Synthesis and optimization of NF/RO membranes for food and beverage industry

For more than 25 years, membrane processes gained an important role in food and beverage processing. These industries represent a significant part of the turnover of the membrane manufacturing industry worldwide. Main applications of these membrane operations are in the dairy industry (milk protein standardization, whey concentration, etc.), followed by beverages (wine and beer fermentation, fruit juices, etc.). In beer industry, the recovery of maturation and fermentation tank bottoms is already being applied on industrial scale. More recently, significant progress is made to implement membrane technology in several steps of beer production and may be used as alternative solution or in combination with conventional separation processes. In general, membrane separation techniques are known to have significant advantages over traditional methods, such as (i) being capable of separating molecules or microorganisms, (ii) lower thermal impact on products, (iii) moderate energy consumption, and (iv) modular design. The challenges lie in the stability of the membrane towards the applied industrial operating conditions (i.e. applied pressure, temperature, ambient atmosphere, etc.) as well as the intrinsic chemical properties of the feed (i.e. pH, intolerance towards certain components in the feed, etc.). The final desired result is an appropriate membrane with the required filtration performance. To efficiently develop such membranes, a novel high throughput set-up was built and validated. This technique allows the screening of multiple membrane samples with similar operating conditions as those applied in the final industrial application (cross-flow mode, pressure, temperature, inert atmosphere, etc.). As a result, a lab-scale membrane with appropriate performances can directly be up-scaled and tested in the final set-up. The final membrane performance is directly linked to the structure of the membrane which depends on multiple factors, including compositional (polymer concentration, solvents, additives) and non-compositional (evaporation time, wet film thickness, annealing time and temperature) parameters during preparation. This multi-parameter optimization is rather complex as well as time and effort consuming. Therefore, combinatorial techniques and high throughput experiments are used to finally obtain a membrane that outperforms the commercially available membranes. At first, cellulose tri-acetate (CTA) membranes were optimized via Genetic Algorithms (GAs) as this technique is well tolerating relatively noisy experimental data. Additionally, these membranes are prepared via the phase inversion synthesis technique, which is easy to be up-scaled. Three subsequent generations of membranes were synthesized and tested. Each generation showed performance improvements compared to the previous generation, such as retention, permeance and ability to cast defect-free membranes (homogeneous vs inhomogeneous mixture ratio improvement). However, the chemical stability of CTA membranes towards the high-alcoholic beer stream was questioned and studied. Therefore, thin-film composite (TFC) membranes were prepared as they are known to have more stable chemical properties. Here, the interfacial polymerization (IP) synthesis technique is used which is more complex to up-scale. Moreover, this technique is combined with the use of hazardous and toxic solvents (i.e. hexane, heptane, toluene, etc.). Use of alternative solvents, such as ionic liquids, is studied instead. These green designer solvents have gained interest because of their special properties. To further increase the chemical stability of these membranes, epoxide based top-layers are studied and tested in combination with preparation in ionic liquids. Promising results for these TFC membranes were obtained. One of the support layers that is used during IP is polysulfone (PSU) based. As these supports were chemically unstable and TFC development was continued with cross-linked polyimide (XL-PI) as support. To improve the chemical stability of PSU, cross-linking was induced via photo-irradiation. Three different UV curing conditions were studied and in depth evaluated. One promising UV curing condition using a 365 nm UV-LED light was selected for future use in a continuous membrane casting line. All the studies performed in the above mentioned chapters were set-up keeping in mind future potential up-scaling of the complete procedure to enable continuous membrane casting production.

Publication year:2021

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