Wednesday, July 15, 2015

Process Wednesday: double cone dryers

I've not had a ton of experience with double cone dryers, but I know that they are a common means of drying compound. They are certainly unique-looking - I presume that it took many years to come up with that shape.

McConville's "The Pilot Plant Real Book" mentions that the agitation is gentle and you get decent product homogeneity, which is good. I presume that you load up the product in one handway, turn on the vacuum (and the heat?) and allow it to slowly rotate and dry product. As the video shows, they don't rotate very fast. When you're done, you get your bag ready and open it up and let the product fall down? Dunno.

I am going to guess that cleaning one of these things is a bear, but again, not much personal experience with them. 


  1. They need to be rotated slowly so that the wet solids in the rising cone have the chance to fall in to the descending cone. If you rotate too fast the cake is pressed into one cone and you end up with a piece of rock. The right rate of turning is even more important for larger dryers as they can become imbalanced if the solids don't tumble inside.

    When the crystals are very fragile the rotary dryer can produce a lot of fines. You can get an indication of trouble ahead while watching the crystals under a microscope. Scoop some crystals with mother liquor onto the microscope slide and take a picture before applying any pressure to the crystals. Next move the crystals around with the slightest touch of a spatula or needle or apply a cover slide. If you see a pile of fines you will be better off using a tray dryer for this batch. For the next batch consider developing another form.

  2. Is there a big dent in the side of that thing?

    SJ is right on that you have to be careful about the particles. Put the wet cake in when it's still too wet, start tumbling, and you might be doing wet granulation that gives you pea-sized to basketball-sized rocks. Rotate too fast when it's dry and you might be doing low impact milling. These decisions on what equipment to use for drying and how to simulate the drying process in the lab are right in the wheelhouse of the engineers and solid state chemists.

    1. It's just fiiiiiiiiiiine, we can take $500 off the sale price.

      Thanks, SJ and Ken, for your contributions.

  3. Ken is right about deliquoring the filter cake. It is easier to do in a centrifuge than in a Nutsche filter.

    The dent seems shallow. Perhaps only the jacket is a bit narrow in that spot and the vessel is fine.

    CJ, is this thing glass lined or Hastelloy? Or, SS? If it is GL I would spark test inside the vessel around the dent.

    On another note emptying that thing is a ton of joy. A tray dryer is a toy to empty, a Rosenmund is a bother, and the rotary dryer is a workout. Without a laminar flow enclosure avoid drying anything more toxic than kitchen salt. Fines + static electricity + vertical travel. You get the drift (cough, cough...).