Irreversibilities and nonidealities in desalination systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 207-220).Energy requirements for desalination systems must be reduced to meet increasing global demand for fresh water. This thesis identifies thermodynamic limits for the energetic performance of desalination systems and establishes the importance of irreversibilities and solution composition to the actual performance obtained. Least work of separation for a desalination system is derived and generalized to apply to all chemical separation processes driven by some combination of work, heat, and chemical energy (fuel) input. At infinitesimal recovery, least work reduces to the minimum least work of separation: the true exergetic value of the product and a useful benchmark for evaluating energetic efficiency of separation processes. All separation processes are subject to these energy requirements; several cases relevant to established and emerging desalination technologies are considered. The effect of nonidealities in electrolyte solutions on least work is analyzed through comparing the ideal solution approximation, Debye-Hückel theory, Pitzer's ionic interaction model, and Pitzer-Kim's model for mixed electrolytes. Error introduced by using incorrect property models is quantified. Least work is a strong function of ionic composition; therefore, standard property databases should not be used for solutions of different or unknown composition. Second Law efficiency for chemical separation processes is defined using the minimum least work and characterizes energetic efficiency. A methodology is shown for evaluating Second Law efficiency based on primary energy inputs. Additionally, entropy generation mechanisms common in desalination processes are analyzed to illustrate the effect of irreversibility. Formulations for these mechanisms are applied to six desalination systems and primary sources of loss are identified. An economics-based Second Law efficiency is defined by analogy to the energetic parameter. Because real-world systems are constrained by economic factors, a performance parameter based on both energetics and economics is useful. By converting all thermodynamic quantities to economic quantities, the cost of irreversibilities can be compared to other economic factors including capital and operating expenses. By applying these methodologies and results, one can properly characterize the energetic performance and thermodynamic irreversibilities of chemical separation processes, make better decisions during technology selection and design of new systems, and critically evaluate claimed performance improvements of novel systems.by Karan H. Mistry.Ph.D

Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies driven by different combinations of heat, work, and chemical energy. This paper develops a consistent basis for comparing the energy consumption of such technologies using Second Law efficiency. The Second Law efficiency for a chemical separation process is defined in terms of the useful exergy output, which is the minimum least work of separation required to extract a unit of product from a feed stream of a given composition. For a desalination process, this is the minimum least work of separation for producing one kilogram of product water from feed of a given salinity. While definitions in terms of work and heat input have been proposed before, this work generalizes the Second Law efficiency to allow for systems that operate on a combination of energy inputs, including fuel. The generalized equation is then evaluated through a parametric study considering work input, heat inputs at various temperatures, and various chemical fuel inputs. Further, since most modern, large-scale desalination plants operate in cogeneration schemes, a methodology for correctly evaluating Second Law efficiency for the desalination plant based on primary energy inputs is demonstrated. It is shown that, from a strictly energetic point of view and based on currently available technology, cogeneration using electricity to power a reverse osmosis system is energetically superior to thermal systems such as multiple effect distillation and multistage flash distillation, despite the very low grade heat input normally applied in those systems.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R13-CW-10

An improved model for multiple effect distillation

Increasing global demand for fresh water is driving research and development of advanced desalination technologies. As a result, a detailed model of multiple effect distillation (MED) is developed that is flexible, simple to implement, and suitable for use in optimization of water and power cogeneration systems. The MED system is modeled in a modular method in which each of the subcomponents is modeled individually and then instantiated as necessary in order to piece together the complete plant model. Modular development allows for studying various MED configurations (such as forward feed, parallel feed, etc.) with minimal code duplication. Use of equation-oriented solvers, such as Engineering Equation Solver and JACOBIAN, rather than sequential solvers, simplifies the coding complexity dramatically and also reduces the number of required approximations and assumptions. The developed model is compared with four prominent forward feed MED models from literature. Through parametric analysis, it is found that the present model compares very well with the simple model provided by El-Sayed and Silver while providing substantially more detail in regard to the various temperature profiles within the MED system. Further, the model is easier to implement than the detailed El-Dessouky model while relying on fewer assumptions. The increased detail of the model allows for proper sensitivities to key variables related to input, operating, and design conditions necessary for use in a cogeneration or hybrid system optimization process.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R13-CW-10

Optimization of multi-pressure himidification-dehumidification desalination using thermal vapor compression and hybridization

Conference site: http://www.ishmt2011.iitm.ac.in/Humidification-dehumidification (HD or HDH) desalination, and specifically HD driven by a thermal vapor compressor (TVC), is a thermal desalination method that has the potential to produce potable water efficiently in order to address the growing demand for water. This article presents a numerical study and optimization of two HD-TVC cycle configurations in order to determine the best achievable thermal performance. Through the use of nonlinear programming, it is found that the simplest configuration of HD-TVC has performance comparable to a traditional single-stage, single-pressure HD cycle (GOR 0.8–2.0), while the hybridized HD-TVC cycle with reverse osmosis (RO) has thermal performance that is competitive with existing large scale desalination systems (GOR 11.8–28.3).Center for Clean Water and Clean Energy at MIT and KFUPMNumerica TechnologyKing Fahd University of Petroleum and Mineral

