Sciencemadness Discussion Board

What size column for my flask?

angelhair - 3-11-2008 at 16:36

Can some one suggest an appropiate size column, width and height for a 12L RBF, because the largest that I can find is only a B24, about 600mm height and 25mm width.

not_important - 3-11-2008 at 17:10

What are you separating, how many theoretical plates does that take?
knowing that and the HETP for a given column type, you'll be able to calculate the column length.

The flask volume has little to do with the question, except for rather small volumes; size is more important for the head source rating. It does play an important role determine how many hours of days you'll spend fractionally distilling.

angelhair - 6-11-2008 at 16:07

yes I understand what you're saying. I don't have anything specific to separate, just general solvent purifications, acetone IPA, benzene toluene xylene' etc.

I have a book called chemical separations but the section about columns and plates is writen very badly, there are lots of formulas but no explanations. The only thing useful to me is a table listing deg C of separation and the coresponding number of plates needed.

Since my question is too vague, can you suggest a book that explains it in simple terms. I have a copy Perry's chemical engineering but the section on fraction columns is way too complex for me.

not_important - 6-11-2008 at 18:17

That table is likely your best guideline, although a proper one would list low and high boiling points rather than just the difference. Figure out what your closest separation is likely to be and use that number of plates.

angelhair - 10-11-2008 at 01:00

Where can I find information that tells me how many milimeters equals number of plates. I haven't found that in my books. Also I am assuming that speed of distillation, still with good separation, is a factor of width.

Basicly, if I have say 6L of toluene and xylene or something with closer bp I don't want it to take more than 8-10 hrs.

watson.fawkes - 10-11-2008 at 06:42

Quote:
Originally posted by angelhair
Where can I find information that tells me how many milimeters equals number of plates. I haven't found that in my books.
That's because it depends on how your column is packed. A theoretical plate is a measure (roughly) of the intimacy of contact between solid and liquid phases. Since this is how heat is transferred between a condensing vapor phase and an evaporating liquid phase, it's the rate-determining step. The specification for your column (not a reference book) should give this figure.

chemrox - 10-11-2008 at 08:38

If it's separations your doing the flask size is irrelevant. The column length will be a function of column design and the boiling point differences in the materials you're separating. There's a decent discussion in Vogels 3rd edition. Snyder columns are good basic fractionating columns. I have a six cell one I use for separating liquids that boil 40 deg or more apart. An important part of the setup is reflux ratio. You need a partial takeoff head to control that. There are some nice solvent heads made... there's quite a bit to fractional distillation particularly when the materials boil closely.