The Separation of Hexamethylenediamine from Aqueous Solution by Solvent Extraction with Butanol

Abstract

This paper details an investigation into the separation of hexamethylenediamine (HMD) from dilute aqueous solutions by solvent extraction. This separation is potentially useful in (i) reclaiming HMD from waste generated in the production of nylon, (ii) for extracting HMD produced by fermentation, or (iii) reclaiming it from waste emanating from the depolymerization of nylon during recycling. Butanol was identified as a suitable solvent for the preferential dissolution of HMD from an aqueous solution, and it was found that the addition of sodium hydroxide resulted in improved separation. It was deduced that HMD has a bridging effect on the partial miscibility of water and butanol, and approximately 10 wt% of HMD was sufficient to allow for complete miscibility and the avoidance of phase separation. However, the addition of NaOH to the system reversed this bridging effect and reduced the solubility of butanol in the aqueous phase whilst also increasing the partitioning of HMD between the organic and aqueous phase. A model for an equilibrium mixer-settler unit was developed with three important findings emerging. Firstly, the amount of water which could be dissolved in the organic phase was found to increase with the amount of HMD present, although this effect was neutralised by the presence of sufficient NaOH. Secondly, the amount of butanol which could be dissolved in the aqueous phase was found to decrease as the amount of NaOH present increased but was largely independent of the amount of HMD. Lastly, the partitioning coefficient of HMD was found to decrease as the amount of HMD increased but increased as the molar ratio of NaOH to HMD was raised. Fitted equations for these three findings allowed closure to be achieved. This model predicted a set of operating conditions giving 99.95% extraction of a very dilute aqueous HMD solution (1 wt%), including a doubling of the HMD concentration in the organic product.