Non-filtration, particle-fluid separations
11/01/2001
by Robert P. Donovan
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The physical, particle-fluid separations described last month convert an initial particle-fluid mixture, or flow stream, into two or more particle-fluid mixtures or output streams. At least one of those output streams has higher particle concentration than the feed stream and at least one has lower particle concentration.
Devices based on these non-filter, particle-fluid separation mechanisms differ from fibrous or membrane filters in that reduced particle concentration is achieved not by capturing particles and removing them from the feed stream but rather by introducing mass-dependent forces that cause particles to move differently than the fluid molecules and thus be separated.
While reducing particle concentration in a fluid flow stream is a desirable result in creating environments suitable for manufacturing particle-sensitive products, the opposite effect, the increase of particle concentration, can also be desirable in measuring particle concentrations, especially at low concentrations. One prime example is the measurement of particle concentrations inside process equipment operating at sub-atmospheric pressures.
 Figure 1: Aerodynamic lens [from Ref. 2, used with permission]. |
The low-pressure environment makes collecting a representative, statistically significant sample more difficult than at atmospheric pressure because of the increased difficulty in focusing widely dispersed particles into a narrow particle beam that can be directed into a laser beam for counting. At atmospheric pressures, sheath air flows are available to help shape and focus particle beams. But at sub-atmospheric pressures inside process equipment, this option is not generally available—and no one has successfully hydrodynamically focused particle beams in liquids.
Fortunately, aerodynamic lenses1 have been developed that can focus particles in a selected mass range into suitable narrow beams at pressures as low as 1 torr and even somewhat lower. These lenses can be structures as simple orifices in a thin baffle plate through which an aerosol sample is drawn from the low-pressure chamber.
On the downstream side of the initial orifice, and of each subsequent orifice, the gases (the small dots in Figure 1) rapidly expand outward to fill the flow tube while the mass of the selected particles (the large dots) keeps them closer to the centerline of the tube. The selected particles are those having the right mass for the tube diameter and orifice diameter, orifice, at the average flow velocity through the orifice, V, as determined by the particle Stokes number, Stk1:
Sτk = τV/dorifice
τ = the particle relaxation time, a sensitive function of particle diameter
For just the one "right" value of Stk, particles after accelerating through just one orifice will be focused on the centerline of the tube. Particles of smaller Stokes number will diverge somewhat from the centerline after passing through the orifice while particles of Stokes numbers near to zero will follow the gas streamlines. Particles with Stokes number larger than the "right" value will either collide with the baffle containing the orifice or will cross the tube centerline and follow a trajectory off the centerline but on the opposite side of the centerline on which it entered the orifice.
Multiple lenses in series, as shown in the figure, allow a range of particle Stokes numbers (and hence particle sizes) to be focused along the tube center line.
As pointed out in Ref. 2, this capability improves both the accuracy and precision of particle counting in sub-atmospheric processing equipment and largely eliminates the uncertainty associated with optimally locating the sensing portion of an optical particle counter within such process equipment. Too bad hydrodynamic lenses can't be developed to provide the same particle focusing capability in liquids.
Robert Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L&M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.
References
- Liu, P., P. J. Ziemann, D. B. Kittelson and P. H. McMurry, "Generating Particle Beams of Controlled Dimensions and Divergence: I. Theory of Particle Motion in Aerodynamic Lenses and Nozzle Expansions", Aerosol Science and Technology 22: 293-313 (1995).
- Liu, P., N. P. Rao, D. B. Kittelson and P. H. McMurry, "Optimizing the Detection Efficiency of a Low Pressure, In-Situ Particle Monitor Using Aerodynamic Focusing Lenses", 1996 Proceedings of the Institute of Environmental Sciences, pp. 217-224 (IEST, 940 E. Northwest Highway, Mount Prospect, IL 60056; www.iest.org).