Utilização de membranas de contato (Hollow fiber) para extração e re-extração de ácido capróico

Abstract

Caproic acid or hexanoic acid has a wide range of applications in the pharmaceutical, chemical and food industries. The utilization of a fermentative production route has been studied as an alternative to petrochemical route, which relies on non-renewable resources. In the fermentative route, it is essential to provide a continuous caproic acid extraction from the biotic medium, because the accumulation of this acid inhibits microorganisms. In this study, the extraction and re-extraction of caproic acid by contact membranes are investigated in detail. This extraction system involves reactive and diffusive processes in two stages: i) the extraction step, where caproic acid passes through selectivity to an extraction solution (mineral oil + 3% trioctylphosphine (TOPO)); ii) the re-extraction step, where the caproic acid passes through affinity to an alkaline solution (0.5M H3BO3, pH ≥ 9.0). This type of extraction system has advantages over conventional liquid-liquid extraction of organic acids, because the membrane has hydrophobic characteristics, and acts as a physical barrier, which prevents the liquid phases dispersion. LIQUICEL two membranes are used (model 2.5 x 8.0 Extra-Flow), with surface area of 1.4 m², volume of the inner hull of 0.15 L (150 ml) and volume of the outer hull of 0.40 L (400ml). Each membrane has 800 hollow fibers of polypropylene (internal diameter 0.24 mm, length 15 cm, 0.03 mm thick wall and 0.03 mM of pore size of the membrane with 40% porosity). The mass transfer rates of membrane system were determined for different operating conditions from the effluent using a bioreactor and a pure (caproic acid diluted in water) effluents. In these experiments the caproic acid concentrations ranged between 0.1 and 2.5 g.L-1, which is a representative range for reactors operated at pH 5.5. The flow rates of the acidic and alkaline solutions ranged from 9,2 a 219,4 m.d-1. During the assays, pH, and caproic acid concentrations were monitored. The experiments using synthetic effluent showed that mass transfer rates in the extraction membranes are 9,5 g.m-2.h-1, and achieved extraction efficiency of 96%. Using the bioreactor effluent, the mass transfer rates were 3,67 g.m-2.h-1, and extraction effiency was 43%. Additionally, from these tests, it is possible to observe that the extraction rates are directly proportional to the liquid velocity in the external parts of the membranes. These results confirm that the maximum rates of extraction and re-extraction of the contact membrane system are in a feasible range to be used together with anaerobic reactors for producing caproic acid.