Influence of Solvent and Acid Properties on the Relative Volatility and Separation Selectivity for Extractive Distillation of Close-Boiling Acids

Abstract

The increasing need for sustainable processes stimulates the production and recovery of renewable organic acids. The purification of these acids is often difficult because of similar acid volatilities but can be improved through extractive distillation. Generic insights into solvent effects on the separation efficiency for close-boiling acids are however lacking. This study provides insights into the effect of acidity, the acidity difference between the acids, the hydrogen-bonding strength of the solvent, and the solvent-to-feed ratio on organic acid separation efficiency. For an acetic acid−formic acid (AA−FA) mixture, the addition of high-boiling organic acids increases the relative volatility of FA over AA significantly. The addition of a Lewis base reverses the relative volatility, which depends on the applied solvent-to-feed ratio and the Lewis base BF 3 affinity. For several binary acid mixtures (such as AA−FA and monochloroacetic acid−dichloroacetic acid), where the acids have a relatively big difference in acidity (ΔpK a ≥ 1), the separation selectivity appears practically independent of the acid strength of the individual acids and increases with increasing BF 3 affinity of the Lewis base. For acid mixtures with a lower ΔpK a , a lower separation selectivity is obtained, as observed for separation of the pivalic acid−butyric acid and valeric acid−2-methyl butyric acid mixtures. When one of the acids in the mixture contains a secondary ketone group (i.e., levulinic acid in a levulinic acid−octanoic acid mixture), the strongest acid based on pK a is not necessarily attracted most by the added Lewis base. This, at first sight, unexpected behavior is most likely the result of complex intra-and intermolecular interactions and is quantitatively in line with COSMO-RS-based selectivity predictions.

Recovery Techniques Enabling Circular Chemistry from Wastewater

“…For affinity-based separation techniques (e.g., extractive distillation, liquid-liquid extraction, and adsorption) to work, their similar molecular properties such as pK a and both having a double C=C bond are expected to limit the operational window. For extractive distillation, the difference in pK a may either be too small or just enough [123]. The difference in hydrocarbon backbone length may result in a feasible difference in distribution over a polar phase and an apolar phase, suggesting that liquid-liquid extraction might offer opportunities for fractionation.…”

Isobaric Vapor–Liquid Equilibrium Prediction from Excess Molar Enthalpy Using Cubic Equations of State and PC-SAFT

“…For highly nonideal systems, the better performing modification of UNIFAC (Dortmund) still does not always provide an accurate description of the VLE, and especially for such systems alternative approaches are desired. 13 For systems with inaccurate UNIFAC or COSMO-RS predictions or for cases where the group parameters are yet to be determined, using a method that requires considerably less experimental labor than required for VLE measurements could be an option to obtain an accurate VLE description. The required measurements for this method should ideally be simple, fast, and not be limited to certain types of compounds.…”

“…Also, more recently, Jirasek et al prospected the use of machine learning to predict infinite diluted activity coefficients. For highly nonideal systems, the better performing modification of UNIFAC (Dortmund) still does not always provide an accurate description of the VLE , and especially for such systems alternative approaches are desired . For systems with inaccurate UNIFAC or COSMO-RS predictions or for cases where the group parameters are yet to be determined, using a method that requires considerably less experimental labor than required for VLE measurements could be an option to obtain an accurate VLE description.…”