Vacuum distillation 2

[edtihcc]

Hi,
I am working on a batch vacuum distillation process that separates methanol,water and an unknown ingredient sensitive to temperature that degrades above 35 deg celsius.
There are foaming and bumping problems especially severe after the contents have been transferred in the batch reactor and start pulling a vacuum to about 0.14mbarA.
How do I go about trying to reduce the bumping and foaming problems?
THe temperature is jacket controlled and it fluctuates severely during the bumping. I am not sure if the bumping causes it or it causes the bumping. Does anyone know more about vacuum distillation and answers to these?

 

[KWHUI]

The simplest way to handle this problem is to work with a chemical manufacture who sells surface active agents and biocides. I believe that the foaming problem can be resolved when you mix the methanol and water mixture with a particular surfactant.

 

[JoeChem]

I have had some experience with this problem when trying to batch distill under very low pressures. Can be a real pain. The bumping and temperature fluctuations are caused by rapid evaporation of low boiling materials in the mixture. Non-condensibles may be present as well. The foaming is obviously a surface tension problem exacerbated by low pressures/rapid evaporation.

The only way I could deal with this situation was to lower the pressure slowly, in steps, where at each step some time was allowed for the mixture to agitate and "degas". When the light boiling materials were removed and the bumping/foaming ceased, the pressure lowered further and degassing step repeated. Eventually, the desired pressure set point was reached. Typically, the process was carried out at ambient temp or below as no heat was applied.

Once the desired pressure was established some heat was applied to begin th distillation.

Worked well most of the time...just added cycle time to process.

JoeChem

 

[edtihcc]

Thanks for the helpful tip.
To joechem, so the best way is to ramp up the vacuum slowly? Any idea when u did that, did u like double the cycle time or worse? Because I am suppose to improve cycle time and distillation efficiency including solving the bumping problem.

There is only methanol,water and a non-volatile ingredient in it. I'm not trying to separate methanol from water, but methanol and water from the non-volatile ingredient.

How do you perform degassing? By agitation at higher speeds?

I won't be able to add in surfactants as the other ingredient is for consumption.

 

[JoeChem]

Yes, the approach is to lower the pressure slowly. This allows time for any non-condensibles or light boiling materials to escape or evaporate away less rapidly. This "degassing" is simply agitating the mixture normally and spending some time at a fixed pressure and letting events calm down before lowering the pressure further. The additional cycle time varies depending on the system but never doubled.

Some comments more specific to your system...

The operating pressure specified (0.14 mBar or 0.1 mmHg) is very low. No doubt maximum removal of solvent (methanol) is required. One cannot attempt to lower the pressure to 0.1 mmHg in this system rapidly. The boiling point of the solvent is too low for this to be practical. Better to do in "steps".

Suggestion:

Slowly lower the pressure to about 40-50 mmHg and apply some heat. This should boilup the solvent at a temperature less than 35 C (bp methanol @ 40 mmHg = 12 C). As solvent is removed and the temperature increases, slowly lower the pressure further to keep mixture temperature below 35 C. If you do not have good pressure control, this may be problematic. If so, back off on the heat first and then lower pressure and resume heat once pressure is stabilized. Eventually, you will reach the desired final pressure.

Hopefully this is helpful.

JoeChem

 

[Compositepro]

Bumping is a very common problem in laboratory vacuum distillation where the factors that cause it are not well controlled. The vapor volume generated during boiling is more than a thousand times what it is at ambient presure. Tiny changes in temperature or pressure take conditions from no boiling to vigorous boiling. Superheating of liquids is common.

Bubbling a tiny amount of inert gas through the liquid helps prevent superheating by nucleating bubbles and the gas load on the vacuum pump helps regulate the presure.

Restricting the vapor flow to the condenser can also help because sudden boiling will increase the pressure in the vessel and regulate the boiling rate. The amount of restriction needed depends on the absolute pressure at which you are operating, however, and the presure changes during pump down.

