On the effect of flow restrictions on the nucleation behavior of molecules in tubular flow Nucleators

Abstract

This paper discusses theoretical and experimental considerations of organic molecules nucleating inside tubular reactors or nucleators. Temperature evolution of these liquid systems is experimentally shown for different flow rates as a function of distance when these nucleators are immersed into a water bath set at spontaneous nucleating conditions. When different restrictions in the flow path are introduced before the cooling phase of the liquid; important differences on the nucleation rates are observed. For this study, Aspirin was dissolved in a blend of water and ethanol in a 50/50 vol%. At a concentration of 200 mg/mL and a nucleation temperature of 10 °C, demonstrated to be close to the metastable zonewidth, these flow restrictions show an antagonistic effect on the nucleation rate. One restriction, placed right before the nucleator enters the cooling bath, induces a reduction in nucleation rate. Putting more restrictions into the flow path with an equal separation of 5 cm in between, the nucleation recovers back to its initial value when a second restriction of an expansion ratio 2 is applied. Restrictions with an expansion ratio of 4 exceed this nucleation rate up to an order of magnitude when more than 2 restrictions are put in place. At a higher kinetic driving force defined KDF, at a concentration of 300 mg/mL of Aspirin, the influence of the restriction becomes invariant as a function of the expansion ratio. For all experiments, the nucleation rates is highly increased with the number of restrictions introduced into the flow path. A thermal gradient difference by using the restrictions on the cooling rate of the liquid flowing inside the tubing was not observed experimentally. Therefore only hydrodynamic changes of the flow seems a plausible cause for this nucleation rate - restriction dependence as it is expected that finite amplitude perturbations, amplified by the use of restrictions, cause vortex shedding in current experimental setup.