Scott presents two papers at the American Institute of Chemical Engineers (AIChE) 2005 Conference entitled, "Crosslinking of High Free Volume Polymers for the Separation of Organic Vapors from Permanent Gases", co-authors Scott Kelman and Benny D. Freeman and "CO2/C2H6 Separation Using Solubility Selective Membrane Materials", co-authors Scott Kelman, Haiqing Lin, and Benny D. Freeman.
Crosslinking of High Free Volume Polymers for the Separation of Organic Vapors from Permanent Gases
Abstract
High free volume polymer membranes are often very weakly size-sieving and, consequently, can remove large gas or vapor molecules from a gas mixture with smaller molecules. This capability finds application in reverse-selective gas separations such as VOC removal from permanent gas streams and monomer recovery from the exhaust of polymerization reactors. Poly(1-trimethysilyl-1-propyne) (PTMSP) is a stiff chain, high free volume glassy polymer well known for its very high gas permeability [1]. PTMSP also has outstanding vapor/gas selectivity. For example, the n-C4H10/CH4 mixed gas selectivity at 25oC is 35, which is the highest value ever reported for this gas pair [2]. Such properties make PTMSP an interesting material for vapor/gas separations. However, gas permeabilities in PTMSP are sensitive to processing history and time [3]. PTMSP undergoes significant physical aging, which is the gradual relaxation of non-equilibrium excess free volume in glassy polymers. PTMSP is also soluble in many organic compounds, leading to potential dissolution of the membrane in process streams where its separation properties are of greatest interest. These phenomena compromise the practical utility of PTMSP.
This study investigates the effect of crosslinking PTMSP on transport properties and physical aging. PTMSP films are crosslinked using bis azides, which have been shown to crosslink PTMSP [4]. The crosslinking chemistry is discussed, and the extent of crosslinking is correlated with the transport properties of this polymer. When PTMSP is crosslinked, it becomes insoluble in common PTMSP solvents such as toluene, cyclohexane and tetrahydrafuran. Thus, there is a significant increase in the chemical stability due to crosslinking. The initial permeability of PTMSP decreased with increasing crosslinking due to the loss in fractional free volume (FFV) upon crosslinking. The O2/N2 selectivity increased as the FFV decreased, showing that crosslinked PTMSP is more size selective than uncrosslinked PTMSP. A strong correlation between permeability and 1/FFV was found. The sorption properties of PTMSP were unaffected by crosslinking, so the decrease in permeability was due to a decrease in diffusion coefficients.
Nanoparticles such as fumed silica and titanium oxide were added to crosslinked PTMSP films and permeability of these films increased by up to 200% compared to crosslinked films with no nanoparticles. A systematic study of the effect of type, shape and amount of nanoparticles added to crosslinked PTMSP has been conducted.The crosslinked PTMSP N2, O2 and CH4 permeability stability is improved and films have been tested for up to 250 days. The increased stability may be due to the crosslinks constraining the PTMSP chains and not allowing them to relax the excess, non-equilibrium FFV that is inherent in PTMSP. Over the same time scale, n-Butane permeability increased by 20%. This result is interesting and could be due to n-butane conditioning the membrane. Further research is required to fully understand the time dependence of permeation properties of crosslinked PTMSP.Mixed gas experiments data show that crosslinked PTMSP displays enhanced mixed gas selectivities, similar to those in uncrosslinked PTMSP. The effect of vapor/gas composition, temperature and pressure on mixed gas permeation properties of crosslinked PTMSP has been studied, and the results from this study will be presented.
[1] K. Nagai, T. Masuda, T. Nakagawa, B. D. Freeman and I. Pinnau, Poly(1-Trimethylsilyl-1-Propyne) and Related Polymers: Synthesis, Properties and Functions, Progress in Polymer Science, 26 (2001) 721-798.
[2] I. Pinnau and L. G. Toy, Transport of Organic Vapors through Poly(1-Trimethylsilyl-1-Propyne), J. Membrane Sci., 116 (1996) 199-209.
