BATCH REVERSE OSMOSIS: EXPERIMENTAL RESULTS, MODEL VALIDATION, AND DESIGN IMPLICATIONS

In theory, batch reverse osmosis (RO) systems can achieve the lowest practical energy consumption by varying feed pressure over time. However, few batch RO syste ms have been built and operated. We have tested a bench-scale prototype of a true batch RO system using a bladder and a 2.5” (6.35 cm) spiral wound membrane element. Some practical issues in implementing batch RO include system start-up time, system depressurization, osmotic backwash during the reset phases, and lower permeate quality. This study is the first to validate batch models by measuring the hydraulic work of both the high pressure pump and the circulation pump. The experimental measurements agree well with the model (error ≤ 3 %) after accounting for concentration polarization. We used the validated model to calculate the energy savings of true batch systems at higher salinities and recovery ratios. We find that the energy savings achievable by true batch systems are less than previously thought, but still significant at relatively high recoveries. At 50% recovery of seawater feed, a batch RO plant could save 15% of the energy consumed by a continuous RO plant while still maintaining the same effective flux. Further studies should identify the additional costs associated with batch RO in order to identify the operating conditions where batch RO will be an economically favorable option compared to conventional continuous RO

Can Batch Reverse Osmosis Make Desalination More Affordable and Sustainable?

Reverse osmosis (RO) desalination can help to ensure secure water resources, but the process remains costly. From 2007-2017, global desalination capacity nearly doubled, from 47 to 92 million m3 /day, with RO accounting for two thirds of installed capacity. Despite this growth, the total volume of treated water accounts for less than half a percent of global freshwater consumption. To be part of a sustainable water supply, RO must be made cheaper. RO energy consumption can never fall below the thermodynamic least work of separation, which is 1 kWh/m3 for 50% recovery of seawater. Practically speaking, RO energy consumption will not reach the thermodynamic limit but may be further reduced through improvements in system design. Batch RO is the most energy-efficient RO process. It saves energy because the feed pressure varies over time with the osmotic pressure. In this thesis we further develop the batch RO technology to identify its benefits and limitations. We demonstrated the first batch RO system using a flexible bladder and validated theoretical models of energy consumption and water production. Next, we investigated practical losses associated with batch operation. This work shows that current batch RO designs are not attractive due to the combined inefficiencies of salt retention and water loss. Incomplete flushing of brine from cycle-to-cycle leads to an elevated feed salinity relative to the feed intake, boosting energy consumption by about ∼10%. De-pressurization during the reset phases of the batch RO cycle leads to water loss via osmosis. This water loss is significant (∼10%) under seawater conditions. We introduce an improved batch RO design which rapidly flushes the system to reduce downtime and water loss. Unfortunately, there does not appear to be a practical way to avoid the salt retention penalty. Batch RO has more economic value in increasing plant productivity, rather than reducing energy consumption. We conclude that batch RO is a promising technology and identify future directions for research and commercialization.Ph.D

Saving energy with an optimized two-stage reverse osmosis system

In a two-stage reverse osmosis (RO) system of finite size, there are two degrees of freedom not present in a single-stage RO system: distribution of RO elements between the two stages (system design), and feed pressures (system operation). In this study, we investigate the optimal system design and operation of a two-stage RO system with a mass-balance model and establish a lower bound for the energy savings achieved by the optimized two-stage system compared to a single-stage system. A two-stage RO system may consume more or less energy than a single-stage RO system of the same size and freshwater productivity, depending on the first-stage feed pressure and second-stage feed pressure. To minimize energy consumption, feed pressures should be chosen to minimize spatial variance in flux. The optimal element configuration places at least half the elements in the first stage; the exact configuration depends on feed salinity, recovery ratio, and membrane permeability. The greatest energy savings are achieved with a two-stage RO system that has both optimal element configuration and feed pressures. More energy can be saved by adding a stage when the thermodynamic least work of separation is larger. For a given feed salinity, energy savings from adding a second stage grow as recovery ratio increases. Brackish water feeds must be taken to high recovery ratios to achieve substantial energy savings; comparable savings can be achieved at lower recovery ratios for higher salinity feeds. We find that significant energy can be saved with the simplest two-stage RO design, at a system flux similar to today's RO plants and accounting for the effects of concentration polarization.King Fahd University of Petroleum and Minerals (project number R15-CW-11

Stochastic time-optimal path-planning in uncertain, strong, and dynamic flows

© 2018 Elsevier B.V. Accounting for uncertainty in optimal path planning is essential for many applications. We present and apply stochastic level-set partial differential equations that govern the stochastic time-optimal reachability fronts and time-optimal paths for vehicles navigating in uncertain, strong, and dynamic flow fields. To solve these equations efficiently, we obtain and employ their dynamically orthogonal reduced-order projections, maintaining accuracy while achieving several orders of magnitude in computational speed-up when compared to classic Monte Carlo methods. We utilize the new equations to complete stochastic reachability and time-optimal path planning in three test cases: (i) a canonical stochastic steady-front with uncertain flow strength, (ii) a stochastic barotropic quasi-geostrophic double-gyre circulation, and (iii) a stochastic flow past a circular island. For all the three test cases, we analyze the results with a focus on studying the effect of flow uncertainty on the reachability fronts and time-optimal paths, and their probabilistic properties. With the first test case, we demonstrate the approach and verify the accuracy of our solutions by comparing them with the Monte Carlo solutions. With the second, we show that different flow field realizations can result in paths with high spatial dissimilarity but with similar arrival times. With the third, we provide an example where time-optimal path variability can be very high and sensitive to uncertainty in eddy shedding direction downstream of the island

