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Solids Handling: Why Didn’t I Think of That?

May 21, 2024
Challenging current operations may spur significant improvements

What we see depends mainly on what we look for. In industry, we often fall into a routine set by operational procedures to ensure a safe workplace and cost-effectiveness. We rarely question the current practice or review alternatives. In solids processing, many myths and superstitions are perpetuated because we don’t fully understand the way nature works or the person who developed the procedure failed to explain it completely.

Ball Mill Vs. Fluid Bed Dryer

We had been making a product in a ball mill for over 40 years and had a commanding position in the market. The process was messy, but the product was not toxic or hazardous, so no one challenged the operations. One of our chemists found an alternate route that combined the reaction and drying steps. It had another advantage: the equipment was easy to clean. The product was very cohesive even when dry and went through a tacky phase during drying. To make matters worse, inlet-moisture control wasn’t very good. 

An alternative technology — a fluid bed dryer— was suggested to replace the ball mills. The plant wouldn’t even consider the option until the cost estimates came in and then, reluctantly, agreed to a test. Although the ball mill posed major operational problems, the plant argued it knew how to address the difficulties while it would have to start the learning curve from scratch with a fluid bed. Never mind how much the ball mills cost to replace or maintain. However, after taking the lead engineer to a plant that had a fluid bed in similar service, the fluid bed dryer was selected. It didn’t hurt that the plant was in Italy.

New Controller Removes Dust

Sometimes, the smallest change will have the greatest impact on the performance of a process. Customers were complaining about the dustiness of our product. It was a lacrimator. We used screw conveyors to move the product in the plant and a blender to meet the PSD. I recalled from past experience that this type of blender rounds the larger particles, which increased the density and makes them less sensitive to attrition. The fines could be collected in an elutriator and returned to the agglomeration step. The amount of rounding and fines reduction was proportional to the amount of fill and only required a new controller. Luckily, I was looking in the right place.

Twin-Screw Mixer Evens Out Moisture 

In another situation, the product from a flash dryer would come out burnt or decomposed. The dryer had a very robust control system, but the plant was tuning it to respond to inlet-moisture changes caused by the upstream centrifuge. The plant recycled some dry product from a baghouse. While the overall moisture was acceptable, fine particles were much dryer and were slow to absorb moisture. These particles never adequately mixed with the other particles and decomposed. Not only should a cyclone have been ahead of the baghouse so material wetter than the fines could be returned to the inlet but also centrifuge control needed improvement. The plant was tuning the wrong part of the process. The solution was to insert a twin-screw mixer between the centrifuge and dryer to even out the moisture fed to the dryer.

Drying Time Down The Drain 

A centrifuge produced a wet cake of about 20–25% moisture, which required over a day to dry the solids. To meet high product demand, we considered buying another dryer or a compression filter to squeeze out more water. One day when the centrifuge drain had plugged up, we observed that just before the product was cut, water stopped flowing at the drain. However, as the peeler cut the solids, water gushed out. Nobody had considered slowing down the cut because it always had been done that way. After a few hours of playing with the peeler settings, we were getting moistures of 6–8%. Drying time was 4–5 hours and the product was fluffier. Why nobody thought to look at the drain rather than the peeler output shows how we can be distracted by “normal” operations.

These cases show that challenging current operations can result in significant cost reductions and improved performance of solids processing. Looking at places that no one has explored is not only a mission in Star Trek, but an ongoing operation in running a chemical plant.

About the Author

Tom Blackwood, Solids Advice columnist | Contributing Editor

Tom Blackwood, a veteran engineer who has dealt extensively with solids over the course of his career, contributes regularly to Chemical Processing and serves as the Solid Advice columnist.

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