Effect of feed salinity on the performance of humidification dehumidification desalination

Humidification dehumidification (HDH) is a thermal desalination technology that imitates the rain cycle in an engineered setting. It can be advantageous is small-scale, decentralized applications. In addition, the components used in HDH systems are fairly robust, and can treat highly saline water. The technology has recently been commercialized in order to treat highly saline produced water from hydraulically fractured oil and gas wells. That plant has proved HDH’s ability to treat water that most current seawater desalination technologies are unable to treat. The major disadvantage of HDH is its low energy efficiency compared to other desalination technologies when treating seawater. Previous studies have shown that the system’s energy efficiency can be improved greatly by varying the water-to-air mass flow rate ratio within the system. This translates into operating two or more adjacent stages at different mass flow rate ratios, which is done by extracting an air stream from an intermediate location in the humidifier and injecting it at an intermediate location in the dehumidifier. Previous models have used fixed effectiveness or fixed pinch approaches to evaluate the benefits of multi-staging, but these do not take account of the size of the system. In physical systems, what remains constant when going from a single-stage to a multi-stage system is the total size of the system and not the effectiveness or the pinch. Therefore, comparing systems with the same total heat exchanger area is the best way to understand the effect of extraction/injection and whether its implementation is beneficial or not. In this paper, a numerical heat and mass transfer model is used to simulate the operation of HDH at various feed salinities. For each case, the performance of the single-stage system is compared to that of a two-stage system of the same size at different values of feed salinity. The ability of HDH to treat feeds with varying salinity is also studied.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Entropy generation analysis of electrodialysis

Electrodialysis (ED) is a desalination technology with many applications. In order to better understand how the energetic performance of this technology can be improved, the various losses in the system should be quantified and characterized. This can be done by looking at the entropy generation in ED systems. In this paper, we implement an ED model based on the Maxwell-Stefan transport model, which is the closest model to fundamental equations. We study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In addition, we study the effect of the channel height, membrane thickness, and cell-pair voltage on the specific rate of entropy generation. We express the second-law efficiency of ED as the product of current and voltage utilization rates, and study its variation with current density. Further, we define the useful voltage that is used beneficially for separation. We derive the rate of entropy generation that is due to the passage of ions through a voltage drop, and we investigate whether voltage drops themselves can provide a good estimate of entropy generation.Kuwait Foundation for the Advancement of Sciences (KFAS) (Project No. P31475EC01

A Numerical Solution Algorithm for a Heat and Mass Transfer Model of a Desalination System Based on Packed-Bed Humidification and Bubble Column Dehumidification

The humidification-dehumidification (HDH) desalination system can be advantageous in small-scale, off-grid applications. The main drawback of this technology has been its low energy efficiency, which results in high water production costs. Previous studies have approached this issue through thermodynamic balancing of the system; however, most theoretical work on the balancing of HDH has followed a fixed-effectiveness approach that does not explicitly consider transport processes in the components. Fixing the effectiveness of the heat and mass exchangers allows them to be modeled without explicitly sizing the components and gives insight on how the cycle design can be improved. However, linking the findings of fixed-effectiveness models to actual systems can be challenging, as the performance of the components depends mainly on the available surface areas and the flow rates of the air and water streams. In this study, we present a robust numerical solution algorithm for a heat and mass tranfer model of a complete humidification-dehumidification system consisting of a packed-bed humidifier and a multi-tray bubble column dehumidifier. We look at the effect of varying the water-to-air mass flow rate ratio on the energy efficiency of the system, and we compare the results to those reached following a fixed-effectiveness approach. In addition, we study the effect of the top and bottom temperatures on the performance of the system. We recommended the implementation a control system that varies the mass flow rate ratio in order to keep the system balanced in off-design conditions, especially with varying top temperature.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Use of multiple extractions and injections to thermodynamically balance the humidification dehumidification desalination system

Humidification dehumidification (HDH) desalination systems are well suited for small scale, off-grid desalination. These systems are very robust and can tolerate a wide range of feed salinities, making them a good candidate for treating produced water from hydraulically fractured natural gas wells. A primary engineering challenge for these systems is their high thermal energy consumption. In this study, we examine the use of multiple air extractions and injections to thermodynamically balance the HDH system, so as to make it more energy efficient. The effect of the number of extractions on several performance parameters is studied. In addition, we study the effect of the enthalpy pinch, which is a measure of performance for a heat and mass exchanger, on these performance parameters. Finally, we present results that can be used as guidelines in designing HDH systems. These results include the identification of appropriate temperatures for the extracted/injected air streams, the division of the heat duty between stages, and the value of the mass flow rate ratio in each stage at various values of enthalpy pinch.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Thermodynamic balancing of a fixed-size two-stage humidification dehumidification desalination system

