Reactive Extraction of Propionic Acid with Aliquat 336 Dissolved in 1-Decanol and n-Dodecane

This paper deals with propionic acid extraction by Aliquat 336, a quaternary amine dissolved in ndodecane and 1-decanol. Chemical equilibrium experiments have been carried out to investigate the effect of modifier, 1-decanol on the extraction efficiency of Aliquat 336 in ndodecane at fixed initial acid concentration of 0.27 gmol/l. Extraction efficiency is determined in terms of distribution coefficients and degree of extraction. The effect of extractant (Aliquat 336) concentration, effect of diluents (1-decanol and n-dodecane) composition have also been derived. It was found that distribution coefficients increased with increasing the 1-decanol concentration in the diluents. The maximum distribution coefficient obtained with Aliquat 336 dissolved in pure 1-decanol is 6.4. The extraction equilibrium constants have been determined. The number of exatractant, Aliquat 336 molecules in the acid:amine complex has been estimated through mathematical model. INTRODUCTION The carboxylic acids are most widely used in the field of food and beverages as an acidulant and also in pharmaceutical and chemical industries. Fermentation technology for the production of organic acids in particular has been known for more than a century and acids have been produced in aqueous solutions. In the recent past, the interest towards propionic acid recovery from fermentation broths and waste streams increased. This interest is caused by growing demand of pure, naturally produced propionic acid used in chemical, pharmaceutical and food industries. Among various available alternate processes for simultaneous removal of the product, extraction is often the most suitable one. Reactive extraction with a specified extractant giving a higher distribution coefficient has been proposed as a promising technique for the recovery of carboxylic and hydroxyl-carboxylic acids [1]. Reactive Extraction is developed to intensify separation by solvent extraction and represents a connection between chemical (solute and extractant reaction) and physical phenomena (diffusion and solubilization of the system components). Some of the advantages of reactive extraction are increased reactor productivity, ease in reactor pH control without requiring base addition, and use of a high-concentration substrate as the process feed to reduce process wastes and production costs. This method may also allow the process to produce and recover the fermentation products in one continuous step and reduce the down stream processing load and the recovery costs [2]. Long-chain aliphatic amines are effective extractants for the separation of carboxylic acids from dilute aqueous solution [3-5]. The amines have been favored because of lower cost and generally higher distribution coefficient. Among different amines, extraction with primary amines is characterized by a large mutual solubility of the aqueous and organic phase; secondary amines have the highest reported distribution coefficient, but tend to form amides in the downstream regeneration by distillation; quaternary amines extract carboxylic acids at both acidic and basic pH via an anion exchange mechanism [4]. Consequently, quaternary and tertiary amines are the most attractive for extractive fermentation on the basis of their low aqueous solubility and intermediate basicity, the latter providing for reasonable extracting power along with the possibility of stripping. 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 [5]. Generally, the amine extractants are dissolved in a diluent such as a ketone, an alcohol, hydrocarbons, etc., that is, an organic solvent that dilutes the extractant to the desired concentration and controls the viscosity and density of the solvent phase. Many factors have been found to influence the equilibrium extraction characteristics of these systems. Three important variables are the nature of the acid extracted, the concentration of the extractant, and the type of diluent [6-8]. Zhong et al. [9] studied the reactive extraction of propionic acid using Alamine 304-1 in 2-octanol, 1-dodecanol, and Withohol 85 NF as diluents at various amine volume fractions from (0 to 100) % and found extraction to be maximum at the amine volume fraction between (20 and 40) %. Studies on the propionic acid reactive extraction using trioctylamine in various diluents (hexanol, butyl acetate, and petroleum ether) show the positive effect of the different diluents on the extraction [10]. Uslu et al. [11] and Kumar et al. [12] studied reactive extraction of propionic acid using Aliquat 336 (quaternary amine) dissolved in five pure solvents (cyclohexane, hexane, toluene, methyl isobutyl ketone, 1-octanol and ethyl acetate) and binary solvents (hexane + MIBK, hexane + toluene, and MIBK + toluene) under various experimental conditions. In all cases, 1:1 acid-amine complexes were formed with no overloading. However a study on the same acid with the tertiary amine (Alamine 336) diluted in toluene at 298.15 K indicated more propionic acid to be transferred to the organic phase than would be expected from the 1:1 stoichiometry of the reaction, thus 2:1 and 3:1 acid-amine complexes were considered [13]. This indicates that complexation depends on the extractant-diluent system chosen. Polar diluents that enhance the extracting power of amines are more favorable than nonpolar diluents as shown in the extraction of citric, lactic, and succinic acid [14]. The aim of this study is to show the influence of inert and active (modifier) diluent concentrations on the extraction efficiency and determination of the equilibrium extraction constant and the number of extractant reacting molecules. The maximum distribution coefficient obtained with Aliquat 336 dissolved in pure 1-decanol is 6.4, but 1-decanol is also showing the toxicity towards the bacterium used for propionic acid fermentation. So combination of non toxic (n-dodecane) and toxic (1-decanol) have been used with Aliquat 336. THEORY For simplicity, we used apparent equilibrium constants, expressed in terms of species concentrations. The physical extraction of propionic acid is neglected, as well as the acid dimerization and the water co-extraction. The interaction between acid and amine is realised trough hydrogen bonding of non-dissociated acid molecule: HA N R HA N R : 3 3 ⇔ + (1) or by ion-pair formation: − + − + ⇔ + + A NH R A H N R 3 3 (2) If more than one amine molecule takes part in the complex formation, the process description is as follows: HA N R N nR HA n ) ( 3 3 ⇔ + (3) n n E R N HA N R HA K ] ][ [ ] ) ( [