Benny D. Freeman

Reverse osmosis desalination: Water sources, technology, and today's challenges

Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.

Gas sorption, diffusion, and permeation in poly(dimethylsiloxane)

The permeability of poly(dimethylsiloxane) [PDMS] to H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8, and solubility of these penetrants were determined as a function of pressure at 35 °C. Permeability coefficients of perfluorinated penetrants (CF4, C2F6, and C3F8) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH4, C2H6, and C3H8), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients. This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon-based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms. The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS. Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C3F8/PDMS segment pair requires 460 J/mol more energy than the formation of a C3H8/PDMS pair. A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed. These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids. From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration. Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons.

Elucidating the structure of poly(dopamine)

Herein we propose a new structure for poly(dopamine), a synthetic eumelanin that has found broad utility as an antifouling agent. Commercially available 3-hydroxytyramine hydrochloride (dopamine HCl) was polymerized under aerobic, aqueous conditions using tris(hydroxymethyl)aminomethane (TRIS) as a basic polymerization initiator, affording a darkly colored powder product upon isolation. The polymer was analyzed using a variety of solid state spectroscopic and crystallographic techniques. Collectively, the data showed that in contrast to previously proposed models, poly(dopamine) is not a covalent polymer but instead a supramolecular aggregate of monomers (consisting primarily of 5,6-dihydroxyindoline and its dione derivative) that are held together through a combination of charge transfer, π-stacking, and hydrogen bonding interactions.

Gas transport properties of poly(ether-b-amide) segmented block copolymers

The permeation properties of H2, N2, and CO2 were determined at 35 °C and pressures up to 15 atm in phase-separated polyether-b-polyamide segmented block copolymers. These polymers contain poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon-6 [PA6] or nylon-12 [PA12] as the hard polyamide phase. Extremely high values of polar (or quadrupolar)/nonpolar gas selectivities, coupled with high CO2 permeability coefficients, were observed. CO2/H2 selectivities as high as 9.8 and CO2/N2 selectivities as high as 56 were obtained in polymers with CO2 permeability coefficients of approximately 220 × 10−10 cm3(STP) cm/(cm2 s cmHg). As the amount of polyether increases, permeability increases. Gas permeability is higher in polymers with less polar constituents, PTMEO and PA12, than in those containing the more polar PEO and PA6 units. CO2/N2 and CO2/H2 selectivities are higher in polymers with higher concentrations of polar groups. These high selectivity values derive from large solubility selectivities in favor of CO2. Because CO2 is larger than H2 and has, therefore, a lower diffusion coefficient than H2, the weak size-sieving ability of the rubbery polyether phase, which is the locus of most of the gas permeation, also contributes to high CO2/H2 selectivity.

Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies

The design and fabrication of ultrathin polymer layers are of increasing importance because of the rapid development of nanoscience and nanotechnology. Confined, two-dimensional crystallization of polymers presents challenges and opportunities due to the long-chain, covalently bonded nature of the macromolecule. Using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, we discovered a morphology that emerges as confined polyethylene oxide (PEO) layers are made progressively thinner. When the thickness is confined to 20 nanometers, the PEO crystallizes as single, high-aspect-ratio lamellae that resemble single crystals. Unexpectedly, the crystallization habit imparts two orders of magnitude reduction in the gas permeability.

Gas sorption and characterization of poly(ether-b-amide) segmented block copolymers

The solubilities of He, H2, N2, O2, CO2, CH4, C2H6, C3H8, and n-C4H10 were determined at 35°C and pressures up to 27 atmospheres in a systematic series of phase separated polyether–polyamide segmented block copolymers containing either poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon 6 [PA6] or nylon 12 [PA12] as the hard polyamide phase. Sorption isotherms are linear for the least soluble gases (He, H2, N2, O2, and CH4), convex to the pressure axis for more soluble penetrants (CO2, C3H8, and n-C4H10) and slightly concave to the pressure axis for ethane. These polymers exhibit high CO2/N2 and CO2/H2 solubility selectivity. This property appears to derive mainly from high carbon dioxide solubility, which is ascribed to the strong affinity of the polar ether linkages for CO2. As the amount of the polyether phase in the copolymers increases, gas solubility increases. The solubility of all gases is higher in polymers with less polar constituents, PTMEO and PA12, than in polymers with more polar PEO and PA6 units. CO2/N2 and CO2/H2 solubility selectivity, however, are higher in polymers with higher concentrations of polar repeat units. The sorption data are complemented with physical characterization (differential scanning calorimetry, elemental analysis, and wide angle X-ray diffraction) of the various block copolymers.

