Modelling of Electrically Driven Membrane Processes

The present work is centered on applications of electro-membrane processes mainly intended for the agricultural or food industry. The applications are divided among three areas:

- The recovery of citric acid from lime juice waste.
- Inhibition of Anjou pear juice browning.
- Recovery and production of lactic acid from a fermenter.

The recovery of citric acid encompasses a theoretical and experimental evaluation of electrodialytic separation of the citrate ion at different valences by changing pH, and the resulting impact on process parameters like current efficiency and energy consumption. These results suggested that the electrodialytic recovery of citrate at valence –2 (HCit^2-) was superior in respect to the chosen parameters. Results from in situ experiments on stripped lime juice containing 4% citric acid from a lime processing factory in Mexico is included. The in situ experiments were largely inhibited by membrane fouling from organic matter. A minor study in microfiltration and ultrafiltration of the lime juice waste is included as well as a more extensive study on concentration of citrate by nanofiltration. The intentions of applying electrodialysis as a direct measure to recover citric acid from the stripped lime juice had to be renounced because of the fouling problems involved. Extensive pretreatment of the waste has to be implemented before electrodialysis can be considered further. Nanofiltration seems a feasible alternative for concentration of the citrate-rich waste stream. 
The inhibition of enzymatic browning (caused by polyphenol oxidase (PPO)) of Anjou pear juice by acidification was studied. Experiments with acidification to below pH 2.5 of juice by electrodialysis with bipolar membranes and subsequent adjustment back to the juice’s original pH-level by the same method “reversed” after a short period of storage demonstrated that it was possible to inhibit and almost eliminate the enzymatic browning caused by PPO significantly. 
Lengthy investigations in electro-membrane processes for lactate recovery from fermentation broth lead to a new process with a built-in anti-fouling mechanism, entitled reverse electro-enhanced dialysis process, which has been submitted as patent application. The membrane setup in this process resembles a Donnan dialysis setup with anion-exchange membranes, but current is added for increased flux, and the electrical migration is controlled by competitive transport between lactate and hydroxide ions at different pH-levels. Current is reversed at short intervals for destabilization of fouling build-up. The reverse electro-enhanced dialysis process was extensively tested on both model solutions and fermentation broths, simulating continuous fermentation operation. Lactate fluxes combined with high retention of sugars, calcium, and magnesium was found to be satisfactory. Process factors like current efficiency and energy consumption was evaluated and found to be initially unsatisfying. It
was estimated that optimization of equipment and of operation parameters by computer simulations could improve the economical evaluation of the process. The process was able to run for prolonged operation times when current was reversed compared to similar runs without current reversal. The efficiency loss incorporated by the frequent current reversals in the reverse electro-enhanced
dialysis process was examined by constructing a mathematical model, which was implemented into a computer program. The constructed program was subsequently able to estimate values for current efficiency, overall flux, energy consumption and necessary membrane area for a part of a reverse electro-enhanced dialysis process. A plant able to produce 10,000 tons lactic acid (88%) annually was designed, incorporating the reverse electro-enhanced dialysis process and process parameters estimated from experiments and simulations. The extraction of lactate from fermentation broth through the reverse electro-enhanced dialysis unit was combined with an electrodialysis with bipolar membranes, an electrodialytic desalination and an evaporator system for final purification. An economical evaluation of the designed plant was conducted. Initial investment was estimated to DKK 145.5 million. Assuming the lowest current market price of 88%-lactic acid of DKK 9.50/kg and comparing it to estimated running costs of DKK 49.2 million per year still yielded an annual profit of DKK 45.8 million and an estimated break-even time of 6.3 years.