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 solutio n simulating a wastewater stream in nylon manufacturing has been carried ou t using freeze concentration and reverse osmosis as separation processes. T he 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 degrees C and at a transmembrane pressure in the range of 1.3 to 6.0 MPa. The feed flow rate w as 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 obser ved at 20 degrees C, while values below even 50% were found at 40 degrees C . The optimum performance for freeze concentration was determined, the best conditions being -10 degrees C of subcooling temperature and 1012 kg/hm of feed flow. A model based on the heat transfer balance allows to predict th e rate of ice crystallization. An economic analysis reveals that although f reeze concentration consumes as much as five times the energy of reverse os mosis, which is compensated by the high costs of membrane replacement in re verse osmosis.