Product testing

It is often a requirement that the drying process does not alter the chemical composition or such physical properties as surface area and crystal structure. Thermal stability is especially important because all gas-suspension dryers use heat to evaporate the liquid. Thermoplastic materials can cause particular problems because they often become sticky at just about the same temperature at which they become dry.

Feeds are usually characterized in terms of viscosity, both at rest and under shear, particle size, moisture content and chemical composition. Final products are specified in terms of particle size, bulk density, moisture content, flowability and dustiness.

As such, simple heat and mass balances along with handbook data and particle trajectory calculation are never enough to select a system, let alone design one. Because of this, all drying processes are somewhat empirical. There must be either existing commercial experience, previous pilot-scale data, or results from process testing. If a new product requires testing, a benchscale feasibility test is usually performed to characterize the feed.

Someone with experience in gas-suspension drying can usually predict whether the feed can best be spray-dried or handled by fluid bed or flash dryer. This depends on whether the feed can be atomized into droplets. If the material appears suitable for spray drying, but the product specification requires particle sizes or bulk densities outside of the usual spray-drying capability, then one of the hybrid systems would be a likely candidate.

If not, one needs to fluidize the material in a bench-scale fluid bed and have a drying curve established. The drying curve usually provides enough information to choose between flash and fluid bed.

Once a potential solution to the drying problem is selected, pilot scale testing is recommended. This can sometimes be carried out in laboratory-sized units or may require a large semiworks system. Although the initial goal of such testing is to determine a set of process parameters that produce acceptable product, much more can be gained from this effort.

During such optimization tests, one may ask: At what inlet temperature, for instance, does the product begin to degrade? Only when this question is answered can one arrive at an efficient design. Another important question: What is the largest droplet size that can be produced by a centrifugal atomizer without forming a "mud ring?" Again, in order to determine this, one has to push the limits until a mud ring does form.

Users and designers of drying systems have depended heavily on pilot runs for establishing the process parameters required to design and operate an efficient process. There is, however, a new tool available that enables the experienced process engineer to make even greater use of his or her pilot plant data. The tool is a set of computational fluid-dynamic programs that can simulate the flow direction, velocity and temperature of gas streams through the drying equipment.

The programs can also predict the path of particles and droplets within the system. Although this capability doesn't replace feasibility and pilot-scale testwork, it reduces the overall amount of testing that needs to be done in scaling up a process from pilot to commercial scale.

References

The author

Fred V. Shaw is national sales manager for the chemical div. of Niro Inc. (9165 Rumsey Road, Columbia, MD 21405; tel. (410) 997-8700). He has been with the company for more than 13 years. Previous positions include process and environmental engineer for Badger Engineers (Cambridge, Mass.), and process, project and technical service engineer for the Davison Div. of W. R. Grace & Co. ( Baltimore, Md.). A member of the American Ceramic Soc., he received his bachelor's degree in chemical engineering the University of Detroit in 1970, and an MBA from Loyola College of Maryland in 1980.

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