Salting-out Extraction Of Short-Chain Diols And Carboxylic Acids

Salting-out extraction is a separation method used to extract hydrophilic target products from aqueous solution with the aid of organic solvent as the extractant and inorganic salt as the salting-out reagent. As a novel separation method, it has many advantages such as low cost, low viscosity, fast phase separation, continuous operation and easy recovery of phase-forming components. It has been successfully applied to the separation of bio-based chemicals, biomolecules and nature products. However, the knowledge of how to determine the salting-out extraction system and operational conditions, and select the corresponding equipment restrict its further application in industry. In order to strengthen the understanding of this system, some rules on the selection of phase-forming components were given, and then ethanol/ammonium sulfate system was selected as the representative salting-out extraction system. For ethanol/ammonium sulfate salting-out extraction system, the complete phase diagram was first obtained. Subsequently, the kinetics of phase separation and the partition behavior of short-chain diols and carboxylic acids on the basis of the phase diagram data were investigated. Finally, this technology was successfully applied to the downstream processes of simulated butyric acid solution and 1,3-propanediol fermentation broth. The main results are as follows:(1) The phase-forming ability of different inorganic salts was determined by the Gibbs free energy of hydration of ions, which was consistent with the phase separation rate of the system. However, obvious difference was observed between the phase-forming ability and salting-out effect of different inorganic salts. The phase-forming ability of different organic solvents was proportional to their hydrophobicity.(2) The complete phase diagram of ethanol/ammonium sulfate salting-out extraction system was plotted and five regions were observed. The region located above the binodal curve is liquid miscible region. Liquid-liquid equilibrium region lies between the binodal curve and solid-liquid equilibrium line, where salting-out extraction process can be optimized. Below the binodal curve and solid-liquid equilibrium line, there are liquid-solid and liquid-liquid-solid regions, which are of interest in the design of recovery and purification methods for salts. Experiment on the kinetics of phase separation in the ethanol/ammonium sulfate salting-out extraction system showed that the rate of phase separation was increased as the tie line length and interphase area of equipment increased, and the shortest separation time was obtained at the phase inversion point for all the tie lines at a constant phase volume ratio. The phase continuity was determined not only by the phase volume ratio, but also by the degree of mixing, which has little effect on the separation time. Most importantly, selecting butanol as the target product, ethanol/ammonium sulfate system achieved complete phase equilibrium and extraction equilibrium within 2 minutes when a proper phase volume ratio was chosen.(4) Based on the obtained phase diagram data of ethanol/ammonium sulfate salting-out extraction system, experiment on the partition behavior of a series of short-chain diols and carboxylic acids revealed that the partition coefficient of all the studied products were correlated with tie line length and their hydrophobicity. In addition, the partition coefficient was increased as the temperature and product concentration, and this effect was increased with the product hydrophobicity. It should be noted that the pH is a vital parameter for carboxylic acids due to the undissociated acid was apt to extract into the top phase, while its influence on the partition behavior of diols was not obvious.(5) Salting-out and salting-out extraction two-step separation obtained a butyric acid yield of 99.4%, most of the acetic acid (88.5%) was recovered simultaneously. For salting-out, the concentration and selectivity of butyric acid was high, however, the salting-out effect was decreased with the concentration of butyric acid and inorganic salt. Thus, this method was efficient for the butyric acid solution with higher concentration (?50g/L). In contrast, salting-out extraction has many advantages such as high recovery, fast phase separation and easy operation. The extraction efficiency was increased as the hydrophobicity of solvent decreased, and increased as the tie line length and the concentration of acid increased. However, the selectivity of butyric acid was low. Therefore, a two-step separation method which combines the advantage of salting-out and salting-out extraction, achieved a high yield of organic acid.(6) Based on the kinetics of phase separation in the ethanol/ammonium sulfate salting-out extraction system, the continuous extraction of 1,3-propanediol in an experimental packed column was achieved. The results indicated that the type of extraction column, the size of packing and the flow rate of dispersed phase affected the extraction effect of 1,3-propanediol. It was found that the column packed with Dixon 3×3 mm packing showed good extraction efficiency (K=10.86, Y=90.30%) and stability for both simulated solution and real fermentation broth of 1,3-propanediol at low flow rate of dispersed phase (1 mL/min). In addition,92.0% of phosphate could be recovered when 0.5 volume of anhydrous ethanol was added into the raffinate phase at pH 4.0.In conclusion, based on the selection principles of phase-forming components and phase diagram data of salting-out extraction system, the study of kinetics of phase separation and partition behavior not only strengthen the understanding of this novel extraction system, but also provide necessary guidance for developing efficient downstream method of various products.