Mixing in continuous crystallizers

Abstract

The review is concerned primarily with some aspects of mixing in continuous crystallizers and recognizes the central importance of the interplay between the mixing and crystallization (or precipitation) processes in the design and performance evaluation of industrial crystallizers. The paper adopts a unifying treatment from a chemical reaction engineering viewpoint in which the Lagrangian approach to mixing in crystallizer systems is emphasized. The concepts of macro-and micromixing applied to crystallization configurations have been introduced with due emphasis on the modeling efforts. The present state of knowledge in various related areas of continuous mixing is assessed to help direct future trends in research. N. S. Tavare Department of Chemical EngineeringUniversity of Manchester Institute of Science and TechnologyManchester, M60 lQD, England SCOPEIn recent years notable progress has been achieved in applying formalized unifying chemical reaction engineering principles to the analysis, simulation, and design of crystallizer configurations, but one significant gap in our understanding of crystallization processes is their interaction with mixing process. Using an analogy with the mixing description conventionally used in chemical reactors, this paper attempts to formalize the interplay between mixing and crystallization from the Lagrangian perspective. The mixing process is generally characterized by two distinct and independent elements, macro-and micromixing. The objectives of this review are to report the present state of the art and to direct further work on the characterization of mixing and its effects on the overall performance in conventional crystallizer configurations. Such a review may help in evaluating some of the research trends in this field. CONCLUSIONS AND SIGNIFICANCEThe present article attempts a comprehensive treatment of mixing in continuous crystallizers from the Lagrangian perspective and has resulted in a recognized and straightforward chemical reaction engineering approach to the analysis of crystallization processes. The first part of the review deals with macromixing aspects, which covers major developments in analysis of residence time and crystal size distributions (RTD and CSD) with particular reference to modeling work. Clearly, CSD modeling is and will remain a popular and central topic of crystallization research. Both RTD and CSD studies should become more detailed and widespread for their greater engineering utility in the design, performance evaluation, and control of industrial units.The second part of the review highlights the relatively new and important area of micromixing, i.e., mixing at the molecular scale. A small number of process simulation studies at extreme levels of micromixing have clearly demonstrated the enormous effects of the micromixing level on overall crystallizer performance. This may provide a comparatively sensitive and useful indicator of intermediate levels of micromixing. Characterization of real vessels of crystallization systems with complex no...

Agglomeration in a continuous MSMPR crystallizer

“…Agglomeration processes can be represented in the population balance by birth and death functions formulated on the basis of empirical description and also preserving the internal consistency. Although the precise mechanisms of agglomeration processes are difficult to conceive, a simple mechanism based on two body collisions has generally been used in formulating the rate models (see, for example, Table 4, Tavare, 1986). The agglomeration model proposed by Liao and Hulburt (1976) used characteristic length as an additive property and provided an analytical solution for a continuous MSMPR crystallizer.…”

“…As the presence of agglomerated particles in crystallizing systems is fairly common, considerable attention has been paid in many recent studies dealing with agglomeration of crystals during precipitation (or crystallization) in understanding the factors that contribute to their formation and to the development of theories to account for their characterization (see, for example, Table 4 of Tavare, 1986). studied the tendency for aggregation and disruption of aggregates of calcium oxalate dihydrate crystals in urine-like mother liquor in a two-stage MSMPR (mixed suspension mixed product removal) crystallizer and Couette flow aggregator operated in series.…”

Mixing, reaction, and precipitation: Interaction by exchange with mean micromixing models

“…Danckwerts (1958) has specifically pointed out the significance of micromixing in a precipitation system. Process simulation studies concerning the extreme levels of micromixing (Becker and Larson, 1969;Tavare, 1984, 1985;Tavare, 1986Tavare, , 1989) have clearly demonstrated this enormous effect and stressed the importance of characterizing micromixing effects from both theoretical and practical viewpoints in a real vessel at some intermediate level. The kinetic events for crystallization systems are generally nonlinear; crystallizers are usually operated at relatively high yields; the residence time distribution (RTD) function is closer to an exponentially decaying type representing a fully backmixed vessel; the physical nature of the crystallization process allows the practical possibility of segregation and therefore micromixing effects may tend to be important in real crystallizers.…”

“…They used a micromixing model based on continuous mass transfer between a point and its environment to describe the molecular dissipation zone as originally proposed by Costa and Trevissoi (1972a,b) and demonstrated the influence of mixing on a precipitation process. Tavare (1986), in his intensive review on mixing in continuous crystallizers, cataloged the major types of models that have appeared in the chemical reaction engineering literature in order to judge their applicability to crystallizer configurations. As there is no general acceptance as to what constitutes a sufficient description of micromixing, a host of models describing specific mixing histories has been proposed for chemical reactors, each being an attempt to represent conceivable extreme or intermediate micromixing states with or without some mechanistic implications.…”

Population balance modelling of polydispersed particles in reactive flows

“…The next logical step was therefore to refine the simplified reaction engineering models by introducing micromixing models. A considerable body of work was devoted to the development of mixing models based on turbulence phenomenology and to their application to continuous and semibatch precipitation -see Baldyga et al [12], [178], [179], David and Marcant [38], [144], David [37] and others; for a comprehensive review, see Tavare [240]. Even in stirred reactors, the turbulent flow field can exhibit large-scale features and inhomogeneities rendering the task of accurately describing them with a simplified model impossible.…”