Solvent developments for liquid-liquid extraction of carboxylic acids in perspective

The growing desire to produce organic acids through fermentative routes, as a starting point for bio-based plastics, has revived the scientific attention on carboxylic acid removal from aqueous streams. One of the main technologies to recover carboxylic acids from diluted aqueous streams is liquid-liquid extraction (LLX). In this review, solvent developments for LLX of carboxylic acids are reviewed. In the past decades, a significant number of research papers have appeared, describing completely new solvents such as ionic liquids, as well as improvements of the traditional state-of-the-art solvent systems comprising of amines and organophosphorous extractants in diluents. The state-of-the-art technology for acid extractions has long been using trioctylamine (TOA) - or Alamine 336, a commercial mixture of trialkyl amines - as the complexating agent. However, with dropping acid concentrations, the economic feasibility of the TOA-based processes is compromised. This review discusses three main categories of solvents, i.e. composite solvents containing nitrogen-based extractants, phosphorous-based extractants and ionic liquids, and includes a discussion on solvent property models that may aid solvent selection. Furthermore, regeneration strategies are discussed, aiming to provide direction towards regenerations that do not further dilute streams that are already diluted before the LLX process. The main conclusion with respect to solvent regeneration when back-extraction is applied, is that solvent-swing strategies should be applied that maximize the ratio between the acid distribution coefficient in the forward extraction and the distribution coefficient in the back-extraction at minimal energy cost. This appears to be through evaporation of part of the diluent after the primary extraction.