GOALI: Toward a Fundamental Understanding of Elutriation in Fluidized Beds

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Abstract CTS-0318999 C. Hrenya, University of Colorado, Boulder Gas-solid flows are ubiquitous, though not well understood. A study of solids processing plants indicates that 80 percent of the plants experienced solids handling problems such as pipe blockage and that the performance of such operations was typically only 40-50 percent 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 percent and in some cases may differ by a factor of more than a hundred. Furthermore, even the qualitative nature of elutriation may be anti-intuitive. For example, the addition of fines to a fluidized 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). In order to better predict the elutriation phenomenon, a combined modeling and experimental effort is being proposed. Attention will be focused on a binary system composed of two species that differ in both size and density. The impetus for this choice is twofold: (i) this system is of specific interest to Millennium Chemicals (one of the two industrial partners on this proposal), and (ii) the binary system represents the simplest form of a polydisperse system. The modeling effort will be based on a kinetic-theory analogy. Preliminary work has indicated that an equipartition of energy between unlike particles, which is a common assumption of existing kinetic-theory models, does not exist. Preliminary calculations have also indicated that non-equipartition gives rise to additional components of the driving force for size/density segregation; the existence of such driving forces has not been previously documented. Because the segregation (or de-mixing) of unlike particles in both the bed and freeboard section will impact system hydrodynamics (and thus elutriation), an investigation on the impact of the non-equipartition effects is needed. Thus, the first part of the proposed study will involve the derivation of a kinetic theory for binary systems in which non-equipartition (and non-Maxwellian) effects are included. The resulting continnum heory will then be incorporated into two CFD (computational fluid dynamics) codes: a standard Eulerian-Eulerian framework (both phases are treated as a continuum) and a new multi-phase particle-in-cell (MP-PIC) formulation developed by Arena-Flow (an industrial partner on this proposal). A unique feature of the latter formulation is the combined Eulerian and Lagrangian approach to the particle phase, which allows for a computationally efficient description of systems with multiple particle types. Unlike previous studies, the experimental portion of the proposed work will involve elutriation measurements on beds composed of two Group B species which differ in size and density. Both overall and local measurements of fluxes for each species will be obtained. Application of the models to the experimental system will represent the first elutriation predictions without empricism (i.e., no adjustable parameters). The resulting comparisons between model predictions and experimental data will indicate the impact of non-equipartition effects on bed hydrodynamics, and the viability of the Eulerian-Eulerian and MP-PIC frameworks in predicting elutriation, which is of immense interest to both industrial partners. This work is motivated by the combined and complementary interests of each partner: the University of Colorado (polydisperse particulate systems), Millennium Chemicals (elutriation), and Arena-Flow (MP-PIC method). The proposed effort is designed to ensure continual interaction and exchange between the partners, with annual meetings and extended visits to both industrial sites by both the PI and PhD students. Broader impacts of the work include the following: (i) a more fundamental understanding of polydisperse systems, which is needed before current treatments can be extended beyond two particle sizes, (ii) training of graduate students in the area of particle technology, which has been identified as a national need, and (iii) sharing of learned information with both the technical community and the student community (via incorporation into coursework and outreach).