MD membrane development and benchmark study of commercial and semi-commercial membranes

Membrane distillation (MD) is an emerging separation technology, mainly used for desalination [1]. The membrane used in membrane distillation should be hydrophobic with sufficiently small pore size to avoid wetting. The microporous structure and membrane thickness greatly affects flux and energy efficiency. In this contribution, a benchmark study is performed for more than 20 different (semi-) commercial hydrophobic membranes. The membranes were evaluated based on flux, energy efficiency, liquid entry pressure, retention, stability and costs. No membrane was available showing a combination of the optimal performance of all these parameters. Most important factor is the high cost of almost all membranes. One way to reduce the cost of the membranes is the hydrophobization of a cheap hydrophilic membrane with different hydrophobic coatings. Overall treatment of a hydrophilic membrane with a hydrophobic coating results in a single layer hydrophobic membrane structure, while the treatment of only one side of membrane results in a dual layer hydrophilic/hydrophobic membrane structure. The coated membranes have the same membrane morphology as the commercial hydrophilic membrane enabling a comparison between one layer and dual layer membranes. All membranes have a water contact angle above 110° and a liquid entry pressure above 2 bar, indicating excellent wetting resistance. DCMD-experiments are carried out in counter-current, with a flow velocity of 0.13 m/s and feed and permeate temperature of 60°C and 45°C respectively. These experiments reveal an increase of flux and energy efficiency of the dual layer membranes compared to the single layer membranes for all combinations of coating materials and supports. For the best performing combination, the flux increases from 12 kg/m2.h for a single layer membrane to 22 kg/m2.h for a dual layer membrane, while energy efficiency increases from 34% to 68%. Moreover, the flux of the best performing dual layer membrane is 20% higher compared to the benchmark, while energy efficiency is comparable.status: publishe