Thermodynamic Analysis of a Reverse Osmosis Desalination System Using Forward Osmosis for Energy Recovery

Thermodynamic analysis is applied to assess the energy efficiency of hybrid desalination cycles that are driven by simultaneous mixed inputs, including heat, electrical work, and chemical energy. A seawater desalination cycle using work and a chemical input stream is analyzed using seawater properties. Two system models, a reversible separator and an irreversible component based model, are developed to find the least work required to operate the system with and without osmotic recovery. The component based model represents a proposed desalination system which uses a reverse osmosis membrane for solute separation, a pressure exchanger for recovering a fraction of the flow work associated with the pressurized discharge brine, and a forward osmosis (FO) module for recovering some of the chemical energy contained within the concentrated discharge brine. The energy attained by the addition of the chemical input stream serves to lower the amount of electrical work required for operation. For this analysis, a wastewater stream of varying solute concentration, ranging from feed to brackish water salinity, is considered as the chemical stream. Unlike other models available in the literature, the FO exchanger is numerically simulated as a mass exchanger of given size which accounts for changing stream concentration, and consequently, stream-wise variations of osmotic pressure throughout the length of the unit. A parametric study is performed on the models by varying input conditions. For the reversible case it is found that significant work reductions can be made through the use of an energy recovery device when the inlet wastewater salinity used is less than the feed salinity of 35 g/kg. For the irreversible case with a typical recovery ratio and feed salinity, significant work reductions were only noted for a wastewater inlet of less than half of the feed salinity due to pump work losses. In the irreversible case, the use of a numerical model to simulate the FO exchanger resulted in a maximum work reduction when the pressure difference between streams was around one half of the osmotic pressure difference as opposed to the precise value of one half found in zero-dimensional exchanger models.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08

Osmotic mass exchangers for power generation and energy recovery : analysis and analogy to heat exchangers

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 151-156).Desalination is an important separation process which can provide water scarce regions with clean water for drinking or for agricultural use. Thermal distillation has historically been the dominant method for obtaining pure water, but today, reverse osmosis (RO) produces a greater percentage of the total desalinated water worldwide by a large margin. Fundamentally, an RO system is a membrane-based osmotic mass exchanger. Another type of membrane-based osmotic process, a subset of forward osmosis (FO) called pressure retarded osmosis (PRO), currently exhibits promise for making desalination more energy efficient and is receiving attention in the literature. PRO exchangers are capable of producing power from two streams of different salinity and recovering energy from the brine stream of any desalination process when paired with water pumps and turbines. RO and PRO exchangers are essentially mass exchangers with a hydraulic or osmotic pressure difference across a membrane acting as the predominant driving potential. Using a simple resistance model for mass transfer applied across an ideal RO and PRO membrane, closed form expressions are developed which relate the performance of a one-dimensional membrane as a function of membrane properties, membrane area, inlet salinities, operating conditions, and flow configuration. These closed form expressions are analogous to the effectiveness versus number of transfer unit (c-NTU) models which have been used for decades in the rating and sizing of heat exchangers. The closed form expressions, along with numerical simulations for validating the models, are used to determine the limits of permeate flux in one-dimensional RO, PRO, and FO membranes; analyze the power performance of a one-dimensional PRO membrane; and determine the viability of using a PRO-based energy recovery device to reduce the net power consumption for RO desalination. The closed-form solutions for determining the performance of the RO and PRO membranes require that osmotic pressure be defined as a linear function of salinity. It is found that for a seawater RO process with a typical recovery ratio of 50% or less, the maximum error associated with linearization is less than 6.1%. For brackish water desalination, where processes typically operate at very high recovery ratios but have brine salinities lower than those encountered in seawater desalination, the error does not exceed 1.8%. For PRO membranes, using varying linearization curves, maximum errors for flux performance of less than 5.5% are incurred by the linear approximation. It is also found that the maximum Second Law efficiency of the power achievable from a one-dimensional PRO membrane is 66.48%. For large membrane areas, the maximum power for a PRO membrane occurs at a hydraulic pressure difference that is not equal to exactly one-half the osmotic pressure difference as reported in literature for zero-dimensional PRO membranes. For PRO membranes used for brine chemical energy recovery from an RO plant treating a feed stream of 35 g/kg, it is found that a wastewater salinity of less than 20 g/kg is required to recover power. Because the membranes within this study have been assumed as ideal, the performance results for flux, power, and power recovery can serve as informative upper bounds.by Leonardo David Banchik.S.M

