Recovery of sodium citrate and citric acid by one step membrane electrodialysis process

Abstract

The main objective of this research was to investigate the performance of electrodialysis using various types of ion exchange membranes and the behavior of the mass transfer phenomena occurring in electrodialysis process for the recovery of citric acid and sodium citrate. An understanding of the membranes properties is a crucial step in their selection for a practical application. Thus, the properties such as thickness, ion exchange capacity, water content, and permselectivity were evaluated. The membranes behavior was also studied in term of current - voltage responses at different current densities for the limiting current density determination. It was found that the limiting current density increases linearly with the increases of feed concentration. At the feed sodium citrate concentration of 15.0 g dm-3, the AFN anion exchange membrane and CMX.SB cation exchange membrane exhibited the highest limiting current density with the value of 7.20 A dm-2 and 8.2 A dm-2, respectively. For performance of electrodialysis using ion exchange membranes in the citric acid and sodium citrate recovery process, the effect of operating parameters such as current density, feed concentration, and water transport was studied. From the experiment results indicated that the AFN anion exchange membrane and the C66-10F cation exchange membrane performed the best for the recovery of sodium citrate with the flux of 4.346 kg m-2 hr-1 and 3.55 kg m-2 hr-1 at the current density of 5.0 A dm-2. The combination of type C66.10F-BP.I-AFN was the best ion exchange membranes for the production of citric acid from sodium citrate. At the current density of 10.0 A dm-2, the highest citric acid concentration was obtained, which was 97.8 g dm-3 with the energy consumption of 12.02 W hr m-2 kg-1. It was also found that the water transport due to the electroosmosis in the electrodialysis process would limit the achievable of the maximum concentration of citric acid and sodium citrate. The highest water transport rate of ion exchange membranes was the membrane type AMI 701-CMI 7000 with the values of 0.168 ml min-1. In the laboratory-scale electrodialysis, the study also demonstrated that over 90% of citric acid and sodium citrate could be recovered and concentrated using electrodialysis approach. The productivity rate achieved for citric acid was from 0.525 kg m2 hr-1 to 0.707 kg m-2 hr-1 for current densities of 2.16 A dm-2 to 4.32 A dm-2. However, the productivity rate of sodium citrate was found to be higher than citric acid, with the value ranges from 1.438 kg m-2 hr-l to 1.587 kg m-2 hr -1 under the range of current densities from 1.72 A dm-2 to 5.17 A dm-2. A limiting current density model was developed in this study and the calculated results were shown in good agreement with the experimental results. After that, a complete ion transport model, based on the description of the different mass transfer phenomena involved in the electrodialysis system, was also developed. The model was able to elucidate the electrodialysis process and determine the operating parameters, which controls the performance of electrodialysis system. The optimum experimental conditions in the electrodialysis system were also evaluated, which was based on the developed ion transport model. Finally, a preliminary pilot scale plant design for the recovery of sodium citrate and citric acid was carried out.