Effect of air gap interval on polyvinylidene fluoride hollow fiber membrane spinning for CO 2 and CH 4 gas separation

Improper control of air gap interval during hollow fiber membranes (HFMs) spinning may lead to structural defects such as inner lumen deformations and macrovoids. In the current work, PVDF HFMs were prepared by manipulating air gap intervals at 5, 10, 15, 20, and 25 cm, using dry-wet spinning mechanism. The changes in its properties, including contact angle, mechanical strength, and most importantly the morphological structure that is usually crucial for gas separation performance have been determined. The morphology was evaluated using SEM, and the inner lumen defects of HFMs were reduced with the increment of air gap interval during the spinning process. Subsequently, the CO 2 gas permeance was observed to increase from 5 to 15 cm air gap distance and almost constant at 20 cm air gap interval, then increase tremendously beyond this point. Furthermore, CO 2 /CH 4 ideal selectivity was observed to be improved and reached the highest end at PVDF-AG15 and dropped beyond this point. Therefore, varying air gap distance is considered as a practical approach for better gas separation. However, macrovoids will form if the air gap length is overlong. Thus, optimum air gap length during PVDF HFMs spinning is vital for morphology and gas separation performance.