Research Article Equilibrium and Kinetics of the Extraction of Propionic Acid Using Tri-n-Octylphosphineoxide Organophosphorous compounds have been widely used in inorganic analysis for the extraction and separation of inorganic acids or metal species. Since these compounds can form hydrogen bonds to proton donors, they can also be used for the extraction of acidic organic compounds. Therefore, the reactive extraction of propionic acid using tri-n-octylphosphine oxide (TOPO) in hexane was stu- died. Equilibrium and kinetics experiments were performed. The extraction of propionic acid using n-heptane, light liquid paraffin, heavy liquid paraffin and hexane was studied and hexane was found to be most suitable diluent. The equili- brium complexation constant for the propionic acid-TOPO complex was determined to be 0.702 m 3 /kmol. The extraction was found to be first order in propionic acid and first order in TOPO with the overall rate constant as 46.91 (m 3 /kmol) 2 /s. Keywords: Equilibrium, Kinetics, Organophosphorous compounds, Propionic acid, Reactive extraction Received: December 13, 2007; revised: January 25, 2008; accepted: January 29, 2008 DOI: 10.1002/ceat.200700490 1 Introduction Propionic acid is a colorless, clear, slightly pungent liquid with molecular formula CH 3 CH 2 COOH and a pKa value of 4.67. Propionic acid and its products have been widely used in food, pharmaceutical and chemical industries. Calcium, sodium and ammonium salts of propionic acid are broad spectrum preser- vatives because of their bactericidal, fungicidal, insecticidal and antiviral effects and due to their neutral taste and smell [1]. Industrially, propionic acid is produced via a petrochemical route [2]. With the rising cost of the petrochemical products, research is now focused on the production through fermenta- tion technology. Conventional fermentation technologies for propionic acid production from glucose or lactose are limited by low reactor productivity (< 1 g/(L h)), low product yield (< 50 wt %) and low product concentration (40 g/L). The low productivity and low product concentration can be attributed to the fact that propionic acid bacteria are strongly inhibited by propionic acid [3]. Therefore, the recovery of propionic acid from fermentation broth and waste streams is of great concern in the generation of products. The conventional meth- od of recovery involves the addition of calcium hydroxide to produce calcium propionate. Propionic acid is separated by the addition of sulphuric acid and the resultant acid produced requires purification and concentration [4].The main problem in the conventional recovery method is the waste generation and significant lime and sulphuric acid consumption. These disadvantages have resulted in searches for alternative methods of recovery. Solvent extraction, membrane bioreactors, liquid surfactant membrane extraction, adsorption, direct distillation, electro- dialysis, reactive extraction, reverse osmosis and anion ex- change are some of the techniques that can be used for propio- nic acid recovery [5]. All of these processes have their own advantages and disadvantages. Reactive extraction has an ad- vantage over the others since it has been successfully applied for extractions from dilute solution, which is commonly found in waste streams and fermentation broths. Reactive extraction employs an extractant-diluent as the solvent phase and in- volves the reversible reaction of the acid with an extractant. Reactive extraction with specified extractant giving a higher distribution coefficient has been proposed as a promising tech- nique for the recovery of carboxylic acids. Extractive fermenta- tion is a relatively new technology that has been shown to be advantageous for alcohols and organic acid fermentation [6]. The advantages for extractive fermentation include high reac- tor productivity, ease in reactor pH control without requiring the addition of base, and use of a high concentration substrate  2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Amit Keshav 1 Kailas L. Wasewar 1 Shri Chand 1 1 Department of Chemical Engineering, Indian Institute of Technology (IIT) Roorkee, India. – Correspondence: Dr. K. L. Wasewar (k_wasewar@rediffmail.com), Department of Chemical Engineering, Indian Institute of Technology (IIT) Roorkee, Uttrakhand-247667, India. 1290 Chem. Eng. Technol. 2008, 31, No. 9, 1290–1295