Impact of the Fluid Flow Conditions on the Formation Rate of Carbon Dioxide Hydrates in a Semi-Batch Stirred Tank Reactor S. Dou € ıeb URPEI, Dept. of Chemical Engineering,  Ecole Polytechnique de Montr eal, Station CV, Montreal, H3C 3A7, Canada TIPs, Universit e Libre de Bruxelles, Av. F.D. Roosevelt 50, CP 165/67, 1050 Brussels, Belgium L. Fradette and F. Bertrand URPEI, Dept. of Chemical Engineering,  Ecole Polytechnique de Montr eal, Station CV, Montreal, H3C 3A7, Canada B. Haut TIPs, Universit e Libre de Bruxelles, Av. F.D. Roosevelt 50, CP 165/67, 1050 Brussels, Belgium DOI 10.1002/aic.14952 Published online in Wiley Online Library (wileyonlinelibrary.com) CO 2 hydrate formation experiments are performed in a 20 L semi-batch stirred tank reactor using three different impel- lers (a down-pumping pitched blade turbine, a Maxblend TM , and a Dispersimax TM ) at various rotational speeds to exam- ine the impact of the flow conditions on the CO 2 hydrate formation rate. An original mathematical model of the CO 2 hydrate formation process that assigns a resistance to each of its constitutive steps is established. For each experimental condition, the formation rate is measured and the rate-limiting step is determined on the basis of the respective values of the resistances. The efficiencies of the three considered impellers are compared and, for each impeller, the influence of the rotational speed on the rate-limiting step is discussed. For instance, it is shown that a formation rate limitation due to heat transfer can occur at the relatively small scale used to perform our experiments. V C 2015 American Institute of Chemical Engineers AIChE J, 00: 000–000, 2015 Keywords: hydrates, mixing, crystal growth (industrial crystallization), mathematical modeling, heat transfer Introduction Carbon dioxide (CO 2 ) capture and storage (CCS) has become a major research focus due to the potential of CCS for mitigating anthropogenic CO 2 emissions. CCS is aimed at large point emission sources such as fossil fuel power plants and major industrial CO 2 -emitters such as cement kiln plants and refineries. In this method, CO 2 is first separated (“captured”) from flue/fuel gases, transported and then either isolated from the atmosphere in a long-term storage location or used industrially. 1,2 Reducing the cost of separation, which is the principal financial impediment to CCS projects, is one of the main challenges to making industrial CCS deployment economically feasible. 1–3 Conventional separation technolo- gies based on chemical absorption, physical adsorption, mem- brane separation, and cryogenic distillation are very energy intensive and suffer from other drawbacks such as chemical degradation, low capacity, or high capital costs. 1,4–6 Hydrate-based CO 2 separation (HBCS) techniques have been receiving increasing attention as promising viable alter- natives to conventional capture technologies. 1,4,7 In these tech- niques, flue/fuel gases containing CO 2 are exposed to liquid water under high pressure, resulting in the formation of CO 2 hydrates. Gas hydrates are ice-like solid compounds in which gas molecules are physically trapped by van der Waals forces in polyhedral cages formed by hydrogen-bonded water mole- cules. 8 Gases that form hydrates when they come into contact with liquid water at the appropriate temperature (typically less than 300 K) and pressure (typically more than 0.6 MPa) are small molecules ( < 0.9 nm), including light hydrocarbon gases such as methane and CO 2 , rare gases such as argon and krypton, and diatomic gases such as nitrogen and oxygen. 9 The basis of CO 2 separation by gas hydrate formation is that, when a flue/fuel gas is put in contact with liquid water in con- ditions leading to the formation of hydrates, CO 2 is preferen- tially incorporated into the cages of the formed hydrates, compared with the other molecules of the gas. This results in a CO 2 -rich hydrate phase and a CO 2 -poor residual gas phase. 3,4 This formation, taking place within the liquid water phase, can be described as a crystallization in solution process that involves a series of mass and heat transfer steps. 10 Relative to the other separation techniques, the advantages of HBCS techniques are the high storage capacity, low invest- ment, small energy penalty, simple process, and environmen- tal friendliness. 4,11,12 Furthermore, the decomposition of the Additional Supporting Information may be found in the online version of this article. Correspondence concerning this article should be addressed to L. Fradette at louis.fradette@polymtl.ca V C 2015 American Institute of Chemical Engineers AIChE Journal 1 2015 Vol. 00, No. 00