Evaluation of moisture diffusivities in various membranes

An experimental set-up was designed and constructed to investigate the moisture permeation through a membrane. The test section consists mainly of an airflow channel, a membrane, and a water tank, with the membrane being sandwiched between the channel and water tank. An air gap exists between the membrane and the water in the water tank to avoid any possible wetting of the membrane by the water. Moisture transfers from the water to the airstream in the channel through the air gap and membrane. The moisture transfer process was described using a serial resistance model, which involves the convective moisture resistance in the channel, the moisture resistance through membrane, and the moisture resistance caused by the air gap. Special methods were developed to determine the convective and air gap resistances, with the membrane resistance obtained by subtracting such two resistances from the total resistance. Tests were conducted on three membranes including the PVDF (poly vinylidene fluoride), PES (poly ether sulfone) and cellulose membranes for airflow rates from 3.0 to 5.0 l/min, yielding Reynolds numbers from 117 to 195. The results showed that the moisture diffusivities in these membranes were in the order of 10−7 to 10−6 kg/m s, with the PVDF yielding the maximum diffusivity and the PES giving the minimum diffusivity.