An old adage says, “To get what you want, stop doing what isn’t working.” In industry, we often fall into a routine set by operational procedures aimed at ensuring a safe workplace and cost effectiveness. We rarely question the current practice and review alternatives. In solids processing, many myths and superstitions get perpetuated because we don’t fully understand the way nature works or the person who developed the procedure failed to explain it completely.
For instance, in planning an expansion, the choice of dryer came up for review. The existing operation used rotary dryers that often over-dried the product or even melted it on the lifters, requiring a significant amount of downtime. 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. The plant wouldn’t even consider the option until the cost estimates came in and then, reluctantly, agreed to a test. Although the rotary dryers 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 rotary drying was costing the site! Fortunately, the fluid bed dryer eventually was selected.
In another situation, the product from a flash dryer would come out burnt or decomposed. The dryer had a very effective control system. However, the plant continually was tuning it to respond to inlet-moisture changes because the upstream centrifuge produced erratic moisture content. 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.
Many years ago, a plant installed a fluid bed dryer that from day one was plagued by sporadic fires. The manufacturer suggested raising the gas flow and maintaining that fixed value, which solved the problem; that high velocity was enshrined in the safety manual. However, losses from the dryer and attrition were very high. This soon became a cost issue. An evaluation of the fluidization grid showed the tuyeres were spaced too far apart and the gas jets were impinging on each other, which caused the high attrition. The fires stemmed from not having enough pressure drop over the grid, not low velocity in the bed. A new grid with the correct pressure drop operating at a lower overall gas flow rate eliminated the fires, attrition and excessive product losses due to entrainment. It took a while to get the green light, though, because changing a safety policy is tough and not for the faint of heart.
In another case, 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. Problem solved!
My company participated in an industry study on project management when I was a young engineer. I wasn’t surprised to learn that while the majority of projects in the chemical industry were completed on-time and near budget, those that involved solids seldom were on-time or anywhere near budget. A couple never started up.
TOM BLACKWOOD is a Chemical Processing contributing editor and our resident powders and solids expert. You can email him at TBlackwood@putman.net