Highly Chlorine‐Tolerant Polymers for Desalination

Consequently, membrane manufacturers recommend thatfeed-water chlorine concentration to such membranes belimitedtoconcentrationslowerthan0.1ppm.Tomeettheseconflicting requirements, water to be purified is oftenchlorinated,todisinfectitandultimatelyinhibitbiofoulingof the membranes, then dechlorinated before being fed tomembrane desalination units. After passing through themembranes, the water is then rechlorinated before beingsenttothedistributionnetwork.

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

Filtering through to whats important Membranes are widely used for gas and liquid separations. Historical analysis of a range of gas pair separations indicated that there was an upper bound on the trade-off between membrane permeability, which limits flow rates, and the selectivity, which limits the quality of the separation process. Park et al. review the advances that have been made in attempts to break past this upper bound. Some inspiration has come from biological membranes. The authors also highlight cases where the challenges lie in areas other than improved separation performance. Science, this issue p. eaab0530 BACKGROUND Synthetic membranes are used for desalination, dialysis, sterile filtration, food processing, dehydration of air and other industrial, medical, and environmental applications due to low energy requirements, compact design, and mechanical simplicity. New applications are emerging from the water-energy nexus, shale gas extraction, and environmental needs such as carbon capture. All membranes exhibit a trade-off between permeability—i.e., how fast molecules pass through a membrane material—and selectivity—i.e., to what extent the desired molecules are separated from the rest. However, biological membranes such as aquaporins and ion channels are both highly permeable and highly selective. Separation based on size difference is common, but there are other ways to either block one component or enhance transport of another through a membrane. Based on increasing molecular understanding of both biological and synthetic membranes, key design criteria for new membranes have emerged: (i) properly sized free-volume elements (or pores), (ii) narrow free-volume element (or pore size) distribution, (iii) a thin active layer, and (iv) highly tuned interactions between permeants of interest and the membrane. Here, we discuss the permeability/selectivity trade-off, highlight similarities and differences between synthetic and biological membranes, describe challenges for existing membranes, and identify fruitful areas of future research. ADVANCES Many organic, inorganic, and hybrid materials have emerged as potential membranes. In addition to polymers, used for most membranes today, materials such as carbon molecular sieves, ceramics, zeolites, various nanomaterials (e.g., graphene, graphene oxide, and metal organic frameworks), and their mixtures with polymers have been explored. Simultaneously, global challenges such as climate change and rapid population growth stimulate the search for efficient water purification and energy-generation technologies, many of which are membrane-based. Additional driving forces include wastewater reuse from shale gas extraction and improvement of chemical and petrochemical separation processes by increasing the use of light hydrocarbons for chemicals manufacturing. OUTLOOK Opportunities for advancing membranes include (i) more mechanically, chemically, and thermally robust materials; (ii) judiciously higher permeability and selectivity for applications where such improvements matter; and (iii) more emphasis on fundamental structure/property/processing relations. There is a pressing need for membranes with improved selectivity, rather than membranes with improved permeability, especially for water purification. Modeling at all length scales is needed to develop a coherent molecular understanding of membrane properties, provide insight for future materials design, and clarify the fundamental basis for trade-off behavior. Basic molecular-level understanding of thermodynamic and diffusion properties of water and ions in charged membranes for desalination and energy applications such as fuel cells is largely incomplete. Fundamental understanding of membrane structure optimization to control transport of minor species (e.g., trace-organic contaminants in desalination membranes, neutral compounds in charged membranes, and heavy hydrocarbons in membranes for natural gas separation) is needed. Laboratory evaluation of membranes is often conducted with highly idealized mixtures, so separation performance in real applications with complex mixtures is poorly understood. Lack of systematic understanding of methodologies to scale promising membranes from the few square centimeters needed for laboratory studies to the thousands of square meters needed for large applications stymies membrane deployment. Nevertheless, opportunities for membranes in both existing and emerging applications, together with an expanding set of membrane materials, hold great promise for membranes to effectively address separations needs. From intrinsic permeability/selectivity trade-off to practical performance in membranes. Polymer membranes for liquid and gas separation applications obey a permeability/selectivity trade-off—highly permeable membranes have low selectivity and vice versa—largely due to broad distributions of free-volume elements (or pores in porous membranes) and nonspecific interactions between small solutes and polymers. We highlight materials approaches to overcome this trade-off, including the development of inorganic, isoporous, mixed matrix, and aquaporin membranes. Further, materials must be processed into thin, typically supported membranes, fashioned into high surface/volume ratio modules, and used in optimized processes. Thus, factors that govern the practical feasibility of membranes such as mechanical strength, module design, and operating conditions are also discussed. Increasing demands for energy-efficient separations in applications ranging from water purification to petroleum refining, chemicals production, and carbon capture have stimulated a vigorous search for novel, high-performance separation membranes. Synthetic membranes suffer a ubiquitous, pernicious trade-off: highly permeable membranes lack selectivity and vice versa. However, materials with both high permeability and high selectivity are beginning to emerge. For example, design features from biological membranes have been applied to break the permeability-selectivity trade-off. We review the basis for the permeability-selectivity trade-off, state-of-the-art approaches to membrane materials design to overcome the trade-off, and factors other than permeability and selectivity that govern membrane performance and, in turn, influence membrane design.

