Investigating the Effect of Electromagnetic Field on Saline Water as a Tool for Enhancing the Efficiency of Thermal Desalination Process

Abstract

Desalination has played a primary role in generating fresh water in the UAE and other Gulf countries, and one of the recent promising desalination technologies is membrane distillation (MD). It is a process where the saline feed is heated and then passed to a hydrophobic microporous membrane, in which coolant water is passed by the other side. However, scaling due to the precipitation of sparingly soluble salts, i.e., CaCO3, under high temperatures is one of the main constraints limiting the efficiency of the MD process. Therefore, this work aims to examine the possibility of increasing the efficiency of the MD process by exposing saline water to an electromagnetic field (EMF) and studying the effects of the EMF on CaCO3 formation under different conditions of supersaturation (SPI 3.3 and SPI 3.9) and salinity (10,000 ppm NaCl and 35,000 ppm NaCl). Two sets consisting of two runs were carried out under the same conditions, in which one run was with EMF on and another one was without EMF. The MD unit was in full circulation with flow rates of 100 mL/min and 300 mL/min and stable temperatures of 50 °C and 10 °C for the hot side and the cold side, respectively. The conductivity of the feed, the conductivity of the permeate, Ca2+ and alkalinity concentrations, and amount of permeate produced and CaCO3 formed were all monitored. Furthermore, the morphology of CaCO3 was examined by employing scanning electron microscope (SEM) and X-ray powder diffraction (XRD). In all experiments, the produced permeate content is higher with the presence of EMF than the one in the run without EMF, even when the solution contains extremely high NaCl concentration at 35,000 ppm. The effects of EMF on the water evaporation are reflected via the difference in permeate amount from the run with and without EMF, which drops with the increase of NaCl concentration due to the abundance of ion-water hydration shell in the presence of NaCl. Moreover, EMF has a significant effect on increasing nucleation and crystal transformation of CaCO3 under a low supersaturated solution with SPI 3.3, whereas the effects of EMF on nucleation are less with the increasing of the supersaturated level of the solution due to its fast nucleation. In the presence of NaCl at 10,000 ppm and 35,000 ppm, it is possible to observe the involvement of EMF in crystal transformation.
Especially in the presence of 35,000 ppm of NaCl, a new phase of crystal is detected in the experiment with EMF, which is suggested as a coprecipitation of CaCO3 and NaCl. The energy consumption by the MD system with EMF has shown to be less compared to the system without EMF in all experiments except with the 35,000 ppm NaCl. In a nutshell, the results from this study provide a fundamental understanding of the role of EMF on CaCO3 formation under various conditions of concentrations and salinities, which directly affects the efficiency of the MD process.

Date of Award	Apr 2023
Original language	American English
Supervisor	Emad Alhseinat (Supervisor)