Two-stage reverse osmosis: optimal element configuration and energy savings

RO desalination can help to ensure secure water resources now and in the future, but the process remains energy intensive. Improving RO’s energy efficiency is thus an important step towards achieving a sustainable water supply. While innovations in membrane and pump technology are not likely to substantially decrease the energy consumption of the RO process, improved system designs have real potential to bring RO closer to its thermodynamic performance limit. Two-stage systems can substantially lower RO energy consumption. In a fixed size two-stage reverse osmosis (RO) system with eight membrane elements, the elements can be shared between the two stages in seven distinct element configurations. In this work, we investigate the optimal element configuration (system design) of a two-stage RO system. We isolate the energetic benefits of staging by comparing the energy consumption of a two-stage RO system to that of a single-stage RO system with the same system size and freshwater productivity. The optimal element configuration will place at least half of the elements in the first stage; the exact configuration depends on feed salinity, recovery ratio, and membrane permeability. Previous studies on the energetic benefits of two-stage RO have not accounted for both the system size and the effects of concentration polarization. We evaluate systems with an average system flux comparable to today's systems and account for frictional losses and the effects of concentration polarization. This results in a more realistic evaluation of the energetic benefits of two-stage RO. More energy can be saved by adding a stage when the thermodynamic least work of separation is larger. Therefore, energy savings from adding a second stage grow as recovery ratio increases. Significant energy can be saved with high salinity feeds at relatively low recovery ratios. We find that significant energy can be saved with the simplest two-stage RO design, at a system flux similar to today's RO plants and accounting for the effects of concentration polarization. We perform a brief economic analysis to compare the relative capital expenses to the reduction in specific energy consumption (SEC) associated with a two-stage RO plant. We find that two-stage RO is probably not viable for seawater desalination at today’s typical recovery ratios. If recovery ratios can be pushed up to 60%, two-stage RO could become viable with favorable financing terms and high cost of electricity

The Need for Accurate Osmotic Pressure and Mass Transfer Resistances in Modeling Osmotically Driven Membrane Processes

The widely used van ’t Hoff linear relation for predicting the osmotic pressure of NaCl solutions may result in errors in the evaluation of key system parameters, which depend on osmotic pressure, in pressure-retarded osmosis and forward osmosis. In this paper, the linear van ’t Hoff approach is compared to the solutions using OLI Stream Analyzer, which gives the real osmotic pressure values. Various dilutions of NaCl solutions, including the lower solute concentrations typical of river water, are considered. Our results indicate that the disparity in the predicted osmotic pressure of the two considered methods can reach 30%, depending on the solute concentration, while that in the predicted power density can exceed over 50%. New experimental results are obtained for NanoH2O and Porifera membranes, and theoretical equations are also developed. Results show that discrepancies arise when using the van ’t Hoff equation, compared to the OLI method. At higher NaCl concentrations (C > 1.5 M), the deviation between the linear approach and the real values increases gradually, likely indicative of a larger error in van ’t Hoff predictions. The difference in structural parameter values predicted by the two evaluation methods is also significant; it can exceed the typical 50–70% range, depending on the operating conditions. We find that the external mass transfer coefficients should be considered in the evaluation of the structural parameter in order to avoid overestimating its value. Consequently, measured water flux and predicted structural parameter values from our own and literature measurements are recalculated with the OLI software to account for external mass transfer coefficients

True batch reverse osmosis prototype: model validation and energy savings

In this study, we tested a bench-scale prototype of a true batch reverse osmosis (RO) system using a flexible bladder and a 2.5 in. (6.4cm) spiral wound membrane element. In theory, batch RO systems can achieve the lowest practical energy consumption by varying feed pressure over time. However, this is the first study to validate batch models by measuring the hydraulic work of both the high pressure pump and the circulation pump. The experimental measurements agree well with the model (error < 3%) after accounting for concentration polarization. We used the validated model to calculate the energy savings of true batch systems at higher salinities and recovery ratios. Previous studies assumed that a batch RO plant would operate at the same flux and steady-state feed salinity as a comparable continuous RO plant. In order to match the permeate production of a continuous RO plant, a batch RO plant must operate at an elevated flux to offset its intermittent permeate production. A batch RO plant will operate at a steady-state feed salinity higher than the plant’s intake feed salinity due to salt retention between batch cycles. As a result of these practical inefficiencies, the energy savings achievable by true batch systems are less than previously thought, but still significant at relatively high recoveries. At 50% recovery of seawater feed, a batch system could save 11% of the energy consumed by a continuous RO system while still maintaining the same level of permeate production. We have demonstrated the successful operation of a true batch system and shown that it can indeed reduce energy consumption. Keywords: batch reserve osmosis; true batch; energy efficiency; energy savings; system desig

Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation

In theory, the batch reverse osmosis (RO) process achieves the lowest practical energy consumption by varying pressure over time. However, few batch RO systems have been built and operated. We have designed, built, and operated the first “true” batch RO prototype using a flexible bladder. The flexible bladder serves as the high-pressure variable-volume tank that is inherent to true batch RO designs (as opposed to batch RO with energy recovery devices). We experimentally validated a model of batch RO energy consumption (≤2.7% difference) by measuring the hydraulic work of the high pressure and circulation pumps. We find that batch RO energy consumption will be greater than expected mostly due to salt retention, a problem neglected by most previous studies. However, despite operating at elevated salinity and flux conditions, batch RO can still save energy relative to single-stage and multi-stage continuous systems. For a seawater desalination plant (35 g/kg intake, 50% recovery, 15 L m [superscript−2]  h [superscript−1]), our newly-validated model predicts that batch RO would save 11% energy compared to a single-stage continuous RO plant. Our work demonstrates that batch RO is an energy-efficient process with the potential to reduce the cost of water desalination. Keywords: Desalination; Reverse osmosis; Batch reverse osmosis; Salt retention; Energy efficienc