Identification of diffusion modes of solutes in spiral-wound reverse osmosis membranes by interpreting tracing experiments by a plug flow model

When you measure the rejection of a solute for a membrane, it only gives a global appreciation of the membrane capacity to reject this solute. This work analyzes the stimulus response experiments of different tracers in order to show their behaviour in composite polyamide reverse osmosis membranes. One new and three worn-out membranes have been tested to study the modification of solute transport in the membranes with wear. A dispersed plug flow model was used to identify the permeation modes of the solutes in the device. The distributions registered in the rejection side are unimodal and the residence time as well as the variance of the distributions obtained with the new membrane are smaller. The distributions obtained in the permeate of the new membrane are unimodal for the stimulus response experiments obtained with tracers like sodium chloride and hydrochloric acid. The stimulus experiment shows two maxima for sodium hydroxide. The presence of a second maximum for the NaOH tracing could be explained by the presence of a classical diffusion mode of NaOH in the polymer by "jump" added with acid-base reactions. Tn cases of worn-out membranes several maxima appear characteristic of permeation modes. The best model to represent the permeation of the solute is the dispersed plug flow model characterized by the Peclet number Pe and the tracer space time tau in the device. Whatever the tracer, we can note the presence of three modes. The fourth, which is due to acid-base reactions, is present only in the NaOH tracings. The first mode is a remnant of the permeation that occurs in the new membrane. Modes 2 and 3 indicate the salt leakage. These new modes are to be linked with the decreasing of the density of worn-out membranes. Consequently, the rejection capacity of the membranes decreases.