Master Assignment

Master Assignment-Fabrication of improved FO hollow fiber supports

In this project, we are cooperating with a membrane company in Denmark (Aquaporin A/S ) to fabricate Forward osmosis (FO) membranes. FO is a green technology which helps to produce pure water with low energy cost. Recently much attention has been paid to biomimetic membranes based on aquaporins [Figure 1]. Aquaporins are cell proteins which act as small water channels to allow fast transport of only water molecules, meanwhile blocking all other types of molecules. FO membranes consist of a thin, selective separating layer on top of a porous support. Both layers are important for the properties of the membrane such as its water flux, reverse salt flux and salt rejection. So far RO-type supports mostly have been used for FO membranes, which suffer from low FO water flux and high internal concentration polarization (ICP) due to their dense and thick structure. However, these thick supports are unnecessary due to the low pressure used in FO. Therefore, it seems essential to fabricate an ideal and improved support for FO. ICP occurs in the support which is mainly characterized by its physical properties such as porosity, tortuosity, and wall thickness. These properties are known as the intrinsic structural factor (m) [eq-1]. Therefore, fabricating an improved support which reduces the ICP (low s-value) as much as possible is one of the main objectives in designing FO supports.

Hollow fibers are the best choice for FO supports because of high control on their structure in the fabrication step and their high surface area. Hollow fiber supports are being fabricated using a solvent containing a base-polymer and a non-solvent. These two are brought in contact with each in a spinning machine to form the hollow fiber.

While the tortuosity and porosity of the support cannot be improved to a high extent (they are normally in a specific range), the only parameter left to be improved is the wall thickness of the hollow fiber support. Based on S-value equation, a thinner support would lead to a lower S-value and consequently a higher FO water flux. Although the FO support does not require high stability, it must withstand a certain operation pressure (~4bar). Therefore, the goal of this project is to test different polymers to fabricate very thin and stable fibers. The influence of wall thickness on FO water flux (S-value) can be more precisely checked by varying it from very thick to very thin.

Several experiments are expected to be carried out in this assignment such as: Hollow fiber fabrication, pure water permeability, pore size measurements, SEM analysis, interfacial polymerization coating, FO performance, and low-pressure RO experiments.

Reference:

[1] Tzahi Y. Cath, Amy E. Childress, Menachem Elimelech; Forward osmosis: Principles, applications, and recent developments; Journal of Membrane Science 281 (2006) 70–87

[2] Robert J. Petersen; Composite reverse osmosis and nanofiltration membranes; Journal of Membrane Science; 83 (1993) 81-150