A major cake mix manufacturer was experiencing unacceptable variations in several of its final mixes. Jenike & Johanson was called in to solve this segregation problem. From our observations and plant processing data, we determined the chief source of ingredient demixing to be an often overlooked one: the pneumatic conveying system.

Pneumatic conveying is widely used to convey bulk solids, due to its many advantages. It is a versatile, safe, relatively simple, inherently clean, and automation-friendly method of material transfer. On the other hand, pneumatic conveying introduces new handling concerns, especially with mixtures. A common—and costly—problem associated with pneumatic transport is particle segregation or demixing. Demixing can occur by air entrainment (fluidization), sifting [220K QuickTime video], or particle entrainment in an airstream.

One of these segregation mechanisms was occurring at the end of the existing transfer pipeline, where entrained particles become decoupled from the airstream. The mixture's smallest particles, the easiest to move, tended to settle out farther from the inlet than did those of greater size and mass.

The orientation of the pipeline as it entered the hold bin was another complication. Fluidization segregation, caused by a vertical, center-fill inlet configuration, resulted in a vertical striation, with fines concentrated at the top surface.

Armed with observations from our site visit and our lab test data which defined the precise handling characteristics of the materials, we were able to design a solution we knew would work without wasting time and money using a trial-and-error approach.

First, we recommended making the mix less free-flowing, and therefore less apt to segregate, by adding one of the major ingredients which is normally added further downstream in the process. Then we recommended increasing the diameter of the conveying pipe at two locations. These successive diameter increases reduced air velocity and the segregation caused by particle entrainment.

To minimize the fluidization segregation, we recommended rerouting the pipeline to enter the bin horizontally, bend vertically upward, and discharge its mixture against a target box. This allows the particles to settle out more gently across the pile's top surface.

Finally, we recommended that the hold bin be modified to ensure a mass flow pattern.

Our changes brought the segregation problems under control, and the number of our client's customer complaints dropped dramatically.

Pneumatic conveying is a highly useful technology, but one with problems all its own. Solving—or, more importantly, preventing—these problems requires a thorough understanding of the flow characteristics of the bulk solids being conveyed as well as of the effects of air-driven transfer on ingredient mixtures.