Abstract
Electrified downstream processes for biotechnologically produced carboxylic acids reduce waste salt generation significantly and make biotechnological production ecologically and economically more attractive. In order to design, optimize, scale-up and control electrochemically induced crystallization processes, knowledge of the metastable zone width (MSZW) is essential. An optical observation approach of nucleation processes close to the electrode and determination of the MSZW is presented. This work presents a method for MSZW measurements for electrochemically induced pH-shift crystallization processes by monitoring the nucleation, the saturation pH value and saturation concentration for different current densities. The measured MSZWs for electrochemically induced pH-shift crystallization are narrow due to the foreign surface, gas bubbles and electrode surface, and rising current densities lead to even smaller MSZW. Nucleation parameters are estimated from MSWZ data, adapting the classical approach of Nývlt to electrochemically induced crystallization.
Highlights
Produced carboxylic acids, including succinic acid (SA), are considered promising alternatives to petrochemically produced platform chemicals
Using Equation (15) with the measured metastable zone width (MSZW) data allows an estimation of nucleation kinetics for the electrochemically induced crystallization
For the determination of the MSZW, the solutions were prepared with an electrolyte concentration of 0.5 M Na2SO4 and pH values of 3.4, 3.6, 3.9 and 4.1, in the same manner as the solutions for the determination of the solubility curves to ensure that each experiment started with a saturated solution
Summary
Produced carboxylic acids, including succinic acid (SA), are considered promising alternatives to petrochemically produced platform chemicals. Different measurement techniques have been used in the literature to detect nucleation, either based on a change in the physical properties of the mother solution, for example by infrared spectroscopy, ultrasound or conductivity, or based on optical detection of the grown nuclei, such as by naked eye methods, imaging procedures, turbidity or focused beam reflectance measurement [13,14,16] Most of these detection methods are not suitable for electrochemically induced crystallization processes due to the emerging gas phase, which disturbs the measurement signal. The pH (Equation (4)) is lowered at the anode and increased at the cathode by electrochemical water splitting (Equations (1) and (2)) This pH-shift influences the Crystals 2021, 11, 1090 dissociation of succinic acid, which can dissociate twice due to two carboxylic functional groups. PH values smaller than pKa1 are targeted for the electrochemically induced crystallization in order to generate supersaturation and produce the protonated succinic acid (Figure 1). If the concentration of succinic acid (c) exceeds the solubility concentration (c∗) of the protonatetd species, the system will be metastable and eventually nucleation will occur
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