Heat Flux and Effectiveness in Bubble Colum Dehumidifiers for HDH Desalination

Humidification-dehumidification is a promising technology for decentralized, small-scale desalination, but high energy consumption and large dehumidifier size are disadvantages. Direct-contact dehumidification in bubble columns has previously been shown to decrease dehumidifier volume by an order of magnitude. In a bubble column dehumidifier, warm, moist air is bubbled though a column of fresh water cooled by heat exchange with seawater feed. The concentration gradient from the warm bubble center to the cool bubble surface drives radial mass diffusion, and water vapor condenses on the surface of the bubble. In this paper, a parallel-flow effectiveness is defined to complement heat flux when assessing the performance of a single-stage bubble column dehumidifier. A bubble column dehumidifier is tested using significantly smaller cooling coils than those tested in previous work. Experimental results are presented in terms of heat flux and effectiveness in order to better understand the factors influencing bubble column dehumidifier performance. It is found that the heat flux can be raised dramatically by reducing the coil area, but that this gain is accompanied by a loss of effectiveness. Increasing air temperature leads to increased heat flux but decreased effectiveness. Because the gas-side pressure drop increases with increasing column liquid height, significantly lower column liquid heights are tested than those used in previous work. The critical liquid height is found to be below 4 cm for the sparger and flow rate tested. Additional heat transfer in the air gap is explored, but found to be minimal for well-designed columns with low temperature pinch. These findings will inform the design of bubble column dehumidifiers for high heat recovery and low capital cost.Center for Clean Water and Clean Energy at MIT and KFUPMMassachusetts Institute of Technology (Pappalardo Fellowship)Massachusetts Institute of Technology (Fort and Beth Flowers Family Fellowship)National Science Foundation (U.S.) (Graduate Research Fellowship Program under Grant No. 1122374

Measurements of Heat Transfer Coefficients to Cylinders in Shallow Bubble Columns

High heat transfer coefficients and large interfacial areas make bubble columns ideal for dehumidification. However, the effect of geometry on the heat transfer coefficients outside cooling coils in shallow bubble columns, such as those used in multi-stage bubble column dehumidifiers, is poorly understood. The generally-overlooked entry and coalescing regions become important in shallow bubble columns, and there is disagreement on the effects of the coil and column diameters. In this paper, a method is presented for measuring the heat transfer coefficient between coil and liquid in a shallow bubble column. Horizontal cylindrical probes are used to measure the heat transfer coefficient over a range of gas velocities. The liquid depth and the diameter, height, and horizontal position of the cylinder are also varied. Existing correlations for tall columns tend to underpredict the heat transfer coefficient and do not account for all effects of geometry. The highest heat transfer coefficients (above 8000 W/m[superscript 2]−K) are recorded on cylinders placed 4 cm high. No significant effect of cylinder diameter is observed. Based on the results, recommendations are made regarding bubble column dehumidifier design.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)Flowers Family FellowshipMIT Department of Physics Pappalardo Program (Fellowship)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

Experiments and modeling of bubble column dehumidifier performance

Humidification–dehumidification is a promising technology for decentralized, small-scale desalination, but conventional dehumidifiers are expensive due to the large surface area required. Direct-contact dehumidification in bubble columns has been shown to significantly decrease dehumidifier size and cost. In this paper, the heat flux and parallel-flow effectiveness of a bubble column dehumidifier are investigated experimentally using significantly smaller cooling coils than in previous work. In addition, a model is developed which predicts the heat transfer rate with an average error of less than 3%. It is found that heat flux rises and effectiveness decreases with decreasing coil area. Increasing air flow rate and air temperature both lead to increased heat flux but decreased effectiveness. Neither bubble-on-coil impact nor column height are found to significantly affect heat flux or effectiveness. The conflicting findings of previous research on bubble-on-coil impact are explained by the other trends identified in this work. Modeling results for salt water temperature and tube diameter are presented. Additional heat transfer in the air gap above the column liquid is explored, but found to be minimal for well-designed columns with low temperature pinch. These findings will inform the design of bubble column dehumidifiers for high heat recovery and low capital cost.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)Flowers Family FellowshipMIT Department of Physics Pappalardo Program (Fellowship)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

