Issue link: https://www.e-digitaleditions.com/i/1302781
8 / November 2020 powderbulk.com W ell, 2020 has been an interesting year to say the least. Markets are volatile and change almost daily. The workforce is rarely at 100 percent, supply chain and delivery schedules are a guess at best, your continuous process is more like a batch process, and 2021 is still a big mystery. Yet, all of us are facing these challenges, which is why the economy will survive and flourish in time. We're all in the same boat, and that boat is heading in the right direction — albeit by way of tacking and adjusting to the wind. As we need to be more flexible, your process also needs to be more flexible, and it starts with startups and shutdowns. First, you need to take the mystery out of the startup by accounting for the physics of time, tran- sient logistics, and equipment and particles at rest. Even at rest, your particles are changing. Solids can decay or degrade with time, resulting in material that may be more resistant to flow. Moisture is another issue. Just because you shut down your process with solids con- tents being dried, doesn't mean they'll stay dry. If they don't, particles become sticky and less likely to flow. Stress consolidation is another time-driven problem. Particles can rearrange themselves into a more com- pact configuration, increasing frictional forces (effective angle of internal friction), making flow more difficult. If you suspect these problems, consider not leaving materials in the affected unit when shutting down. Keep the moisture or oxygen out with a nitrogen or inert gas purge. You don't need much gas, just enough to have a slight positive pressure on the unit. The man- aging of consolidation is a bit trickier. While difficult to prevent, you can limit it by reducing bed height or adding flow aids such as aerators or air cannons. (Vibrating flow aids aren't recommended here because vibration could consolidate the bed even more.) Under- standing what your bulk solids are doing while at rest will help you know what needs to be done to get your process up and running quickly and on-spec. Also realize that your tried-and-true shutdown and startup procedures aren't likely to be as effective as they've been in the past. Chances are they were designed for infrequent cycles. You and your team need to discuss what frequent startups and shutdowns are doing to your productivity. Is shutting down with units completely full or empty the best scenario? Are valves best left in the open or closed position? What is cycling doing to the equipment with respect to mechanical stresses? How are motors, belts, and chains holding up? Should startup be done more slowly or faster? There are many questions to ask, and if you want the best answers, work with your team. Ask ques- tions. Listen to hunches. Do whatever it takes to spark meaningful discussions. This also applies to your safety procedures. These probably also were designed for less frequent start- ups and shutdowns. Again, as a team discuss current procedures and maybe adding new ones. Should addi- tional training be added? These discussions could be done during downtimes. Moving parts like to keep moving. Frequent startups and shutdowns put a lot of stress on your equipment — not just on rotary valves and compressors, but on that computer for your process control, too. Your old maintenance schedule is probably not applicable and even getting in the way. It may need to be revised with some maintenance occurring on a more frequent basis and during downtimes. Instruments may need recal- ibration with each startup. Your thermocouples are probably just fine, but your pH sensor may not be. Does your PLC require a buffer of data for process control? The message here is that regardless of what your maintenance or operational schedules were, chances are they need to be reevaluated. It's the new normal. You're going to have to rethink a lot of tried-and-true practices. However, if you evaluate your bulk solids behavior over time, reevaluate your startup and shutdown pro- cedures, and develop maintenance and operational schedules for today's world, you'll have a process and a team more adaptable, reliable, and productive for 2021. INDUSTRY PERSPECTIVE PBE Ray Cocco, president, Particulate Solid Research Inc. Particles, physics, and the pandemic