BACKGROUND

Numerous chemical processes involve mixtures of solids, liquids, and gases. The mixtures may be intentionally created for a particular reaction to occur, or they may be the result of a chemical reaction. Many of these processes require that the liquids and gases be separated from the solids at some point further in the process.

When the main product of the process is a bulk solid, it is important that the liquid or the gas be removed in an efficient manner that is not detrimental to the material being produced. Some commercial dryers can be effective at doing this but can also be very expensive. Depending on the application, these dryers can cost up to a million dollars.

THE PROBLEM

At a Dow Chemical facility, flaked material during part of the manufacturing process was produced with excess surface moisture within the product. For further processing, most of this water would have to be removed. Dow's engineers decided to investigate whether a new surge hopper could be designed to dry the product by forcing hot, dry gas countercurrent to the flowing material, which would be much less costly than using commercially available dryers.

Process requirements dictated that the bin dry the products uniformly to less than 1000 ppm water, thus removing approximately 10% water by weight. The drying also had to be done in less than 60 minutes, to assure product stability.

THE SOLUTION

For the dryer to be effective, two major design considerations had to be met. First, the velocity profile of the product in the dryer had to be uniform to control drying time. The way to accomplish this was to ensure a mass flow pattern well within the mass flow limits and to maintain a level of material in the cylinder section.

The second major consideration was to keep the superficial gas velocity low enough to prevent the product from becoming locally fluidized.

Jenike & Johanson determined the basic dimensions of the dryer from these requirements and the material's flow properties. The resulting cylinder section was relatively short and wide, which would make uniform air distribution difficult. We modified the design to provide a larger height-to-diameter ratio, to ensure a more uniform distribution of drying air.

Jenike & Johanson then developed three air plenum design options. Even though our designs were based on sound principles of bulk solids flow, the design and use of the dryer was unique, so we developed three scale model designs, corresponding to the three full scale options. These designs were modified slightly from exact scale models to account for the flow properties associated with the smaller dryers. Dow selected the recommended option, then built and tested the scale model.

THE RESULT

According to Karl Jacob of Dow's Solids Processing Lab, "The pilot design worked as predicted, confirming uniform material flow without any product carryover while meeting moisture specifications.We saw no signs of product quality degradation. The pilot design basically confirmed Jenike & Johanson's design and again highlighted their expertise. This has assured us that a full scale production unit will be highly successful, thereby saving substantial amounts of money and time over commercially available equipment. Jenike & Johanson continues to show us they are a leader in the area of solids handling."

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