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 wo rk 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 modifica tion 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 devic e. The distributions registered in the rejection side are unimodal and the residence time as well as the variance of the distributions obtained with t he new membrane are smaller. The distributions obtained in the permeate of the new membrane are unimodal for the stimulus response experiments obtaine d with tracers like sodium chloride and hydrochloric acid. The stimulus exp eriment shows two maxima for sodium hydroxide. The presence of a second max imum 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 react ions. Tn cases of worn-out membranes several maxima appear characteristic o f permeation modes. The best model to represent the permeation of the solut e is the dispersed plug flow model characterized by the Peclet number Pe an d 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 reactio ns, is present only in the NaOH tracings. The first mode is a remnant of th e permeation that occurs in the new membrane. Modes 2 and 3 indicate the sa lt leakage. These new modes are to be linked with the decreasing of the den sity of worn-out membranes. Consequently, the rejection capacity of the mem branes decreases.