Experimental evaluation of the thermal polarization in direct contact membrane distillation using electrospun nanofiber membranes doped with molecular probes

Membrane distillation (MD) has recently gained considerable attention as a valid process for the production of fresh-water due to its ability to exploit low grade waste heat for operation and to ensure a nearly feed concentration-independent production of high-purity distillate. Limitations have been related to polarization phenomena negatively affecting the thermal efficiency of the process and, as a consequence, its productivity. Several theoretical models have been developed to predict the impact of the operating conditions of the process on the thermal polarization, but there is a lack of experimental validation. In this study, electrospun nanofiber membranes (ENMs) made of Poly(vinylidene fluoride) (PVDF) and doped with (1, 10-phenanthroline) ruthenium (II) Ru(phen) 3 were tested at different operating conditions (i.e., temperature and velocity of the feed) in direct contact membrane distillation (DCMD). The temperature sensitive luminophore, Ru(phen) 3 , allowed the on-line and non-invasive mapping of the temperature at the membrane surface during the process and the experimental evaluation of the effect of the temperature and velocity of the feed on the thermal polarization

Development of fluorescent thermoresponsive nanoparticles for temperature monitoring on membrane surfaces

In this work, tris(phenantroline)ruthenium(II) chloride (Ru(phen)3) was immobilized in silica nanoparticles prepared according to the Stöber method. Efforts were devoted on the optimization of the nano-thermometer in terms of size, polydispersity, intensity of the emission and temperature sensitivity. In particular, the immobilization of the luminophore in an external thin shell made of silica grown in a second step on bare silica nanoparticles allowed producing fluorescent monodisperse silica nanoparticles (420 ± 20 nm). A systematic study was addressed to maximize the intensity of the emission of the fluorescent nanoparticles by adjusting the concentration of Ru(phen)32+ in the shell from 0.2 to 24 wt.%, whereas the thickness of the shell is affected by the amount of silica precursor employed. The luminescent activity of the doped nanoparticles was found to be sensitive to the temperature. In fact, the intensity of the emission linearly decreased by increasing the temperature from 20 °C to 65 °C. The thermoresponsive nanoparticles were functionalized with long aliphatic chains in order to obtain hydrophobic nanoparticles. The developed nanoparticles were immobilized via dip-coating procedure on the surface of hydrophobic porous membranes, such as Polyvinylidene fluoride (PVDF) prepared via Non-Solvent Induced Phase Separation (NIPS), providing local information about the membrane surface temperature

Microfluidic devices as gas – Ionic liquid membrane contactors for CO2 removal from anaesthesia gases

This work proposes a microfluidic gas – ionic liquid contactor for CO2 removal from anaesthesia gas, containing Xe. The working principle involves the transport of CO2 through a polymer flat membrane followed by its capture and enzymatic bioconversion in the ionic liquid solvent. Microfluidic devices enable a rapid and inexpensive screening of potential CO2 absorbers. The alveolar – type design of the ionic liquid chamber was adopted to reduce mass transfer limitations of CO2 through the liquid phase. Polydimethylsiloxane (PDMS) was the chosen polymer for dense membrane, as well as for the microfluidic device fabrication, mainly due to the high permeability of gases, O2 and CO2, and low cost. The selected ionic liquid was cholinium propionate (CP) with a water activity of 0.753, due to its high affinity towards CO2 and biocompatibility with the enzyme used for CO2 conversion to bicarbonate, carbonic anhydrase (CA). The CO2 and Xe permeability and CO2/Xe selectivity were determined in the microfluidic devices developed and compared to those exhibited by free standing PDMS membranes mounted on a standard permeation cell. The performance of the microfluidic devices as gas – ionic liquid contactors was evaluated for a given solvent flow rate with pure gas streams of CO2 and Xe. The obtained results show that cholinium propionate with or without the enzyme has no effect on the Xe transport, but remarkably enhances the affinity towards carbon dioxide leading to enhancement factor up to 1.9 in the presence of 0.1 mg CA/gIL

A non-invasive optical method for mapping temperature polarization in direct contact membrane distillation

Membrane Distillation (MD) is a thermal membrane process allowing for a theoretical 100% rejection of non-volatile compounds (i.e. ions, macromolecules, colloids, cells), whereas vapour molecules permeate through a micro-porous hydrophobic membrane due to a difference of vapour pressure established across the membrane-self. The effective driving force and, then, the vapour trans-membrane flux is affected by temperature polarization phenomena occurring in the boundary layers adjacent to the membrane. The temperature values at the membrane surface are usually difficult to measure and only recently some invasive techniques were adopted for this scope. The aim of this work was to introduce luminescent molecular probing as an innovative technology for non-invasive and in-situ monitoring of thermal polarization in MD. Tris(phenantroline)ruthenium(II) chloride (Ru(phen)3) was selected as temperature sensitive luminescent probe and immobilized in a flat poly(vinylidene fluoride) electrospun nanofibrous membrane (PVDF ENM). Experiments showed the key role of the Ru(phen)3 and Lithium Chloride (LiCl) in the preparation of homogeneous PVDF ENM due to their ionic nature that improved the electrical conductivity of the polymeric solution favouring the electrospinning. Furthermore, PVDF ENM showed a good performance in Direct Contact Membrane Distillation (DCMD) process. The immobilization of the molecular probe allowed to optically monitoring the membrane surface temperature during DCMD experiments. On the other hand, the employment of an IR-camera permitted the evaluation of the temperature of the bulk of liquid streams. Therefore, the combination of these two optical techniques enabled to evaluate, in a direct and non-invasive way, the thermal polarization along the membrane module during DCMD experiments

Enhanced protein crystallization on Nafion membranes modified by low-cost Surface patterning techniques

In this work, the influence of surface topography on protein crystallization over Nafion is investigated. Two types of Nafion-based membranes were modified by soft lithographic techniques in order to create different topographies at the micro and nano scale and subsequently tested. From an analysis of the induction time, nucleation, and crystal growth rate of trypsin from bovine pancreas, all of the patterned Nafion-based membranes show an enhanced nucleation and crystal growth. To provide additional insight into the experimental observations, the wettability properties of the prepared samples and the ratio of the Gibbs free energy of heterogeneous nucleation to homogeneous nucleation were evaluated. The crystallization outcome results from the combined effect of both the structural and chemical properties of the nucleant Nafion surface.This work was supported by the Associate Laboratory for Green Chemistry-LAQV, which is financed by national funds from the FCT/MCTES (UID/QUI/50006/2019). M.P. thanks the Education, Audiovisual and Culture Executive Agency (EACEA) for a Ph.D. grant under the Program “Erasmus Mundus Doctorate in Membrane Engineering”-EUDIME (FPA 2011-2014, http://www.eudime.unical.it).Peer reviewe