Understand flow patterns in glass-lined reactors

CEP November 2004 www.cepmagazine.org 21 GLASS-LINED REACTORS ARE ESSENTIAL process equipment in the pharmaceutical and specialty chemicals industries. A typical glass-lined reactor (Figure 1) includes a retreat curve impeller (RCI; Figure 2) near the bottom of the vessel and usually a single baffle mounted through a nozzle in the vessel head. The RCI with rounded blade corners may limit harmful turbulence effects while maintaining circulation throughout the vessel. Glass lining (the term “lining” is used to refer to the glass coating on the agitator and the inside of the tank) provides corrosion resistance, is easy to clean, and eliminates product contamination. The retreat curve of the RCI blades provides better radial flow than radial flow impellers with similar power characteristics. The impeller is placed near the bottom of the vessel to maximize the allowable range of liquid levels and to produce circulation from the bottom to the top of the vessel. The baffle (occasionally two baffles) is mounted from a nozzle in the top head because mounting to the side of a glass-lined vessel is difficult. The impeller and baffle always have a rounded cross-section without sharp corners because high stresses in the glass can cause the brittle coating to fail. In recent years, improvements in mixing technology and glass formulations have led to new impeller designs. Many of the impeller styles offered in metal alloy mixers are now available in glass-lined reactors. However, despite the development of new impeller technology, little has been published about the performance characteristics of the mixing equipment. Even such basic information as power number is rarely found for either retreat curve or newer types of glasslined impellers. Computational fluid dynamics (CFD) can describe the three-dimensional flow characteristics of a glass-lined reactor. With proper validation, CFD can be used with confidence to study equipment and process variations. Recent articles on the application of CFD modeling to Understand Flow Patterns in Glass-Lined Reactors