(71e) A Novel Static Mixer to Ensure a Minimum Processing Time for Thermophyllic Sludge Digestion

The focus is on sterilizations and similar reactions where all portions of the fluid must experience some minimum residence time. An important application is the sterilization and partial digestion of sewage sludge so that human pathogens are destroyed and the effluent can be used as a fertilizer. The minimum residence time corresponds to the first appearance time measured by tracer experiments. Short, fat reactors are desired for economic reasons, but such reactors can have a first appearance time less than expected for fully developed laminar flow. This is a classic problem in entrance length flows. Static mixers can increase the minimum residence time, but conventional, proprietary designs are ill suited for the purpose. CFD was used to test a simple design that uses axisymmetric baffles to slow the fastest moving fluid elements and thus to increase the first appearance time.