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Crude Glycerol Purification


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#1 seanjeremiah

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Posted 22 October 2014 - 07:26 AM

Hello engineers,

 

What would be a practical method to purify glycerol to a grade suitable for catalytic hydrogenolysis?

The composition of the crude glycerol by weight percentage is as follows:
 

Glycerol      22.9
Methanol    10.9

Water         18.2
Soap           26.2
FAME          21.3
Glycerides    1.2
FFAs            1.0
Ash               3.0

TOTAL       102.9*

*inorganic salts are double counted in soap and ash content

This is a hypothetical composition from a biodiesel plant given to us as part of a class project on producing value added products from glycerol.

Any input is much appreciated. Thank you very much!


Edited by seanjeremiah, 22 October 2014 - 07:32 AM.


#2 MrShorty

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Posted 22 October 2014 - 09:55 AM

Early in my career, my mentor would do an interesting thing with me. A lot of time when I would ask a similar, open ended question, he would turn it back to me and tell me, "come up with a proposal to do <blank> that you think will work, then propose that to me and we can talk about the strengths and weaknesses of your proposal." If no one else responds, I would suggest that you come up with an idea and propose it. State why you think it will work and state the reasons that you would be hesitant to try your proposal. I often find that I get better discussion when I propose something for critique than when I ask "open ended" questions.



#3 P.K.Rao

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Posted 22 October 2014 - 10:48 PM

The first thing you have to do is removing soaps by precipitating them with a water soluble aluminium compound such as aluminium chloride and  filter. Then steam it to remove methanol. Distil under vacuum to recover glycerine rejecting the water  cut..



#4 seanjeremiah

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Posted 27 October 2014 - 01:30 AM

Early in my career, my mentor would do an interesting thing with me. A lot of time when I would ask a similar, open ended question, he would turn it back to me and tell me, "come up with a proposal to do <blank> that you think will work, then propose that to me and we can talk about the strengths and weaknesses of your proposal." If no one else responds, I would suggest that you come up with an idea and propose it. State why you think it will work and state the reasons that you would be hesitant to try your proposal. I often find that I get better discussion when I propose something for critique than when I ask "open ended" questions.

 

Thanks for the advice MrShorty. I should explained what I have already accomplished. I started out from scratch again as I am doubtful about my current design. Please see the attached PFDs.

 

Here's what I had planned.

 

Given the composition if crude glycerol, the first process would be to remove ash from the feed. This is achieved using a 0.45 um micro-filter. This process is followed by recovering a fraction of the FAME using a liquid-liquid hydrocyclone. The FAME is fed back to the biodiesel production plant.

 

The remaining FAME is now converted into soap via the following saponification process:

 

R-COOCH3 + NaOH à R-COONa + CH3OH

Methyl Oleate + Sodium Hydroxide à Sodium Oleate + Methanol

 

The overhead product of from the reactor is largely methanol and water. This too is fed back to the biodiesel plant. The liquid product is then fed to a second reactor where the following reaction occurs:

 

R-COONa + HCl à R-COOH + NaCl

Sodium Oleate + Hydrochloric acid à Oleic acid + Sodium chloride

 

By the acidification process, soap is converted to the free fatty acid, oleic acid, and salt. Since glycerol (SG: 1.26) is more dense than oleic acid (SG: 0.89), the two phases are easily separated using a phase separator.

 

In order to remove salt which remains dissolved in the glycerol, glycerol is evaporated under vacuum conditions at 71.3 kPa and 230°C. The vapour product is fed to a distillation column, also operating under vacuum conditions to remove methanol and water.

 

The product of distillation is 99.9% glycerol with trace amounts of salt and water.

Attached Files



#5 seanjeremiah

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Posted 27 October 2014 - 01:47 AM

The first thing you have to do is removing soaps by precipitating them with a water soluble aluminium compound such as aluminium chloride and  filter. Then steam it to remove methanol. Distil under vacuum to recover glycerine rejecting the water  cut..

Thank you for your reply. 

I had not considered flocculation using aluminium chloride as I was unsure if the same process used to coagulate mud in raw water would work on soaps in this glycerol-oil mixture. I will read read more on this. Perhaps you could provide some additional information.

 

Please excuse my lack of knowledge. What do you mean by steaming the glycerol? I understand that evaporation can be used to recover methanol. Must the steam come in contact with the glycerol or would it be better for the glycerol to be simply heated to 65 C to allow the methanol to evaporate.

Also, I understand what you mean by distilling to recover the glycerol.But what do you mean by rejecting the water cut?



#6 P.K.Rao

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Posted 27 October 2014 - 03:58 AM   Best Answer

In this process, aluminium chloride is not used for flocculation. It is used for precipitating soaps. If they are not removed, during distillation, there would be a lot foaming and distillation would become practically impossible. Soaps which are present as sodium salts of fatty acids are precipitated as aluminium soaps. You can recover the aluminium soaps also by washing and drying. Aluminium soaps are used in grease manufacture.

You have to distill crude glycerine under vacuum. If you distill under atmospheric pressure, or if the pressure is high, glycerol will crack. 1-2 mm Hg is OK. Under 1-2 mm Hg, methanol can not be condensed, It will go as  vapor and lost. So steam the crude glycerol so that it would go with steam which can be condensed along with methanol and there is a possibility to recover the methanol. You may study in laboratory and optimize the conditions. Alternately distill under a higher pressure till methanol is removed and then lower the pressure gradually to remove water and obtain glycerol.

When you distill under  vacuum (after removing methanol, ofcourse), water is the first to evaporate because its boiling point is 100 deg C and that of glycerol is 290 deg C (under amospheric pressure). You have to separate this water. Otherwise, recovered glycerol would be of low purity. You may try other ways also, if you find or  if you know any without affecting glycerol purity or recovery.

P.K.Rao


Edited by P.K.Rao, 27 October 2014 - 07:57 AM.





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