Mechanical vapor compressio--Membrane distillation hybrids for reduced specific energy consumption

The energy efficiency of membrane distillation (MD) systems is low when compared to other thermal desalination systems. This leads to high water production costs when conventional fuels such as natural gas are used. In MD, separation of pure product water from feedwater is driven by differences in vapor pressure between the streams. Thus, the process can occur at low temperature and ambient pressure. As a result, MD is most frequently paired with waste or renewable sources of low temperature heat energy that can be economically more feasible. MD systems with internal heat regeneration have been compared to and modeled similar to counter-flow heat exchangers. In this study, MD is used to replace the preheater heat exchanger used for thermal energy recovery from the brine stream in mechanical vapor compression (MVC). Using MD in place of the heat exchanger results not only in effectively free thermal energy for MD, but also subsidized cost of capital, since the MD module is replacing expensive heat exchanger equipment. The MVC–MD hybrid system can lead to about 6% decrease in cost of water, compared to a stand-alone MVC system. The savings increase with: an increase in MVC operating temperature, a decrease in MVC recovery ratio, and with a decrease in MD capital cost. The conductive gap configuration of MD leads to maximum savings, followed by air gap and permeate gap systems, over a range of operating conditions, assuming equal specific cost of capital for these configurations.Masdar Institute of Science and Technology/MIT/Abu Dhabi, UAE (Cooperative agreement, Reference no.02/MI/MI/ CP/11/07633/GEN/G/00

ULTRAPERMEABLE MEMBRANES FOR BATCH DESALINATION: MAXIMUM DESALINATION ENERGY EFFICIENCY, AND COST ANALYSIS

Reducing the energy consumption of membrane desalination is critical to reducing its cost of water and minimizing desalination’s CO₂ emissions. Hybrids of reverse osmosis (RO) with ul trapermeable membranes promise to address the efficiency, rejection, and fouling issues. In a batch reverse osmosis (BRO) process, salinity is varied over time so that the applied pressure better matches osmotic pressure, increasing efficiency. In this paper, the impact of ultrapermeable membranes in BRO are modelled, and a cost analysis is performed. The results show energetic advantages for the BRO over the best continuous RO configurations. Batch RO systems offer significant cost savings, and saves more energy than the use of ultrapermeable membranes in continuous RO systems. The two combined, BRO and ultrapermeable membranes, has the potential for the most efficient desalination systems yet proposed. However, low membrane cost is needed for ultrapermeable membranes to be viable

On the present and future economic viability of stand-alone pressure-retarded osmosis

Pressure-retarded osmosis is a renewable method of power production from salinity gradients which has generated significant academic and commercial interest but, to date, has not been successfully implemented on a large scale. In this work, we investigate lower bound cost scenarios for power generation with PRO to evaluate its economic viability. We build a comprehensive economic model for PRO with assumptions that minimize the cost of power production, thereby conclusively identifying the operating conditions that are not economically viable. With the current state-of-the art PRO membranes, we estimate the minimum levelized cost of electricity for PRO of US$1.2/kWh for seawater and river water pairing,$0.44/kWh for reverse osmosis brine and wastewater, and $0.066/kWh for nearly saturated water (26$0.074/kWh. We show two methods for reducing this cost via economies of scale and reducing the membrane structural parameter. We find that the latter method reduces the levelized cost of electricity significantly more than increasing the membrane permeability coefficient.National Science Foundation (U.S.) (Graduate Research Fellowship Program, Grant No.1122374) )Kuwait Foundation for the Advancement of Sciences (KFAS) (Project No. P31475EC01

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

Membrane distillation model based on heat exchanger theory and configuration comparison

Improving the energy efficiency of membrane distillation (MD) is essential for its widespread adoption for renewable energy driven desalination systems. Here, an energy efficiency framework for membrane distillation modules is developed based on heat exchanger theory, and with this an accurate but vastly simplified numerical model for MD efficiency and flux is derived. This heat exchanger analogy shows that membrane distillation systems may be characterized using non-dimensional parameters from counter-flow heat exchanger (HX) theory such as effectiveness (εε) and number of transfer units (NTU). Along with the commonly used MD thermal efficiency (ηη), “MD effectiveness” ε should be used to understand the energy efficiency (measured as gained output ratio, GOR) and water vapor flux of single stage membrane distillation systems. GOR increases linearly with ηη (due to decreasing conduction losses), but increases more rapidly with an increase in εε (better heat recovery). Using the proposed theoretical framework, the performance of different single stage MD configurations is compared for seawater desalination. The gap between the membrane and the condensing surface constitutes the major resistance in both air gap (AGMD) and permeate gap (PGMD) systems (75% of the total in AGMD and 50% in PGMD). Reducing the gap resistance by increasing gap conductance (conductive gap MD (CGMD)), leads to an increase in εε through an increase in NTU, and only a small decrease in ηη, resulting in about two times higher overall GOR. GOR of direct contact MD (DCMD) is limited by the size of the external heat exchanger, and can be as high as that of CGMD only if the heat exchanger area is about 7 times larger than the membrane. While MD membrane design should focus on increasing the membrane’s permeability and reducing its conductance to achieve higher ηη, module design for seawater desalination should focus on increasing ε by reducing the major resistance to heat transfer. A simplified model to predict system GOR and water vapor flux of PGMD, CGMD and DCMD, without employing finite difference discretization, is presented. Computationally, the simplified HX model is several orders of magnitude faster than full numerical models and the results from the simplified model are within 11% of the results from more detailed simulations over a wide range of operating conditions.Masdar Institute of Science and Technology/MIT/Abu Dhabi, UAE (Cooperative agreement, Reference no.02/MI/MI/ CP/11/07633/GEN/G/00

