Browsing by Subject "Desalination"

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    Convection-Enhanced Evaporation: Modeling and Optimal Control for Modular Cost-Effective Brine Management
    (2022-11) Kaddoura, Mustafa
    This dissertation proposes mathematical modeling and novel cost-optimal control methods for Convection-Enhanced Evaporation (CEE) systems. CEE is the approach of evaporating water from saline films (brine) on packed evaporation surfaces by air convection, and actively controlling the operation variables to minimize the process cost. The developed approach represents a modular, cost-effective solution for brine management in decentralized and/or small scale desalination plants and industrial processes which currently lack safe and effective brine management options. Forced convection across packed, wetted evaporation surfaces is induced either by means of a fan, natural wind, or a combination of both (hybrid approach). As air flows over the liquid films, the difference in vapor pressure between the air and liquid surfaces induces evaporation. The work contains three major parts. The first part develops a generalized mathematical model of CEE systems to simulate the heat and mass transfer associated with convection-driven evaporation of saline films. The model is derived from the fundamental conservation equations of mass and energy, solved numerically using the finite difference method to predict the evaporation rate and the spatial distribution of humidity, temperature and salinity along the evaporation surfaces based on ambient condition, liquid (brine) inlet condition, and design configuration. The model-predicted performance is in good agreement with experimental pilot CEE system performance and with values published in the literature. The developed model is used to explore and compare the performance of three design aspects: (1) the liquid-air flow configuration (cross-flow vs parallel-flow), (2) the alignment and wetting of the surfaces (vertically aligned with double-sided wet surfaces vs horizontally aligned with single-sided wet surfaces), and (3) hybrid wind-fan operation, a novel operation model aimed at reducing the electrical energy demand of the fan by harnessing the natural drying power of the wind. The second part of this dissertation focuses on cost optimization. It proposes a method for formulating objective functions using cost ratios to generalize the optimization results to applications with varying material and energy prices and scenarios. The problem of identifying the cost-optimal operating settings was then solved as a multi-objective optimization using the genetic algorithm. The optimization revealed and characterized two distinct operation modes: "all-electric" mode, and "heating" mode. Finally, the last part of this dissertation proposes a data-driven optimal control method. The controller is based on a large dataset consisting of Pareto fronts, obtained in advance by solving a set of optimization problems. The method allows three optimal operation strategies: (1) real-time selection of operating variables, (2) predictive scheduled operation, and (3) hybrid wind-fan operation. The effectiveness of the proposed strategies was assessed through two case studies with distinct geographical locations and weathers. The results showed significant costsaving potential relative to static operation. The presented control strategies enable CEE to adjust its operation under various weather conditions. The models and methods developed in this dissertation are conducive to study and control of other configurations of CEE systems. They have the potential to be applied to other desalination and renewable energy systems, particularly those involving a trade-off between thermal and electric energy demand.
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    Modeling of fluid flow in spiral wound reverse osmosis membranes
    (2013-07) Srivathsan, G.
    The research performed here is motivated by the need to understand and quantify the phenomena that underlie the purification of impure water by the reverse osmosis process. Despite the fact that reverse osmosis is a well-established method of water purification, the design and implementation of the process has been based on vastly oversimplified models. Oversimplified reverse-osmosis (RO) models lead to inefficient RO element performance estimation.Reverse osmosis is based on profoundly interacting fluid flow and mass transfer phenomena. These phenomena are modeled without approximation, and the model is implemented by numerical simulation. The numerous oversimplifications of prior models have been eliminated. In particular, the linearity that marked those models has been demonstrated to be invalid. Although the flow is laminar, it is not friction dominated. Instead, pressure drop non-linearity exists because of inertial effects. A second factor promoting non-linearity is the continuous bleeding-off of product water from the salt-containing feed stream. The flow phenomena at the entry and exit of the individual reverse osmosis elements have been clarified. The associated pressure drops were found to be remarkably small.The simulations spanned the entire range of operating conditions of actual reverse osmosis installations. The species conservation for salt took account of both advection and diffusion. Mass transfer coefficients at the membrane surface were determined, again for all practical operating conditions. The main outcomes of the simulations were the true portrayal of the in-element pressure losses and mass transfer coefficients. Experiments were performed to support the simulation model. The attainment of marginal levels of agreement motivated careful examination of the physical interactions of the feed spacer and the RO membrane. Upon investigation, it was found that the feed spacer penetrated into the membrane, with the outcome that the dimensions of the actual flow passage were less than that based on the dimensions of the feed spacer alone. When the simulation was repeated with the actual flow passage dimensions, good agreement was achieved between the simulations and the experimental data.Based on the new information extracted from both the simulations and the experiments, a new methodology was developed for the accurate simulation of typical reverse osmosis elements. The new methodology supersedes that which has been standard in the past. All of the oversimplifications and omissions have been avoided in favor of a logic-based application of the underlying physical phenomena. The outcome of the research work is a thorough understanding of the reverse osmosis desalination operation.