An Air-Lift Crystallizer Can Suppress Secondary Nucleation at a Higher Supersaturation Compared to a Stirred Crystallizer

Secondary nucleation is suppressed in an air-lift crystallizer at levels of supersaturation where in a stirred crystallizer a clear contribution of secondary nucleation is visible. A comparison of batch crystallization of l-ascorbic acid in an air-lift crystallizer and in a stirred crystallizer is presented. The results demonstrate that at low supersaturation, secondary nucleation can be suppressed in both the air-lift crystallizer and the stirred crystallizer. At higher supersaturation, nucleation starts to dominate in the air-lift crystallizer. At an intermediate level of supersaturation, a clear contribution of secondary nucleation in the final product obtained from the stirred crystallizer is visible. However, experiments with similar conditions in the air-lift crystallizer show a significantly smaller contribution of secondary nucleation. The observed enlargement of the operating window in terms of supersaturation where secondary nucleation is suppressed in an air-lift crystallizer may have important practical consequences. Air-lift crystallizers can potentially operate with a higher crystal growth rate and the operating window for design and automated control can be extended

Adsorption of Volatile Organic Compounds. Experimental and Theoretical Study

The adsorption of traces of five volatile organic compounds (VOCs) comprising butanal, 2-ethyl-2-hexenal, 2,6-dimethylcyclohexanone, 2,4,6-trimethylanisole, and 2,4,6-trimethylphenol from liquid toluene was investigated. Twenty-one commercial adsorbents of different classes were tested in batch adsorption experiments using the six-component mixture. The Na form of FAU zeolite (NaY) performed by far the best for the overall removal of these compounds from toluene, although removal of all six compounds to very low concentrations (<50 ppm) might not be feasible (in a single step). To further investigate the adsorption behavior of this zeolite, the ideal adsorbed solution theory (IAST) combined with Monte Carlo (MC) simulations was used. A force field was developed for these compounds that allows the computation of pure-component adsorption isotherms in zeolites using MC simulations. The pure-component isotherms are used as input in an IAST model to predict multicomponent adsorption behavior in zeolites. Simulations of binary and six-component mixture are compared to experimentally obtained adsorption isotherms. We show that (1) NaY zeolite performs best for the overall adsorption of the selected compounds from liquid toluene and (2) a combined molecular simulation–IAST approach can be used for this system to predict the adsorption behavior in NaY reasonably well

Minimization of Attrition and Breakage in an Airlift Crystallizer

Minimization of secondary nucleation by attrition in industrial crystallizers is a major challenge. In this work, a novel airlift crystallizer has been designed, constructed, and experimentally tested aiming at the reduction of attrition by using air for mixing instead of a stirrer or a circulation pump. It is experimentally demonstrated that in this crystallizer ideal growth, i.e., growth of crystals without any nucleation, can be approached up to a seeding load of 0.5% and crystal size of up to 600 μm. Attrition is considerably decreased in an airlift crystallizer compared to conventional impeller-mixed crystallizers. This air-mixed crystallizer enables the production of crystals of high quality and offers a large flexibility of the final crystal size by manipulating the air flow rate and the sparger design. Comparison of different designs showed a large effect of a gas disengagement zone on the performance of the crystallizer, especially when large crystals were desired. The disengagement zone allows high circulation velocities and thus good mixing without entrainment of the gas bubbles in the downcomer, approaching a uniform suspension of the crystals

Multiparameter Investigation of Laser-Induced Nucleation of Supersaturated Aqueous KCl Solutions

Various mechanisms have been proposed to explain the nonphotochemical laser-induced nucleation (NPLIN). Identifying the dominant mechanism requires addressing a large set of experimental parameters with a statistically significant number of samples, forced by the stochastic nature of nucleation. In this study, with aqueous KCl system, we focus on the nucleation probability as a function of laser wavelength, laser intensity, and sample supersaturation, whereas the influence of filtration and the laser-induced radiation pressure on NPLIN activity is also studied. To account for the nucleation stochasticity, we used 80–100 samples. The NPLIN probability showed an increase with increasing laser intensity. The results are different from the previous report, as a supersaturation independent intensity threshold is not observed. No dependence of the NPLIN probability on the laser wavelength (355, 532, and 1064 nm) was observed. Filtration of samples reduced the nucleation probability suggesting a pronounced role of impurities on NPLIN. The magnitude and the propagation velocity of the laser-induced radiation pressure were quantified using a pressure sensor under laser intensities ranging from 0.5 to 80 MW/cm². No correlation was found between the radiation pressure and NPLIN at our unfocused laser beam intensities ruling out the radiation pressure as a possible cause for nucleation