 

[madhavank007]

Dear Friend
igot some tips from my collgs
bumping is mainly due to High volume and low boiling liquid
so during initial stage of vac distillation
switch off the stirrer maintain relatively low tempt
once substatial amount of solvent got distilled
u can increase the tempt
As u know water itself will started to distill at 20 deg c at good vacuum
so increase the tempt slowly
later switch on the agitator



K Madhavan

 

[edtihcc]

Thank you so much for all the tips...

How do you go about calculating the bp of methanol & water at low pressures such as the vacuum pressure? Since in most cases the actual composition of the solution is not known.
This is cause i was trying to determine what is the max. pressure i can go without exceeding the degradation temperature.

Also I noticed from past data that other than the fast pressure ramping,jacket temperature fluctuates alot during times when there was severe bumping.
How do I know if the fluctuations in jacket temperature (hence poor jacket control) is the cause of the bumping or it was due to bumping that I see temperature fluctuations?


Thanks!

 

[quark]

I don't think you can go for fast agitation, this may worsen your situation. Slow application of vacuum , as suggested by Joechem, seems to be good idea. Slow agitation with a propeller agitator will take care of the entrapped gases or evaporation.

If the rate of evaporation increases during bumping, you do experience fluctuations in jacket temperature.(one indication for this is decreasing vacuum)


Regards,

 

[25362]

It is possible some information could be obtained from a search through Google just by asking for methanol-water phase diagram at subatmospheric pressures.

 

[edtihcc]

To 252362, there is another unknown component in the distillation process..with that I think it will affect the boiling point right?

QUestion:
Why is it necessary to maintain the vacuum pump speed during the distillation process to increase distillation rate? Should it be pulling at a lower speed instead after vacuum has been established?

 

[25362]

If the unknown product degrades thermally, it may be worth to consider an alternative, such as those shown in

http://www.vta-deg.de/english/vakuumdestillation.htm

good luck.

 

[edtihcc]

I made a typo error..
the vacuum pressure is at 0.14 bara
not 0.14mbara..

 

[25362]

At 0.14 bara water boils at above 50[sup]o[/sup]C. So, how can one distill it at temperatures below 35[sup]o[/sup]C after the methanol has been chased away ? Could it be that the pressure is 0.014 bara ?

 

[edtihcc]

Yes it boils at 0.14mbara.
This is because it is a mixture of methanol and water with the unknown dissolved solid. The composition of water is about 10% only.

I suspect that the condenser is limiting the vacuum pump from establishing a better vacuum in the system.
Is there anyway I can investigate it?

 

[25362]

You have the specifications of the vacuum pump. Check the delta P of the condenser, and verify whether there is a leak of air or the formation of non condensables in the process, that could limit the vacuum pump capabilities.

 

[spekuljak]

The foam in distillation. If it is not produced by entraped gases but tensioactive agent (more heavy than methanol and water in this case), there are three ways to figth it, a) antifoam agent, b)a better reboiler design (falling film, recirculation), c)to break the foam by means small live steam jets (three or four installed in the column perimeter, from outside to inside). This last case we succesfully applied in cases of grappe fruit desulfitation, high foamly system.

 

[edtihcc]

The foaming is due to the Ammonia in the mother liquor..
other than degassing..is there other way of removing the ammonia to minimize the foaming?
I can't add anti-foaming agents in because the mother liquor that i'm distilling is used for consumption.
Also, i'm not using a reboiler..but a jacketed reactor to do the distillation.

 

[spekuljak]

Sometime the pH change allow to diminish or avoid the foam. In my experience, methanol-water in alcaline condition produces foam, but neutralization the liquor (pH 6,9 - 7,1) the foam dissapear. Be carefully with acidification, if the column material are steel or stainless steel, because the corrosion could be important at stronger acidic values of pH.

 

[carlos81]

I'm sorry to insist on the anti-foaming agents. But there's no way you can separate the agent after the distillation process?