9 replies to this topic

[prabhugmr]

I am studying masters degree in chemical engineering. During a discussion with my friend, who was working in ethanol distillery, we discussed about the impurities in ethanol. the impurities with high molecular weight than ethanol like, propanol, isobutanol, n-butanol, isopropanol were getting collected in ethanol during distillation.

 

Is there any simple method by which in simple distillation column at lower temperature, these components can be removed upto 50ppm least.

[Muhammad Waqas]

Water Scrubbing

[latexman]

The alcohols heavier than ethanol in the fermentation process are called "fusel oils".

 

I believe a small liquid purge stream located where these components concentrate in the distillation column is how the concentration of these components are reduced in the distillate.

[prabhugmr]

During distillation, my friemd couldnot able to separate the high impurities. He told me that multiple distillation with temperature in reboiler as low as possible would reduce the high boil content in distillate. Isnt it?

 

What is water scrubbing? How it removes high impurities? Can you expalin

[katmar]

The problem with some of these higher alcohols is that they make azeotropes with water.  For example, isobutanol and water form an azeotrope with a boiling point of 89.7 °C.

 

In a usual ethanol rectifier the top product would be close to the azeotropic ethanol-water mixture (approx 95% ethanol by mass) and the bottom product would be nearly pure water. What this means for the higher alcohols is that at the top of the column their volatility is much less than the ethanol, so they are "pushed" down the column.  On the other hand, at the bottom of the column the excess of water results in the higher alcohols forming azeotropes and being "pushed" up the column.  The higher alcohols become trapped in the column and small purge streams are taken off just below and just above the feed point to remove them.

 

In a potable alcohol (i.e. for beverages) plant the common practice is to first distill the alcohol to around 95% to make a crude or industrial grade product, and then use the water scrubbing column mentioned by Muhammad, followed by a redistillation to 95%.

 

The water column is not really a scrubber.  It is often called a wash column or a hydro-selection column.  In this column the spirit is diluted down to around 15% with recycled water.  This large excess of water allows all the alcohols to form their azeotropes and the increase in volatility of the higher alcohols is greater than the increase in the volatility of the ethanol.  The result is that although there is still a large amount of ethanol in the top product, the ratio of the higher alcohols to the ethanol is much higher in the top product than in the feed. This top product is taken to a recovery column where the higher alcohols are taken out as side stream purges as mentioned above.

[Bobby Strain]

I guess these higher alcohols are not produced by the strains of yeast used to make drinking beer. I know from experience with fuel alcohol production that the fusel oil is really stinky stuff. If it come into contact with skin, you can't get rid of the smell for days!

 

Bobby

[katmar]

A large part of the procedure for avoiding the impurities when fermenting beer is to keep the temperature low. Beer fermentation is done at 10°C or even below.  Fermentation for producing neutral spirit (eg for vodka) is done at 30 to 35°C.  A vodka fermentation will be completed in 2 to 4 days, but beer is fermented for much longer. 

 

I don't know whether the yeast itself produces more impurities at the higher temperatures, or if there are other wild organisms that thrive at the higher temperatures and produce them. Good hygiene certainly helps keep the levels lower.

 

When you concentrate the neutral spirit of course you concentrate the impurites as well, and the vodka producers take the attitude that since they are going to have to deal with the impurities anyway they might as well take advantage of the rapid fermentation and just deal with slightly higher levels of impurities.  Beer producers don't have this option. Even in vodka, the impurity levels are low enough that they do not have any significant impact on the raw material consumption.

 

Fusel oil is terrible stuff, but in small quantities it is what turns a vodka into a rum - and makes rum drinkers smell so bad!

[prabhugmr]

So water scrubbing means that when industrial alcohol is produced it needs to be diluted with water in a column where high boils could go along with water in the bottom and to product containing water and ethanol will be again separated in another column. Am I right sir.

[P.K.Rao]

Please visit the following sites:

http://pac.iupac.org...f/5706x0855.pdf

http://lib.dr.iastat...xt=abe_eng_conf

Edited by P.K.Rao, 19 December 2014 - 09:45 AM.

[katmar]

The high boilers go mainly in the top product from the wash column.  You are correct that the bottom product needs to be rectified again.  This second rectifier is very similar to the first (industrial) rectifier and is usually known as the secondary or potable rectifier.

 

The top product from the wash column goes to a recovery column where the higher alcohols are removed as side streams and the top product is a low grade or industrial alcohol.