Browsing by Subject "Gas separation membranes"
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Item Carbon dioxide removal from natural gas by membranes in the presence of heavy hydrocarbons and by aqueous diglycolamine®/morpholine(2004) Al-Juaied, Mohammed Awad; Rochelle, Gary T.; Koros, William J.Item Crosslinked hollow fiber membranes for natural gas purification and their manufacture from novel polymers(2004) Wallace, David William; Koros, William J.; Paul, Donald R.Item Crosslinking and stabilization of high fractional free volume polymers for the separation of organic vapors from permanent gases(2008-05) Kelman, Scott Douglas, 1979-; Freeman, B. D. (Benny D.)The removal of higher hydrocarbons from natural gas streams is an important separation that has been identified as a growth area for polymer membranes. An ideal membrane material for this separation would be more permeable to higher hydrocarbons (i.e., C3+ compounds) than to CH₄. This allows the CH₄ rich permeate to be retained at or near feed pressure, thus minimizing the requirement for repressurization followingmembrane separation. A polymer which demonstrates the ability to separate vapor from gases with high efficiency is poly [1-(trimethylsilyl)-1-propyne] (PTMSP). PTMSP is a stiff chain, high free volume glassy polymer well known for its very high gas permeability and outstanding vapor/gas selectivity. However, PTMSP is soluble in many organic compounds, leading to potential dissolution of the membrane in process streams where its separation properties are of greatest interest. PTMSP also undergoes significant physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers. Crosslinking PTMSP with bis(azide)s was undertaken in an attempt to increase the solvent resistance and physical stability of the polymer. A fundamental investigation into crosslinking PTMSP with a bis(azide) crosslinker was the focus of this thesis. Pure gas transport measurements were conducted with N₂, O₂, CH₄, C₂H6, C₃H₈, and n-C₄H₁₀ over temperatures raging from -20°C to 35°C and pressures ranging from 0 to 20 atm. Mixed gas permeation experiments were conducted using a 98 mol % CH₄, and 2 mol % n-C₄H₁₀ mixture. The mixed gas permeation experiments were conducted at temperatures ranging from -20°C to 35°C, and pressures ranging from 4 to 18 atm. Inorganic nanoparticles such as fumed silica (FS) were added to uncrosslinked and crosslinked PTMSP, and the effects of their addition on the transport properties were investigated. Crosslinking PTMSP with bis(azide)s increases its solvent resistance, and crosslinked films are insoluble in common PTMSP solvents such as toluene. At all temperatures, the initial pure and mixed gas permeabilities of crosslinked PTMSP films are less than those of uncrosslinked PTMSP. This decrease in permeability is consistent with the fractional free volume (FFV) decrease that accompanies crosslinking. Pure gas solubility coefficients are relatively unaffected by the crosslinking process, so the decrease in permeability is caused by decreases in diffusivity. The addition of FS nanoparticles increases the initial pure and mixed gas permeabilities of uncrosslinked and crosslinked PTMSP. The pure gas permeabilities and solubilities of all PTMSP films increase when the temperature decreases, while the diffusivities decrease. The rates of change in pure gas transport properties with temperature is similar for all films, so the temperature dependence of pure gas transport properties of PTMSP is unaffected by the addition of crosslinks or FS. The aging of uncrosslinked and crosslinked PTMSP films was investigated by monitoring N₂, O₂ and CH₄ permeabilities and FFV over time. The FFV and permeabilities of crosslinked films decreased over time, so crosslinking did not arrest the physical aging of PTMSP, as has been previously reported, and these differences in aging observations are likely to be a consequence of differences in post film casting thermaltreatments. The addition of 10 wt % polysiloxysilsesquioxanes (POSS) nanoparticles decreases the permeabilities of uncrosslinked and crosslinked PTMSP by approximately 70 %, and the permeability and FFV values of the resulting nanocomposite films were stable over the course of 200 days. In all PTMSP films, the mixed gas permeabilities of n-C₄H₁₀ increase with decreasing temperature, while the mixed gas CH₄ permeabilities decrease with decreasing temperature. As a result, the mixed gas n-C₄H₁₀/CH₄ permeability selectivities increase with decreasing temperatures. The addition of crosslinks and FS nanoparticles to PTMSP decreases the mixed gas n-C₄H₁₀/CH₄ permeability selectivities, and changes in the free volume characteristics of PTMSP caused by crosslinking and FS nanoparticles are thought to reduce the blocking of CH₄ permeation by n-C₄H₁₀.Item Dynamic behavior of ultra-thin polymer films deposited on surface acoustic wave(SAW) devices: A feasibility study of saw applications(1996-08) Ahuja, Ashish; Narayan, Raghu S.; James, Darryl; Tock, Richard W.The main objective of this research effort was the investigation of relevant applications that a cutting edge technology afforded. While the work did not attempt to delve into mathematical interpretation of SAW device responses, it did seek to evaluate the usefulness of the SAW technology from an engineering perspective.Item Effects of materials, processing, and operating conditions on the morphology and gas transport properties of mixed matrix membranes(2004) Moore, Theodore Thomas; Koros, William J.Gas separation membranes are currently based on polymers, which are limited by a trade-off between permeability (productivity) and selectivity. Zeolites offer significantly higher