Analytical Modeling of a Bubble Column Dehumidifier

Bubble column dehumidifiers are a compact, inexpensive alternative to conventional fin-tube dehumidifiers for humidification-dehumidification (HDH) desalination, a technology that has promising applications in small-scale desalination and industrial water remediation. In this paper, algebraic equations for relevant mean heat and mass transfer driving forces are developed for improved modeling of bubble column dehumidifiers. Because mixing in the column ensures a uniform liquid temperature, the bubble column can be modeled as two single stream heat exchangers in contact with the column liquid: the seawater side, for which a log-mean temperature difference is appropriate, and the gas side, which has a varying heat capacity and mass exchange. Under typical conditions, a log-mean mass fraction difference is shown to drive latent heat transfer, and an expression for the mean temperature difference of the moist gas stream is presented. These expressions will facilitate modeling of bubble column heat and mass exchangers.National Science Foundation (U.S.)Flowers Family FellowshipMIT Department of Physics Pappalardo Program (Fellowship)Center for Clean Water and Clean Energy at MIT and KFUP

Heat transfer to a horizontal cylinder in a shallow bubble column

Heat transfer coefficient correlations for tall bubble columns are unable to predict heat transfer in shallow bubble columns, which have unique geometry and fluid dynamics. In this work, the heat transfer coefficient is measured on the surface of a horizontal cylinder immersed in a shallow air–water bubble column. Superficial velocity, liquid depth, and cylinder height and horizontal position with respect to the sparger orifices are varied. The heat transfer coefficient is found to increase with height until reaching a critical height, and a dimensionless, semi-theoretical correlation is developed that incorporates superficial velocity, liquid properties, and height. Additionally, the more minor effects of flow regime, column region, and bubble impact are discussed. Notably, the heat transfer coefficient can be as high in the region of bubble coalescence as in the bulk of the column, but only if bubbles impact the cylinder. The correlation and discussion provide a framework for modeling and designing shallow, coil-cooled bubble columns.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)Flowers Family FellowshipNational Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

Raising forward osmosis brine concentration efficiency through flow rate optimization

An exergetic efficiency is defined in order to compare brine concentration processes including forward osmosis (FO) across a wide range of salinities. We find that existing FO pilot plants have lower efficiency than reverse osmosis plants in the brackish and seawater salinity ranges. High salinity FO, in its current form, is still less efficient than mechanical vapor compression. We show that efficiency is the product of FO exchanger and draw regenerator efficiencies, and therefore FO system energy efficiency benefits from improvements to both. The mass flow rate ratio (between draw and feed flow rates) is identified as a crucial parameter in the design of efficient FO systems because of its effect on exchanger efficiency. We demonstrate a method of thermodynamically balancing an FO system by choosing flow rates that lead to equal osmotic pressure differences at both ends of the exchanger, and show the method's potential to increase the efficiency of current systems by 3–21%.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)Hugh Hampton Young Memorial Fellowshi

In situ visualization of organic fouling and cleaning mechanisms in reverse osmosis and forward osmosis

Fouling models rely on knowledge of foulant accumulation and removal mechanisms. In this study, a fouling visualization apparatus is developed to elucidate centimeter-scale mechanisms of organic fouling and cleaning in reverse osmosis (RO) and forward osmosis (FO). Alginate is used as a model organic foulant and dyed with methylene blue, which is shown not to affect fouling or cleaning, and to sufficiently highlight the gel for visualization at low salinity (up to 1% NaCl). When cleaning by increasing the cross-flow velocity, with or without reverse permeation, foulant peels off the membrane in discrete pieces in both RO and FO. Videos of cleaning show that foulant cake swelling and wrinkling can facilitate gel detachment and removal. Despite their effectiveness in slowing fouling, spacers can hinder removal of detached foulant pieces by obstructing their path. Finally, photographs point to a new mechanism of internal fouling in FO: vapor formation due to sub-atmospheric pressure within the membrane. Awareness of these mechanisms allows for better modeling of fouling and motivates optimization of swelling-inducing cleaning procedures.Center for Clean Water and Clean Energy at MIT and KFUPM M (Project #R4-CW- 11)Martin Family Society of Fellows for Sustainability (Martin Fellowship for Sustainability

