Assessment of the main engineering parameters controlling the electrodialytic recovery of sodium propionate from aqueous solutions Marcello Fidaleo, Mauro Moresi * Department of Food Science and Technology, University of Tuscia, Via San Camillo de Lellis, 01100 Viterbo, Italy Received 21 October 2004; accepted 10 May 2005 Available online 24 August 2005 Abstract The main engineering parameters (i.e. ion transport numbers in solution and electro-membranes; effective solute and water trans- port numbers; effective membrane surface area, membrane surface resistances, limiting current intensity and mass transfer coeffi- cient) controlling the recovery of sodium propionate from model solutions by electrodialysis were determined in accordance with a sequential experimental procedure. Such parameters allowed a satisfactorily simulation of training and validation tests carried out under constant- or variable-current intensity. The performance of this ED process was characterised in terms of a current efficiency (X) of about 98% in constant- and variable- current regions, a water transport number (t W ) of about 15, and a specific energy consumption (e) increasing from 0.22 to 0.35 kW h kg 1 for a solute recovery yield of 95% as the current density (j) was increased from 200 to 400 A m 2 . The specific resis- tance of the cationic or anionic membranes were found to be practically coincident or at least four times greater than that provided by the manufacturer in aqueous solutions of sodium chloride, respectively. These parameters are to be used to design and/or opti- mise ED stacks involved in the downstream processing of propionic acid fermentation broths. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Electrodialysis; Electro-membrane resistance; Limiting electric current; Sodium propionate; Specific energy consumption; Transport numbers 1. Introduction Propionic acid (CH 3 CH 2 COOH) is a naturally occur- ring carboxylic acid with physical properties intermedi- ate between those of the smaller carboxylic acids, formic and acetic acids, and the larger fatty acids. Its current industrial production (about 200,000 metric tons, Mg) is mainly based on the oxo process, which in- volves reacting ethylene and carbon monoxide to pro- duce propionaldehyde, to be further oxidised in the presence of cobalt or manganese ions at 40–50 °C. Quite smaller amounts (approximately 6,800 Mg) of propionic acid are obtained as a coproduct of acetic acid manufac- ture via the liquid-phase oxidation of n-butane (Anony- mous, 2002). The present availability of propionic acid appears to be more than adequate for the near future, since its world demand (about 102,000 Mg) is presently running about 50% of capacity and growth is projected at 1.8% annually through 2006 (Anonymous, 2002). Its current market price ranges from €0.84 to €0.90 per kg and is expected to increase, largely because of escalating oil prices. In 2004 about 45% of the overall consumption of propionic acid is expected to be used as such or as ammonium propionate either as a solution or on an absorbent (vermiculite) to prevent mould in animal feeds, including hay, wet corn, silage, and grains. Approximately 21% of propionic acid (E 280) is ex- pected to be converted into sodium (E 281) and calcium 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.05.010 * Corresponding author. Tel.: +39 0761 35 74 94; fax: +39 0761 35 74 98. E-mail address: mmoresi@unitus.it (M. Moresi). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 76 (2006) 218–231