One of the biggest problems encountered in solids processing is sampling particulate solids containing fine particles. Individual particles may differ in chemical composition as well as in physical size and shape. While many vendors offer equipment to address these issues, somehow designers of a process often ignore or forget about this aspect of process control.
There are legitimate reasons to minimize sample ports: contamination concerns and, maybe, cost. Sometimes, no sample is needed because of downstream processing. However, when sampling is necessary, obtaining a representative sample can be a problem for materials with wide particle-size distributions. The quantity required for a representative sample may become unreasonable. In addition, after taking a sample, subsequent handling and processing can compromise the sample integrity.
Thief Sampler Trouble
One of the most-common ways to sample is via a thief sampler. The use of this device generally is required because sampling methods weren’t built into the process design.
Unfortunately, it also is the most-abused sampling method, especially when materials contain fine particles. In such cases, it frequently gives the wrong results. For instance, we used to go to great lengths to maintain the correct particle-size distribution during manufacture. Nevertheless, a customer using a thief sampler on material in a newly arrived bulk delivery truck rejected the shipment.
Part of the problem was the way we loaded the truck from a silo. The other was having short delivery distances. At the end of a loading cycle, the chute was shaken to dislodge the final solids. Unfortunately, this freed a large fraction of fine particles that then dropped onto the top of the solids in the truck; so grab samples taken by the customer contained an excess of fine particles. Even though our silo was designed for mass flow, some of the fines collected on the chute due to electrostatic and cohesive forces. By placing a very small high-speed blender between the silo and truck nozzles, we could minimize the accumulation of fines — so that, even using a thief sampler, the customer got consistent results. It’s interesting to note that trucks traveling longer distances never had this problem due to the sifting that took place during transport. We should have anticipated that the shorter distance would exacerbate the problem.
Feeding a process with multiple ingredients that widely differ in particle-size distribution presents a similar set of problems. How do you blend ⅛-in. particles with 100-µ ones — or should you even try? Unless you plan to coat the large particles, don’t try!
An extrusion process in which seven chemicals were used to produce a pellet product exemplifies the problems that can arise. Loss-in-weight feeders provided the components to the extruder in the correct ratio. The process also included a blender — because during process development one had been used to mix the feed to the extruder in batch mode. The design team kept this blender in the flowsheet partly to provide a sampling point. In all of the batch studies, the blender effectively fluidized the mixture and emptied completely. The designers thought, “a blender is a blender,” and chose a different type for the continuous process. Unfortunately, fines accumulated along the shell of the blender and occasionally would slough off, upsetting the composition out of the extruder. Removing the blender restored a consistent extruder product. Sometimes, simpler is better — and, yes, there are significant differences in blender design.
These examples show how complex the handling of fine solids can be, especially if you don’t follow the particles’ path and behavior.
• In the first case, the supplier of the product should have considered the short transport distance along with the tendency of the product to have a trailing dust cloud. Many integrating samplers that could have provided a composite sample to be sent with the truck are available; using one would have eliminated the need for a thief sample at the customer.
• The second case shows one of the hazards of converting a process from batch to continuous, and demonstrates that you have to trust your process design (i.e., the loss-in-weight feeders) to provide the correct mix.
Always remember that fine particles have a tendency not to follow the path of the other particles and to be more susceptible to segregation.
TOM BLACKWOOD is a Chemical Processing Contributing Editor. You can email him at TBlackwood@putman.net