(123e) Model-Based Development of a Novel Apparatus for Electrochemically Induced Crystallization of (di-)Carboxylic Acids

Bio-based platform chemicals, such as carboxylic, amino or benzoic acids present a promising alternative to petrochemical synthesis routes. However, the downstream processing is still demanding and accounts for up to 60% of the total production costs despite intensive research [1]. The target molecules are biotechnologically produced at neutral pH and mostly present in the carboxylate form after fermentation. For separation by e.g. reactive extraction, crystallization or precipitation, multiple adjustments of the pH value are necessary. For this purpose, acids and bases are used, which produce neutral salts as by-products, reduce the economic efficiency and increase the carbon footprint of bio-based platform chemicals.

Method:

This contribution presents the model-based development of a novel electrochemical cell for the pH shift crystallization of carboxylic acids. Instead of additives, the adjustment of the pH, which triggers the crystallization of the acid, is conducted by employing water-splitting electrolysis [2-4]. The scale-up of the electrochemical crystallization requires novel electrolysis cells that operate reliable even when dispersed solid crystals are present in the electrolyte. We applied Computational Fluid Dynamics (CFD) to design suitable geometries for the electrolyte chambers, which can cope with liquid-gas as well as liquid-gas-solid containing electrolytes. Especially the diameter of the bubbles strongly influences the gas-liquid interaction, but literature regarding the diameter of hydrogen and oxygen gas bubbles inside electrolyser with small gaps is sparse [5]. Therefore, we experimentally investigated the influence of operating parameters on the diameter of oxygen bubbles in the anodic chamber of a 100cm² electrolysis cell. Using a high-speed camera with a transparent electrolysis design and a neural network for bubble recognition, the effect of different current densities, volumetric flow rates and electrolyte concentrations on the bubble diameter is quantified [6].

Discussion & Conclusion:

Preliminary simulation and literature [5] indicates that the fluid pattern is governed by the nucleation and induced convection of the rising gas bubbles. Based on these findings, a three-phase (gas-solid-liquid) Euler-Lagrange simulation algorithm was created inside the OpenFOAM environment to describe the effect of the disperse phases on the continuous phase. With this approach different prototypes for electrolysis cells were developed, 3D-printed and investigated in the laboratory with respect to their hydrodynamic behavior. The simulation results are validated with the experimentally measured residence time distributions and predicted the increase of Bodenstein number due to the bubble generation. The information gained from the experiments and simulation is used in a model aided feedback loop to generate geometric designs with improved performance regarding the fluid, gas and particle residence time. The designs are subsequently tested by rapid prototyping. Different concepts for gas-liquid flows like co-current or crosscurrent are presented and their advantages and disadvantages are discussed. With the developed CFD model and the rapid prototyping aided workflow we were able to design cell geometries tailored for the electrochemical crystallization. The new designs achieved stable operation with particles present inside the electrolysis cell. Besides, a classification according to particle size could be observed, when using a crosscurrent approach. While large crystals were successfully withdrawn from the suspension outlet at the bottom of the chamber, fine particles exited with the liquid. This enables the automatic separation of product and the recovery of fine particles, which can then be returned to the electrolysis cell with fresh feed solution and be used as seeds. We envision the new modules developed for electrochemically induced crystallization to overcome the waste burden and mitigate the environmental footprint of industrial biotechnology by providing a technology for waste free recovery of succinic acid.