Improved Batch Reverse Osmosis Configuration for Better Energy Effiency

Recent progress in batch and semi-batch reverse osmosis processes such as CCRO have shown the promise to be the most efficient desalination systems. Despite their progress, it is critical to further increase their efficiencies, and reduce the downtime between cycles that worsens their cost performance. In this study, we model in new detail a further improved batch desalination system that uses a high pressure feed tank with a reciprocating piston. A high-pressure pump fills the inactive side with the following cycle’s feedwater, providing two main benefits. First, no tank emptying step is needed because feed is already present, thus reducing downtime. Second, the tank fully empties each cycle, thus avoiding the small energy losses from brine mixing with the new feed that past best designs had. The modeling methodology is the most thorough yet for batch processes, as it uses a discretized module that includes transient mass transport equations for salt boundary layers, membrane permeability effects, and minute salt permeation through the membrane. Comparing the new configuration to standard reverse osmosis with and without energy recovery, the new process vastly outperforms, with the potential to be below 2 kWh/m3 for seawater. The new process has less downtime too, around 2% of cycle time, compared with 10% for CCRO or 16% from past batch studies

Investigating Membrane Material Alternatives for Air Revitalization in Space

Recently, NASA’s ultimate goal has been to launch a crewed Mars mission. However, the current system used for carbon dioxide (CO2) removal in air revitalization in the International Space Station (ISS) is not equipped to handle beyond low-earth-orbit missions. The Carbon Dioxide Removal Assembly (CDRA) is a complex system that relies heavily on sorbent materials and faces challenges in reliability, energy efficiency, and material degradation. Although the CDRA has operated well in the ISS for the past two decades, health effects from high CO2 levels are amongst the most common complaints from and challenges for astronauts. Recent developments in membrane technology prove to be a promising alternative to sorbent-based systems for CO2 removal. Maintaining high selectivity for CO2 with a reasonable permeability, at such low partial pressures and in the presence of water, is among the main challenges of using membranes in this application. In this work, we have created a membrane-based model with appropriate conditions to identify the membrane technology for this application. We expect to determine a working range of critical parameters such as permeability, selectivity, and membrane area for successful CO2 separation. We will also be comparing the thermodynamic efficiency of a membrane-based process to that of the CDRA to pin-point areas of improvement

Long-Running Comparison of Feed-Water Scaling in Membrane Distillation

Membrane distillation (MD) has shown promise for concentrating a wide variety of brines, but the knowledge is limited on how different brines impact salt scaling, flux decline, and subsequent wetting. Furthermore, past studies have lacked critical details and analysis to enable a physical understanding, including the length of experiments, the inclusion of salt kinetics, impact of antiscalants, and variability between feed-water types. To address this gap, we examined the system performance, water recovery, scale formation, and saturation index of a lab-scale vacuum membrane distillation (VMD) in long-running test runs approaching 200 h. The tests provided a comparison of a variety of relevant feed solutions, including a synthetic seawater reverse osmosis brine with a salinity of 8.0 g/L, tap water, and NaCl, and included an antiscalant. Saturation modeling indicated that calcite and aragonite were the main foulants contributing to permeate flux reduction. The longer operation times than typical studies revealed several insights. First, scaling could reduce permeate flux dramatically, seen here as 49% for the synthetic brine, when reaching a high recovery ratio of 91%. Second, salt crystallization on the membrane surface could have a long-delayed but subsequently significant impact, as the permeate flux experienced a precipitous decline only after 72 h of continuous operation. Several scaling-resistant impacts were observed as well. Although use of an antiscalant did not reduce the decrease in flux, it extended membrane operational time before surface foulants caused membrane wetting. Additionally, numerous calcium, magnesium, and carbonate salts, as well as silica, reached very high saturation indices (\u3e1). Despite this, scaling without wetting was often observed, and scaling was consistently reversible and easily washed. Under heavy scaling conditions, many areas lacked deposits, which enabled continued operation; existing MD performance models lack this effect by assuming uniform layers. This work implies that longer times are needed for MD fouling experiments, and provides further scaling-resistant evidence for MD

Wetting prevention in membrane distillation through superhydrophobicity and recharging an air layer on the membrane surface

