Static mixer assisted membrane filtration

  • Anwendung statischer Mischer in der Membranfiltration

Weik, Sarah; Wessling, Matthias (Thesis advisor); Panglisch, Stefan (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019

Abstract

Membrane filtration, and ultrafiltration in particular, is an essential process for (drinking) water purification and wastewater treatment. As all pressure-driven membrane processes, ultrafiltration suffers from fouling phenomena that diminish its efficiency. Various fouling countermeasures have been developed over the years. The approach of altering the membrane surface to make the membrane insusceptible to fouling is extensively studied in literature. Despite some progress, a membrane which is immune to all kinds of foulants has not been developed yet. This thesis follows a different approach, aiming at influencing the process conditions and the hydrodynamics in particular, instead of altering the membrane surface. Changing the hydrodynamics is a rather universal approach and can - depending on the actual measure - be implemented in a wide range of membrane modules and applications. Fouling countermeasures influencing the hydrodynamics range from the simple application of turbulent flow to sophisticated ideas such as vibrating membrane modules. However, most of the ideas or approaches discussed in research are too complicated, too energy intensive or too expensive to apply them in industrial processes. Static mixers inserted in the flow channel of tubular membranes are a simple and easy to apply measure to influence the hydrodynamics inside the module and to mitigate fouling. They deflect and redistribute the fluid, induce secondary flows, and enhance mixing. At the same time, shear rate and particle back-transport are improved. As all these phenomena help to mitigate fouling, static inserts are a promising antifouling measure. This thesis presents a comprehensive investigation of static mixers in membrane filtration. The effects of design and position of static mixers on fouling, retention, pressure loss and energy consumption are analyzed. Various mixer designs were 3D-printed and investigated in humic acid and silica filtration. The analyzed geometries of the static inserts range from plain and modified twisted tapes over screw-threaded inserts to well-known designs as the Kenics static mixer. Furthermore, new mixer geometries such as the ``turning chips'' insert were developed. When taking into account fouling mitigation, energy demand, retention and material consumption, the turning chips mixer turned out to be the best design of all geometries investigated in this thesis. Combining static mixers with air sparging proved even more effective in fouling mitigation. In particular, the newly developed aerating mixer which combines aeration and mixing in one device, showed promising results in fouling mitigation. Due to the energy needed for the air compression, air-sparged systems required more energy than non-sparged systems. The mixing effectiveness and mass transfer enhancement of static mixers were analyzed in the aqueous ab- and desorption of CO2. The results indicate that using static mixers in membrane reactors to improve yield and selectivity of chemical reactions is a promising approach. All in all, it could be shown that static mixers are a useful, easy-to-apply fouling countermeasure which can significantly increase the efficiency of membrane processes. They enable the process operation at laminar flow, thus considerably reducing the energy demand. Developing criteria how to select the mixer design suitable for a specific application can further simplify the usage of static mixers in membrane filtration.

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