selectivities than polymers; however their properties make them prohibitively expensive to process into membranes. Organic-inorganic, or “mixed matrix”, materials may provide the basis for the next generation of economical, high performance membranes. The topic of this research is mixed matrix materials comprising a dispersion of zeolites in a polymer matrix. A major limitation of mixed matrix technology is the inability to prepare membranes from selected polymers and sieves with properties approaching the theoretical predictions. This difficulty is related largely to undesirable properties of the polymer- sieve interface, and this work seeks to understand and control these interfacial properties. First, an understanding of membrane formation is presented to explain how nonideal interfacial morphologies form. Factors affecting this process include: polymer flexibility, polymer – sieve affinity, and membrane preparation conditions. Membrane preparation conditions affect the propensity for stress to accumulate at the polymer- sieve interface, and depending on the severity of the stress, the likelihood that the polymer – sieve interface will fail. The next part of this work details experiments undertaken to better understand the factors affecting membrane morphology and transport properties. Factors ranging from material selection (e.g. silane coupling agent selection), dope formulation (e.g. polymer “priming”, sieve settling), and membrane preparation conditions (e.g. casting surface, temperature) were investigated. The final part of this work considers additional effects on mixed matrix properties caused by contaminants and minor feed components. A framework has been developed to account for the effects of potential impurities in the feed gas on the polymer, zeolite, and mixed matrix membrane. Based on this framework and results with model impurities, it appears that strongly sorbing components selectively displace the desired gases from the zeolite, preventing improved selectivity in mixed matrix membranes. This work has developed a better understanding of the factors that affect mixed matrix membrane performance and identified new ones that require additional study. After further development, this technology should allow for the increased application of membranes for the separation of gases and possibly also vapors and liquids.Item Fundamentals of gas sorption and transport in thermally rearranged polyimides(2014-05) Smith, Zachary Pace; Freeman, B. D. (Benny D.); Paul, Donald R.; Willson, Carlton G; Sanchez, Isaac C; Hill, Anita JThermally rearranged polymers are formed from the solid-state thermal reaction of polyimides and polyamides that contain reactive groups ortho position to their diamine. These polymers have shown outstanding transport properties for gas separation applications. The thrust of this work is to critically examine the chemical and morphological structure of these polymers and to identify the fundamental contributions of gas sorption to permeability. To accomplish this goal, a series of TR polymers and TR polymer precursors have been synthesized and investigated for transport properties. As a function of conversion, diffusivity increases more dramatically than sorption, which explains the outstanding permeabilities observed for these samples. Modifications to the polymer backbone structure, which can be achieved by adding rigid functional groups such as hexafluoroisopropylidene-functional linking groups, can further be used to improve permeabilities. The precursor used to form TR polymers has dramatic effects on the final polymer transport properties. Despite having nearly identical polymer structure, TR polymers formed from polyamide precursors have lower combinations of permeability and selectivity than TR polymers formed from polyimide precursors. In addition to structure-property studies with TR polymers, this thesis also present comparisons of permeability, diffusivity, and sorption of sparingly soluble gases (i.e., hydrogen and helium) for hydrocarbon-based polymer, highly fluorinated polymers, perfluoropolymers, and a silicon-based polymer. An explanation for the unique transport properties of perfluoropolymers is presented from the standpoint of the solution-diffusion model, whereby perfluoropolymers have uniquely different solubility selectivities than hydrocarbon-based polymers. Additionally, a large database of sorption, diffusion, and permeability coefficients is used to determine the contributions of free volume on solubility selectivity in polymers.Item Gas transport properties of poly(n-alkyl acrylate) blends and modeling of modified atmosphere storage using selective and non-selective membranes(2007-12) Kirkland, Bertha Shontae, 1976-; Paul, Donald R.The gas transport properties of side-chain crystalline poly(n-alkyl acrylate) and poly(m-alkyl acrylate) blends are determined as a function of temperature for varying side-chain lengths, n and m, and blend compositions. The side chains of poly(n-alkyl acrylate)s crystallize independently of the main chain for n [is greater than or equal to] 10 which leads to an extraordinary increase in the permeability at the melting temperature of the crystallites. The compatibility of these polymers are examined and macroscopic homogeneity is observed for a small range of n and m when the difference /n - m/ is between 2 - 4 methylene units. Thermal analysis shows that the blend components crystallize independently of one another; at the same time, the crystallization of each component is hindered by the presence the other component. The permeation responses of these blends show two distinct permeation jumps as the crystallites from each component melt