DUGi

Concentració d’àcids grassos volàtils mitjançant destil·lació per membrana

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This study investigates the use of direct contact membrane distillation (DCMD) to concentrate volatile fatty acids (VFAs), which are fermentation products of agricultural food industry waste streams, with the aim of reusing them for the production of polyhydroxyalkanoates. These streams contain VFA in low concentrations, mixed with compounds such as ethanol and salts, which makes direct recovery difficult. DCMD is a temperature-dependent process that involves passing water vapour through a hydrophobic membrane from a hot fluid next to the feed to a cold fluid next to the permeate (draw). This work aims to extract water and other compounds while VFA is concentrated. Initially, two membranes, each with different support (polyethylene terephthalate and polypropylene), are compared by testing with water and then water with a concentration of 35 g·L -1 of NaCl at 50ºC, 60ºC and 70ºC. Water flux and ion permeation were measured. Water flux increases with temperature and the membrane with the polypropylene support has better ion retention (as well as slightly higher flux values). For this reason, this membrane was selected for the rest of the tests, with solutions of 70 g·L -1 and 140 g·L -1 NaCl at 50ºC and 70ºC. At these concentrations, it can be seen that 140 g·L -1 ion permeation is higher than at 70 g·L -1 , but the flux is lower. Tests are then carried out on a VFA stock solution (2.5 g·L -1 ), at pH 3, 5 and 7 at 50ºC and 70ºC, and the samples are analysed by gas chromatography. The results show that pH is the key factor: at pH 7 is where the best retention and concentration of VFA in the feed is achieved, as the undissociated form predominates. On the other hand, at pH 3 VFA losses to the draw are high due to the high dissociated fraction. At pH 5 there is an intermediate behaviour. Despite increasing the water flow, temperature does not affect the VFA balance. In parallel, the behaviour of ethanol was analysed under the same conditions, observing that it permeates almost completely into the draw (losses higher than 60%) in all cases so that it is efficiently removed from the feed. In summary, DCMD is an effective technique to concentrate VFA as long as the pH and temperature conditions are adjusted. In addition, the polypropylene membrane presents a good balance between flux and ionic selectivity, and at pH 7 it effectively concentrates VFA

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