Passive vapor generation systems with interfacial solar heat localization enable high-efficiency low-cost desalination. In particular, recent progress combining interfacial solar heating and ...vaporization enthalpy recycling through a capillary-fed multistage architecture, known as the thermally-localized multistage solar still (TMSS), significantly improves the performance of passive solar desalination. Yet, state-of-the-art experimental demonstrations of solar-to-vapor conversion efficiency are still limited since the dominant factors and the general design principle for TMSS were not well-understood. In this work, we show optimizing the overall heat and mass transport in a multistage configuration plays a key role for further improving the performance. This understanding also increases the flexibility of material choices for the TMSS design. Using a low-cost and free-of-salt accumulation TMSS architecture, we experimentally demonstrated a record-high solar-to-vapor conversion efficiency of 385% with a production rate of 5.78 L m −2 h −1 under one-sun illumination, where more than 75% of the total production was collected through condensation. This work not only significantly improves the performance of existing passive solar desalination technologies for portable and affordable drinking water, but also provides a comprehensive physical understanding and optimization principle for TMSS systems.
•Thermodynamic principles are applied to systematically compare three technologies.•Merits and limits of standalone versus integrated designs are identified.•Effect of climate conditions on ...performance and technology selection is evaluated.•Integrated desiccant/membrane technologies outperform current state-of-the-art VCS.
Recently, next-generation HVAC technologies have gained attention as potential alternatives to the conventional vapor-compression system (VCS) for dehumidification and cooling. Previous studies have primarily focused on analyzing a specific technology or its application to a particular climate. A comparison of these technologies is necessary to elucidate the reasons and conditions under which one technology might outperform the rest. In this study, we apply a uniform framework based on fundamental thermodynamic principles to assess and compare different HVAC technologies from an energy conversion standpoint. The thermodynamic least work of dehumidification and cooling is formally defined as a thermodynamic benchmark, while VCS performance is chosen as the industry benchmark against which other technologies, namely desiccant-based cooling system (DCS) and membrane-based cooling system (MCS), are compared. The effect of outdoor temperature and humidity on device performance is investigated, and key insights underlying the dehumidification and cooling process are elucidated. In spite of the great potential of DCS and MCS technologies, our results underscore the need for improved system-level design and integration if DCS or MCS are to compete with VCS. Our findings have significant implications for the design and operation of next-generation HVAC technologies and shed light on potential avenues to achieve higher efficiencies in dehumidification and cooling applications.
Faced with a pressing need for membranes with a higher permeability and selectivity, the field of membrane technology can benefit from a systematic framework for designing membranes with the ...necessary physical characteristics. In this work, we present an approach through which transport modeling is employed in fabricating specialized nanofiltration membranes, that experimentally demonstrate enhanced selectivity. Specifically, the Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE) is used to probe for membrane properties desirable in desalination pretreatment. Nanofiltration membranes are systematically fabricated in-house using layer-by-layer (LbL) deposition to validate model predictions and to develop a new specialized membrane for this application. The new membrane presents a 30% increase in permeability and a 50% reduction in permeate hardness relative to state-of-the-art NF membranes. Our results indicate that a ‘specialized’ tight membrane can outperform looser counterparts in both permeability and selectivity. Given the possibility of extending this framework to other applications, the work furthers our understanding of the relationships governing membrane form and function, while having broad potential implications for future nanofiltration membranes used in chemical separation and purification.
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•Modeling can probe for membrane properties desirable in desalination pretreatment.•Nanofiltration membranes are systematically prepared to validate model predictions.•Pore size affects rejection & selectivity more than membrane charge or thickness.•New membrane achieves 30% higher permeability and 50% lower permeate hardness.
