Exploring the potentialities of membrane distillation: Membranes, configurations and applications

Membrane distillation (MD) is based on vapor transport through a hydrophobic porous membrane, retaining the liquid phase. The driving force for this vapor transport is the vapor pressure difference, induced by a temperature difference over the membrane. Originally, the focus of this technology was mainly on seawater desalination, while currently interests in high salinity applications outside the scope of traditional desalination techniques are arising. The process was already invented in 1963 and the number of publications is growing exponentially. Nevertheless, the process is not yet available on industrial level.

This PhD aims to reduce the gap between research and industrial applications, by gaining new fundamental insights, by providing a membrane with improved properties and by increasing the credibility of the technology during application testing at lab and pilot scale. The first part of this dissertation is mainly focused on the membranes and configurations. A critical review coherently summarizes the proliferation of publications on membrane design in MD. Additionally, over 20 commercial membranes were evaluated using a standardized characterization protocol. Importantly, it was shown that the salinity strongly determines the selection of the membrane thickness at lab scale. Moreover, the membrane selection also depends on the configuration and supported membranes show reduced fluxes in air gap and permeate gap membrane distillation. The comparison of configurations also revealed contradictory conclusions at lab and pilot scale, due to the limited inflow energy at pilot scale.

In the second part, multiple coating methods were evaluated, showing an outstanding performance of the plasma coatings. The membrane structure itself was improved in terms of porosity, pore size, tunable thickness and a reduced compressibility using the phase inversion process. The exposure to humid air before coagulation, an increased coagulation bath temperature and a sufficiently high dope viscosity were proven to be important factors for the formation of a porous structure.

In the final part, novel application areas were explored. A brief overview of wetting and fouling in membrane distillation was given and a systematic approach for application testing was proposed. The first case investigated the applicability of MD in the presence of surfactants and oil. The contact angle and surface tension provided a quick and valid method to predict membrane wetting for different membrane surface chemistries. Less hydrophobic membranes (PE) were found more susceptible towards membrane wetting compared to PTFE membranes. It was also shown that oleophobicity can improve the wetting resistance, especially in the presence of oil traces. In the second case, MD was successfully applied for the recovery of cheese pickle brines. FTIR combined with SEM/EDX were shown to be valuable characterization techniques for membrane autopsy to detect the type of membrane fouling, allowing to propose suitable cleaning strategies. The short term lab and pilot experiments combined with simulations show the technical and economic feasibility of membrane distillation for the treatment and recovery of cheese pickle brines.