Vacuum distillation
Vacuum distillation
(OP)
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?
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?





RE: Vacuum distillation
RE: Vacuum distillation
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
RE: Vacuum distillation
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.
RE: Vacuum distillation
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
RE: Vacuum distillation
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.
RE: Vacuum distillation
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
RE: Vacuum distillation
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!
RE: Vacuum distillation
If the rate of evaporation increases during bumping, you do experience fluctuations in jacket temperature.(one indication for this is decreasing vacuum)
Regards,
RE: Vacuum distillation
RE: Vacuum distillation
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?
RE: Vacuum distillation
http://www.vta-deg.de/english/vakuumdestillation.htm
good luck.
RE: Vacuum distillation
the vacuum pressure is at 0.14 bara
not 0.14mbara..
RE: Vacuum distillation
RE: Vacuum distillation
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?
RE: Vacuum distillation
RE: Vacuum distillation
RE: Vacuum distillation
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.
RE: Vacuum distillation
RE: Vacuum distillation
RE: Vacuum distillation
we don't wanna cause any side reactions or contamination to the substance...
only way is to change agitation speed or other operating conditions to improve the situation.
How do u know what is the best agitation speed to operate it at to reduce foam?