Sciencemadness Discussion Board

Column lengths& the newbie

Funkerman23 - 19-2-2012 at 20:04

I have been wandering about the web looking for 19/22 glass and a heating mantle, and as I often do,& ran into the usual suppliers... but they each had very different lengths for their Vigreux columns.. For proof of theory and analysis work would I be better off with a 200mm length and adjust my number for that or stack the 100mm ones as needed? To condense the many things I could say I know the ideal column length is Rxn dependent but this is more of a "rule of thumb" question( and ideals are just that; perfect idea in an imperfect word): BUT please ..if you have any comments, suggestions or advice feel free to post it. I am here to learn.

zoombafu - 19-2-2012 at 20:17

I have a 200mm and It serves me well. I think it depends on what you are distilling, so if there are any experiments in particular that you are looking at doing, you should check out the notes for a specific column length.

Halcyon - 19-2-2012 at 23:08

I'm assuming that the trade off is between the level of distinction between compounds and the temperature required down in the flask for a given temp at the distillation head?

ie a long column will give better fractionation, but requires you to heat the flask more, which could cause problems in temp-sensitive compounds?

zoombafu - 19-2-2012 at 23:12

Quote: Originally posted by Halcyon  
I'm assuming that the trade off is between the level of distinction between compounds and the temperature required down in the flask for a given temp at the distillation head?

ie a long column will give better fractionation, but requires you to heat the flask more, which could cause problems in temp-sensitive compounds?


That is pretty much correct. Because if you think of the vapor traveling up the column and reaching the top, there is going to be a larger length for the vapor to cool down and condense before making it over. If you are distilling two compounds that have very similar boiling points then you want that length as it will make it easier to regulate only the one compound you want coming over.

Endimion17 - 20-2-2012 at 07:15

If your column is too long, you can use several insulating jackets of different insulation power. Regular aluminium foil, rock wool, rock wool in foil, etc. Using several jackets, you can effectively get several columns of different power of fractionation.
I rarely use a naked Vigreux column. The heat loss is too great, and my best heating source is a cooking oil bath on a small hot plate.

Hexavalent - 20-2-2012 at 07:36

Depending on your budget, you may wish to purchase a small one, perhaps 200mm, and a larger one, perhaps 40mm+. Best of both worlds.

Dr.Bob - 20-2-2012 at 14:47

I would think 200 mm is a good start. There is a limit as to what is a reasonable height for a certain diameter, and 200 mm should be fine, but for 19/22, I would think 400 mm is pushing your luck unless it is vacuum jacketed. You have to be able to fit enough vapor into the column to provide the heat needed to reflux the material in the column, and once you get too ling, it becomes hard to do so without overloading the column and having liquid bump through it. I have only seen one or two 19/22 vigreux columns and none that were vacuum jacketed, so I would stick to 200 mm if you can find one cheaply. I would try asking jaidormi who also has a lot of glassware for sale.

Bob

Endimion17 - 20-2-2012 at 16:28

400 mm for a 19 isn't pushing luck, it's madness. There's a great chance it will be flooded nearly every single time. And forget the vacuum jacket, it's too costly and unnecessary.
Listen to Dr.Bob and order 200, 250 tops.