Humidification dehumidification (HDH) is a desalination technology that has shown promise in small scale, decentralized applications. Previous studies on the multi-staging of HDH have used fixed-effectiveness models which do not explicitly account for transport processes in the components. However, to fully understand the effect of the variation of the mass flow rate ratio, it is necessary to implement heat and mass transfer models of the HDH system. In this paper, we model an HDH system consisting of a packed-bed humidifier and a multi-tray bubble column dehumidifier. We study the effect of the mass flow rate ratio on the performance of a fixed-size system, and we consider its effect on the entropy generation and the driving forces for heat and mass transfer. In addition, we define a generalized energy effectiveness for heat and mass exchangers. We also implement an air extraction/injection and simulate a wide range of operating conditions. We define criteria for the best system performance, and we study the effect of the distribution of available area between separate stages. We also present a thorough explanation of why the direction of extraction should always be from the humidifier to the dehumidifier.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Thermodynamic analysis of brine management methods: Zero-discharge desalination and salinity-gradient power production

Growing desalination capacity worldwide has made management of discharge brines an increasingly urgent environmental challenge. An important step in understanding how to choose between different brine management processes is to study the energetics of these processes. In this paper, we analyze two different ways of managing highly saline brines. The first method is complete separation with production of salts (i.e., zero-discharge desalination or ZDD). Thermodynamic limits of the ZDD process were calculated. This result was applied to the state-of-the-art industrial ZDD process to quantify how close these systems are to the thermodynamic limit, and to compare the energy consumption of the brine concentration step to the crystallization step. We conclude that the brine concentration step has more potential for improvement compared to the crystallization step. The second brine management method considered is salinity-gradient power generation through pressure-retarded osmosis (PRO), which utilizes the brine's high concentration to produce useful work while reducing its concentration by mixing the brine with a lower salinity stream in a controlled manner. We model the PRO system coupled with a desalination system using a detailed numerical optimization, which resulted in about 0.42 kW h/m3 of energy saving.Kuwait Foundation for the Advancement of Sciences (KFAS) (Project No. P31475EC01

Numerical fixed-effectiveness and fixed-area models of the humidification dehumidification desalination system with air extractions and injections/

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 101-104).The humidification dehumidification (HDH) desalination system can be advantageous in small-scale, off-grid applications. This system is very robust and can tolerate a wide range of feed salinities, making it a good candidate for treating produced water from hydraulically fractured natural gas wells. The main drawback of this technology has been its low energy efficiency, which results in high water production costs. This work focuses on the thermodynamic balancing of HDH. The first part uses a fixed-effectiveness approach to model the use of multiple air extractions and injections to thermodynamically balance the HDH system, so as to make it more energy efficient. The effect of the number of extractions on several performance parameters is studied. In addition, we study the effect of the enthalpy pinch, which is a measure of performance for a heat and mass exchanger, on these performance parameters. Further, we present results that can be used as guidelines in designing HDH systems. These results include the identification of appropriate temperatures for the extracted/injected air streams, the division of the heat duty between stages, and the value of the mass flow rate ratio in each stage at various values of enthalpy pinch. Fixing the effectiveness of the heat and mass exchangers allows them to be modeled without explicitly sizing the components and gives insight on how the cycle design can be improved. However, linking the findings of fixed-effectiveness models to actual systems can be challenging, as the performance of the components depends mainly on the available surface areas and the flow rates of the air and water streams. In the second part of this study, we present a robust numerical solution algorithm for a heat and mass transfer model of a complete humidification-dehumidification system consisting of a packed-bed humidifier and a multi-tray bubble column dehumidifier. We look at the effect of varying the water-to-air mass flow rate ratio on the energy efficiency of the system. In addition, we study the effect of the top and bottom temperatures on the performance of the system. We recommended the implementation a control system that varies the mass flow rate ratio in order to keep the system balanced in off-design conditions, especially with varying top temperature. Finally we consider a single air extraction, and look at the effect of the location of extraction, and its direction. We define the criteria for achieving a completely balanced system.by Karim Malek Chehayeb.S.M

