Characterization of the use of vacuum enhancement in commercial pilot-scale air gap membrane distillation modules with different designs

Abstract

The use of vacuum enhancement in air gap membrane distillation (AGMD) has been shown to improve the permeate production and the thermal efficiency. This work presents the characterization of that improvement for different operating conditions (input temperatures and feed flow rate) in three commercial multi-envelope AGMD modules with different internal designs. Modules had different number and length of internal circulation channels, yielding different feed velocities and residence times, and thus allowing to investigate the role of these. Experiments were performed with simulated seawater in solar membrane distillation pilot plants, and the decrease of permeate flux (PFlux) and specific thermal energy consumption (STEC) were analysed for each operating condition. The greatest impact of vacuum enhancement in the performance improvement was observed in the modules with the longest residence time operated at low hot temperature. The lower the driving force, the more important the role of vacuum enhancement in the improvement of vapour diffusion was, independently of feed velocity. Increases of PFlux up to 88% and reductions of STEC down to 70% compared to AGMD operation were quantified. Moreover, vacuum enhancement led to the reduction of hydraulic pressure drop, decreasing 26% the specific electrical consumption.