PVDF-rGO Thermally Insulative Membranes for Efficient Dehumidification

Conventional vapor compression dehumidification is an energy extensive process as it operates below the dew point temperature to condense moisture, whereas membrane-based dehumidification saves energy using a selective membrane to avoid the phase change. Passive membrane dehumidification (PMD) is one the most promising technologies that uses water vapor pressure differential between feed and permeate stream to remove the moisture. However, current passive systems cannot efficiently reject humidity without losing heat, due to their poor membrane thermal insulation. In this work, we fabricated a highly thermally insulating support layer of polyvinylidene difluoride (PVDF) and reduced graphene oxide (RGO), and a hydrophilic selective layer (or, active layer) of PEBAX 1657 and graphene oxide (GO) using casting and phase inversion method. Dimethylformamide (DMF) with 10% PVDF was mixed with DMF-RGO (0%, 0.5%, 1%, and 1.5% of RGO relative to PVDF) solution (80^oC and 600 rpm for 45 minutes) and then cast using a casting knife on a glass plate. The support layer was then subjected to a DI water bath for 24 hours and dried for another 24 hours inside a controlled oven (25^oC and RH = 35%). For active layer, 1.5 g of PEBAX 1657 pellets in 50 mL water/ethanol solution (30/70 by volume) was prepared (80^oC and 800 rpm for 2.5 hours) and then mixed with 10 mL of GO solution (0.4% by weight in water) in a container. The active layer solution was then cast on a side of the PVDF/RGO support layer (from herein, the membrane can be referred to as PVDF/RGO/PEBAX/GO/(RGO %)). The thermal conductivity of the membrane support layer was found to be 0.041 W/m-K and 0.084 W/m-K for the PVDF/RGO/0.5 and PVDF/RGO/1.0 samples, respectively. In comparison, the PVDF without any RGO has a higher thermal conductivity of 0.114 W/m-K that matches the value mentioned in other literatures. For dehumidification performance analysis, water vapor permeance was measured using ASTM E96 Standard – Wet Cup Method, and air permeance was measured following ISO 15101-1:2007 standard. The reported water vapor permeance for the fabricated membranes varied in a range of 2100 – 5000 GPU while polyvinyl alcohol (PVA) based active layer membrane reached water vapor up to 1050 GPU . As the amount of RGO in the support layer increased, the pores became smaller and thus showed a lower water vapor permeance. The air permeance for the membrane was almost constant and thus, the selectivity (ratio of water vapor permeance/air permeance, range of 480 – 1000) decreased in the order of water vapor permeance. Scanning electron microscopy (SEM) and capillary flow porometry (CFM) were also used to measure the pore size and porosity of the fabricated membrane. Due to its high water vapor permeance and selectivity, this thermally insulated membrane can be used in PMD/SMD systems to improve efficiency. In the future, we will investigate the energy performance of these thermally insulated membrane modules in the proposed system under different operating conditions.