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; however, they can also be very expensive. Depending on the application, these dryers can cost up to a million dollars, which is a substantial investment for any size process.

A flaked material is produced at one of Dow Chemical's North American facilities. During part of the manufacturing process, excess surface moisture is present with the product. For further processing to be done, most of this water must 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. The approach, if successful, would be much less costly than using commercially available dryers. Drying and storage could be combined in a single vessel, and the bin could be installed in much less time than it would take for delivery of a commercial dryer.

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.

Jenike & Johanson was asked to evaluate the feasibility of designing a hopper dryer to meet Dow's requirements, and to recommend the functional design of this new dryer.

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, and to maintain a level of material in the cylinder section. With mass flow, all material is in motion whenever any is withdrawn, eliminating areas of nonmoving material. If the level in the dryer were to approach the converging section (conical in this case), the material would flow faster in the center due to friction along the walls, and the converging geometry. Material higher up in the cylinder section would not be affected by the converging section and would thus maintain a uniform velocity profile.

The second major consideration was to keep the superficial gas velocity low enough to prevent the product from becoming locally fluidized and/or airborne. Based on heat balance equations, Dow engineers estimated the amount of drying air that would be necessary to remove the required amount of moisture. They also estimated, based on experience, the maximum superficial gas velocity which would minimize the amount of airborne particles.

Our engineers determined the basic dimensions of the dryer from these requirements and the material's flow properties, which we had previously measured. The resulting cylinder section was relatively short and wide, which would make uniform air distribution difficult. We therefore modified the design to provide a larger height-to-diameter ratio, to ensure a more uniform distribution of drying air. To make these changes, some of the requirements had to be relaxed slightly. This was acceptable because they were based on potential maximum operating conditions, not the normal expected conditions.

With the overall dimensions set, we next addressed the method of introducing the air by developing three air plenum design options. We had frequently recommended similar designs in the past for air permeation systems; however, they utilized much lower quantities of air.

Even though our designs were based on sound principles of bulk solids flow, the design and use of the dryer was unique. Other questions remained, such as the actual limit to the superficial gas velocity. Another concern was whether proper drying would occur in the cylinder section, where there is little interparticle motion. To best answer these questions, we recommended building a scale model of the dryer.

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.

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."

If you have a need to dry, heat, purge, or condition a bulk solid in a process vessel, call Jenike & Johanson. We can determine how to improve the efficiency of your existing equipment, or design new systems that will work reliably.