A comparative study of reverse osmosis and freeze concentration for the removal of valeric acid from wastewaters

The recovery of valeric (n-pentanoic) acid from a synthetic aqueous solution simulating a wastewater stream in nylon manufacturing has been carried out using freeze concentration and reverse osmosis as separation processes. The concentration of valeric acid in aqueous solutions was in the range of 0.5¯25 g/L. Reverse osmosis was carried out at 20 and 40°C and at a transmembrane pressure in the range of 1.3 to 6.0 MPa. The feed flow rate was 2 m/s in all the experiments. Although no membrane fouling was observed under the experimental conditions tested, a strong interaction of the acid with the membrane was noticeable. Rejections of the order of 90% were observed at 20°C, while values below even 50% were found at 40°C. The optimum performance for freeze concentration was determined, the best conditions being -10°C of subcooling temperature and 1012 kg/hm of feed flow. A model based on the heat transfer balance allows to predict the rate of ice crystallization. An economic analysis reveals that although freeze concenetration consumes as much as five times the energy of reverse osmosis, which is compensated by the high costs of membrane replacement in reverse osmosis.