Mixed Gas Permeation of Syngas Components in Poly (dimethylsiloxane) and Poly (1-trimethylsilyl-1-propyne) at Elevated Temperatures:

Abstract The permeability of poly(dimethylsiloxane) (PDMS) and poly(1-trimethylsilyl-1-propyne) (PTMSP) to a simulated syngas feed containing H 2 , CO, CO 2 , and H 2 S was determined as a function of temperature up to 240°C (464°F). The permeation properties of rarely studied CO and H 2 S were found to be consistent with their molecular properties (i.e. critical temperature) in both rubbery PDMS and high free volume, glassy PTMSP. At room temperature, PDMS and PTMSP are more permeable to the more condensable gases CO 2 and H 2 S than to H 2 . However, both polymers become hydrogen selective at elevated temperatures. Activation energies of permeation are highest for H 2 in both polymers and decrease regularly with increasing gas condensability. PTMSP exhibits evidence of accelerated physical aging at high temperature.

Sorption and transport of hydrocarbon and perfluorocarbon gases in poly(1‐trimethylsilyl‐1‐propyne)

Pure gas solubility and permeability of H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8 in poly(1-trimethylsilyl-1-propyne) (PTMSP) were determined as a function of pressure at 35°C. Permeability coefficients of the perfluorinated penetrants are approximately an order of magnitude lower than those of their hydrocarbon analogs, and lower even than those of the permanent gases. In striking contrast to hydrocarbon penetrants, PTMSP permeability to fluorocarbon penetrants decreases with increasing penetrant size. This unusual size-sieving behavior in PTMSP is attributed to low perfluorocarbon solubilities in PTMSP coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. In general, perfluorocarbon penetrants are less soluble than their hydrocarbon analogs in PTMSP. The difference in hydrocarbon and perfluorocarbon solubilities in high free volume, hydrocarbon-rich PTMSP is much smaller than in hydrocarbon liquids and liquidlike polydimethylsiloxane. The low solubility of perfluorocarbon penetrants is ascribed to the large size of the fluorocarbons, which inhibits their dissolution into the densified regions of the polymer matrix and reduces the number of penetrant molecules that can be accommodated in Langmuir sites. From the permeability and sorption data, diffusion coefficients were calculated as a function of penetrant concentration. With the exception of H2 and the C3 analogs, all of the penetrants exhibit a maximum in their concentration-dependent diffusion coefficients. Resolution of diffusion coefficients into a mobility factor and a thermodynamic factor reveals that it is the interplay between these two terms that causes the maxima. The mobility of the smaller penetrants (H2, O2, N2, CH4, and CO2) decreases monotonically with increasing penetrant concentration, suggesting that the net free volume of the polymer–penetrant mixture decreases as additional penetrant is added to PTMSP. For larger penetrants mobility either: (1) remains constant at low concentrations and then decreases at higher penetrant concentrations (C2H6, CF4, and C2F6); (2) remains constant for all concentrations examined (C3H8); or (3) increases monotonically with increasing penetrant concentration (C3F8). Presumably these results reflect the varying effects of these penetrants on the net free volume of the polymer–penetrant system.