Predicting the lowest effluent concentration in capacitive deionization

This research article published by Elsevier B.V., 2013Capacitive deionization (CDI) is a promising technology for desalination of brackish water with different applications such as in the pharmaceutical industry, semiconductor manufacturing, and domestic use. The CDI cell utilizes an electric potential across two electrodes in which one of the electrodes becomes positively charged and the other becomes negatively charged. Cations and anions are attracted towards the anode and cathode, respectively. The adsorption and desorption mechanism within the CDI cell determines the amount of salt in the effluent stream. Modeling the dynamic response of the effluent concentration is vital to understanding the water purity level. In this paper, the equations predicting the lowest concentration time and lowest concentration have been found using the adsorption cycle mathematical model. During purification process the effluent concentration reaches the highest purity level after a certain period of time. We define the time it takes to reach the highest purity level as lowest concentration time and the corresponding instantaneous effluent purer water is what we call lowest concentration. While the lowest concentration depends on all of the CDI operating parameters i.e., applied potential, capacitance, flow rate, feed concentration, dead volume, and spacer volume, the lowest concentration time depends only on flow rate, dead volume, and capacitance. Using a genetic algorithm, it was found that seawater (32,702 ppm) could be desalinated to as low as 2.1 ppm; which is within the standards for drinking water set by the world health organization

Desalination using capacitive deionization at constant current

This research article published by Elsevier B.V., 2013Capacitive deionization (CDI) is an emerging technology of desalinating brackish/seawater to attain freshwater. The process involves polarization of the two electrodes electrically using direct current; thus the cations and anions are attracted towards the oppositely charged electrode. So far most of the experiments/models involve the charging of the CDI cell at constant voltage. However, charging at constant voltage leads to having a shorter time in a given CDI cell cycle when the system has reached its lowest effluent concentration. This is undesired phenomena. To overcome this problem desalination process is preferred to be performed at constant current. The dynamic response model to describe the variation of the effluent concentration with time under constant current charging has been derived and validated. Also, the effect of processing parameters such as applied current, flow rate, CDI cell dead volume, and capacitance on the lowest effluent concentration is analyzed

Modeling the capacitive deionization batch mode operation for desalination

This research article published by Elsevier, 2014Capacitive deionization (CDI) is an emerging desalination technology in which saline water flows through a pair of polarized/biased electrodes. The cations and anions are attracted towards the negative and positive electrodes, respectively. In CDI operation there are two possible modes: single pass and batch mode. In single pass operation, saline water passes only once through the CDI cell, whereas in batch mode operation, the fixed volume of saline water is recycled continuously until a steady state is reached. This paper presents the transient response of the CDI cell under batch mode operation. The model is developed by taking into account single pass CDI operation and the mixing phenomena that occur in the recycling tank. The developed model was successfully validated using experimental data, and the model helped to derive the equation for predicting the steady state of the CDI cell for the given operating parameters: flow rate, saline water quantity, CDI capacitance, CDI resistance, spacer volume, dead volume, applied potential, and initial concentration of the saline water

Integrating reverse electrodialysis with constant current operating capacitive deionization

This research article published by Elsevier Ltd., 2014The presence of a salinity gradient between saline water streams may result in the production of electricity via either reverse electrodialysis (RED) or forward osmosis. While the former system generates electricity because of the ionic current, the latter process produces electricity due to the osmotic pressure. In this study, RED is coupled with capacitive deionization (CDI) so that highly pure water, fresh water and electricity could be generated simultaneously. A CDI cell is operated at constant current, and it generated ultrapure water and two streams (a lower salinity stream of approximately 17.4 mol NaCl per m3 and a high salinity stream of approximately 512.8 mol NaCl per m3) to be fed to the RED stack from a 15,000 ppm CDI feed concentration. The performed simulation reveals that, the total power generated from the RED using infinitely divided electrodes is 0.57 W/m2 electrode area. The use of RED in a CDI plant introduces a new approach to minimize CDI brine concentration, which would otherwise have a negative impact on the environment if it were disposed directly without prior treatment

Crustacean derived calcium phosphate systems: Application in defluoridation of drinking water in East African rift valley

This research published by Elsevier B.V., 2018Calcium phosphate adsorbents, derived from prawns and crabs shell biomass wastes have been developed using wet chemistry and low temperature treatment. The adsorbents were characterized by X-ray diffractometry and Fourier transform infrared spectroscopy. Batch adsorption test were carried out to investigate their effectiveness in adsorption of fluoride from ground and surface waters. Adsorption capacities were compared with bone char and synthetic hydroxyapatite (CCHA). Results indicate that prawns derived adsorbent (PHA) formed hexagonal structure with phases identifiable with hydroxyapatite while crabs based adsorbent (CHA) formed predominantly monoclinic structure with crystalline phase characteristic of brushite. Vibrational analysis and kinetic studies predicted defluoridation occurred mainly by ion exchange and ion adsorption mechanisms. Defluoridation capacity of the adsorbents was found to be superior compared to bone char and CCHA. CHA was the most effective with efficiencies above 92% and highest capacity of 13.6 mg/g in field water with fluoride concentration of 5–70 mg/L. PHA had highest capacity of 8.5 mg/g which was still better than 2.6 mg/g recorded by CCHA and bone char. Adsorption was best described by pseudo 2nd order kinetics. The findings indicate that crustacean derived calcium phosphate systems have better potential for defluoridation than traditional bone char and synthetic systems

Hybrid CV-CC operation of capacitive deionization in comparison with constant current and constant voltage

