Coagulation鈥揂gglomeration of Fractal-like Particles: Structure and Self-Preserving Size Distribution

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• 作者：Eirini Goudeli ; Maximilian L. Eggersdorfer ; Sotiris E. Pratsinis
• 刊名：Langmuir
• 出版年：2015
• 出版时间：February 3, 2015
• 年：2015
• 卷：31
• 期：4
• 页码：1320-1327
• 全文大小：475K
• ISSN：1520-5827

文摘

Agglomeration occurs in environmental and industrial processes, especially at low temperatures where particle sintering or coalescence is rather slow. Here, the growth and structure of particles undergoing agglomeration (coagulation in the absence of coalescence, condensation, or surface growth) are investigated from the free molecular to the continuum regime by discrete element modeling (DEM). Particles coagulating in the free molecular regime follow ballistic trajectories described by an event-driven method, whereas in the near-continuum (gas-slip) and continuum regimes, Langevin dynamics describe their diffusive motion. Agglomerates containing about 10鈥?0 primary particles, on the average, attain their asymptotic fractal dimension, D_f, of 1.91 or 1.78 by ballistic or diffusion-limited cluster鈥揷luster agglomeration, corresponding to coagulation in the free molecular or continuum regimes, respectively. A correlation is proposed for the asymptotic evolution of agglomerate D_f as a function of the average number of constituent primary particles, n虆_p. Agglomerates exhibit considerably broader self-preserving size distribution (SPSD) by coagulation than spherical particles: the number-based geometric standard deviations of the SPSD agglomerate radius of gyration in the free molecular and continuum regimes are 2.27 and 1.95, respectively, compared to 鈭?.45 for spheres. In the transition regime, agglomerates exhibit a quasi-SPSD whose geometric standard deviation passes through a minimum at Knudsen number Kn 鈮?0.2. In contrast, the asymptotic D_f shifts linearly from 1.91 in the free molecular regime to 1.78 in the continuum regime. Population balance models using the radius of gyration as collision radius underestimate (up to about 80%) the small tail of the SPSD and slightly overpredict the overall agglomerate coagulation rate, as they do not account for cluster interpenetration during coagulation. In the continuum regime, when a recently developed agglomeration rate is used in population balance equations, the resulting SPSD is in excellent agreement with that obtained by DEM.