Effectiveness-mass transfer units (ε-MTU) model of a reverse osmosis membrane mass exchanger

A strong analogy exists between heat exchangers and osmotic mass exchangers. The effectiveness-number of transfer units (ε-NTU) method is well-known for the sizing and rating of heat exchangers. A similar method, called the effectiveness-mass transfer units (ε-MTU) method, is developed for reverse osmosis (RO) mass exchangers. Governing equations for an RO mass exchanger are nondimensionalized assuming ideal membrane characteristics and a linearized form of the osmotic pressure function for seawater. A closed form solution is found which relates three dimensionless groups: the number of mass transfer units, which is an effective size of the exchanger; a pressure ratio, which relates osmotic and hydraulic pressures; and the recovery ratio, which is the ratio of permeate to inlet feed flow rates. A novel performance parameter, the effectiveness of an RO exchanger, is defined as a ratio of the recovery ratio to the maximum recovery ratio. A one-dimensional numerical model is developed to correct for the effects of feed-side external concentration polarization and nonlinearities in osmotic pressure as a function of salinity. A comparison of model results to experimental data found in the literature resulted in an average error of less than 7.8%. The analytical ε-MTU model can be used for design or performance evaluation of RO membrane mass exchangers.Center for Clean Water and Clean Energy at MIT and KFUPMNational Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374

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

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

Advances in membrane-based oil/water separation

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 117-124).Oil is a widespread pollutant from oil spills to industrial oily wastewater in the oil and gas, metalworking, textile and paper, food processing, cosmetics, and pharmaceutical industries. A wastewater of particular concern is produced water, an oily waste stream from hydrocarbon extraction activities. Worldwide, over 2.4 billion US gallons of produced water is generated every day. Membrane technologies have emerged as the preferred method for treating these wastewaters; this has allowed operators to reclaim and reuse fresh water for potable, industrial, and agricultural use and to meet waste discharge regulations. Yet, despite their technological predominance, membranes can become severely fouled and irreversibly damaged when bulk and small stabilized oil droplets, emulsions, are present in intake streams. In this thesis, we seek to mitigate these deleterious effects through several means. First we seek to better understand fouling by oil-in-water emulsions on conventional polymeric ultrafiltration membranes. We investigate the decrease in water production over time using model and actual produced water samples with varying solution zeta potentials and make meaningful recommendations to operators based on our observations. Next, we develop a robust multifunctional membrane which can in one step degrade organic pollutants and separate bulk and surfactant-stabilized oil/water mixtures while achieving high fluxes, high oil rejection, and high degradation efficiencies. Finally, we investigate the potential of novel in-air hydrophilic/oleophobic microfiltration and reverse osmosis membranes for their anti-oil fouling performance relative to conventional hydrophilic/oleophilic membranes. Contrary to claims in literature of superior performance, we find that in-air oleophobicity does not aid in underwater anti-fouling due to surface reconstruction of mobile perfluoroalkyl chains in the presence of water. Based on these observations, we discuss opportunities for future research on oil anti-fouling membranes using fluorinated moieties.by Leonardo David Banchik.Ph. D

Effectiveness–mass transfer units (ε–MTU) model of an ideal pressure retarded osmosis membrane mass exchanger

Many membrane-based systems, such as reverse osmosis (RO), forward osmosis (FO) and pressure retarded osmosis (PRO), are being used in desalination, water treatment, and energy production. These systems work on the basis of mass transfer through a semi-permeable membrane which allows for the permeation of water while rejecting salts and other substances. The membrane-based devices are essentially mass exchangers which are analogous to heat exchangers. The driving potentials in these mass exchangers are the concentration and pressure differences, whereas in heat exchangers the driving potential is the temperature difference. Closed form solutions of the permeation rate through an ideal PRO mass exchanger are obtained for parallel and counter flow configurations. The recovery ratio (RR) is obtained as a function of dimensionless parameters such as the number of mass transfer units (MTU), mass flow rate ratio (MR), and osmotic pressure ratio (SR). The resulting mathematical expressions form an effectiveness–MTU model for osmotic mass exchangers. These expressions are analogous to those for heat exchangers and can be used as an initial design for PRO membrane based mass exchange devices.Center for Clean Water and Clean Energy at MIT and KFUP

