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Removing Polymer from Centrifuge

08/01/2014 2:38 PM

Hello, here is an issue I would like to discuss with you: one of the products my company manufactures is an Acrylic polymer (PMMA). The whole chemical process takes place in an aqueous suspension in a 1000 liter reactor. Once the polymerization is finished the suspension (water + newly formed polymer granules) is emptied into a centrifuge in order to separate water from particles. The process ends when water is recycled and particles are dried in an oven @ 60ºC.

The procedure I would like to improve removal of the polymer from the centrifuge.

The centrifuge is about 700 mm (70 cm) high and 1000 mm (1 meter) in diameter. A big nylon bag placed inside the "basket" acts as a sieve to dramatically reduce polymer loss during centrifugation. When the centrifugation comes to an end, the inner walls of the bag are completely covered wit approx 100 mm of polymer. The polymer is removed from the centrifuge by means of an oar-like plastic shovel to prevent tearing the bag (which actually happens more often than expected)

I would like to improve this process in order to reduce the contact between the person who operates the machine and the product (possible contamination of the product) and to speed up the centrifugation.

I have thought of using vacuum, but it seems a bit noisy and messy to me. Another alternative may be a flexible Archimedes screw .... but well, no concrete idea yet.

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Guru

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#1

Re: Removing polymer from centrifuge

08/01/2014 5:05 PM

A question: Is the centrifuge interior larger in diameter, smaller in diameter or the same diameter as the centrifuge opening? (A photo would be helpful but not strictly necessary)

Thanks

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#3
In reply to #1

Re: Removing polymer from centrifuge

08/01/2014 7:43 PM

The drum (or basket) of the centrifuge is cylindrical. The diameter of the opening at the top is a 30% smaller than the drum. I evaluated the possibility of de-attaching the bag from the drum, to lift it with a small winch and to open it at the bottom on a suitable tray.... but the opening is to small. Enlarging it would probably make the operation of the machine dangerous

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#4
In reply to #3

Re: Removing polymer from centrifuge

08/01/2014 8:14 PM

In my reply where I said: to lift it with a small winch and to open it at the bottom on a suitable tray I actually meant making an opening (with a lid) at the bottom of the bag in order to unload the content of the bag on the tray that later on gets into the drying chamber.

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#5
In reply to #3

Re: Removing polymer from centrifuge

08/01/2014 8:50 PM

Can the bag be removed then emptied by turning it inside-out, possibly by means of a 'flexible handle' sown into the inside-bottom of the bag?

Might a different bag material, reinforced Teflon perhaps, aid in reducing adhesion of product to the bag? How much product clings the bag? Would it tend to build up or would the amount be fairly constant so that, over multiple runs, the amount emptied after the first run remains roughly the same?

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#7
In reply to #5

Re: Removing polymer from centrifuge

08/01/2014 9:29 PM

What you mean was one of my first ideas: to get it out of the machine and to empty it turning it inside-out.. but the bag is made of a sturdy polyethylene mesh (it actually looks like the jumbo bags used for transporting 1 m3 sand on trucks (more or less like this one)

The problem is that when lifting a batch of polymer (approx 1/3 of the capacity of the jumbo bag) it adopts a pear shaped form and get´s stuck at the opening of the machine

There is no significant adhesion of the polymer to the bag.

I also have thought of -instead doing the process in a single batch, to do it in smaller amounts, but it is unpractical, because it prevents me from immediately starting a new process in the reactor (process in the reactor takes about 6 hours) and such a delay means to have to add a new shift of personell for controlling the process. I also searched for continuos centrifuges, but we cannot afford buying them, as they are pretty expensive

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#9
In reply to #7

Re: Removing polymer from centrifuge

08/02/2014 12:39 AM

You need a "rigid" bag that is small enough to lift out of the mouth of the centrifuge.

Or, a porous bucket that holds the bag so it can't bulge when lifted.

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#10
In reply to #7

Re: Removing polymer from centrifuge

08/02/2014 4:50 AM

Does the centrifuging stage pose the 'bottleneck' in your process? If you could eliminate it altogether how would you do it? What other techniques quickly remove bulk water? Drain then run through a heated vacuum dryer? (heated because the remaining water will freeze if you don't). Is it possible to mount the (or a) centrifuge on a gimbal which allows the contents to be dumped, somewhat akin to a cement mixer?

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#13
In reply to #10

Re: Removing polymer from centrifuge

08/02/2014 1:12 PM

Mounting on a gimble would also be my choice. Given that the capacity is 1000 litres and the base looks to be cast iron the whole centrifuge with motor is going to weigh in the order of ¾ton. You would need a rigid welded frame mounted on maybe 6" bearings with the centrifuge attached by anti vibration mountings. It would need to invert 180º and be fitted with a vibrator so that the polymer could be vibrated out onto the tray. Balance point just above the centre of gravity for easy invertion and strong locking bolts in both the upright and inverted positions. If you have compressed air available flipping the machine would be safer and quicker if done with pneumatic cylinders. Should be able to get unloading times down to 1-2 minutes with zero chance of damaging the filter bag.

