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Particulate segregation from vacuum vapor line
2

Particulate segregation from vacuum vapor line

Particulate segregation from vacuum vapor line

(OP)
We are conducting an R&D experiment to dry material in a vacuum dryer (about 200 mmHg absolute). It is proprietary so I cannot include any specifics. Much more particulate exits the dryer with the solvent vapor than expected. We have mechanical filters installed in the vacuum line but they are entirely ineffective.
I am interested in anyone's input for what the best available technology might be to remove the particulate from the vapor stream right at the dryer. I first envisioned a fabric filter, but I'm concerned about keeping everything hot enough to prevent condensation. A second idea is a small electrostatic precipitator, but I do know know how the efficiency will be affected in a vacuum. I believe the velocity and efficiency is too low, and mechanical loss would be too great for a cyclone.

Brad Waybright

It's all okay as long as it's okay.

RE: Particulate segregation from vacuum vapor line

I think a cyclone could work well for for you, but why do you have particles coming out of your dryer? This means you have agitation of the dryer bed due to boiling or high vapor velocity leaving the bed. The simple thing to do is to not entrain particles in the vapor flow in the first place, which is easy to do with a tray dryer. Drying rate is controlled by controlling heat input or pressure. I suspect that your real issue is that you apply too much vacuum too quickly so that there is very intense boiling when you initially apply vacuum. Pressure in the dryer should be reduced slowly to a point where the boiling vapor flow begins, but is not great enough to entrain particles. Continue to reduce pressure slowly as the bed temperature reduces due to boiling. You can do this with a vacuum pressure regulator, but a simple restrictive orifice (needle valve) will be somewhat self regulating. The vapor flow will be limited, and then as the material dries and cools the pressure will fall in the drying chamber to maintain boiling of the solvent. At a certain point you can open the vacuum valve completely because the drying rate will no longer be determined by your vacuum pump capacity but by the rate of heat flow into your bed.

RE: Particulate segregation from vacuum vapor line

(OP)
This is a continuous process and similar to a ribbon dryer so the product is constantly moving. The system is sized for the capacity to dry the material and can't be limited to minimize reentrainment. Those two factors dictate the exhaust velocity and previous assumptions suggested minimal carryover.

Brad Waybright

It's all okay as long as it's okay.

RE: Particulate segregation from vacuum vapor line

Then you can only prevent particle entrainment by lowering feed rate, or perhaps slowing the ribbon speed. If that is unacceptable, then you need a cyclone and filter. This just means that the process was not properly designed in the first place. You could increase the volume of the ribbon blender by adding a chamber above it to allow for particle settling. None of the solutions will be trivial in terms of expense and added maintenance and cleaning.

RE: Particulate segregation from vacuum vapor line

(OP)
This R&D project wasn't really meant to be a test of our vacuum system, but the main problem is that the solvent vapors are intended to be condensed and returned to the process. The vent lines are heated all the way to the condenser. The dryer works a little bit more like our fluid bed dryer which has a big baghouse on top. Instead of introducing heated air from below, the product dries naturally in the high temperature, low pressure environment while being gently blended. The material is very sticky when solvent is present so any condensation on filter bags (or any other filter/separator material) is a bad thing. We have 'X' amount of material with 'Y' amount of solvent to remove. Those are both constants. I do not believe it is possible to eliminate reentrainment of all particulate from the dryer atmosphere. My primary question regards whether a precipitator will provide the required efficiency in the scarce atmosphere as I believe the ability to generate a corona will be greatly diminished. Yes/no?

Brad Waybright

It's all okay as long as it's okay.

RE: Particulate segregation from vacuum vapor line

At the level of vacuum you are at, it will not affect ionization significantly. However, they are prone to sparking.

RE: Particulate segregation from vacuum vapor line

How about a liquid solvent spray contacting tower? Run at a high solvent recirculation rate that may prevent this sticky consistency, then pump it out from the bottom of the tower for recycling? If this may work, it probably means running at a much higher solvent rate right through this process due to this recyling.

RE: Particulate segregation from vacuum vapor line

(OP)
That's not an option. I want to retain all of the particulate in the dryer and to condense the solvent and return that to the process too. There should be nothing in the exhaust stream except solvent vapor and a small amount of nitrogen used to keep the dryer atmosphere inert.

Brad Waybright

It's all okay as long as it's okay.

RE: Particulate segregation from vacuum vapor line

Well, can you tell us what happens upstream of the dryer and at what location and how does the pressure drop down to 200mmHg abs, presuming you are drying by reducing vapor space pressure and not by thermal means. If you are dropping the pressure all in one go, that may be the reason why you get all this particulate entrainment into the vaporised solvent, as a result of this sudden decompression. Will dropping the pressure in 2 or 3 stages, with some residence time in each of the stages help?

RE: Particulate segregation from vacuum vapor line

(OP)
That was already discussed. It is a continuous process. The dryer vacuum is held constant while material is added. My question isn't about how to prevent dust entrainment. I do not believe the process can be modified to prevent it. I'm simply trying to determine the best method to keep the dust in the dryer.

Brad Waybright

It's all okay as long as it's okay.

RE: Particulate segregation from vacuum vapor line

The fact that this is some gooey paste in the vapor stream leaves you with no choices other than to modify the existing process scheme if you ask me.

RE: Particulate segregation from vacuum vapor line

What is the particle size of the dust entrained in the vapor space? That answer will significantly affect the options for removing the dust.

RE: Particulate segregation from vacuum vapor line

Most People would use bag filters with N2 blow back to push the dried material back into the dryer. If you are getting particles through the bag reduce the pore size on the bags. Work with WL Gore on the bag porosity issue. You may need two stages of bag filters. The second stage may not be able to be recovered and sent back to the dryer.

Brad

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