Theoretical framework for predicting inorganic fouling in membrane distillation and experimental validation with calcium sulfate

A methodology for predicting scaling in membrane distillation (MD), which considers thermodynamics, kinetics, and fluid mechanics, is developed and experimentally validated with calcium sulfate. The theory predicts the incidence of scaling as a function of temperature, concentration, and flow conditions by comparing the nucleation induction time to the residence time and applying an experimental correction factor. The relevant residence time is identified by considering a volume of solution near the membrane surface that contains enough ions to form a nucleus of critical size. The theory is validated with fouling experiments using calcium sulfate as a model scalant over a range of temperatures (40–70 °C), saturation indices, and flow rates. Although the model is validated with a bench-scale MD system, it is hoped to be compatible with large-scale systems that may have significant changes in concentration, temperature, and flow rate along the flow direction. At lower temperatures, the saturation index can be as high as 0.4–0.5 without scaling, but the safe concentration limit decreases with increasing temperature. Increasing the feed flow rate reduces concentration polarization and fluid residence time, both of which decrease the likelihood of fouling. The model is translated into easily readable maps outlining safe operating regimes for MD. The theory and maps can be used to choose safe operating conditions in MD over a wide range of conditions and system geometries.National Science Foundation (U.S.) (1122374

EFFECT OF PRACTICAL LOSSES ON OPTIMAL DESIGN OF BATCH RO SYSTEMS

Batch reverse osmosis (BRO) systems may enable a significant reduction in energy consumption for desalination and water reuse. BRO systems operate with variable pressure, by applying only slightly more pressure than is needed to overcome the osmotic pressure and produce reverse water flux. This study explains, quantifies, and optimizes the energy-saving performance of realistic batch designs implemented using pressure exchangers and unpressurized tanks. The effects of additional design parameters such as feed tank volume at the end of the cycle, volume of water in the pipes, per-pass recovery, cycle operating time, and cycle reset time on the performance of BRO are captured. Loss mechanisms including hydraulic pressure drop and concentration polarization as well as friction and mixing in the energy recovery devices are considered. At low cycle-reset time (10% of productive time) and low piping volumes (12% of volume inside membrane elements), about 13% energy savings is possible compared to a continuous system operating at the same overall pure water productivity. Under these conditions, we also show that the ideal per-pass recovery is close to 50%, similar to single-stage RO. This recovery reduces the need for system redesign with additional pressure vessels in parallel, contrary to predictions in the literature. The projected savings in terms of the overall cost of water is around 3%. Additionally, advanced ultra-permeable membranes, such as those based on graphene or graphene oxide, are expected to lead to more significant energy savings in BRO than in single-stage RO

Energy consumption in desalinating produced water from shale oil and gas extraction

On-site treatment and reuse is an increasingly preferred option for produced water management in unconventional oil and gas extraction. This paper analyzes and compares the energetics of several desalination technologies at the high salinities and diverse compositions commonly encountered in produced water from shale formations to guide technology selection and to inform further system development. Produced water properties are modeled using Pitzer's equations, and emphasis is placed on how these properties drive differences in system thermodynamics at salinities significantly above the oceanic range. Models of mechanical vapor compression, multi-effect distillation, forward osmosis, humidification–dehumidification, membrane distillation, and a hypothetical high pressure reverse osmosis system show that for a fixed brine salinity, evaporative system energetics tend to be less sensitive to changes in feed salinity. Consequently, second law efficiencies of evaporative systems tend to be higher when treating typical produced waters to near-saturation than when treating seawater. In addition, if realized for high-salinity produced waters, reverse osmosis has the potential to achieve very high efficiencies. The results suggest a different energetic paradigm in comparing membrane and evaporative systems for high salinity wastewater treatment than has been commonly accepted for lower salinity water.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)Center for Clean Water and Clean Energy at MIT and KFUPM (Project R13-CW-10)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement 02/MI/MI/CP/11/07633/GEN/G/00