Simple method for balancing direct contact membrane distillation

A simple theoretical method for maximizing efficiency via real-time balancing of direct contact membrane distillation (DCMD) systems is presented. The method is applicable under variable operating conditions. Balancing involves measuring only the flow rates of feed stream out of the module and the cold water flow into the module, as well as the salinity of the feed. A valve or variable frequency drive is used to set the condensate water flow into the module so that the heat capacity rates of the hot and cold streams are equal. This method is much simpler and more general than what is proposed in the literature, which generally requires more measurements and a complicated expression. Balancing leads to 20–50% improvement in efficiency (GOR) compared to equal inflow of both feed and pure water streams, which is the common practice. Real-time balancing is particularly useful for variations in feed salinity, whereas the improvement by real-time balancing is low for changes in system top or bottom temperatures.MIT & Masdar Institute Cooperative Program (Reference No. 02/MI/MI/CP/11/07633/GEN/G/00

Multistage vacuum membrane distillation (MSVMD) systems for high salinity applications

Multistage membrane distillation (MD) systems can have significantly higher efficiencies than their single stage counterparts. However, multistage MD system design has received limited attention. In this paper, the performance of a multistage vacuum membrane distillation (MSVMD) which is thermodynamically similar to a multi-stage flash distillation (MSF) is evaluated for desalination, brine concentration, and produced water reclamation applications. A wide range of solution concentrations were accurately modeled by implementing Pitzer's equations for NaCl-solution properties. The viability of MSVMD use for zero liquid discharge (ZLD) applications is investigated, by considering discharge salinities close to NaCl saturation conditions. Energy efficiency (gained output ratio or GOR), second law efficiency, and the specific membrane area were used to quantify the performance of the system. At high salinities, the increased boiling point elevation of the feed stream resulted in lower fluxes, larger heating requirements and lower GOR values. The second law efficiency, however, is higher under these conditions since the least heat for separation increases faster than the system's specific energy consumption with increase in salinity. Under high salinity conditions, the relative significance of irreversible losses is lower. Results indicate that MSVMD systems can be as efficient as a conventional MSF system, while using reasonable membrane areas and for a wide range of feed salinities. Given MD's advantages over MSF such as lower capital requirement and scalability, MSVMD can be an attractive alternative to conventional thermal desalination systems.MIT & Masdar Institute Cooperative Program (Reference No. 02/MI/MI/CP/11/07633/GEN/G/00)Massachusetts Institute of Technology. Department of Mechanical Engineering (Rohsenow fellowship

Comprehensive condensation flow regimes in air gap membrane distillation: Visualization and energy efficiency

The thermal performance of air gap membrane distillation (AGMD) desalination is dominated by heat and mass transfer across the air gap between the membrane and the condensing surface. However, little is known about the impact of condensate flow patterns in some design variations of the air gap. In this study, air gap membrane distillation experiments were performed at various inlet temperatures, varying module inclination angle, condensing surface hydrophobicity, and gap spacer design to identify the effect of each on the permeate production rate and thermal efficiency of the system. Additionally, this study is one of the first with enhanced visualization of flow patterns within the air gap itself, by using a transparent, high thermal conductivity sapphire plate as the condenser surface. System-level numerical modeling is used to further understand the impact of these flow regimes on overall energy efficiency, including flux and GOR. A brief review of membrane distillation condensation regimes is provided as well. For tilting the AGMD flat-plate module, permeate flux was barely influenced except at extreme positive angles ( > 80°), and moderate negative angles ( 1 mm). Meanwhile, the hydrophilic surface for small gap sizes ( < 3 mm) often had pinned regions of water around the hydrophilic surface and plastic spacer. Overall, the various results imply that the common assumption of a laminar condensate film poorly describes the flow patterns in real systems for all tilt angles and most spacer designs. Real system performance is likely to be between that of pure AGMD and permeate gap membrane distillation (PGMD) variants, and modeling shows that enhanced condensing in air gaps may improve system energy efficiency significantly, with strong relative advantages at high salinity. Keywords: Membrane distillation; Hydrophobic surface; Air gap; Condensation; Visualizatio

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

Visualization of droplet condensation in membrane distillation desalination with surface modification: hydrophilicity, hydrophobicity, and wicking spacers

Condensation performance is a key target for improving the energy efficiency of thermal desalination technologies such as air gap membrane distillation (AGMD). This study includes the first visualization of condensation in AGMD, through the use of a high conductivity, transparent sapphire condenser surface. The study examines how flow patterns are affected by several novel modifications, including varied surface hydrophobicity, module tilt angle, and gap spacer design. The experimental results were analyzed with numerical modeling. While the orientation of the mesh spacer, which holds the air gap apart, was found to have no substantial effect on the permeate production rate, the surface's hydrophobicity or hydrophilicity did result in different rates. The hydrophobic surface exhibited fewer droplets bridging the gap, more spherical droplets, and better droplet shedding. For gap sizes less than ~3 mm, the hydrophilic surface frequently had regions of water pinned around the surface itself and the plastic spacer. While the flow patterns observed were more complex than the film condensation typically used to model the process, the simplified numerical modelling yielded good agreement with the data when an adjustment factor was used to account for the gap size