Equilibrium and Kinetic Studies On Reactive Extraction of Propionic Acid Using Tri-n-Octylamine In 1-Decanol + Cyclohexane (1:1 v/v)

Equilibrium and Kinetic Studies on Reactive Extraction of Propionic Acid using Tri-n-Octylamine in 1-Decanol + Cyclohexane (1:1 v/v)

Sushil Kumar¹ and B V Babu²

¹Assistant Professor, Chemical Engineering Department, Birla Institute of Technology and Science

(BITS), PILANI, Rajasthan, 333 031, India

E-mail: skumar@bits-pilani.ac.in  

²Director, Institute of Engineering and Technology (IET), JK Lakshmipat University (JKLU),

Ajmer Road JAIPUR - 302 026

Phone: +91-141-2168-330 /344 / 167

E-mail: director.iet@jklu.edu.in; profbvbabu@gmail.com  

Abstract

Propionic acid is most widely used in the field of food and beverages as an acidulant and also in pharmaceutical and chemical industries. Due to growing demand of pure, naturally produced propionic acid, the interest towards propionic acid recovery from fermentation broths and aqueous effluents is increased. Specific uses of propionic acid draw attention toward a better recovery process, which will increase the productivity and reduce the overall production cost. A reactive extraction process exploits reversible chemical complexation in the extractant phase, provides an effective separation, especially for dilute solutions such as the aqueous solution of propionic acid in a fermentation broth. Long-chain aliphatic amines are effective extractants for the separation of carboxylic acids from dilute aqueous solution. The specific chemical interactions between the amines and the acid molecules to form acid-amine complexes in the extractant phase allow more acid to be extracted from the aqueous phase. The specific affinity of long-chain tertiary amines for propionic acid gives high selectivity for this type of solute with respect to water and eventually non-acidic species in the mixture. The present study is aimed at the recovery of propionic acid from aqueous solution by reactive extraction with tri-octyl-amine (TOA) dissolved in a mixture of decane-1-ol (an active diluent) + cyclohexane (an inactive diluent).

The equilibrium and kinetic extraction experiments are carried out to investigate the effect of modifier composition (20 - 80%), the effect of initial acid concentration (0.0675 to 0.675 mol L^-1) and the effect of agitation speed (250 – 500 rpm) on the extraction efficiency at a constant temperature of 298 K. To analyze the reactive extraction efficiency, the loading ratio (Z), distribution coefficient (K_D) and degree of extraction (E) are calculated using equilibrium data. The higher modifier (1-decanol) concentrations lead to an increase in the extraction efficiency (degree of extraction) of acid. When, TOA is used as an extractant in a mixture of an inert diluent and an active diluent (modifier) in reactive extraction, the solubility of extracted species increases in the organic phase. The values of estimated equilibrium constant (K_E) and stoichiometry (n) depends not only on the concentration of modifier in the presence of inert diluent, but also on the volume phase ratio between modifier and diluent. To see the effect, the volume phase ratio between modifier and inert diluent is taken one in this study. The degree of extraction (%) decreases significantly when the concentration of propionic acid is increased with TOA in cyclohexane/1-decanol (1:1 v/v). A mathematical model based on mass action law and a population-based search algorithm (differential evolution, DE) is proposed and used to estimate the extraction equilibrium constants (K_E) and stoichiometry of reactive extraction. Individual equilibrium constants for simultaneous formation of (1:1) and (2:1) acid:amine complexes are also determined.

The shape and the position of the evaluated kinetic curves for agitations above 350 min^-1 is almost the same. Hence, according to these preliminary measurements, the selected level of agitation of 350 min^-1 excludes the influence of the agitation on kinetics. This means that the effect of mass transfer on the overall kinetics is minimized and presumably the interfacial chemical kinetics can be determined from the experimental results. In this simulation, the best pair of rate constants for a given particular set of individual orders individual orders - α', β' and γ' are determined. The experimental kinetic data for the extraction of propionic acid are interpreted by a forward reaction rate with α' = 1.68 and β' = 1.0 and the reverse reaction rate with γ' = 1.0, a very good fit is obtained with almost constant values for both the rate constants, k₁ and k₂, for the whole range of acid (0.27 – 0.54 mol.L^-1) and a constant concentration of amine (0.457 mol.L^-1).

 

Keywords: Reactive extraction; propionic acid; tri-n-octyl-amine (TOA); equilibria and kinetics; modifier; differential evolution (DE).