New membranes for membrane distillation process
Creators: name: PAGLIERO, MARCELLO
Publisher: name: Universit� degli studi di Genova
Year: 2021

description: The PhD research program was focused of the development of new hydrophobic membranes suitable for membrane distillation (MD) operation. In particular, the preparation of polymeric flat sheet membranes via nonsolvent induced phase separation (NIPS) technique was investigated. This method allows to fine-tune a large number of variables in order to obtain membranes with an ample variety of different morphologies and properties. Therefore, a systematic study on the important factors affecting the membrane structure, which in turn determines the distillation performance, was carried out. The selected polymer was polyvinylidene fluoride (PVDF), the solvent was dimethylformamide (DMF).
First, the effect of the dope solution composition was evaluated. The polymer amount was found to be a key element in defining the porosity and the pore size of the final membrane. Moreover, a minimum critical concentration required to obtain a proper structure was identified on the basis of the dope solution viscosity. In fact, at lower concentrations brittle or defective films were produced.
Another important preparation parameter thoroughly investigated was the coagulation bath strength. Harsh nonsolvents induce fast precipitation creating a dense skin above a macrovoid-dominated layer, while weak coagulation media promote a delayed demixing and generate uniform and symmetric structures. Using a semi crystalline polymer such as PVDF, the precipitation rate becomes even more important because it also influences the crystallization of the polymer. The strength of the coagulation bath was regulated by adding different amounts of ethanol to the water bath, from 0% up to 96% v/v. Optimization of this parameter allowed to prepare almost superhydrophobic membranes that were able to withstand the pressure and temperature conditions during vacuum membrane distillation (VMD) tests and to deliver high distillate fluxes and total salt rejection when treating a concentrated NaCl solution.
A different approach to improve the membrane performance was exploited in further phase of activity. Different kinds of pore forming agents ? such as polyethylene glycols and lithium chloride ? were added to the dope solution in order to enhance the porosity and control the pore size. Since the support material can act as an added mass transfer resistance, it was decided to cast these membranes without any reinforcement. However, the absence of any rigid support caused severe shrinkage phenomena during the drying of the membranes leading to an almost complete collapse of the porous structure. It was found that the structure could be preserved by simply clamping the wet membrane on a stiff planar surface and leaving it to dry at room temperature.
The amount and type of the additive had impacts on both kinetic and thermodynamic factors governing the phase separation process. By adjusting the dope solution composition, it was possible to favour one or the other to obtain membranes with the desired structure and performance. These unsupported membranes were not able to bear the pressure difference normally applied during VMD, therefore they were tested using a direct contact membrane distillation (DCMD) setup.
Since the presence of the support material is mandatory for VMD application, the effect of different kinds of supports was investigated. In particular, several commercial nonwovens were used to prepare PVDF membranes based on the knowledge acquired during the first phase of the PhD activity. Moreover, along with the nonwovens, three polymeric nets, characterized by different structure and made with different polymers, were tested as possible supports. While the commercial nonwovens did not alter too much the performance and morphology of the PVDF membranes, using the nets some remarkable effects were registered. The alternation between holes and crests of the nets caused the formation of membranes with zones having different porosities. The VMD tests highlighted the better performance of the nonwoven casted membranes. However, the patterned surface of the net supported membranes resulted in lower flux decline when a concentrated NaCl solution was used as feed.
Polymeric membranes are the most studied type for MD application both for the easy processability and the low production costs. Moreover, the commonly used polymers can withstand the normal operation conditions for desalination or wastewater treatment applications. However, the possibility of producing membranes able to resist higher temperatures and pressures would open the way to new MD applications. One of the possible paths to reach this goal is the exploitation of ceramic membranes. Ceramic material are nevertheless naturally hydrophilic and surface modification procedures must be carried out in order to turn such membranes hydrophobic. Therefore, the reaction between the surface hydroxyl groups of alumina commercial membranes and a silanizing agent was exploited. By changing the reaction conditions, it was possible to obtain highly hydrophobic membranes without affecting the initial pore size and porosity. This functionalizing surface layer proved to be stable up to 400�C which would allow to cover any possible MD operation condition.