Chemical engineers at Case Western Reserve University, Cleveland, have modeled the filtration of particulate suspensions in acoustically driven porous media.
In their work, researchers Dr. Sanjay Gupta and Donald L. Feke studied the retention of suspended solids within a porous medium that was subjected to resonant ultrasonic fields. They observed that a simplified analysis of particle trajectories toward cylindrical collectors, in response to acoustic and hydrodynamic forces, provides insight into the fundamental physical phenomena that govern the acoustically enhanced filtration process.
According to Gupta, now employed by the Nestle Corp., many chemical, material, and biological processes involve multiphase systems where a fluid phase is the carrier for a particulate or immiscible liquid phase. Often, finally divided dispersions are deliberately created to aid heat- and mass- transfer rates by utilizing the large surface-areato-volume ratio. "Normally, at one or more stages in these processes, there is a need for separation of the dispersed from the continuous phase, and various methods have been developed to do this."
Conventional approaches, he notes, include physical screening techniques (mechanical sieves, beds of filtration media, or filter membranes) and gravity-driven methods (settling or flotation) that accomplish the desired separation using the density difference between the dispersed and continuous phase. More-advanced schemes utilize centrifugal, electrical, or magnetic fields to enhance the quality or rate of separation.
The researchers chose a blend of acoustic and physical screening for their study, using an unconsolidated bed of millimeter-scale glass beads and aluminum meshes of various pore sizes. A highly porous polymeric foam was selected as the filtration medium. In contrast to glass and aluminum, this material, Feke said, has an acoustic impedance close to that of water, which was used as the suspended medium in the demonstration. Then, a resonant ultrasonic wave field within the porous medium was used.
They found that the acoustic field allows the collection of suspended solids two or three orders of magnitude smaller than the pore size of the porous medium. Gupta noted that the "collection of micron-sized solids can be achieved without the large pressuredrop penalty usually associated with filtration of such solids. Deactivating the acoustic field and flushing the collected solids from the porous medium with processing fluid cleans and regenerates the filtration medium."
Eventually, however, the porous medium becomes saturated with particles and instabilities in the collection phenomenon arise, the researchers found.
More information about their work appears in the May issue the AIChE Journal.
Комментариев нет:
Отправить комментарий