ENERGY EFFECTIVENESS OF SIMULTANEOUS HEAT AND MASS EXCHANGE DEVICES

Simultaneous heat and mass exchange devices such as cooling towers, humidifiers and dehumidifiers are widely used in the power generation, desalination, air conditioning, and refrigeration industries. For design and rating of these components it is useful to define their performance by an effectiveness. In this paper, several different effectiveness definitions that have been used in literature are critically reviewed and an energy based effectiveness which can be applied to all types of heat and mass exchangers is defined. The validity and the limitations of the various effectiveness definitions are demonstrated by way of several examples including direct and indirect contact, parallel and counterflow heat and mass exchangers. The limiting case of a simple heat exchanger is also discussed. The importance of thermal balancing in minimizing entropy production and its implications for optimization and design of these devices is dealt with in detail. The application of the energy effectiveness to heat-exchanger-like "-NTU correlations is also examined using a detailed numerical model.King Fahd University of Petroleum and Mineral

Prognosis of Component Degradation Under Uncertainty: A Method for Early Stage Design of a Complex Engineering System

This paper proposes a method that dynamically improves a statistical model of system degradation by incorporating uncertainty. The method is illustrated by a case example of fouling, or degradation, in a heat exchanger in a cogeneration desalination plant. The goal of the proposed method is to select the best model from several representative condenser fouling models including linear, falling rate, and asymptotic fouling, and to validate and improve model parameters over the duration of operation. Maximum likelihood estimation (MLE) was applied to obtain a stochastic distribution of condenser fouling. Akaike’s Information Criterion (AIC) and the Bayesian Information Criterion (BIC) were then computed at time intervals to assess the accuracy of the MLE results. The degradation model was further evaluated by estimating future prognoses and then cross-validating with real world fouling data. The results show the accuracy of a prognosis can be improved substantially by continuously updating fouling model parameters. The proposed method is a step toward facilitating prognosis of engineering systems in the early design stages by improving the prediction of future component degradation.Center for Clean Water and Clean Energy at MIT and KFUPMNatural Sciences and Engineering Research Council of Canad

Second law analysis and optimization of HD desalination cycles

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 139-143).Humidification-Dehumidification (HD) desalination is a thermal distillation method that has the potential to be driven using solar heating. It is a promising technology that can potentially bring safe drinking water to people of the developing world. Surprisingly, few systematic efforts have been made to find the best HD cycles or to improve and optimize existing cycles. This thesis applies irreversibility analysis to characterize HD desalination cycles and to identify how to further improve cycles and components. It is shown that minimizing the specific entropy generation of the cycle maximizes the gained output ratio (GOR). It is also shown that each cycle has one limiting component that cannot be substantially improved and a second component that should be the target of efforts to minimize entropy generation. Finally, the failure of exergy analysis to yield conclusive results for on-design HD cycle analysis is discussed briefly. Following the Second Law analysis, an optimization effort is performed using nonlinear programming techniques in order to optimize HD desalination cycles for operating conditions that result in maximum GOR. Closed air, open water and open air, open water cycles with either air and water heating were considered in this analysis. Numerical optimization resulted in substantial improvement in GOR for all four cycle types considered. It was found that the GOR of the cycles decreases with increasing component terminal temperature difference (TTD) and that different cycles perform best at different temperature differences. Optimization also revealed that some counterintuitive design configurations can result in superior performance under the appropriate operating conditions. Other topics discussed include the behavior of exergy for pure substances and psychrometric mixtures as well as the effect of salinity on the performance of HD cycles.by Karan H. Mistry.S.M

Energy effectiveness of simultaneous heat and mass exchange devices

Simultaneous heat and mass exchange devices such as cooling towers, humidifiers and dehumidifiers are widely used in the power generation, desalination, air conditioning, and refrigeration industries. For design and rating of these components it is useful to define their performance by an effectiveness. In this paper, several different effectiveness definitions that have been used in literature are critically reviewed and an energy based effectiveness which can be applied to all types of heat and mass exchangers is defined. The validity and the limitations of the various effectiveness definitions are demonstrated by way of several examples including direct and indirect contact, parallel and counterflow heat and mass exchangers. The limiting case of a simple heat exchanger is also discussed. The importance of thermal balancing in minimizing entropy production and its implications for optimization and design of these devices is dealt with in detail. The application of the energy effectiveness to heat-exchanger-like "-NTU correlations is also examined using a detailed numerical model.King Fahd University of Petroleum and Mineral

Entropy Generation of Desalination Powered by Variable Temperature Waste Heat

Powering desalination by waste heat is often proposed to mitigate energy consumption and environmental impact; however, thorough technology comparisons are lacking in the literature. This work numerically models the efficiency of six representative desalination technologies powered by waste heat at 50, 70, 90, and 120 °C, where applicable. Entropy generation and Second Law efficiency analysis are applied for the systems and their components. The technologies considered are thermal desalination by multistage flash (MSF), multiple effect distillation (MED), multistage vacuum membrane distillation (MSVMD), humidification-dehumidification (HDH), and organic Rankine cycles (ORCs) paired with mechanical technologies of reverse osmosis (RO) and mechanical vapor compression (MVC). The most efficient technology was RO, followed by MED. Performances among MSF, MSVMD, and MVC were similar but the relative performance varied with waste heat temperature or system size. Entropy generation in thermal technologies increases at lower waste heat temperatures largely in the feed or brine portions of the various heat exchangers used. This occurs largely because lower temperatures reduce recovery, increasing the relative flow rates of feed and brine. However, HDH (without extractions) had the reverse trend, only being competitive at lower temperatures. For the mechanical technologies, the energy efficiency only varies with temperature because of the significant losses from the ORC