at their respective melting temperatures. Blends with continuous permeation responses are found to have higher effective activation energies than observed for common polymers. Thermal analysis proved to be a useful tool to help predict the permeation response for poly(alkyl acrylates); thus the thermal behavior of poly(n-alkyl acrylate) blended with n-aliphatic materials and random copolymers of poly(n-alkyl acrylates) are briefly examined. A bulk modified atmospheric storage design is proposed where produce is stored in a rigid chamber that is equipped with both selective and non-selective membrane modules that help regulate the oxygen entering and the carbon dioxide leaving the produce compartment. The design enables control of the atmosphere inside the chamber by modulating gas flow, i.e. the gas flow rate and composition, through the non-selective membrane by delivering fresh air upstream of the non-selective membrane. The model shows that the choice of materials for the selective and non-selective membranes dictate the range of concentrations achievable; however, the air flow rate allows the control between these ranges. The method to design a practical chamber from this model is also described.Item Improving polyimide membrane resistance to carbon dioxide plasticization in natural gas separations(2002) Wind, John David; Paul, Donald R.; Koros, William J.Polyimide membranes have been widely applied for gas separations due to their attractive permeability, selectivity, and processing characteristics. Their use for natural gas and hydrocarbon separations is limited by plasticization-induced selectivity losses in feeds with significant partial pressures of CO2 and C3+ hydrocarbons. This project focuses on understanding CO2-induced plasticization of polyimide membranes and how it can be controlled by thermal annealing and crosslinking. Covalent and ionic crosslinking are investigated as approaches for suppressing plasticization, while retaining attractive transport properties. A novel covalent crosslinking protocol has been developed, which offers significant advantages over the traditional post-treatment that was initially used. The twostep crosslinking treatment allows for spectroscopic characterization of the reaction yields in the monoesterification and transesterification reactions. These crosslinking reactions occur at temperatures well below the glass transition and no additives are required in the casting solution, making the approach attractive for the eventual production of asymmetric hollow fibers. The ionically crosslinked membranes are not as stable against CO2 plasticization as the covalently crosslinked materials. By varying the ionic crosslinking density, the effects on long-term sorption and permeation at high CO2 pressures were investigated. From STEM images, it does not appear that heterogeneity in the ion distribution is the cause of the membrane plasticization. With covalent crosslinking, the copolymer composition, crosslinking agent, and thermal treatment are important factors in determining the final membrane transport properties. The crosslinking reaction is accompanied by a heat treatment that can also lead to stabilization of aromatic polyimides. These effects were decoupled by systematic variations in the polymer structure and thermal treatment. In a plasticized membrane, the sorption, diffusion, and swelling processes are all interdependent. The key to controlling plasticization is to control the membrane swelling, since this is related to the increase in polymer chain segmental mobility facilitated by the CO2 sorption. Mixed gas separations demonstrate the non-ideal factors that must be accounted for when modeling membrane performance over a wide range of pressures. The separation performance at practically relevant feed conditions is intrinsically better and more stable than the commercial polymeric membranes currently used for natural gas separations.Item Integral-skin formation in hollow fiber membranes for gas separations(2001-12) Carruthers, Seth Blue; Koros, William J., 1947-; Willson, C.G., 1939-The morphologies of polymeric integrally-skinned asymmetric gas separation membranes are typically visualized as a thin selective skin region supported by a low resistance porous structure. Improvements in scanning electron microscopy (SEM) now allow for combinatorial analysis of this visualization with gas permeation measurements for previously reported ultra-thin defect-free hollow fiber membranes. The fibers were formed via a dry-jet, wet quench process with a spinning solution comprised of Matrimid polyimide and components of varying volatility. Depending on the formation conditions, the fibers displayed either defect-free skin layers or lower selectivity nodular skin morphologies. Under ideal conditions, defect-free skin thicknesses of 130 nm were characterized by O2, N2 and He permeation in conjunction with SEM studies. A fiber forming technique has allowed for the quick characterization of the skin layer via SEM analysis. The fiber forming technique, high-resolution SEM analysis and gas permeation measurements have allowed for a more complete understanding of defect-free skin formation. Typical solvent exchange techniques did not have a significant influence on the formation of the defect-free skin layer, although a critical point drying method was able to produce membranes with initial gas permeances twice those of conventionally dehydrated hollow fibers. Skin formation was found to primarily be influenced by the evaporation of volatile components from the nascent skin layer in the air gap. Phase separation of the polymer solution in the nascent skin layer was found to be detrimental to