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#16
In reply to #13

Re: Removing polymer from centrifuge

08/02/2014 7:25 PM

Great ideas! Thanks

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#15
In reply to #10

Re: Removing polymer from centrifuge

08/02/2014 7:23 PM

Hi europium! When proof reading a previous reply to you, pressing some wrong button made it vanish in cyberspace! Well, let´s try again: The entire process has some more steps than in the OP: the polymer is produced in the reactor and dumped into the centrifuge where we spin it to remove excess water. After that some rinsing cycles are made with distilled water inside the centrifuge (number depends on tests made on the residual water). Usually we manage to do two complete polymerisations in a shift. Speeding up this process would allow us to do an additional process in a back-up reactor we also have. Perhaps for being impressed by the "chunkiness" of the centrifuge I never thought of tilting it to unload the machine: next Monday I will take a closer look at it to see if it is feasible. My only concern are the springs, the machine rests on, but I guess it will not be difficult to block them somehow... GREAT IDEA!!!! Thanks

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#8
In reply to #1

Re: Removing polymer from centrifuge

08/01/2014 9:32 PM

The machine looks like this one (image "borrowed" from the web)

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#2

Re: Removing polymer from centrifuge

08/01/2014 6:04 PM

Not sure the present configuration can be modified to speed up the water separation.

Once the surface of the bag is covered by product, then the limiting factor becomes the acrylic, not the bag.

Have you explored a different bag material?

By "vacuum", do you mean to decrease drying time or removal of product after drying.

Interesting problem.

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#6
In reply to #2

Re: Removing polymer from centrifuge

08/01/2014 9:07 PM

I probably have not been to precise in my question: the first drying process is no issue, as it only takes some 3 to 4 minutes to separate water from polymer. When you say: "then the limiting factor becomes the acrylic, not the bag" you are right: the limiting factor is the manual removal of the acrylic from the bag. I was wondering if any of you had a better idea on how to replace the shovel actually used for a more technological method. I have evaluated different alternatives: one was to detach the bag (with the polymer contained within) from the drum, to lift it out of the machine with a small winch attached to the ceiling by means of a rail system that would allow me to to move the hanging bag some meters, in order to empty it by gravity (lid at the bottom) on a tray that will then go to the drying chamber. By using "vacuum" I mean to get the polymer out of the centrifuge using something like a vacuum cleaner, where the product gets aspirated by means of a hose, and transferred into a plastic tank (which could be on a slightly higher level in order to empty it directly on the tray/s by gravity... I discarded this idea, because one can anticipate the constant clogging of the aspirating hose. Some more info: after centrifugation, polymer can be easily detached from the walls of the drum, looking like slightly humid sand. If you take it into your hands and compress it, it will seep no water, but will get hard (just like sand would do). I believe this is called a thixotropic phenomenon.

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#11

Re: Removing polymer from centrifuge

08/02/2014 5:47 AM

Sounds like the only real problem is the size of the top opening. Why not cut it off and weld some screw clamps to it?

Like this:

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#12

Re: Removing polymer from centrifuge

08/02/2014 1:04 PM

Use a vacuum transfer apparatus. It is identical in operation as a shop vacuum. A soft surfaced tube is connected to a hose which leads to a filter and a fan/motor which is located on a receiver container. Centrifuge the acrylic, open the lid, turn on the vacuum collection system, vacuum the acrylic from the centrifuge with the vacuum collection apparatus, shut off the vacuum, dump the granules onto/into a container and dry in the oven. This can be used either by itself or in addition to the removal of the bag after sufficient granules have been removed to take the bag from the centrifuge.

It can be made to an optimum system for your needs. Vacuum can be provided by a plant vacuum system or aspirator, an enclosure which combines the tray and the oven, weight scale, particle sizing and many other processing steps. This also has the advantage that it can be arranged for no human contact with the materials, inert atmospheres, portioning and other conditions.

Have used this method for the transfer of powders from drums to a PK blender and also loading powders into reactors with liquids in them. This eliminated exposure to operators which was very important since the powdered materials contained a mercury compound which would severely burn skin. It also reduced atomized powers in the room air.

Try the method out by putting saw dust in a 5 gallon pail, vacuum it through another pail acting as a powder receiver which is connected to a shop vacuum. The dust is moved from the first to the second pail which represents the final transfer point.

Good Luck, Old Salt

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#14
In reply to #12

Re: Removing polymer from centrifuge

08/02/2014 5:21 PM

This is not a commercial endorsement nor sales pitch. For information only!!!!!

Couldn't remember it before but the vacuum systems utilized on several of our systems were manufactured by VAC-U-MAX of Belleville, NJ. There are probably others that make this type system. Ours was not the complete blender system but just the vacuum collection and material transfer system.

http://www.vac-u-max.com/

Good Luck, Old Salt

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#17
In reply to #12

Re: Removing polymer from centrifuge

08/02/2014 7:31 PM

I have contacted a company that manufactures vacuum equipment for such a purpose. They will be in next Tuesday. Will see what they say! As stated on a previous post I will also evaluate tilting the machine. (europiums idea) Have a great weekend on your boat!

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