Drying Technology Options

I’m currently looking at various drying technologies for a sludge-type material and am curious if anyone has any suggestions for other options to explore that I may not be thinking about.

This material is a bit challenging in that it is a sludge which is quite sticky. I also need to avoid over-heating it; temperature of the material being dried needs to remain below 95C. As if those two items weren’t enough, I also need to dry it to around 0.2% moisture content. The incoming moisture content is around 40% to 50% by weight.

I’ve eliminated a rotary drum or spray dryer due to the sticky nature of the sludge, as well as a flash tube dryer due to the temperature limitation and intensive energy requirements of that technology. I’m currently leaning toward, and moving forward with, a belt dryer - I’m awaiting initial feedback from several vendors to see how long of a belt I’ll need to get down to necessary moisture content.

I appreciate any suggestions or tips anyone may have.

PS: I didn’t see any tags currently available which would be appropriate for this topic; if a moderator would, please add a tag for “drying” - thanks!

That sounds like a tall order to get it to 0.2% from those levels with those constraints. A combination of different methods is likely more feasible than one unit/process that gets you from 50% to 0.2% with that temperature limit.

I’d be surprised if the belt estimation ends up at a reasonable length. That last 0.8% is probably going to take more time on the belt than the initial 49% if your sludge layer has any real thickness to it.

Does the moisture contribute to the physical tack of the sludge? Does it become more friable once it loses some moisture? If it does, you might look into a somewhat rapid, though less complete “pre-dryer” to make the material easier to handle for more traditional drying methods.

If that is the case, sewage dewattering methods could probably get the material from 50% to 25% pretty rapidly and then you can focus on something like a tumbler or drum with the partially dewattered material.

Something like this might work as that “pre-drying” step if your material fits the specs: https://www.elodeusa.com/elode-specification

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Is the incoming moisture content of 40-50% the result of a preceding filter press operation? My experience with latex sludge is that the limit of a filter press is about that range. We basically pump against the filter press with a 3", 90 psig air driven diaphragm pump until it is almost deadheaded. Then, we blow 90 psig dry plant air through the filter cake for a while. My recollection is we can get 35-45% moisture content at best.

The sludge does tend to become more friable as it dries…I’ll see if I can get some feedback from the client regarding if there’s a “tipping point” with the moisture content where it doesn’t stick everything so readily. They’ve run several benchtop trials themselves and hopefully can provide that feedback. I like the idea of multi-stage drying as a potential, reasonable solution.

It’s actually the result of a centrifugal separation process. The 40% to 50% moisture range is based upon initial trial runs…those results show that higher flow rates result in lower moisture content in the sludge, so we expect the range we’re currently forecasting to be conservative.

I added the drying tag.

What comes to mind first, given the heat sensitivity, is a vacuum dryer. I think that is more of a batch drying operation. Maybe a vacuum dryer can be continuous, but you’d need a good airlock coming in and going out. Like, rotary valves or double dump valves or something, maybe both. Google “vacuum dryer” and see if you see something reasonable.

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You didn’t mention any toxicity issues. Is it benign?

My 5th Ed. of Perry’s Handbook has a lot of information on different kinds of dryers. Do you have access to a Perry’s?

I’ll take a look at a vacuum dryer, particularly if multi-stage drying is an option. We do have a central vacuum system as a few vessels are operating under a vacuum.

It is benign - I’d like to elaborate, but I’m bound by a confidentiality agreement.

I’ll ask around and see if I can turn up a copy of Perry’s - I hadn’t even thought of that. I’ve used Perry’s in the past, and always found it very helpful when I needed it. I actually went to Amazon after I read your post and found a 6th edition for a good price and should have it in a couple of weeks (and won’t have to go hunting next time).

@KoachCSR - Take another look at rotary vacuum filters for primary drying. We used them for dewatering thickener underflow from flue gas desulphurization (scrubber) systems at our coal fired electric generating stations. This type filter is a continuous process with a slowly rotating drum picking up underflow from a trough, vacuum dewatering while the drum turns, and slicing off the filter cake with a long knife edge onto a moving conveyor belt. This type equipment is suitable for use with sticky material, here is one manufacturer: Rotary Vacuum Filter

We typically had three of these filters, each about 8’ diameter x 20’ long, turning at about 1 RPM. I led initial plant startup and initial operation of our first system using this equipment; it’s pretty dependable.

Rotary Vacuum Filter


Komline-Sanderson is an old, established company with lots of experience in many kinds of dryers. You couldn’t go wrong asking their, or your favorite dryer vendor, advice on your product. Execute a secrecy agreement with them first though; that’s normal protocol.

Is your slurry closer to a liquid or a solid?

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@SlideRuleEra - I will certainly look deeper into a rotary drum vacuum dryer - thank you for the link!

@Latexman - Thank you for the recommendation for Komline-Sanderson, I’m not familiar with them and will certainly reach out.

This slurry is closer to a solid at the point it leaves the separation process…it basically comes out in “gooey chunks” which we carry on a belt conveyor and plan to flatten out with a scraper. It may be more of a true “slurry” with some added moisture making the vacuum drum dryer mentioned by SlideRuleEra a feasible option.

I know these answers aren’t terribly helpful - I had a discussion with our client today who will run some trials on fresh samples next week and be able to provide some feedback on the behavior of the material at various moisture levels to help inform our approach. Everyone’s input so far has been very helpful in reigning in our available options and identifying design considerations as well.

Glad we could help! Here’s some more resources associated with “Dryers” in Chemical Engineering’s Buyers Guide. Komline-Sanderson is listed there along with many others.

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As an update to any interested, I’m still waiting on feedback from our client on how the material behaves relative to moisture content.

Initial feedback from belt dryer vendors is further driving toward a multi-stage drying solution as our most likely path forward - they are only comfortable getting down to 5% moisture at best. I’m waiting on those lab results before I go out for another round of equipment quotes, but plan to explore the above solutions from you all, as well as a couple I noted in Perry’s.