Anyone who knows me will say: “He’s crazy about fluidization.” That’s the absolute truth. The heat transfer is greater, the mixing can be better if you’re careful, and fluidization often provides the lowest cost option for processing. One of our plant’s operators observed that the only thing his product didn’t stick to was air, so he became a fan of fluidized beds. Not all products are well-suited for this type of operation, though. Attrition and segregation of the product may pose concerns. However, you can design around these limitations or even turn them to your advantage. Here are some examples:
• A granular product having some fine particles was being loaded into drums, which was an easily contained operation. However, when the lid was removed, the excess fines created a dust and handling problem. Fluidizing the product as it was loaded enabled stripping off the fine particles, eliminating the problem.
• Coal fed to a calciner produced an emission of fine particles that would be very expensive to collect at 2,000°F. Rather than install emission controls, the site added a fluidized bed that removed the coal fines before the calciner. This worked well because larger coal particles have less inorganic chemicals than the fines.
• Fine crystals that form in solution often are more reactive than grown crystals that are ground down to the desired size — and thus frequently command a much higher price. So, eliminating the grinding operation, using a fluidized bed and segregating the finer particles during the drying process can boost profits.
Attrition often is cited as a reason not to use a fluid bed. However, particle-particle impact is much more damaging than impact between a particle and a gas or even a particle and a wall. In one study of a cyclone, we found most attrition occurred when the cyclone was removed and the solids discharged directly into the bin. One of the major concerns of designers of fluid beds is maintaining adequate fluidization of the bed; so they use too high a velocity, which can impact attrition. To compensate, they don’t provide enough pressure drop at the fluidization grid to prevent larger solids from settling on the grid, which in a dryer can cause fires or burn the product. Note I said pressure drop, not velocity. High pressure drop ensures uniform distribution of the gas, whereas high velocity may increase particle-particle impact and attrition.
Pneumatic conveying systems, including so-called dense-phase ones, count on fluidization to transport particulate solids. Clearly, dilute-phase systems rely on fluidization — most of their operational problems stem from not maintaining fluidization all along the line. In these systems, we not only are fluidizing the particles but also are accelerating them to some velocity below the gas velocity. Gas velocity is increased at the feed point to help in this process but the effort is wasted if the travel distance before an elbow or diverter isn’t sufficient. Also, we know that putting two elbows close together is a well-known recipe for defluidization, which increases the solids/air ratio and pressure drop. In dense-phase systems, fluidization is less obvious with the typical dune or even plug flow. Some particulate solids need some sort of gas bypass to refluidize and maintain motion down the pipe.
Have you ever tried to coat a large particle with a fine powder? Mechanical devices frequently fail because of the clumping of the fine powder or lack of uniform coverage on the larger particles. Enter the fluid bed coater. It exposes the full surface of the larger particle to the gas that contains the fine particle. Any excess fines can be scrubbed off in the bed and returned for coating.
One of the more important aspects of fluidization is heat transfer. Not only is more surface area available but also convection is more effective than conduction. In addition, fouling of heat transfer surfaces is less of an issue, even when in-bed heat transfer surfaces are involved. By the way, in-bed heat transfer is an often-overlooked technology for high-solvent particulate. It allows use of much lower inlet gas temperatures, which can be especially valuable with heat-sensitive products. So the next time you want to move, dry or dedust a product, get a fluidizing device.
TOM BLACKWOOD is a Chemical Processing Contributing Editor. You can email him at TBlackwood@putman.net