Development of Spinning Disk Encapsulation Processes, 01-9279

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Principal Investigators
Neal K. Vail
Christopher J. Freitas

Inclusive Dates: 10/01/01 - 12/31/02

Background - Microencapsulation is a key technology in many industries, such as food and pharmaceuticals, biomedical and biotechnology, industrial chemicals, and agricultural and veterinary sciences. Active in encapsulation research since 1949, SwRI is recognized for its ability to provide encapsulation solutions using a diversity of encapsulation technologies. Spinning disk technology has been a core component of the Institute's encapsulation technology portfolio since the early 1960s, when the technology was used to prepare coated glass spheres for chromatographic columns. Much of the Institute's external encapsulation business is derived from spinning disk technology. During 1999 - 2001, the Institute conducted more than 50 encapsulation projects, resulting in approximately $2 million revenue. Of these projects, 55 percent used spinning disk technology, accounting for more than 50 percent of all encapsulation-related revenue. Interestingly, the Institute has no strong intellectual property position in spinning disk technology. Yet, approximately 1,300 U.S. patents containing microencapsulation embodiments are issued per year, with nearly 30 percent of these issue patents involving some form of spinning disk technology. For SwRI to remain competitive in the microencapsulation market, it is important to continually improve the Institute's capabilities in spinning disk processing and to develop technological advantages in this area.

Approach - The objective of this research is to develop robust, scalable spinning disk processes that can produce encapsulated products with well-defined characteristics. SwRI's spinning disk technology has remained essentially unchanged for more than 30 years. A limited fundamental basis exists among SwRI researchers either to rationalize current rotating disk designs or to support the choice of operating parameters to prepare encapsulated product. Consequently, spinning disk processing leans toward empirical and anecdotal experience, which leads to limited process control and processing variability between operators. This variability results in inconsistent batch-to-batch product quality at bench-scale sample volumes. Similarly, process scale-up to larger product volumes of equally consistent quality becomes problematic. The research team proposes to develop a fundamental understanding of spinning disk encapsulation processes through simulation and experimentation. Knowledge gained during this research will be used to (1 strengthen and expand this core encapsulation technology, (2 broaden SwRI's intellectual property position in this technology, and (3 provide improved high quality product to clients.

The specific aims of this study include:

  1. Develop a computational model of SwRI's current spinning disk process using computational fluid dynamics methods. Insights gained through the analysis of a suite of simulations will be used to develop enhanced process parameters for fluid film and particle formation processes using model fluids. We will then examine the role of critical fluid properties and process parameters on film and particle formation. The simulation results will be corroborated by physical experiments;
  2. Develop fundamental knowledge and predictive tools that will significantly expand our capability to provide competitive spinning disk processing solutions to our clients. We anticipate developing a fast-running prediction tool that can be used during bench-scale process development to streamline processing activities and to provide high-quality products. Furthermore, we develop enhanced process scale-up capabilities to meet our clients' complete product development requirements;
  3. Develop intellectual property positions in spinning disk technology. The knowledge and predictive tools noted above will add considerably to SwRI's intellectual property portfolio. We will use these tools to develop the Institute's spinning disk capabilities to meet more demanding process design criteria, such as process scale-up and advanced disk design. In the latter case, we will investigate, refine, and test candidate disk designs;
  4. Identify areas for continued spinning disk process development.

Accomplishments - The historical disk design was found to be sensitive to operating conditions and fluid properties with respect to its ability to produce narrow particle-size distributions. Precise control of fluid addition to the disk and fluid surface tension properties was necessary to ensure uniform fluid film formation over the disk surface. Similarly, fluid films were maintained only for disk speeds less than approximately 4,000 revolutions per minute using medium viscosity fluids. Fluids of lower viscosity tended to detach from the disk at lower disk speeds. Narrow particle-size distributions were achieved provided fluid flow rates were maintained sufficiently to ensure fluid filament formation at the disk periphery. Interestingly, narrow particle distributions could be obtained for nonfilament conditions if the disk experienced vibration.

Modifications to the historical disk design to mitigate film formation problems proved valuable. However, the resulting disk generally produced larger average particles with broader distributions over all conditions. The addition of conical tips to the periphery edge of the modified disk design significantly improved disk performance with respect to average particle size and particle-size distribution. The average particle size was smaller for a given set of operating conditions, and the particle-size distribution was nearly two times narrower for all conditions. Particle-size distributions broadened with reduced disk speeds and increased fluid viscosity, probably because of flooding of the conical tips.

Generally, particle size was invariant to the fluid flow rate, but particle-size distribution was not invariant to fluid flow rate as noted above. The predominant process variables with respect to average particle size and particle-size distribution were disk speed and fluid viscosity. A constitutive equation was developed to adequately predict average particle size for conventional and serrated disk for the fluid systems studied.

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