Chemical and Biomedical Engineering - Theses
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ItemMembrane capacitive deionisation as a novel approach to wastewater treatment( 2018)In this thesis, Membrane Capacitive Deionisation (MCDI), an energy-efficient desalination technology, was studied through laboratory experiments and mathematical modelling. An MCDI cell consists of a pair of porous carbon electrodes in front of which ion-exchange membranes are placed. A comprehensive literature review on operational modes of CDI and MCDI, performance evaluation metrics, recent developments on novel carbon electrodes and ion-exchange membranes, and the dynamic ion transport models available for (M)CDI is initially presented. Following that the scope of the thesis is introduced by specifying gaps in demand of more investigation. An MCDI cell built in house is used as the basis for all experiments in this thesis. In the first instance, the properties of the carbon materials, the ion-exchange membranes (IEMs) and the flow channel compartment of the MCDI cell were determined. To understand the underlying physico-chemical properties of the carbon electrodes, the electrode material was characterized using N2 isotherms, Thermogravimetric Analysis (TGA), Fourier Transform Infrared (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). The N2 isotherms showed that the dominant pore diameter is < 2nm, confirming that the overlapping Electrical Double Layers could be assumed as the basis to describe the temporary ion adsorption in the carbon micropores. FTIR and XPS enabled the author to identify the surface functional groups on the carbon source. SEM images taken from the fabricated electrodes provided details on adhesion of the carbon slurry to the current collector and the apparent thickness of the carbon layer as 150 µm. The commercial ion-exchange membranes used in this work were also characterized thoroughly by measuring their water content and ion sorption capacity at different concentrations and in different electrolyte solutions; as well as counter and co-ion permeabilities. The significant outcomes of this section enabled the author to obtain membrane properties to be used in the later ion transport model. As the first research question, the feasibility of regenerating the MCDI cell with a brine stream (a stream at higher concentration compared with the feed concentration) was investigated. The idea of brine management and its effect on water recovery and water productivity was identified as an unsolved research question in the area of MCDI. In this section, the effect of brine concentration on the desorption time and water recovery was first evaluated through experimental work. Following that, an improved MCDI set-up was suggested in which the stream is recirculated via a recycle tank during the desorption period. 40% water recovery enhancement could be achieved employing the aforementioned proposed set-up. In the final part of this section, the dependency of the salt adsorption and water recovery on the residence time within an MCDI cell was investigated using an improved MCDI ion-transport model. To understand the effect of competing ion effects, the feed stream to CDI and MCDI was extended to more realistic solutions involving mixed salt feed streams. In a comprehensive study the performance of the CDI cell was compared with that of MCDI in terms of the adsorption and desorption rate of the various ions, total salt removal and normalized charge efficiency. A slower adsorption rate of divalents was observed in the CDI cell relative to the monovalent ions. Additionally, it was observed that in an MCDI cell, the ion transport through the IEMs is the rate-controlling step out of all transport resistances. In the case of the MCDI, a sharp desorption peak of divalent ions was noticeable both for Ca2+ and SO42-. This behavior could be justified by the sorption results collected in the IEM characterisation section. In the first step toward enhancing the currently available models, the ion-transport model developed in this thesis eliminates the symmetry assumption of the MCDI cell by identifying the controlling IEM, and includes the effect of the ion activity coefficient in the membrane. The mathematical model was then further extended to make it more accurate and capable of predicting the CDI and MCDI behaviors with a wide range of feed solutions. While it is shown that this extended model is capable of predicting the CDI and MCDI cell performance to a great extent, a faster rate of adsorption and desorption was observed in the model. This behavior can be explained by the quasi-steady state assumption which is employed in the mathematical approach. This assumption assumes an instantaneous equilibrium between the macro and micro pores of the carbon as well as the surface of the IEMs with the electrode macropores. Including a time-dependency for these two transport steps will reduce the model rate of change. In the final section, the effect of organic fouling was studied using two model foulants, the sodium salt of alginic acid and humic acid. While the effect of fouling on the salt removal and charge efficiency of the CDI cell was significant, the effect on MCDI was more limited. Fouling on the IEMs could be further reduced by increasing the flow rate during the desorption step, showing that the fouling on the IEMs is predominantly reversible. The effect of foulant concentration and feed pH were also studied. In the case of CDI, where the fouling had a detrimental effect on its desalination performance, the effect of alkaline cleaning on the performance recovery was investigated. It was observed that activated carbon eroded from the electrode material during such alkaline cleaning (4% carbon mass loss). In summary, some of the unresolved aspects of MCDI were tackled in thesis which will provide more insight into this energy efficient technology and pave its way to commercialization. Comprehensive characterisation, an improved ion transport model, a brine management study, a comparison of CDI and MCDI performance in salt mixtures, and finally the effect of organic fouling in MCDI were the main focuses of this work.