Understanding Variation of Experimental Flux and Rejection
Membrane performance can vary due to lot-to-lot variance, making it reasonable to expect performance differences when testing a smaller area of membrane. Polymeric membranes are widely used across various industries and are typically produced in large batches using processes such as phase inversion and interfacial polymerization. Achieving consistent performance across the entire membrane area, and across different batches is challenging due to the complexity of the production process and the scale of manufacturing. The following sections provide a short summary of the membrane production process, focusing on phase inversion and interfacial polymerization techniques.
Phase Inversion Process
The phase inversion process begins with the preparation of a polymer solution, where a polymer is dissolved in a suitable solvent. This solution is then poured onto a flat surface, such as a moving support layer, and evenly spread using a casting knife. Next, the polymer-coated membrane is immersed in a non-solvent bath (e.g., water), which causes the solvent to diffuse out and results in the formation of pores. Finally, the membrane is washed to remove residual chemicals and dried under control conditions to prevent damage.
Interfacial Polymerization Process
Interfacial polymerization is primarily used for fabricating thin-film composite membranes, such as those used in reverse osmosis (RO) and nanofiltration (NF). The process begins by washing the substrate layer, typically made of Polysulfone or Polyethersulfone. In the aqueous phase, a diamine monomer is dissolved in water and applied to the support layer. The organic phase involves applying acid chloride monomers, which react with the aqueous monomers to form a thin, selective film. The membrane is then cured with heat, washed to remove impurities and dried under control conditions.
Challenges in Performance Consistency
As explained, membrane fabrication is a multi-step process influenced by many factors including temperature, time, chemicals purity, and production equipment. Although manufacturers employ quality control procedures, performance variations are expected even when testing production scale spiral-wound membranes. The performance variation is even more common when testing lab-scale membrane coupons due to the small size. The performance variation may affect the flux, rejection rate, and the membrane’s visual appearance.
Useful Tips and Tricks
- Follow pre-conditioning procedures and recommendations stated in the membrane’s data sheet.
- Ensure you have enough membrane coupons to complete all planned experiments without the need to order a new lot.
- Perform each experiment in triplicate and calculate the average to ensure reliable results.
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