Low Carbon Desalination: Status and Research, Development, and Demonstration Needs, Report of a workshop conducted at the Massachusetts Institute of Technology in association with the Global Clean Water Desalination Alliance

Water demand is increasing worldwide as a result of growing populations and rising standards of living. Further, increasing climate variability is disrupting historical patterns of precipitation and water storage. While conservation and reuse efforts have helped to moderate demand for new freshwater resources in some locations, desalination technology is increasingly being used to meet demand worldwide. Currently installed capacity is almost 90 million m3/day (90 billion liters per day) of desalinated water, a value that has been growing rapidly, with growth projected at 12% over the next five years. Energy consumption is the major cost of desalination, accounting for more than 1/3 of the cost of water in modern plants, and energy use also represents the major environmental impact of desalination. Thus, desalination using low-cost energy sources that have low greenhouse gas emission is highly desirable. Participants in the workshop contributed prewritten material on research and development needs that they regarded as critical to the reduction of the global warming potential (GWP) of desalination. These inputs form the bulk of this report. The workshop itself was devoted to a vigorous and wide-ranging discussion of the opportunities and priorities for powering desalination systems with low-carbon energy in the context of current and emerging trends in desalination and energy production. The report summarizes the experts’ assessment of available technologies and their recommendations for research, development, and demonstration (RD&D) of low carbon desalination. A major conclusion of this workshop is that currently available energy and desalination technologies can be effectively combined to reduce desalination’s GWP in the near term. Keywords: Desalination, renewable energy, solar energy, wind power, climate change mitigation, nuclear energy, battery, electric grid, membranes, fouling, CO2, cost analysis, case studiesMassachusetts Institute of Technology. Abdul Latif Jameel World Water and Food Security LabMassachusetts Institute of TechnologyGlobal Clean Water Desalination Alliance (GCWDA

Limits of power production due to finite membrane area in pressure retarded osmosis

Dimensionless analytical expressions for the power attainable from an ideal counterflow pressure retarded osmosis (PRO) system model are developed using a one-dimensional model that accounts for streamwise variations in concentration. This ideal PRO system has no salt permeation or concentration polarization. The expressions show that the optimal hydraulic pressure difference, for which the maximum power is produced, deviates significantly from the classical solution of one-half of the trans-membrane osmotic pressure difference, Δπ/2, as the dimensionless membrane area (MTUπ) increases and the ratio of draw to feed mass flow rates (MR) varies. The overall maximum power attainable from a PRO membrane is found to occur in the limit of infinitely large MTUπ (an effectiveness of unity) and infinite MR. For an ideal PRO system which mixes seawater (35 g/kg) and river water (1.5 g/kg), the overall maximum power of 1.57 kJ/kg of feed can be attained at roughly MTUπ of 15, an MR of 10, and a pressure of 0.83Δπ. Due to economic considerations, a PRO system in practice will have limited membrane area and will operate at an effectiveness of less than unity. The present work can be used to estimate the operating conditions and area required for a PRO system of given performance. The effect of concentration polarization on optimal hydraulic pressure difference and maximum power performance is also investigated using a numerical modelNational Science Foundation (U.S.) (Graduate Research Fellowship Program, Grant no. 1122374)King Fahd University of Petroleum and Mineral

Economic framework for net power density and levelized cost of electricity in pressure-retarded osmosis

Economic analysis is necessary to ascertain the practical viability of a pressure-retarded osmosis (PRO) system for power production, but high complexity and the lack of large scale data has limited such work. In this study, a simple yet powerful economic framework is developed to relate the lower bound of levelized cost of electricity (LCOE) to net power density. A set of simplifying assumptions are used to develop an inverse linear relationship between net power density and LCOE. While net power density can be inferred based on experimentally measured power density, LCOE can be used to judge the economic viability of the PRO system. The minimum required net power density for PRO system to achieve an LCOE of $0.074/kWh (the capacity-weighted average LCOE of solar PV in the U.S.) is found to be 56.4 W/m2. Using this framework, we revisit the commonly cited power density of 5 W/m[superscript 2] to conclude that it is not economically viable because net power density would be even lower. Finally, we demonstrate that fundamental difference exists between power density and net power density, and as a result we recommend using net power density as a performance metric for PRO system. Keywords: Pressure-retarded osmosis; Economic analysis; Levelized cost of electricity; net power densityKuwait Foundation for the Advancement of Sciences (P31475EC01)Massachusetts Institute of Technology. Tata Center for Technology and Desig