Thermodynamic analysis of electrodialysis

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 157-163).The work presented in this thesis is motivated by the water and energy problems our world faces today. Desalination can help alleviate the problem of water shortage by increasing the supply of fresh water. However, for desalination to play a major role in the future, it needs to be done in a sustainable manner. Significant progress towards making desalination technologies sustainable can be made by decreasing their energy consumption. This can be done with the help of a better understanding of the thermodynamics of desalination technologies. In this thesis, we present a thermodynamic analysis of electrodialysis (ED). ED is a desalination technology with many applications, and has shown promise in desalinating brackish water and in concentrating high-salinity brines. In order to better understand how the energetic performance of this technology can be improved, we first study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In the second part, we define a fair set of constraints to allow a fair comparison between different system sizes, designs, and operating conditions. We study the tradeoffs governing the optimal channel height and velocity for brackish-water desalination and for high-salinity brine concentration. In addition, we study the minimum costs associated with the different system sizes, and we compare the differing trends in brackish-water and high-salinity applications. Further, we report optimal values of system size, current density, length, velocity, and cost for the two applications at different unit fixed costs and energy costs. In the third part, we study possible improvements to the energy efficiency of electrodialysis through the use of two electric stages with different voltages, and through the operation using a counterflow configuration. We first look at how a two-stage ED system should be operated for optimal energy efficiency. We then quantify the effect of operating under two voltages in brackish-water desalination and in high-salinity brine concentration. This is done at systems sizes that are shown to be cost effective at different unit fixed costs and energy costs. Finally, we quantify the effect of operating ED in counterflow for the same applications. In the final part, we study the optimal operation of a batch ED system for the desalination of brackish water and seawater, and for the concentration of high-salinity brine. We compare three processes: operation under constant voltage, constant current, and constant entropy generation. We then study the effect of improved operation on the energy consumption and on the system cost of batch ED at different fixed-to-energy cost ratios. It is shown that significant improvements to energy consumption and cost can be made through better system operation, especially in the seawater desalination application.by Karim Malek Chehayeb.Ph. D

On the electrical operation of batch electrodialysis for reduced energy consumption

The cost of desalination using electrodialysis (ED) may be decreased by lowering the electrical energy consumption. This can be done by improving the operating conditions of ED systems. For a fixed application, the total amount of salt that needs to be transported, or the duty, is roughly fixed. For a fixed system size and a fixed duty, the optimal operation can be guided by the theorem of equipartition of entropy generation. This paper examines the improved operation of a batch ED system for the desalination of brackish water and seawater, and for the concentration of high-salinity brine. Energy consumption is compared for three operating conditions: constant applied voltage, constant current, and constant entropy generation. The work then considers the effect of improved operation on the energy consumption and on the non-pumping system costs of batch ED at various cost factors by using a simple cost model. Significant improvements to energy consumption and cost are shown to be achievable through better system operation, especially for the seawater desalination application and when fixed costs are low relative to energy costs.Kuwait Foundation for the Advancement of Sciences (Grant P31475EC01

On the merits of using multi-stage and counterflow electrodialysis for reduced energy consumption

The cost of electrodialysis (ED) systems can be decreased by decreasing their power consumption. Such reductions may be achieved by using degrees of freedom in the system's configuration to obtain a more uniform spatial distribution of the rate of entropy generation, as explained by the theorem of equipartition of entropy generation. In this paper, we study possible improvements to the energy efficiency of electrodialysis through the use of two electric stages with different voltages, and through operation in a counterflow configuration. We first consider how a two-stage ED system should be operated. In particular, we look at how the voltages and current densities should be chosen. In addition, we quantify the effect of operating under two voltages in brackish-water desalination and in high-salinity brine concentration. Finally, we quantify the effect of operating ED in counterflow for the same applications. We show that high ED fixed costs prevent the achievement of significant improvements in energy efficiency. If fixed costs are reduced, and larger systems become cost-effective, we show that a power reduction of up to 29% is possible by going from a single-stage to a two-stage configuration. Keywords: Equipartition of entropy generation; Electrodialysis; Brackish water desalination; Brine concentration; Energy efficienc