Membrane fouling is a ubiquitous challenge in water treatment and desalination systems. Current reverse osmosis (RO) membrane cleaning technology relies on chemical processes, incurring considerable ...costs and generating waste streams. Here, we present a novel chemical-free membrane cleaning method applicable to commercially existing RO spiral-wound membrane modules. The method employs controlled membrane deformation through pressure modulation, which induces shear stresses at the foulant-membrane interface that lead to detachment and removal of the foulants. To investigate the effectiveness of the method, experiments on organic fouling by alginate are conducted on a flat-sheet membrane coupon followed by tests on a commercial spiral-wound module with feeds of varying fouling propensities. Cleaning durations are six-fold lower, and the experimental results demonstrate flux recoveries and cleaning efficiencies comparable to those of chemical cleaning. The experiments on the spiral-wound module indicate that this method will have applicability in industrially-relevant settings. To elucidate the underlying cleaning mechanisms, membrane deformation experiments with no flow are conducted, and in situ visualization techniques are employed for both the flat-sheet and spiral-wound modules. The results show that cleaning is caused by a reduction in shear strength at the foulant-membrane interface after cycles of repeated loading, a behavior typical of fatigue. By enabling more frequent cleanings, deformation-induced cleaning is shown to considerably lower operating costs in an economic case study while offering a more sustainable and environmentally sound solution to membrane cleaning and antifouling in desalination.
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•DIC, a new chemical-free foulant mitigation method, is presented.•DIC uses controlled deformation inducing interfacial stresses and foulant fatigue.•No-flow experiments and in situ visualization elucidate the foulant removal mechanism.•DIC’s effectiveness is demonstrated for organic fouling and on spiral-wound RO.•Performance is competitive with chemicals while cleaning times are six-fold lower.
Given its high surface area to volume ratio and desirable mass transfer characteristics, the hollow fiber module configuration has been central to the development of RO and UF technologies over the ...past five decades. Recent studies have demonstrated the development of a novel class of low-pressure nanofiltration (NF) hollow fiber membranes with great promise for scale-up implementation. Further progress on large-scale deployment, however, has been restrained by the lack of an accurate predictive model, to guide module design and operation. Earlier models targeting hollow fiber modules are only suitable for RO or UF. In this work, we propose a new modeling approach suitable for NF based on the implementation of mass and momentum balances, coupled with a validated membrane transport model based on the extended Nernst-Planck equation to predict module performance at the system-level. Modeling results are validated with respect to synthetic seawater experiments reported in an earlier work. A preliminary module design is proposed, and parametric studies are employed to investigate the effect of varying key system parameters and elucidate the tradeoffs available during design. The model has significant implications for low-pressure nanofiltration, as well as hollow fiber NF module design and operation.
•Model for large-scale hollow fiber nanofiltration modules developed & validated•Model validated with respect to synthetic seawater NF experiments•Preliminary module design proposed for water softening & desalination pretreatment•Parametric studies uncover effect of different system parameters on performance
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, May, 2020
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 145-157).
...As water scarcity continues to intensify around the globe, the need for more efficient and sustainable desalination technologies has never been more pressing. While membrane technology, namely reverse osmosis (RO), currently stands as the most energy efficient desalination technology, it is plagued by fouling undercutting both productivity and permeate quality. To restore performance, desalination plants resort to chemical cleaning, incurring losses in productivity, chemical cost, and membrane replacement all while raising environmental concerns associated with chemical waste. In this work, we explore alternative chemical-free methods of membrane fouling mitigation. First, membrane pretreatment using nanofiltration is investigated as a means of mitigating inorganic fouling in downstream desalination systems. Transport modeling is employed in fabricating specialized nanofiltration membranes for desalination pretreatment.
The Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE) is used to probe for desirable membrane properties, while new membranes are systematically fabricated in-house using layer-by-layer (LbL) deposition to validate model predictions and to develop a new specialized membrane for this application. The new membrane presents a 30% increase in permeability and a 50% reduction in permeate hardness relative to state-of-the-art NF membranes. Apart from proactive pretreatment approaches, reactive approaches remain necessary in handling already fouled RO membranes. To that end, osmotically-induced cleaning (OIC), whereby a RO membrane effectively undergoes osmotic backwashing, is explored. Specifically, the effectiveness of OIC against organic fouling is examined, underlying mechanisms are elucidated, and potential applicability in the presence of spacers is investigated.
