Study on the effect of air–gap length on properties and performance of surface modified PVDF hollow fiber membrane contactor for carbon dioxide absorption

• Different air–gap lengths were used to fabricate the surface modified PVDF HFMs.
• The outer surface roughness of HFMs decreased slightly by increasing air–gap.
• The PVDF HFMs were used in contactor application for CO2 absorption.
• The CO2 absorption flux increased by increasing air–gap to 10 cm then decreased.

Surface modified polyvinylidene fluoride (PVDF) hollow fiber membranes (HFMs) were spun via dry–wet spinning technique at different air–gap lengths (0–20 cm). The morphology of prepared membranes was evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Membranes were also characterized in terms of gas permeation, overall porosity, collapsing pressure, critical water entry pressure (CEPw) and contact angle. To determine the CO2 absorption flux of HFMs, a gas–liquid membrane contactor system was used. Experimental results of this study revealed that by increasing the air–gap distance from 0 to 20 cm, wetting resistance and contact angle of fabricated membranes increased due to enhancement of membrane surface hydrophobicity. The highest helium (He) permeation was achieved for the spun fiber at the air–gap of 10 cm. From CO2 absorption experiment it was found that the hollow fiber spun at the air–gap of 10 cm had the maximum CO2 absorption flux of 1.41 × 10−3 mol/m2 s at the absorbent flow rate of 300 ml/min, which was significantly higher than CO2 absorption flux obtained by other researchers. It was also found that both highest He gas permeance and CO2 absorption flux were controlled by the surface porosity of the hollow fiber due to the maximum values obtained. Thus, the choice of an appropriate air–gap distance for fabrication of surface modified membranes was found to be a promising method to improve CO2 removal in membrane contactor systems.