This research article published by Taylor & Francis Online, 2016Capacitive deionization (CDI) is a technique used to desalinate saline water by means of electrical potential applied to the electrode along both sides of a spacer channel through which water flows. CDI operates either at constant voltage (CV) or at constant current (CC) operation to desalinate saline water. The purity of the water is the main requirement at the outlet of the cell. The lowest effluent concentration is achieved within a very short time by operating the CDI cell at CV, but after that the effluent concentration continues to increase. On the other hand, in CC, the lowest concentration is achieved later as compared with CV, but once it is achieved it continues to remain constant until the target voltage is reached. In this paper, we combine both CV and CC operation to get the lowest concentration for maximum time during the adsorption process so that more desalinated water is produced. We compare hybrid CV-CC and constant voltage and constant current in terms of effluent concentration, energy consumption per ion removal, water recovery, and water quality by varying operational parameters like cell potential. It was observed that ultrapure water can be produced with hybrid CV-CC operation by systematically varying different process parameters like flow rate and cell potential to get better results

Ultrapure water from seawater using integrated reverse osmosis-capacitive deionization system

This research article published by Taylor & Francis Online, 2013The use of water for particular application depends on its purity level. In accordance with the world health organization, water with total dissolved salts (TDS) less than 500 ppm can be considered good for human consumption. Ultrapure water is used in areas such as semiconductor industry, pharmaceuticals, and laboratories. Purification processes like electrodeionization process, thermal processes, and membrane processes are used to produce ultrapure water from very low salinity (10–200 ppm) water source. In this study, seawater is desalinated to produce ultrapure water using the integrated reverse osmosis (RO)-capacitive deionization (CDI). The RO permeate is fed to the CDI cell to generate the high purity water. It has been found that, with the use of RO-CDI integrated system, seawater can be used to produce ultrapure water with TDS less than 2 ppm and potable water with TDS less than 400 ppm by consuming 3.171 kWh/m3 of energy. The proposed integrated RO-CDI system is of significant interest in the areas where ultrapure water along with fresh water is required from seawater

Simulation and optimisation of the pyrolysis of rice husk: Preliminary assessment for gasification applications

This research article published by Elsevier B.V, 2020Thermochemical conversion of biomass into useful products is a promising route to harness biofuels. This process is clean, renewable and can reduce the use of fossil fuel. In this study, SuperPro Designer (SPD) software and response surface methodology (RSM) is used to simulate and optimize rice husk pyrolysis process. The SPD simulator was built to handle kinetics and stoichiometric reaction of lignocellulosic composition of rice husk into final products. The SPD simulation and RSM optimization were performed at a temperature ranging from 350 to 800 °C and residence time of 0.25−60 s. The simulated results were in agreement with product yield published in the literature at an average relative error of 6.8 %. The combined effect of temperature and residence time were analysed by using RSM and analysis of variance (ANOVA). A cubic model for bio-oil and quartic model for char and gas yield were proposed. The desirability function in Design-Expert showed that the optimum bio-oil yield (36.72 %) could be attained at a temperature 588 °C and a residence time 0.25 s while the optimum gas yield (73.25 %) could be achieved at a temperature 798.8 °C and a residence time 15.47 s. These findings therefore revealed that the energy content of the rice husk could be harnessed by pyrolysis/gasification to obtain substantial fuel products

Performance optimization of integrated electrochemical capacitive deionization and reverse electrodialysis model through a series pass desorption process

This research article published by Elsevier, 2017A capacitive deionization (CDI) system is one of the emerging desalination technologies used to purify brackish water. It is an electrochemical technology that uses electrically charged porous electrodes to remove salt ions from water. In this study, we developed a process model by integrating CDI with reverse electrodialysis (RED) for the production of pure water and energy. RED is a power generation technology that uses the mixing entropy of water with high and low salt concentrations. Desalination with low energy consumption and high water recovery (WR) was a design preference for this integrated electrochemical model. CDI system was optimized with a series four pass reverse current desorption (RCD) method to achieve WR of almost 96.7% that was previously 50–80% on average. Moreover, an artificial salinity gradient was also produced for RED to generate energy through this four-pass RCD method of CDI. The concentration gain ratio (CGR), WR of CDI, and power density of RED was numerically assessed with different number of desorption passes and for CDI desorption current. WR and CGR value in CDI increased to 96% and 25, respectively, with the increase of number of desorption passes to four. Two stage RED cell system is used to get energy from salinity gradient produced through CDI. Energy consumption of 1.5 kJ/l for pure water production was reduced to 0.58 kJ/l with this purposed integrated four-pass CDI-RED system. This integrated electrochemical system reduced desalination energy consumption as well reducing environmental pollution with an eco-friendly, renewable power generation method and a reduction in the CDI disposal concentration

Experimental Study of a Lab Scale Hybrid Fixed Bed Gasifier

This research article published by Science Publishing Group, 2020Thermo-chemical conversion technologies (incineration, gasification and pyrolysis) have emerged as potential technologies for municipal solid waste management (MSWM). This is happening due to the increase of the need for clean and sustainable energy as a result of fossil fuel depletion. The increase in municipal solid waste (MSW) generation as well as land scarcity for MSW disposal is another reason in raising the potential for thermal technology. Incineration has been the most common thermo-chemical technology for solid waste disposal. However, due to environmental concern, gasification technology is currently becoming more preferable since it is environmental friendly for MSW disposal as well as energy recovery. The aim of this study is to analyze the flue gases obtained from the hybrid fixed bed gasifier during gasification of MSW. The fire was initiated by wood charcoal and six kilograms of MSW was fed in the gasifier. The combustion was supported by the air supplied by electric blower. The flue gas analyzer, TESTO 327-1 was used to analyze the concentration of CO, CO2 and O2. Results show that after 150 minutes of the gasification process, O2 concentration increased by 17.2% while CO and CO2 decreased by 0.0% and 3.77% respectively. The experimental results show that, during gasification process the O2 concentration was increasing with time while CO and CO2 concentration decreased