Distillation by rotating disks

[Enthalpy]

Hello nice people!

Fractional distillation is accomplished by varied apparatus:
http://en.wikipedia.org/wiki/Fractional_distillation
http://en.wikipedia.org/wiki/Spinning_cone
http://www.solvent--recycling.com/spinning_band_packed_column.html
but I haven't seen precisely the one I imagine:

 

 

The rotating disks expose a good area of liquid film for evaporation and condensation, while the pools are separated excepted for narrow pipes that avoid diffusion. This permits one temperature per pool and disk, and hundreds of disks are feasible. Thanks to the good stage separation at the liquid and the vapour, the theoretical separation efficiency must be attained - which competing columns with many stages don't.

 

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The rotation axis is essentially horizontal; a slight tilt can let the liquid flow gently in the direction opposite to the vapour, possibly with active valves at the pipes - or use pumps.

Limited clearances between the disks and the upper part of the vessel shall minimize vapour diffusion between the stages - use labyrinths or even seals if you prefer. Active valves at the liquid can regulate to zero the pressure differences between the stages.

The vessel can resist over- or under-pressure. Adding a carrier gas to the vapour would allow the liquid to evaporate gently, without bubbles: this makes evaporation more selective. The disks can be corrugated or sintered for increased area. The apparatus can have several feeds and exits as usual.

 

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Take 2mm thick stiff metal if the disks are seriously wide, then you can stack them at 8mm intervals for instance, which means 1000 disks and separation steps in an apparatus only 8m long - this is a difference with competing methods, the reason why I suggested the use for difficult separations. Other methods like particle beds can have more steps but don't separate properly the liquids at each step.

One apparatus can be flexibly divided into several sections aligned, with warmer and cooler ends alternating between them, and many inlets and outlets. That way, when you don't need 1000 successive separation steps but 10, you get the throughput of 100 apparatus in parallel - or the combination you prefer. Now it looks compact even if you evaporate without boiling.

The helices at the shaft between the disks, to prevent mixing the liquid trickling from adjacent disks, don't need to be machined in the shaft; spring wire fit around the shaft should suffice.

 

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What uses? I expect the rotating disks to evaporate and condense the compounds less quickly than other methods do, but:
- The gentle process, without spat nor local pressure and temperature fluctuations, keeps the evaporation selectivity;
- Hundreds of disks cumulate their selectivity;
- The process wastes very little power (one evaporation heat for hundreds of repeated steps) so it can be upscaled;
- The horizontal axis eases upscaling as well.

Hence I hope distillation by rotating disks may find some use where compounds are difficult to separate.

 

Marc Schaefer, aka Enthalpy

[Enthalpy]

Nearly the same setup can clean a gaz from impurities, including gases, fumes and aerosols - or dissolve the gas completely, or load a gas with a vapour. Home air humidifiers built similarly also claim to remove tobacco smoke after all.

The gas and the catching liquid flow transversally as on one sketch. This may reduce the pressure drop but it separates imperfectly the clean liquid from the used one, though wipers can improve that. Several stages would improve the exhaust cleanliness for the same liquid consumption.

 

Or the flows can be axial as on the other sketch. This one separates better the cleanliness stages of both the gas and the liquid but is expected to cost some more pressure. Passages permit the gas and liquid to flow by: holes on the sketch, could be slits, cuts-and-bends, stampings... Offsetting them from one disk to the next lets the gas and liquid pass by the disk faces where the useful action occurs. Many small passages ease the flow between the disks. While keeping a good ratio between the holes diameter, holes spacing and disks spacing, these can be reduced together to shrink the apparatus.

 

Combined designs can let the gas and liquid flow transversally in several steps through the apparatus split in sections.

Such setups accept corrosive, hot, dirty fluids more easily than other designs do with pumps. For instance, the gas can be hot SO₃ + SO₂ + O₂ and the liquid H₂SO₄ dissolving SO₃. I see no reason against a reaction in the same vessel. The disks can carry a catalyst too. A magnetic coupling can rotate the shaft.

 

Marc Schaefer, aka Enthalpy

[Enthalpy]

If using rotating disks to evaporate a solvent and concentrate a solution, a method that looks less prone to clogging, one may prefer to condense the solvent afterwards or catch the odours. On the appended sketch, a second set of rotating disks interleaved with the first set shall do that. It brings a second liquid to close distance to the first one over a big area where the vapour of the first liquid condense.

 

The second liquid can be cold to favour the condensation. It can be of the same nature of the first solvent.

The second liquid can also catch the first solvent's vapour by a strong interaction. Hygroscopic substances are known to absorb water vapour for instance.

One hygroscopic substance is ammonia solution, whose vapour might (or not?) impede the transfomation of urea into poisons if heat is used.

Both liquids can move slowly through the machine, for instance parallel to the rotation axes here, preferably in opposite directions so the second liquid enters the machine fresh at the end where the first liquid exits concentrated.

A good casing lets choose the operating pressure.

The disk profile can have a groove at mid-thickness if this avoids drops falling in the other liquid.

Marc Schaefer, aka Enthalpy