While experimental results demonstrate flux recoveries of up to 30%, the method's effectiveness is shown to be dramatically influenced in the presence of spacers and far from completely eliminating a biofilm or preventing its regrowth once operation is resumed. Given the practical limitations of OIC, we finally present the development of deformation-induced cleaning (DIC), a novel chemical-free fouling mitigation method. applicable to commercially existing spiral-wound membrane modules. The method employs controlled membrane deformation through pressure modulation, which induces shear stresses at the foulant-membrane interface that lead to detachment and removal of the foulants. Experiments on organic fouling by alginate are conducted on a flat-sheet membrane coupon followed by tests on a commercial spiral-wound module. Shutdown durations are shown to be six-fold lower, while flux recoveries are comparable to those of chemical methods.
In-situ visualization is employed alongside bench-scale experiments to elucidate the underlying mechanisms and ultimately devise an optimized chemical-free fouling mitigation strategy. Experiments on a commercial spiral-wound module indicate this method will have applicability in industrially-relevant settings. By enabling more frequent cleanings, DIC considerably lowers operating expenses while offering a more sustainable and environmentally sound solution to membrane fouling mitigation in desalination.
by Omar Labban.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineering
Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 89-96).
Recently, ...interest in nanofiltration (NF) has been surging, as has interest using it as a technology for better brine management and pretreatment in reverse osmosis (RO) plants. Using NF for pretreatment reduces fouling and scaling in RO units, allowing for potentially higher recoveries. This lowers the environmental impact of RO by decreasing the amount of water to be treated per unit volume of water produced, and reducing the volume of RO brine to be managed. This can potentially curb the CO2 emissions resulting from the RO desalination process. A novel class of low-pressure nanofiltration (NF) hollow fiber membranes, particularly suited for water softening and desalination pretreatment have lately been fabricated in-house using layer-by-layer (LbL) deposition with chemical crosslinking. These membranes can operate at exceedingly low pressures (2 bar), while maintaining relatively high rejections of multivalent ions. In spite of their great potential, our understanding as to what makes them superior has been limited, demanding further investigation before any large-scale implementation can be realized. In this study, the Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE) is applied for the first time to these membranes to describe the membrane separation performance, and to explain the observed rejection trends, including negative rejection, and their underlying multi-ionic interactions. Experiments were conducted on a spectrum of feed chemistries, ranging from uncharged solutes to single salts, salt mixtures, and artificial seawater to characterize the membrane and accurately predict its performance. Modeling results were validated with experiments, and then used to elucidate the working principles that underly the low-pressure softening process. An approach based on sensitivity analysis shows that the membrane pore dielectric constant, followed by the pore size, are primarily responsible for the high selectivity of the NF membranes to multivalent ions. Surprisingly, the softening process is found not to be sensitive to changes in membrane charge density. Our findings demonstrate that the unique ability of these membranes to exclusively separate multivalent ions from the solution, while allowing monovalent ions to permeate, is key to making this lowpressure softening process realizable. Given its high surface area to volume ratio and desirable mass transfer characteristics, the hollow fiber module configuration has been central to the development of reverse osmosis (RO) and ultrafiltration (UF) technologies over the past five decades. Following the development of the LbL membrane, interest in their scale-up implementation for softening and desalination pretreatment has been growing. Further progress on large-scale deployment, however, has been restrained by the lack of an accurate predictive model, which is pivotal to guiding module design and operation. Earlier models targeting hollow fiber modules are only suitable for RO or UF technologies, and no appropriate NF models have been presented to characterize the performance of hollow fiber modules at the large-scale. In this work, we propose a new modeling approach based on the implementation of mass and momentum balances, coupled with a suitable membrane transport model, such as the Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE), to predict module performance at the system-level. We then propose a preliminary module design, and employ parametric studies to investigate the effect of varying key system parameters and to elucidate the tradeoffs available to the module designer. The model has significant implications for low-pressure nanofiltration, as well as hollow fiber NF module design and operation. An approach based on comparing the marginal increase in system recovery to the marginal increase in transmembrane pressure (TMP) was used to define an optimal operating point. Our findings reveal that increasing the TMP could potentially increase energy savings under some operating conditions.
by Omar Labban.
S.M.
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