Available online at www.sciencedirect.com Journal of Membrane Science 306 (2007) 355–364 RO module RTD analyses based on directly processing conductivity signals Qingfeng Yang a,∗ , Alexander Drak b , David Hasson b , Raphael Semiat b a School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b GWRI Rabin Desalination Research Laboratory, Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 9 April 2007; received in revised form 9 September 2007; accepted 14 September 2007 Available online 19 September 2007 Abstract 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 E t , or dimensionless exit age distribution function of E θ . 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 ¯ t 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Spiral wound module; Reverse osmosis; Residence time distribution 1. Introduction Reverse osmosis (RO) is widely applied in various fields such as desalination, raw water treatment and ultrapure water production. The design of the vast majority of RO plants is based on spiral wound membrane modules. Although spiral wound modules have an overall favorable performance, there is a wide scope for improving their flow performance and thereby lowering costs. Improved module design could be achieved by better understanding the geometric and operating parameters that determine the complex flow patterns occurring in a spiral wound module. ∗ Corresponding author. Tel.: +86 21 54748942; fax: +86 21 54748942. E-mail address: yangqf@sjtu.edu.cn (Q. Yang). Residence time distribution (RTD) techniques have long been recognized in the chemical industry to be powerful tools for characterizing flow patterns and diagnosing malfunctioning in complex flow equipment such as packed bed catalytic reactors [1]. The basic principle of the technique is to inject a tracer material, such as a dye, a conductive solution or a radioactive compound, in the form of a “pulse” or a “step change” and to determine the exit concentration of the tracer material. The shape of the “response signal” can be used to determine important flow parameters such as the mean residence time of the fluid in the equipment, the presence of dead volumes and/or bypass streams, as well as the intensity of mixing effects along radial and axial directions [1]. One of the main goals in the design of a spiral wound module is to ensure uniformity of flow of the feed as it enters from the space above the membrane roll into the feed flow passages of the membrane roll. In view of other conflicting design require- 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.09.018