Although membrane distillation offers distinctive benefits in some certain areas, i.e., RO concentrate treatment, concentrating solutions in the food industry and solar heat utilization, the occurrence of wetting of the hydrophobic membrane hinders its potential industrial applications. Therefore, wetting prevention is a vital criterion particularly for the treatment of solutions with lower surface tension than water. The present work examines the effect of recharging air bubbles on the membrane surface for the wetting incidence when a surfactant (sodium dodecyl sulfate, SDS) exists in a highly concentrated NaCl aqueous solution. This study shows that the presence of the air bubbles on the surface of the superhydrophobic membrane in a direct contact membrane distillation setup inhibited the occurrence of wetting (similar to 100% salt rejection) even for high concentrations of the surface-active species (up to 0.8 mM SDS) in the feed solution while no undesirable influence on the permeate flux was observed. Introducing air into the feed side of the membrane displaces the liquid which partly tends to penetrate the macro porous structure with air bubbles and therefore increases the liquid entry pressure, and in addition, the simultaneous use of a superhydrophobic membrane enhances the solution contact angle

Temporally Multi-staged Batch Counterflow Reverse Osmosis for High Recovery Desalination

Osmotically assisted reverse osmosis (OARO) or counterflow reverse osmosis (CFRO) are recent RO configurations that uses saline streams on both sides of the membrane in counterflow. This reduces the osmotic pressure difference that needs to be overcome for permeation and allows water recovery from high salinity feeds at regular RO pressure. Batch RO is a new, transient RO configuration that closely follows the osmotic pressure profile of the feed and is marked by high energy efficiency. In this work we extend a transient version of CFRO, Batch CFRO for high recovery (~74%) desalination of seawater using a temporally multi-staged version of the process for the first time. In doing so, we introduce the first configuration to achieve Batch CFRO using entirely available components, including a pressure exchanger rather than high pressure tanks. Using a reduced order model, the terminal salinity of the brine leaving the system is calculated to be 183 g/kg. The key feature of this new configuration is that it is multi-staged in time rather than space. As such it can use the same hollow fiber membrane module for the different stages and hence reduce the component (pumps and pressure exchangers) count of the process. The brine produced in each stage is stored in inexpensive atmospheric pressure tanks. This is in contrast with other multi-stage processes where the number of flow devices usually scale with the number of stages needed for higher recovery and usually leads to high cost. Notably, the choice of membrane type can make a significant difference, as common narrow hollow fibers can experience large pressure drops that become significant. This leads to the conclusion that module design must be key to achieve the top-energy numbers of other batch CFRO configurations by the team, such as spiral wound membranes, turbulence-inducing spacers, or using feed on the shell side of the fibers

A Thermodynamics Analysis for Improvement of Carbon Dioxide Removal Technologies for Space

The carbon dioxide removal assembly (CDRA) has been used for the past two decades to continually remove carbon dioxide (CO2) as part of the air revitalization system onboard the international space station (ISS). The CDRA is an adsorption-based system that relies on sorbent materials that require a significant energy input to be thermally regenerated. Additionally, the system faces challenges in reliability and size/weight, so it is being re-evaluated for viability beyond-lowearth-orbit missions. The CDRA removes CO2 from the cabin air through a cyclical adsorption-desorption process that uses four molecular sieve beds. The main components include two desiccant beds to remove H2O, two CO2 zeolite sorbent beds, an air blower, two resistive heaters, and a cooling heat exchanger. Past studies on the CDRA primarily focus on predictive physics-based modeling of the sorbent beds to understand reliability, performance, and sorbent kinetics, with very few performing a thermodynamic analysis of the entire system. This study aims to improve the understanding of component-level losses of the CDRA using exergy destruction analysis and to quantify the losses. We developed a thermodynamics black-box model using a first and second law balances over each individual component over one operational cycle. The results indicate that the molecular sieve sorbent beds are major contributors to lost work within the CDRA. However, the total exergy destruction in the desiccant beds is greater than the sorbent beds. This indicates that the desiccant beds are the largest contributor of losses. Removing water prior to the removal of CO2 from the flow stream is a necessary step because the zeolite sorbent will preferentially adsorb water. Our findings motivate the use of alternative components that may offer direct separation of water at higher efficiencies

Managing Humidity in Electronics Using Water Vapor-Selective Membranes

Abstract: As outdoor electronics have become prevalent in every aspect of daily life, handling humidity in them is an emerging issue for reliable devices. As water vapor enters the electronics enclosure, there is a risk of condensation, which could prevent the electronics from functioning and further damage the device. Traditionally, the humidity removal for electronics is usually done by heating the air within the enclosure, which can be energy-intensive and less efficient. Vapor selective membrane systems are promising alternatives for air heating dehumidification as they do not require heating energy for water vapor removal. It allows water vapor transport through the membrane while blocking air. The objective of this research in the spring is to design and assemble electronics enclosures with a vapor selective membrane and sensors to monitor the humidity removal progress. Two designs including a vacuum pump and joule pump will be tested. The expected results are testable prototypes and preliminary test data that prove it is possible to accurately monitor the humidity level both inside and outside the enclosure for future testing