GOALI: Segregation and Elutriation of a Binary Mixture

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National Science Foundation - Division of Chemical &Transport Systems Particulate & Multiphase Processes Program (1415)Proposal Number: 0650893Principal Investigators: Hrenya, ChristineAffiliation: University of Colorado at BoulderProposal Title: GOALI: Segregation and Elutriation of a Binary MixtureGas solid flows are ubiquitous, though not well understood. A study of solids processing plants indicates that 80% of the plants experienced solids handling problems such as pipe blockage and that the performance of such operations was typically only 40-50% of design. Furthermore, even "reliable" empirical correlations for fluid catalytic cracking units are often inadequate. The proposed GOALI effort is targeted at advancing the fundamental knowledge associated with a phenomenon that is typically handled empirically, namely elutriation. Elutriation refers to the carryover of fines in a bubbling, gas-fluidized bed. Although more than 50 elutriation correlations have been proposed in the literature over the last two decades, the disagreement between experiments and empirical prediction is typically over 100%, and in some cases may differ by a factor of more than a hundred.Intellectual MeritTo better predict the elutriation phenomenon, a combined modeling and experimental effort is proposed. Research on the bubbling bed (dense) and freeboard (dilute) regions will be pursued in parallel. Two outstanding questions will drive this effort: (i) what physical mechanism(s) lead to the observed segregation (de-mixing) and bubbling patterns in the bubbling bed region, and (ii) what physical mechanisms lead to the counter-intuitive behavior observed in the freeboard region in which the addition of fines to the bed is known to increase the elutriation rate of relatively coarse particles (i.e., particles which have a terminal velocity that is greater than the superficial velocity at which the bed operates)? It is hypothesized that particle particle interactions play a key role in both regions. Similarly, a plausible explanation for the counter-intuitive behavior observed in the freeboard is the expected preferential collision of fine particles with the bottom half of upwardly moving coarse particles (since the fines travel at a higher velocity due to their reduced mass). This additional collisional force on the bottom of the coarse particles is expected to lead to their increased elutriation. The hypotheses will be tested by means of a two-fluid gas-solid model based on a new kinetic theory for mixtures that does not involve any of the assumptions of previous theories (nearly elastic, equipartiton of energy, Maxwellian velocity distribution). Model validation will be carried out with measurements that are both local and species-specific (species velocities and concentrations). A new probe will be developed that is both capable of measuring species velocities and concentrations and is adaptable to an industrial setting. The resulting data will represent the first detailed measurements of segregation and elutriation of two non-cohesive materials, and will be the first to characterize the impact of non-uniform particles on bubbling behavior. Furthermore, the model data comparison will be the first attempt to predict elutriation based on fundamentals (no adjustable parameters).Broader ImpactsThis work is motivated by complementary interests of each partner: the University of Colorado (polydispersity and segregation), Millennium (elutriation modeling), and PSRI (fluidization instrumentation and elutriation). This interdisciplinary team has over 50 years of experience in particle technology. Industry-university interaction will be promoted via industrial internships of PhD students involved in the project, co-advising of PhD students, and regular meetings and telecons. The results will be shared with the public using standard methods (conference presentations, peer-evaluated publications, website, etc.) and via the incorporation of the new model into MFIX (www.mfix.org), an open-source, no-cost CFD code which all of the PI's have extensive experience with. The proposed work will also involve the training student researchers in the area of particle technology, which has been identified as a national need. The PI has a strong record in involving undergraduates in research and promoting a diverse workforce.