The energy challenge of direct contact membrane distillation in low temperature concentration

Direct contact membrane distillation (DCMD) was operated at low temperatures from 25 to 40 °C to suit the purpose of thermally concentrating sensitive liquid foods, especially fruit juices to high solid content concentrate with most of the quality attributes preserved. A lab scale DCMD unit has been set up at the Centre for Plant and Food Science, University of Western Sydney. Hollow fibre modules (HFM) using five types of fibres of polyvinylidene fluoride (PVDF) and Halar material, with mass transfer areas ranging from 281 to 573 cm2 were employed. Experiments for concentration of glucose solutions from 30 to 60% (w/w) were carried out. Results indicated that not only the operating conditions were important, but also the membrane properties. It was found that Halar fibres were performing 2-3 times better than PVDF fibres in term of removing water from the feed, and 3-4 times better in term of energy saving. Results also showed that an increase of the feed inlet temperature from 25 to 40 °C improved the mass flux up to 6 times and energy efficiency (EE) up to 2.5 times depending on the feed concentration. With flux up to 2.88 kg m-2 h-1 for PVDF and 5.83 kg m-2 h-1 for Halar fibres when concentrating 30% glucose solution at 40 °C, DCMD appeared to be an attractive concentration technique, when product quality is the priority. However, with EE from as low as 2.1-14.9%, PVDF fibres employed in the study seemed not to be very suitable for DCMD liquid food concentration under low temperature condition. DCMD in Halar fibres with EE up to 45.6% still encounters the challenge of energy and could only be cost competitive to osmotic distillation and evaporative concentration when cheaper energy sources or heat recovery measures are employed.