Prevent heat-exchanger plugging

Draw from a higher point
It appears that the problem is in pumping the slurry through the heat exchanger due to the high solids loading. A simple solution is to pull the solvent for recovery at a point high enough in the vessel to maintain low solids. Once the solids level rose enough, you could then pump from your current location (presumably the bottom) to the incinerator feed.

The puzzler did not specify the shape of vessel used to hold solvent awaiting recovery. If you are starting from scratch, consider a lamella separator with a conical bottom for pumping out the sludge or, if the solids are sticky, then an oversized conical tank with subsurface feed (below the takeout point for pumping to the evaporator).

Barry Bershad, senior process engineer
Clariant, Coventry, R.I.

Install an automatic filter
As solids seem to limit the capacity of the evaporator, why not install
some way of removing them? An automatic filter is one device that could work here if the solids are not sticky. The filter would discharge a concentrated stream to a collection system whenever the pressure differential across it reached a given set point. In this way, the evaporator could be operated continuously, resulting in greater throughput for this system. Admittedly this solution would require new equipment to be installed.

Jim Darby, process design engineer
LANXESS Inc., Sarnia, Ont.

Consider four options
First of all, I’m not sure of the reasoning behind using a falling film evaporator to concentrate a mother liquor solution from a crystallizer. With the mother liquor being saturated, any increase in concentration by evaporation will result in crystal (solids) formation.

1. Why not send the mother liquor back to the crystallizer rather than the falling film evaporator?
2. If the mother liquor cannot be sent back to the crystallizer and must be processed by the evaporator, then go to continuous operation and add a “product pump” with density control loop and modify the suction piping for the recirculation pump per Figure 1. With the rapid solids settling rate, the new product pump will discharge the slurry to the incinerator, and the recirculation pump will handle “clarified” solution.

3. It is possible that fouling in the heat exchanger is a result of plugging of the distribution plate (if a drilled perforated plate is used). If so, I recommend replacement with a shower-spray-nozzle distributor.
4. With an increase in production requirements, I recommend addition of a submerged-tube forced circulation finisher to operate in parallel in terms of vapor flow and series in terms of liquid flow. The solids concentration in the falling film would be reduced and the product concentration solution would be delivered by the forced circulation effect.

Joseph T. Badyrka, president
Evaporation Technical Services, Inc., Birmingham, Ala.

Try backflushing
Would it be possible to install a backflush system in your evaporator loop? It would work like this: A relatively clean solvent stream would be pumped backwards through the evaporator for about a minute every few hours (as needed). If this backflush idea worked satisfactorily, it could be automated a bit. It could be activated by a pressure switch (or something that detects plugging) or it could be put on an automatic timer.

Mark Otterson, project manager
Noveon, Inc., Avon Lake, Ohio

Go to a continuous crystallizer
Your solvent recovery/batch evaporation description doesn’t state the relative value of those streams or whether the incinerator is dedicated to this stream only. However, incineration is always a net expense. Instead, remove the precipitated solids via continuous filtration from the recirculated reboiler stream, before the heat exchanger. You can then landfill or perhaps even sell the contaminant-containing solids at a discount. The bonus is that you can recover much more of your solvent this way. Even if your solvent or solids have appreciable fuel value, they are almost always more valuable as process materials.