300bucks and boil sassy ar 55c. I don't see why people keep knocking them. I bought one of mine used from a retired refrigeration technician and he never changed the oil the entire time he owned it and it sat in his garage for ten years with that same oil init I changed the oil and ran it a while and changed it again and it runs sassy at 65c. I assume it operates as it did new the newer ritchie has a lower ultimate vacuum rating thatn that old ass one.

[raffike]

Got that one free.Had to buy a kwt motor but that was 3 bucks for me.I'll see if i can get a yellowjacket somewhere.

[hypo]

is the original problem solved already?

a possible (likely) explanation of vacuum getting worse during
distillation is oil impurities:
pump temperature rises -> vapor pressure of impurities increases -> attainable vacuum gets worse.

[hest]

I said it before.
Use a freeztrap (those 10$ on dry ice is spent well.)
Iff (for some reason ) you don't use a freeztrap, let the balast bee at max, after the destilation let the pump run (with a stoppeer in the tube) for 1-2 houer, this blow most of the polutan's out of the oil. A wac.pump. is a delicat instument. Treatet well, it last for a lifetime. Treatet bad it last 6 month.

[sYnThOmAtIc]

His vacuum drifts from 60-150c?? Sounds more like bad internal components of the pump. You should look into a rebuild kit for your pump. Or call the manufacturer and see what they reccomend...


I see your point hest But my pump sucks more vapours then his does and when distilling safrole here in texas at 90f room temp and 55c boiling temp evaporation form the recieving flask has to be maximal and I doubt he is getting enough tgoods in his pump to affect it that wildly. Just by looking at the pic and comparing it to others I have see like it with metal flanges his shold outperform a yellow jacket any day. And I have run lond distillations of several liters with supposedly dirty oil from previous procedures and the most my temp ever drifted wat 5c that was mostly due to the strong vacuum sucking my grease out of my joints and causing leakes which was fixed using teflon sleeves. I just don't see that big a pump drifting that far from a lil imnpurities I'd say its more from mechanical wear or both. The impurity content would have a bigger impact on a worn pump. Just my two bits I may be wrong but jsut think about it.

[ragnaroekk]

No, I don´t think raffike is to stupid to get the right motor but e-motors are buildt in very different ways and have very different characteristics. Perhaps the pump runs to fast - also if the named data on motor and pump look right. Try running slower by an controller or another motor - a controller would be preferred for giving a very nice control over the pump in addition (and is usable for most pumps and else stuff...)
Running to slow by rising/decreasing load is also possible but less probable btw.

[raffike]

Pump was bought used and was returned.He'll buy a new one to be sure.It's not very expensive...