|M.Sc Student||Komlos Chagai|
|Subject||Detection of Fouling on Reverse Osmosis Modules by|
Residence Time Distribution Analyses
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Raphael Semiat|
|Professor Emeritus David Hasson|
|Full Thesis text|
One of the problems in RO units is the possibility of deposition of sparingly soluble scaling salts on the membrane. Scale deposit is unacceptable as it causes loss of permeate product.
The aim of this research was the development of a simple online monitoring method enabling the detection of the presence of fouling deposits, based on the Residence Time Distribution (RTD) technique.
Tests were carried out with both a laboratory scale 2.5'' spiral wound module and an industrial scale 8'' module. The membranes were fouled with commonly encountered scaling species - Mg(OH)2 and CaCO3.
The flow disturbance induced by the presence of a deposit was characterized by measurement of the dispersion coefficient and of the dead volume. Analyses of the data were based on the Exponentially Modified Gaussian (EMG) model.
Experimental results indicated that satisfactory accuracy and reproducibility could only be obtained with the industrial scale module. For clean membranes, dispersion coefficients D were found to increase almost linearly with Re, from approximately D=31 cm2/sec at Re=90 to D= 72 cm2/sec at Re=237. RTD measurements in the presence of a Mg(OH)2 or CaCO3 deposits indicated that dispersion coefficients increase significantly with the fouling level of the membrane.
For Mg(OH)2 deposit characterized by 10% flux decline, dispersion coefficients increased to values of D=49-145 cm2/sec at Re=93-295 in the fouled membrane, and for a deposit characterized by 21% flux decline, dispersion coefficients increased to values of D=80-293 cm2/sec at Re=88-304. Similar trends were obtained in the presence of CaCO3 deposits. For CaCO3 deposit characterized by 7.5% flux decline, dispersion coefficients increased to values of D=39-131 cm2/sec at Re=86-273 in the fouled membrane, and for a deposit characterized by 11% flux decline, dispersion coefficients increased to values of D=57-166 cm2/sec at Re=93-249.
Another useful indication on the presence of a scale deposit was obtained from the dead volume measurements. For clean membranes, RTD data indicated the absence of a dead volume. Dead volumes were observed in the presence of deposits. In the case of Mg(OH)2 scaling characterized by 21% flow decline, dead volumes amounting to 13-23% were detected at Re=88-304. In the case of CaCO3 scaling characterized by 11% flow decline, dead volumes amounting to 14-23% were detected at Re=93-249.
The results of this research clearly indicate that the RTD technique has a potential for practical application as a simple reliable technique for online detection of incipient scaling in spiral wound RO modules.