Tesi di Dottorato

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Membrane crystallization for recovery of valuable compounds from waste streams
(2016-02-26) Quist-Jensen, Cejna Anna; Drioli, Enrico; Macedonio, Francesca; Molinari, Raffaele
Sustainable development and Process intensification strategy are guidelines for industrial processes in perspective. It is becoming more and more common that industry wants to fully exploit their resources due to environmental regulations, economic gain, sustainable standpoint, etc. In this perspective, waste streams have to be turned into resources in the most environmental friendly, economic and sustainable way. Membrane Engineering is already a key-figure to realize this objective. Novel membrane technologies such as membrane distillation (MD), membrane crystallization (MCr), pressure retarded osmosis (PRO), reverse electrodialysis (RED) and forward osmosis (FO), are evolving and are being suggested for a better exploitation of waste streams. This Ph.D. study focusses, particular, on Membrane crystallization (MCr), which is a novel technology for simultaneously production of water and minerals. It has several advantages with respect to conventional crystallizers in terms of purity, controlled kinetics and crystal morphology. Moreover, MCr is able to treat high concentration solutions, which are challenging for other traditional membrane operations. The current Ph.D. work emphasizes on various aspects of membrane crystallization for approaching zero-liquid discharge in industrial processes. Improved membranes, specifically developed for MCr applications, have to be manufactured. In this study, preliminary suggestions on membrane features are given for the requirements in MCr. Lab-made PVDF membranes with different characteristics have been tested and evaluated for their performance in MCr. This study, suggests that membranes with symmetric sponge layer structure and low thickness are favorable. Membrane of asymmetric structure with many macrovoids seems more pronounced to suffer from wetting. Moreover, it has been shown that, membrane crystallization is able to treat several kinds of feed solutions including RO brine, produced water and wastewater containing high amounts of sodium sulfate. The recovered crystals exhibit high purity, good size distribution and controlled growth. Na2SO4 can be recovered as different polymorphs and in this study it has been crystallized in the anhydrous form (Thenardite). Moreover, the process has shown excellent stability in terms of transmembrane flux and maintenance of hydrophobicity of the membrane. In some cases the treatment has been continued for more than 90 hours by only slight cleaning with distillate water. Membrane crystallization, in the direct-contact membrane distillation configuration, can normally treat solutions with very high concentrations. However, its limitations in the recovery of lithium from single salt solutions have been highlighted in this study. Vapor pressure, due to increase in concentration, is reduced significant, that it is not possible to reach LiCl saturation by this configuration. Likewise, combined direct-contact and osmotic distillation configuration have not been able to increase the driving force enough in order to exceed saturation. Instead vacuum membrane distillation has been introduced to eliminate the osmotic phenomena. This configuration has been able to recover LiCl in two different polymorph structures depending on the utilized operative conditions. Furthermore, integrated membrane system, including membrane crystallization, has shown excellent capability to treat orange juice. The quality of the juice has been maintained through ultrafiltration, membrane distillation and membrane crystallization treatment. In this study, the MD/MCr feed temperature is kept below 30 °C causing a relatively low flux. However, it has still been possible to reach from a concentration of 9 °brix to 65 °brix using MD/MCr. The advantages of MD/MCr with respect to isothermal osmotic membrane distillation configuration, is the elimination of the reconcentration stages of the draw solution. All the carried out case studies show that MD/MCr is able to reduce the volume of the waste stream significantly. The obtained results might be used as guidelines for practical application. Moreover, the low temperatures and atmospheric pressures utilized, makes it possible in real industrial processes to use waste or low-grade heat. Unlike other processes, MCr is able to produce two high quality products (i.e. water and salts) and will therefore not produce any additional waste. Hereby, the extended treatment by means of MCr will only positively influence the overall “sustainability” of the entire industrial process.
Evaluation of thermal polarization and membrane characteristics for membrane distillation
(2014-11-11) Alì, Aamer; Drioli, Enrico; Aimar, Pierre; Bouzek, Karel; Fila, Vlastimil; Molinari, Raffaele
The current PhD work emphasizes on various aspects of membrane distillation for approaching zero liquid discharge in seawater desalination. In broader sense, two themes have been discussed in detail: (i) correlation between membrane features and their performance in MD (ii) understanding and control of thermal polarization in MD. Introduction and state-of-the-art studies of MD including progress in membrane development, understanding the transport phenomenon, recent developments in module fabrication, fouling and related phenomenon and innovative applications have been discussed in introductory part of the thesis. The effect of operating conditions and dope compositions on membrane characteristics and correlation between membrane features and their performance has been discussed in subsequent section. It has been established that membrane morphology plays a crucial role in performance of the membrane for real applications. Furthermore, it has been demonstrated that the effect of membrane morphology is different for direct contact and vacuum configurations. Theoretical and experimental aspects of thermal polarization in direct contact membrane distillation have also been investigated. Thermal polarization phenomenon in a flat sheet membrane has been studied by using a specifically designed cell. The effect of operating conditions and solution concentration on thermal polarization has been explored experimentally. It has been observed that increased solution concentration favors the thermal polarization due to resulting poor hydrodynamic at the membrane surface and increase in diffusion resistance to the water vapors migrating from bulk feed phase to the membrane surface. Some active and passive techniques to decrease thermal polarization and possible fouling in membrane distillation have also been discussed in the current study. Thermal polarization can be greatly reduced by inducing secondary flows in the fluid flowing inside the fiber. The induction of secondary flows in the current study has been realized by using the fibers twisted in helical and wavy configurations. Due to improvement of thermal polarization coefficient on up and downstream, the undulating fiber geometries provide high flux and superior performance ratio. Application of intermittent and pulsatile flow to control thermal polarization in MD has also been discussed. It has been inferred that these flows have positive impact on performance ratio and volume based enhancement factors without compromising on packing density of the system. The application of MD for treatment of produced water has also been studied. The effect of membrane features on their performance for the treatment of this complex solution has been discussed. The desirable membrane features for successful application of MD for such treatment have been distinguished. It has been inferred that MD possesses the capability to produce a distillate of excellent quality and is an interesting candidate to recover the minerals present in the produced water. The fouling tendency of the membranes with different characteristics towards different types of feed solutions has also been discussed in this study. It has been shown that the porosity enhanced through the introduction of macrovoids in non-solvent induced phase separation technique creates problems related with wetting and pore scaling during practical application of such membranes. The fouling related issues are less severe in the membranes with sponge like microstructure but the overall porosity of such membranes is relatively less. Thus it has been concluded that there should be an optimum between the high throughput and stable performance of the membranes synthesized through phase inversion techniques. Conclusions of the study and future perspectives have been discussed in the last section of the study.
Applicazione della distillazione a membrana a processi di interese industriale
(2009-11-13) Carnevale, Maria Concetta; Drioli, Enrico; Criscuoli, Alessandra; Molinari, Raffaele

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