Citation: Seckler, M.M. Crystallization in Fluidized Bed Reactors: From Fundamental Knowledge to Full-Scale Applications. Crystals 2022, 12, 1541. https://doi.org/10.3390/ cryst12111541 Academic Editors: Heike Lorenz, Alison Emslie Lewis, Erik Temmel and Jens-Petter Andreassen Received: 13 August 2022 Accepted: 21 October 2022 Published: 28 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). crystals Review Crystallization in Fluidized Bed Reactors: From Fundamental Knowledge to Full-Scale Applications Marcelo Martins Seckler Department of Chemical Engineering, Polytechnic School, University of São Paulo, São Paulo 05508-010, Brazil; marcelo.seckler@usp.br; Tel.: +55-11-9996-35142 Abstract: A review is presented on fifty years of research on crystallization in fluidized bed reactors (FBRs). FBRs are suitable for recovery of slightly soluble compounds from aqueous solutions, as it yields large, millimeter sized particles, which are suitable for reuse and permits low liquid residence times in the timescale of minutes. Full-scale applications for water softening have been applied since the 1980s, and since then, new applications have been developed or are in development for recovery of phosphorus, magnesium, fluoride, metals, sulfate, and boron. Process integration with membrane, adsorption, and biological processes have led to improved processes and environmental indicators. Recently, novel FBR concepts have been proposed, such as the aerated FBR for chemical-free pre- cipitation of calcium carbonate, the seedless FBR to yield pure particulate products, a circulating FBR for economic recovery and extended use of seeds, as well as coupled FBRs for separation of chiral compounds and FBRs in precipitation with supercritical fluids. Advances are reported in the understanding of elementary phenomena in FBRs and on mathematical models for fluid dynam- ics, precipitation kinetics, and FBR systems. Their role is highlighted for process understanding, optimization and control at bench to full-scale. Future challenges are discussed. Keywords: fluidized bed reactor; homogeneous granulation; crystallization from solutions; precipitation; wastewater treatment; water softening; phosphorus removal; struvite; chiral separations 1. Introduction Industrial crystallization from solutions is applied as a separation operation or as a means of synthesizing particulate products. Four crystallization methods exist, depending on how the solution interacts with its environment to promote the formation of the solid phase: cooling, evaporative, antisolvent and chemical reaction crystallization. The choice of the crystallization method is primarily based on thermodynamics of multiphase systems [1], meaning the solubility of the crystallizing compound plays a dominant role. Moderately and highly soluble compounds are preferably processed by either cooling or evaporative crystallization because respectively a high yield and a low energy consumption are feasible. Unacceptably low yields would result for slightly soluble compounds, so for this class of compounds either chemical reaction (precipitation) or antisolvent crystallization is applied. The latter finds use only when high valued compounds are involved, as the cost of separating and recycling the antisolvent is high, so most slightly soluble compounds are processed by chemical reaction crystallization. Precipitation equipment usually involves rapid mixing of the reactants (timescale of seconds or lower) followed by a long period (timescale of hours) when particles are allowed to develop until they meet requirements for downstream separation from the liquid. The rapid mixing step takes place either in a static mixing device or in a high turbulence zone within the crystallizer. The crystallizer may be a gently stirred tank or a static vessel. The logic behind this process arrangement is related to the kinetics of elementary phenomena of crystallization of slightly soluble compounds. In the mixing step fast primary nucleation (timescales << 1 s) and slow crystal growth (typically 10 -8 to 10 -10 ms -1 ) take place, Crystals 2022, 12, 1541. https://doi.org/10.3390/cryst12111541 https://www.mdpi.com/journal/crystals