Effect of resistance to gas accumulation in multi-tank receivers on membrane characterization by the time lag method. Analytical approach for optimization of the receiver

The resistance to gas accumulation in constant volume systems has been evaluated by means of a position-depended time lag. The greater the absolute value of the time lag, the greater the error in membrane diffusivity determined using the constant volume system. The time lag, and thus the error in the diffusion coefficient, depends on the position within the receiver of a constant volume system, and regardless of the actual configuration of the receiver, there is always at least one zero time lag position within the receiver. In this paper we have developed an analytical model that allows assessment of the position-dependent time lag in any configuration of the receiver, including multi-tank receivers commonly used in membrane testing laboratories. The model allows optimization of the position of the pressure transducer(s) in the existing receivers, as well as, to understand the effect of important design parameters such as the diameter of tubes, the length of tubes forming direct connection between a membrane cell and the accumulation tank(s), the distribution of the volume between the tanks, and the position of tanks relative to each other and to the membrane cell. The model was verified experimentally using an existing two-tank receiver equipped with three pressure transducers.