Process Wednesday: solids blending and segregation

A child's delight, a manufacturer's problem.
Credit: zoomdoggle.com

From the pages of Chemical Engineering, an interesting and hilarious demonstration of the difficulties of mixing solids on large-scale by Thomas G. Troxel:

In other applications the sample size subjected to scrutiny is not as clear cut. Take, for example, a box of cereal. In one specific example a manufacturer produced a cereal with a cereal particle and a marshmallow candy particle. The packaging line filled boxes from a series of conveyors that fed a mixture of two components into a gravimetric packaging machine that dispensed the required weight of the blend into each package. The two components of the mixture were of similar size and density and the only blending that occurred was in the handling system along several conveyors and in a surge hopper. Each component was meter onto the conveyor system, at the correct ratio, but there was not a discreet blending process. The manufacturing plants fine tuned their systems to get an acceptable mixture into each box.  

In this case, each box was always within weight tolerance for the stated package quantity, but there was no direct control of the components in any box. In an effort to improve quality, a new packaging line was installed that handled each component individually up to the packaging head where each component was weighed in a series of weigh hoppers, and then each box was filled from several hoppers of each component, selected by the packaging system controlled, to control both the proportion of each component and the total weight in a box. This would seem an ideal solution since it would guarantee both accurate package weight and composition. From the producers point of view, they had improved quality by insuring that every package met their quality standard.  

However, to everyone's surprise, consumer complaints from packages produced at this plant went up. What the plant had gained in compositional accuracy they had sacrificed in uniformity. Consumers found that individual bowls of cereal poured from the new packages had much wider variations of the two components than boxes filled from the old filling line. ...The new system allowed only about one second from the time the ingredients were separate until they were combined in the package. There was very little opportunity to do any blending, and the result was that some boxes were well-blended while others were segregated. 

This doesn't have a ton to do with "classic" process chemistry (how often are raw/fine chemical manufacturers blending solids?) -- I just thought it was kinda funny.

UPDATED: Added "raw/fine" and "classic" to clarify. Formulation and drug delivery, of course, does a lot of solids blending.