Problems with evaporation crystallizer seal

[Pedro_GIC]

Hello, good morning,

I am working as a project engineer on an evaporative crystallization process, at pilot plant level, which uses a vacuum pump to promote evaporation of water at lower temperature etc.

The problem is that the vacuum is not generated because the round lid of the crystallizer is divided into 2 parts and has a large hole through which the agitator shaft passes. Is there any material, glue or mechanical seal that allows the equipment to operate as is?

The other option is basically to have a one-piece cover made with the hole just with the external diameter of the agitator shaft.

I remain attentive, and thank you very much!

 

[pierreick]

Hi,
This is ridiculous, how can you pull vacuum with this set up?
Pilot step is the intermediate step before industrial step where you can get all the parameters defined for future extrapolation. You need to get it right with proper equipment.
For shaft you need mechanical seal(s) with fluid barrier, no opening on crystallizer, a sight glass and a small manhole for cleaning, no bolted lid, probes must be tightened, transfer by pump.
my 2 cents
Pierre

 

[jari001]

What level of vacuum are you trying to maintain?

There's also a hole that tubing seems to pass freely through that's up and to the right from the E-H LT...that's a vacuum leak for sure. At minimum, you need to make a one piece cover with proper agitator seals and proper connection types to ensure vacuum tightness for other instrumentation.

 

[Compositepro]

Vacuum (reduced pressure) will do little to increase "evaporation" unless you reach the boiling point. True evaporation uses air flow to move water vapor away from the liquid surface. Making the clearance between the shaft and hole larger will allow more air flow, and increase evaporation rate. A baffle or duct around the shaft to maximize the air velocity across the liquid surface can significantly increase evaporation rate, and warming this air will as well.

A vacuum pump is designed to reduce pressure, whereas a blower is designed to move air. You may not be using the correct word in English, but a "vacuum pump" would be inappropriate for your process.

 

[Pedro_GIC]

pierreick (Chemical)

Hi,
This is ridiculous, how can you pull vacuum with this set up?
Pilot step is the intermediate step before industrial step where you can get all the parameters defined for future extrapolation. You need to get it right with proper equipment.
For shaft you need mechanical seal(s) with fluid barrier, no opening on crystallizer, a sight glass and a small manhole for cleaning, no bolted lid, probes must be tightened, transfer by pump.
my 2 cents
Pierre

Hello pierreick thank you for replying and thank you for the shaft.

Indeed it is ridiculous, no vacuum is achieved. The container is not airtight.
The vacuum pump only generates pull for the vapours coming out of the suction line going to the condenser. The pull is maintained by the change of volume of the water vapor to liquid, the evaporated water is recovered from the system to know the concentration of the inorganic solution inside the crystallizer and to achieve saturation.

jari001(Chemical)
What level of vacuum are you trying to maintain?

There's also a hole that tubing seems to pass freely through that's up and to the right from the E-H LT...that's a vacuum leak for sure. At minimum, you need to make a one piece cover with proper agitator seals and proper connection types to ensure vacuum tightness for other instrumentation.

Hi jari001, thank you for replying,

These guys use a 7mbar (2 m3/h) vacuum pump at lab level. Don't ask me. It's the only thing they have on hand.

They want to evaporate at 90°C but with that pump and with the tightness issues it doesn't evaporate the water very fast let's say.

Compositepro (Chemical)28 Nov 22 18:26
Vacuum (reduced pressure) will do little to increase "evaporation" unless you reach the boiling point. True evaporation uses air flow to move water vapor away from the liquid surface. Making the clearance between the shaft and hole larger will allow more air flow, and increase evaporation rate. A baffle or duct around the shaft to maximize the air velocity across the liquid surface can significantly increase evaporation rate, and warming this air will as well.

A vacuum pump is designed to reduce pressure, whereas a blower is designed to move air. You may not be using the correct word in English, but a "vacuum pump" would be inappropriate for your process.

Hi Compositepro, thank you for replying,

In the literature they also differentiate between evaporative crystallization and vacuum crystallizer.
I am going to do more research on the subject.

 

[pierreick]

Hi,
If you share some info about the product and the objectives of your project, we may be able to give you meaningful information.
Right now, it's more less plumbing.
Good luck
Pierre

 

[3DDave]

Getting most of the water out of sugar requires over 300F (150C) so if you are stopping at 90C you have a long long wait I expect. The better approach is to supersaturate the solution at a higher temp and let the crystals form in the solution as it cools.

The rate at which the water leaves has less to do with the pressure and everything to do with the temperature, though removing the water by freezing it would likely to the same as the vacuum pump in dehumidifying the atmosphere. As the solution becomes more concentrated the energy required to get the water out rises - which is why candy requires the temp far over the boiling point of water.

I suppose whatever you are crystallizing cannot withstand over 90C, so maybe put the material into anhydrous alcohol to give the water a better place to go and then let the alcohol evaporate which will happen much faster, possibly with an acetone follow-on to remove the alcohol.

 

[Compositepro]

Dave, you appear to be speaking from experience in making sugar candies on the stove. That is amorphous, molten sugar. Sugar crystals do not contain water.

 

[jari001]

Did I miss the post that said the OP's process was related to sugar?

 

[3DDave]

You are right - all candy recipes start with dry sugar and how soft they are when cooled depend on the highest temperature reached. /s

I do note the lack of nucleation sites in the OPs apparatus. A quick review shows that steam is usually used for this process, but as I mentioned, the material in question is probably not one that can withstand higher temperatures.

jari001 - Sugar is an example. You didn't miss anything.