Christina Winterscheid

24 Aug 2012

Dipl.-Ing. Christina Winterscheid

Since 06/2012 - Ph.D. Student, Institute of Chemical Process Engineering, RWTH Aachen University

2009 – 2010 - Student exchange, École Polytechnique Fédérale de Lausanne

2008 – 2009 - Student Assistant, Institute of Thermal Process Engineering, RWTH Aachen University

2006 – 2012 - Diploma of Mechanical Engineering at RWTH Aachen University, Major: Chemical Process Engineering

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Fouling characterization of colloidal silica gel ultrafiltration with flow-field flow fractionation

Colloidal silica gel is a stable aqueous dispersion of amorphous silicon dioxide particles. This environmentally friendly product has a wide range of applications, for example as a de-inking agent during paper recycling, for anti-soiling and abrasion resistant coatings.  Silica gels produced from native silicon dioxide is synthesized with sodium carbonate and sulfuric acid in salty and aqueous slurry. After synthesis silica gels are desalted applying a downstream process such as ultrafiltration. In this study the ultrafiltration process is investigated in detail concerning fouling mechanisms.

The highly viscous silica gel dispersion has a low gelation point around 15-35 wt%. Repulsion forces between the particles, which stabilize the system and prohibit gelation, can be intensified or repressed by changing pH, particle and salt concentration or temperature of the slurry. Due to concentration polarization in the vicinity of the membrane during ultrafiltration, silica gel is deposited on the membrane, causing fouling and reducing membrane performance.

The fouling behavior is quantitatively analyzed using flow-field flow fractionation (flow-FFF), a technique which originally separates particles through hydrodynamic forces and diffusion in a flow channel. The hydrodynamics of flow-FFF resemble the conditions of a cross-flow or dead-end flat sheet filtration module. In flow-FFF a permeate flow through a membrane presses the particle towards the membrane, while the particle diffuse back in opposite direction. Then, a laminar cross flow elutes the particles out of the flow channel. First smaller particles are eluted out of the channel by the laminar flow field, because they have a higher diffusion coefficient. 

Particle-particle and particle-membrane interaction during ultrafiltration are investigated in this study varying surface charge of the membrane, pH and ionic environment of the medium and hydrodynamics in the flow-FFF channel. Agglomeration of particles as well as reversible and irreversible fouling is determined by the average particle size, particle size distribution, recovery rate of the injected silica volume and the critical permeate flux. The membrane surface charge is tuned applying layer-by-layer technique. A strong positively charged polyelectrolyte poly(diallyl dimethyl ammonium chlorid) (PDADMAC)  and strong negatively charged poly(sodium styrene sulfonate) (PSS) are deposited on the negatively charged polyethersulphone membrane (PES). Electrolytes with different ionic strength, sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr) and calcium chloride (CaCl2), influence the stability of the suspension. The impact of salt concentration on attraction and repulsion forces of particles and membrane and therefore on the fouling behavior is studied.

Long-term dead-end filtration experiments including backflush, a high cycle number and an increasing permeate flux will be conducted under the same chemical, physical and hydrodynamic conditions to validate the results of flow-FFF.