RO module RTD analyses based on directly processing conductivity signals

Residence time distribution (RTD) techniques can be used to diagnose the flow characteristics in spiral wound reverse osmosis (RO) modules. However, the methods of processing tracer response conductivity signals and mathematically modeling of RTD curves often involve complicated steps including conductivity-concentration transformation, baseline selection and the use of exit age distribution function of Et, or dimensionless exit age distribution function of Eθ. In this paper, a simple and direct method for processing RTD signals from conductivity data was developed for spiral wound membrane RO system. Two models were tested: axial dispersion (AD) model and exponentially modified Gaussian (EMG) model. The results show that the present method provides a simple, fast and accurate RTD data reduction. The levels of the axial mixing intensities, characterized by the dispersion number D/uL, indicated significant deviations from ideal plug flow in both the laboratory and the industrial size modules. In both the modules, the dispersion coefficient D increased roughly linearly with the Reynolds number. Membrane fouling and worn-out led to an increase in D. Moreover, the values of mean residence time image and D/uL obtained from the EMG model were more stable against the change of the curve tail length, especially for the parameter D/uL. Furthermore, RTD analysis also indicated that the membrane wearing-out could lead to dead zones.