skin formation. The one-phase nascent skin layer is suggested to be kinetically hindered from phase separating due to the relative immobility of the polymer chains before immersion into the quench bath. The polymer chain mobility for different potential membrane forming dopes are compared using polymer physics models. Qualitative evidence suggests that lower molecular weight Matrimid samples may be formed into defect-free membranes if the initial dope is 40% wt. polymer, as suggested by the scaling of the previous defect-free membrane system by the Rouse model.Item Low hydrocarbon solubility polymers: plasticization-resistant membranes for carbon dioxide removal from natural gas(2004) Prabhakar, Rajeev Satish; Freeman, B. D. (Benny D.)Item Rigorous numerical simulation of gas separation by hollow-fiber membranes(Texas Tech University, 1996-12) Clancy, Donald J.The work develops numerically-stable computer models to simulate the performance of cocurrent, and countercurrent membranes. Additionally, a previously unreported gas-flow pattern, a variance of cross flow, is discussed and simulated. These models assume solution-diffusion permeation, and account for gas non-ideality. A stable method is presented for calculating the permeate pressure drop through the hollow bore of the membrane fiber. This analysis also develops an energy balance and calculation procedures for simulating the temperature change which occurs in a hollow-fiber membrane. The cocurrent and counter-current numerical models are then validated by comparing calculated results against operating data obtained from air- and hydrogen-separation applications. The cross-flow numerical model is then correlated against data obtained from a full-scale, carbon-dioxide purification facility. It was noticed that this facility's cellulose acetate membranes exhibited a marked increase in gas permeability as the CO2 partial pressure increased. This led to an investigation and correlation of the effects of C02-induced plasticization of cellulose acetate on the increase in mixed-gas permeability. It was found that the permeability of hydrocarbons increases much more than that of CO2 as the CO2 partial pressure increases. This leads to a decrease in the amount of hydrocarbon that can be recovered from a membrane unit. This work concludes that enhanced oil recovery facilities utilizing cellulose acetate membranes operating at CO2 partial pressures in excess of approximately 150 psia could substantially benefit by avoiding CO2 plasticization effects either by operating at lower CO2 partial pressures, or lowering the degree of acetylation in the cellulose acetate membrane polymer.Item Solubility selective membrane materials for carbon dioxide removal from mixtures with light gases(2005) Lin, Haiqing; Freeman, B. D. (Benny D.)Membrane technology has attracted interest for the selective removal of carbon dioxide from mixtures with light gases such as H2, CH4 and N2. While conventional structure-property correlations have focused mainly on improving the separation performance by increasing polymer size sieving ability (i.e., diffusivity selectivity), this project explores the possibility of harnessing favorable interactions between CO2 and polymers containing polar groups to improve permeability/selectivity properties. Ether oxide groups are discovered to be among the best moieties known to interact with CO2, leading to high CO2 solubility and CO2/light gas solubility selectivity, while still providing polymer chain flexibility, leading to high CO2 diffusivity and favorable CO2/H2 diffusivity selectivity. Poly(ethylene oxide) (PEO) has a high concentration of ether oxygen groups and exhibits high CO2/light gas selectivities. However, gas permeability is low due to the high crystallinity in PEO. Crosslinking and introduction of short chain branching are efficient methods to inhibit crystallization. Three series of crosslinked poly(ethylene oxide) rubbers have been prepared using prepolymer solutions containing: (1) poly(ethylene glycol) diacrylate (PEGDA) and H2O, (2) PEGDA and poly(ethylene glycol) methyl ether acrylate (PEGMEA), and (3) PEGDA and poly(ethylene glycol) acrylate (PEGA). Independent of the prepolymer composition, all of these polymers have similar ethylene oxide (EO) content (approximately 82 wt.%). Crosslink density decreases with decreasing PEGDA content in the prepolymer solution, which is estimated from water swelling experiments and/or dynamic mechanical testing and has essentially no effect on gas transport properties. Increasing PEGMEA content increases the average size of free volume elements, resulting in a decreased glass transition temperature, and increased CO2 permeability and CO2/H2 selectivity. In contrast, the presence of PEGA or water has a negligible impact on these properties. Due to the affinity between EO and CO2, decreasing temperature increases CO2/light gas solubility selectivity. PEGDA/PEGMEA-30 (a copolymer containing 30 wt.% PEGDA and the balance PEGMEA) demonstrates very favorable mixed gas CO2/H2 and CO2/CH4 separation performance. The separation properties improve with decreasing temperature and have excellent resistance to impurities in the feed streams such as high pressure CO2, higher hydrocarbons and water. Interestingly, the highly sorbing impurities can even improve CO2/H2 separation. Pure and mixed gas diffusivity and permeability in these crosslinked PEO rubbers were successfully modeled using a new free volume model, which was based on the Cohen-Turnbull and Fujita free volume models coupled with Chow’s model for estimating glass transition temperatures in polymer/gas mixtures as a function of temperature and penetrant concentration.