In a membrane crystallizer (MCr), the membrane matrix acts as a selective gate for solvent evaporation, modulating the final degree and the rate for the generation of the supersaturation. Hence, acting on the transmembrane flux, either by changing the driving force of the process or by choosing membrane with proper characteristics, allows controlling the crystallization process very precisely. In the last years, MCr has increased in interest and more and more publications can be found in literature. The crucial requirement of membranes to be used in MCr is to be hydrophobic. Polymers such as polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidenefluoride (PVDF) are commonly employed in the preparation of membranes for membrane contactor applications. In this work, the results in the application of different innovative hydrophobic membranes (both inorganic and polymeric) in MCr operations are described. In particular, two different ceramic hollow fiber membranes, three different Hyflon/PVDF composite flat sheet membranes and a hybrid Bi2Se3/PVDF flat sheet composite membrane were used for performing MCr experiments. All the tested membranes showed stable performance, without any wetting, during crystallization. Visual observation of the obtained NaCl crystals showed cubic shape whereas the analysis of their crystal size distribution proved their low dispersion (i.e., CV). Moreover, a secondary nucleation was observed which occurred simultaneously with crystal growth due to the contact of the growing crystals with the different parts of the plant. This effect was more evident for the membranes with the highest trans-membrane flux due to the highest rate in achieving supersaturation degree, supersaturation rate, nucleation and secondary nucleation. Furthermore, the inclusion of Bi2Se3 fillers in PVDF membranes, assisted the crystal-growth for NaCl, leading to a faster crystal growth and a higher uniformity of the crystal size. In conclusion, all the analyzed membranes proved their suitability for MCr process whose performance can be controlled acting on the chemical-physical properties of the membranes and process parameters (temperature, concentration, flowrate, etc.). These aspects would be of undoubted benefit because allow modulating the final properties of the crystals produced both in terms of structure (polymorphism) and morphology (habit, shape, size, and size distribution).
Membrane-assisted Crystalliazation Progresses
E Drioli;F Macedonio
2019
Abstract
In a membrane crystallizer (MCr), the membrane matrix acts as a selective gate for solvent evaporation, modulating the final degree and the rate for the generation of the supersaturation. Hence, acting on the transmembrane flux, either by changing the driving force of the process or by choosing membrane with proper characteristics, allows controlling the crystallization process very precisely. In the last years, MCr has increased in interest and more and more publications can be found in literature. The crucial requirement of membranes to be used in MCr is to be hydrophobic. Polymers such as polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinylidenefluoride (PVDF) are commonly employed in the preparation of membranes for membrane contactor applications. In this work, the results in the application of different innovative hydrophobic membranes (both inorganic and polymeric) in MCr operations are described. In particular, two different ceramic hollow fiber membranes, three different Hyflon/PVDF composite flat sheet membranes and a hybrid Bi2Se3/PVDF flat sheet composite membrane were used for performing MCr experiments. All the tested membranes showed stable performance, without any wetting, during crystallization. Visual observation of the obtained NaCl crystals showed cubic shape whereas the analysis of their crystal size distribution proved their low dispersion (i.e., CV). Moreover, a secondary nucleation was observed which occurred simultaneously with crystal growth due to the contact of the growing crystals with the different parts of the plant. This effect was more evident for the membranes with the highest trans-membrane flux due to the highest rate in achieving supersaturation degree, supersaturation rate, nucleation and secondary nucleation. Furthermore, the inclusion of Bi2Se3 fillers in PVDF membranes, assisted the crystal-growth for NaCl, leading to a faster crystal growth and a higher uniformity of the crystal size. In conclusion, all the analyzed membranes proved their suitability for MCr process whose performance can be controlled acting on the chemical-physical properties of the membranes and process parameters (temperature, concentration, flowrate, etc.). These aspects would be of undoubted benefit because allow modulating the final properties of the crystals produced both in terms of structure (polymorphism) and morphology (habit, shape, size, and size distribution).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