[3] L. C. E. Struik, Physical Aging in Amorphous Polymers and Other Materials, Elsevier, Amsterdam, 1978, pp. 7-9.
[4] J. Jia, PhD Thesis, Michigan State University, 1997.
CO2/C2H6 Separation Using Solubility Selective Membrane Materials
Abstract
Carbon dioxide is an impurity which must be removed from natural gas streams in a process commonly known as natural gas conditioning. Ethane is a major component of natural gas streams, and it forms a minimum pressure azeotrope with carbon dioxide which hinders carbon dioxide separation [1]. At 293K, the carbon dioxide/ethane azeotrope has a carbon dioxide mole fraction of 0.7 [2]. Traditional techniques (e.g., chemical absorption using amine solutions and adsorption using molecular sieves), for breaking the carbon dioxide/ethane azeotrope are capital intensive and require complex process control [3]. This study investigates the effectiveness of membranes to break the carbon dioxide/ethane azeotrope. Membrane technology has been successfully implemented in natural gas processing facilities and has been shown to be effective in separating carbon dioxide from methane [1]. The membrane material selected to break the carbon dioxide/ethane azeotrope should have a high carbon dioxide/ethane selectivity and high carbon dioxide permeability. From previous work in our laboratory, crosslinked poly(ethylene oxide) [XLPEO] shows high carbon dioxide permeability and high carbon dioxide/ethane selectivity. The pure gas permeability and solubility of carbon dioxide and ethane in XLPEO has been measured at temperatures ranging from 253K to 308K and over a pressure range of 0 to 15 atmospheres. The permeability of carbon dioxide and ethane increase as temperature increases, while the gas solubility increases as temperature decreases. At 253K, the permeability of carbon dioxide increases strongly with increasing carbon dioxide partial pressure, since carbon dioxide strongly plasticizes XLPEO. These strong plasticizing effects are not observed at 253K for ethane. Mixed gas permeation experiments were conducted using a gas mixture containing 45.3 mol % carbon dioxide and the balance ethane. The mixed gas results show carbon dioxide strongly plasticizes XLPEO films at lower temperatures. Slight plasticization of the film is seen at 283K and 308K. Carbon dioxide plasticization decreases the mixed gas carbon dioxide/ethane selectivity, relative to pure gas values, because the ethane permeability is enhanced by the plasticization effects of carbon dioxide. The mixed gas selectivity increases as temperature decreases. At 253K and 10 atmospheres pressure the carbon dioxide permeability is 83 Barrers and the mixed gas carbon dioxide/ethane selectivity is 13. The pure gas and mixed gas carbon dioxide permeabilities and carbon dioxide/ethane selectivities were plotted on a Robeson plot and were found to be lie close to the upper bound. The performance of the membrane material was simulated using a computer model developed in our laboratory [4]. The mixed gas experimental conditions and results were used as parameters for the simulation. It was established that XLPEO is effective in breaking the carbon dioxide/ethane azeotrope. When the feed stream to the membrane module contains 45.3 mol% carbon dioxide and the balance ethane, 85% ethane recovery is achieved at an ethane purity of 78%, for a one pass separation at 253K.
[1] G. H. Gall and E. S. Sanders, Membrane Technology Removes Co2 from Liquid Ethane, Oil & Gas Journal, 100 (2002) 48-55.
[2] A. Fredenslund and J. Mollerup, Measurement and Prediction of Equilibrium Ratios for Ethane + Carbon Dioxide System, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 70 (1974) 1653-60.
[3] S. Horstmann, K. Fischer, J. Gmehling and P. Kolar, Experimental Determination of the Critical Line for (Carbon Dioxide + Ethane) and Calculation of Various Thermodynamic Properties for (Carbon Dioxide + N-Alkane) Using the Psrk Model, Journal of Chemical Thermodynamics, 32 (2000) 451-464.
[4] D. T. Coker, B. D. Freeman and G. K. Fleming, Modeling Multicomponent Gas Separation Using Hollow-Fiber Membrane Contactors, AIChE Journal, 44 (1998) 1289-1302.