A luminescent thermosensitive coating for a non-invasive and in-situ study of thermal polarization in hollow fiber membranes

A new non-invasive method to evaluate the thermal polarization is herein proposed for hollow fiber (HFs) membranes during the Direct Contact Membrane Distillation (DCMD) process. The goal was reached by using a temperature-sensitive phosphorescent molecular probe such as Tris(phenanthroline) ruthenium(II) chloride (Ru(phen)3) and an infrared camera (IR) camera, to map, in-situ, the thermal profile directly at the membrane surface which is in contact with the hot feed water solution. The molecular probe was immobilized, by means of a perfluoropolyether (PFPE)-based coating, on the surface of a polypropylene HF in order to obtain a thermosensitive polymerized coating. The produced HFs were fully characterized in order to demonstrate the successful embodiment of the molecular probe on the surface of the membrane and their suitability, in terms of pore size, hydrophobicity and stability, for DCMD application. The luminescent activity of the immobilized Ru(phen)3 complex was measured by a bifurcated optical fiber, one branch was used to excite the molecular probe and the other branch was able to constantly monitor the molecular emission which was dependent on the temperature along the HF surface. The HFs were tested in DCMD at different feed temperatures (40 oC, 50 oC, 60 oC) and constant conditions of streams flow rate and distillate temperature (Qf = 90 L/h; Qp = 24 L/h; Tp = 13 oC). The obtained results clearly demonstrated the possibility of studying the thermal polarization phenomenon occurring during DCMD, with the temperature profile developed on the HF surface monitored, for the first time, at molecular level using a non-invasive technique. This approach can shed the light on better understanding this phenomenon and the possible strategies for its mitigation and counteraction.