09-21-02 02:17
No 358631
Theoretical Plates Required to Separate Mixtures, Based on Boiling-Point Differences of Components
BP difference No. of theoretical plates
108 1
72 2
54 3
43 4
36 5
20 10
10 20
7 30
4 50
2 100
Info taken from a laboratory book
(Hive Addict)
09-21-02 02:38
No 358637
how does this apply to a vigreux column? Does each 'spike' count as a tp? Doesnt seem right to swim... Swim assumes that a column packed with ss wool would obviously contain more theoretical plates...would this help a bee who has too strong a pump and needs to separate chemicals that are within 15-20C of eachother? The vigreux/kick assed pump combo (and swims newbee assed lab skills) just arent cutting it for clean separation.
(Chief Bee)
09-21-02 03:12
No 358647
http://chemweb.stanford.edu/summer2002/c
http://www.chem.ufl.edu/~barbaro/2211L/n
(Hive Bee)
09-21-02 03:28
No 358662
In swim's opinion it is best to buy a distillation column with a given amount of plates, for example perforated plate columns or concentric tube columns where the column is "pre-rated" by the company who makes it. Yes, they are extremely expensive ($1000+) but if you want ultra-pure shit, why not? One could easily get the money for one from like 10g of product. The theoretical plates of a packed column changes depending on how tightly you pack it, what material you use to pack it, etc. You do lose some small amounts of product if you use a column and/or a real distilling head (also expensive), instead of a still head. Many of these systems however tell you how much will be lost in mL. Swim has seen systems where holdup is less than 1 mL at 100 mL per hour. Which would you choose? 100 mL of 90% ketone or 85 mL of 99.5% ketone. swim would choose the latter.
(Stoni's sexual toy)
09-21-02 04:27
No 358674
> In swim's opinion it is best to buy a distillation column
> with a given amount of plates
Theoretical plates and actual plates are two different animals.
> where the column is "pre-rated" by the company who makes it
This rating depends on so many factors that again you won't know how good your separation will be.
> they are extremely expensive ($1000+) but if you want
> ultra-pure shit, why not?
Because you can have the same separation efficiency for a fraction of the price.
I'm not fat just horizontally disproportionate.
(Hive Bee)
09-21-02 04:36
No 358676
>This rating depends on so many factors that again you >won't know how good your separation will be.
If you used a variable reflux ratio distilling head couldn't you duplicate the conditions outlined in their rating? For example, they will tell you that at x mL/hr you will get y amount of theoretical plates. So you would control the ratio so you would get that amount.
>Because you can have the same separation efficiency for a >fraction of the price.
How would you propose doing that? Just use a regular distillation column w/ packing? If so, what kind of packing? And wouldn't it be pretty hard to know how many theoretical plates you are getting? Why would the textbook give these numbers if even some bees say that using a still head (1 theoretical plate) is good enough?
(Stoni's sexual toy)
09-21-02 05:17
No 358683
> If you used a variable reflux ratio distilling head
> couldn't you duplicate the conditions outlined in their
> rating? For example, they will tell you that at x mL/hr
> you will get y amount of theoretical plates.
Yes you could, if they tell you the necessary reflux ratio and throughput, and you manage to reproduce them.
Big ass columns and distillation heads etc. are meant for solvent purification where column holdup and amount of forerun don't matter, not for 50-100g ketone distillations.
They are great to turn used or lower grade solvents into reagent grade or spectroscopy grade, where the difference between 99.5 and 99.8% purity and absence of specific compounds is important.
For our purposes it doesn't really matter if the ketone is 98.3 or 99.5% pure, it certainly isn't worth the huge difference in money and time necessary to convert the former into the latter.
If you have to distill a liter of ketone every other week, and have enough privacy to let that thing run for several days then go ahead and don't let me stop you.
Otherwise that time and money could be used more effectively to clean up your products.
> How would you propose doing that? Just use a regular
> distillation column w/ packing?
Yes.
> If so, what kind of packing?
Depends on many factors. Crushed glass, glass balls, rolled wire mesh, commercially available column filling. Depends on column size, required th. plates, throughput, OTC or not, cost, vacuum or no vacuum distillation, batch size etc.
> And wouldn't it be pretty hard to know how many
> theoretical plates you are getting?
Yes. But you can guesstimate it by consulting tables and formulas, and eventually checking the distillate by GC or any other means and see how pure your product is. Who cares if you achieved 11.5 or 13.2 theoretical plates, as long as the result is ok and repeatable all is well.
> Why would the textbook give these numbers if even some
> bees say that using a still head (1 theoretical plate) is
> good enough?
Those bees don't know what they are talking about.
A stillhead is often better than one th.pl.
I'm not fat just horizontally disproportionate.
(Hive Bee)
10-14-02 04:55
No 368292
TheBlindGenius: I believe I know what book you speak of. It also mentions in the same chapter the use of a twisted platinum screen for spinning band columns beeing far more superior at seperation than fractionating colums. I utfse, but noone seems to have any experience with one.
edit:
I don't think the book stated it as "far superior", rather than a more sufficent, or even better way, or something along those lines. Don't want anybody thinking those were the exact words coming straight from the book.
(Old P2P Cook)
10-14-02 06:20
No 368330
spinning band columns beeing far more superior at seperation than fractionating colums. I utfse, but noone seems to have any experience with one.
In one of the labs that I worked in one of the PhDs bought a spinning band distilling rig. I set it up on the bench and always wanted to take it for a spin (pun intended) but never got around to it.
Baseline Does Not Exist.
(Hive Bee)
10-14-02 15:22
No 368440
Here is the quote you refer to
"The most effective type of column is the spinning-band column. In the most elegant form of this device, a tightly fitting, twisted platinum screen or a Teflon rod with helical threads is rotated rapidly inside the bore of the column (Fig. 10.8). A spinning-band column that is available for microscale work is shown in Figure 10.9. This spinning-band column has a band about 2-3 cm in length and provides 4-5 theoretical plates. It can separate 1-2mL of a mixture with a 300C boiling-point difference. Larger research models of this spinning-band column can provide as many as 20 or 30 theretical plates and can separate mixtures with a boiling-point difference of as little as 5-100C."
BTW, the book also says that a 20-cm Vigreux column "might only have 2.5 theoretical plates," and that a packed column is more effective, but not by a large margin.
(Hive Bee)
10-14-02 15:30
No 368443
by the way, I highly recommend it, especially to those who have limited chemistry experience. It has a very detailed "The Techniques" section about 200 pages long. More detailed than Zubrick, I think.
Introduction to Organic Laboratory Techniques: Small Scale Approach
Pavia, Kriz, Lampman, Engel
Copyright 1998 by Harcourt, Inc.
(Hive Bee)
10-18-02 05:26
No 369808
"Bubbleplate" columns at auction site for a song. 5 plate and 10 plate columns for $25-$50!!! All have silvered vacuum jackets! List price $500 - $2,000. Can separate Ketone into fractions - fire up the hot plate and watch the distillation receiver.
Ain't surplus technology wonderful??!!
(Hive Bee)
10-18-02 09:15
No 369877
Can be achieved by using an empty coulumn or more than one theoretical plate.
This basically means how many times a substance recondenses in the column an equilibrium between condensation and vaporization could take place in an unpacked column as well, having a surface to condense the substance on increases the number of times this happens, in the simplest terms.