5 replies to this topic

[shunmu]

I have to remove X qty of a solvent with boiling point 116 deg C at vacuum.How could i find the time taken for removal of the same theoritically?

The problem is i have temperature restrication cannot heat more & Vacuum can be upto 1 torr.Known parameters: Temperature, Pressure in the equipment, Quantity of solvent

Also vapor pressure of solvent is dependent on the associated fluid in the equipment or not?

[Art Montemayor]

Shunmu:

It would be much simpler and easier to understand your application if you could furnish even a simple PFD (Process Flow Diagram) of your system. I think I understand what you want to do, but I’m not sure. I presume you have a batch operation. I also assume you have a heated vessel with a vacumm device pulling a 1.0 torr vacuum on it. The vessel vapors exit through the overheads and into a vent condenser that is connected to the vacuum device. Is this correct?

If so, the time it takes to remove the solvent is related to the capacity of the vacuum device, the vent condenser, or of the heating device in the vessel. One or the other will be the constraint and will set the maximum rate of flow of the solvent vapors. Usually, it is the heater or heating device in the vessel that is the constraint, although the vent condenser size can also limit the rate at which the solvent is condensed and removed. Seldom is it the vacuum device – if it is sized appropriately.

The vapors generated in the vessel will be vapor pressure-dependent and will be composed of the various liquid fluids found in the vessel – according to their vapor pressures at the operating temperature (160 ^oC).

The vacuum pressure you are creating in the vessel is the summation of all the partial vapor pressures generated by the liquid(s) at the operating pressure (160 ^oC).

[Ali66]

Hello,

Are you saying the boiling temperature of the solvent is 116 C at 1 torr absolute pressure?
If so, you are likely going to need a tight vacuum system and a vacuum pump that can enable obtaining a lower absolute pressure. This may be the limitation on your evaporation rate.
Please clarify.

If the solvent vapor pressure at the max temperature is greater than 1 torr, then likely your evaporation rate is controlled by heat input rate in the reboiler (depends on surface area, delta T and fouling.) This assumes you are able to pull down to 1 torr and vapor pressure of solvent condensate is lower than 1 torr, i.e, vacuum pump mainly removing ingress of air.

Is it a pure solvent or you need to evaporate a solvent from some mixture?

Ali

[shunmu]

Yes you are correct.We have adequate vacuum capacity about 2000m3/hr.99.5% of the solvent is easily removed. Remaining 0.5% only taking much more time. This we are trying with Nitrogen purging to lift the vapors. Let us try that.

Shunmu:

It would be much simpler and easier to understand your application if you could furnish even a simple PFD (Process Flow Diagram) of your system. I think I understand what you want to do, but I’m not sure. I presume you have a batch operation. I also assume you have a heated vessel with a vacumm device pulling a 1.0 torr vacuum on it. The vessel vapors exit through the overheads and into a vent condenser that is connected to the vacuum device. Is this correct?

If so, the time it takes to remove the solvent is related to the capacity of the vacuum device, the vent condenser, or of the heating device in the vessel. One or the other will be the constraint and will set the maximum rate of flow of the solvent vapors. Usually, it is the heater or heating device in the vessel that is the constraint, although the vent condenser size can also limit the rate at which the solvent is condensed and removed. Seldom is it the vacuum device – if it is sized appropriately.

The vapors generated in the vessel will be vapor pressure-dependent and will be composed of the various liquid fluids found in the vessel – according to their vapor pressures at the operating temperature (160 ^oC).

The vacuum pressure you are creating in the vessel is the summation of all the partial vapor pressures generated by the liquid(s) at the operating pressure (160 ^oC).

[shunmu]

Boiling point is 116 deg C at atmospheric pressure.We cannot heat beyond 70 deg C.It decomposes. Vacuum system is hydrotested.

[Ali66]

shunmu,

For lack of knowing the solvent specifically these are general comments.

It is not clear why for a solvent/liquid that boils at 116 C at atmospheric pressure that you'd need to go down to 1 torr when you are able to heat to 70 C? Likely even higher absolute pressure should be adequate for most of the evaporation/mass removal.

If you are at 1 torr, what is your condensate temperature? In order for you to be still pulling the solvent away from the equipment upstream of condenser your overhead condensate temperature need to correspond to a solvent vapor pressure lower than the working absolute pressure ( 1 torr). Is your condensate refrigerated cold? Unless you are pulling the last bit of solvent through the vacuum pump itself.

If you use Nitrogen to "lift" the residual vapor you need to ensure capture of the same vapor from this Nitrogen before it is released into atmosphere.
1 torr absolute pressure is reflective of very low density/very small mass per unit volume. If necessery a few purge/pressure and release can take care of it as long as vented amount of vacuum exhaust is handled appropriately if materail is hazerdeous or harmful to the environment.

Ali

Boiling point is 116 deg C at atmospheric pressure.We cannot heat beyond 70 deg C.It decomposes. Vacuum system is hydrotested.