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The effects of temperature and humidity on the permeation properties of membrane transport media used… Engarnevis, Amin; Romani, Sarah; Sylvester, Alexander; Huizing, Ryan; Green, Sheldon; Rogak, Steven 2017-06

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 Amin Engarnevis is a PhD candidate in the Department of Mechanical Engineering, University of British Columbia, Vancouver, BC. Sarah Romani is an undergraduate research fellow in the Department of Mechanical Engineering, University of British Columbia, Vancouver, BC. Alexander Sylvester is an MASc student in the Department of Mechanical Engineering, University of British Columbia, Vancouver, BC. Ryan Huizing is the director of research and development at dPoint Technologies, Vancouver, BC. Sheldon Green is the head of the Department of Mechanical Engineering, University of British Columbia, Vancouver, BC. Steven Rogak is a professor in the Department of Mechanical Engineering, University of British Columbia, Vancouver, BC.   The effects of temperature and humidity on the permeation properties of membrane transport media used in energy recovery ventilators   Amin Engarnevis                                    Sarah Romani                          Alexander Sylvester Student Member ASHRAE                             Student member ASHRAE  Ryan Huizing, P.Eng.      Sheldon Green, PhD, P.Eng.     Steven Rogak, PhD, P.Eng.                      Member ASHRAE                                                  Member ASHRAE ABSTRACT We report on an experimental study of the effects of relative humidity and temperature on the transport of water vapor and CO2 through a series of standard polymeric materials to determine their potential for use as membrane media in energy recovery ventilators (ERVs). Results are reported for four polymers of two major types (glassy and rubbery). The selectivity of water vapor over CO2 is also evaluated from permeation experiments.   Permeability results show that rubbery membrane samples, with glass transition temperatures well below the temperature range of experiments (30°C to 50°C), have a higher water vapor permeability and a much higher CO2 permeability compared to glassy membrane samples. This is hypothesized to be mainly due to the higher diffusivity of water vapor and CO2 in rubbery polymers with higher chain flexibility leading to a much lower selectivity for rubbery membrane samples.  In all polymer samples, water vapor permeability increases with relative humidity (up to an order of magnitude) and decreases with temperature. This is attributed to the negative enthalpy of water vapor sorption dominating its activation energy of permeation, which is associated with a lower solubility at higher temperatures. In contrast, CO2 permeability increases with temperature because of the positive activation energy of CO2 permeation.  CO2 permeability decreases slightly with increasing relative humidity (up to 30%), which is hypothesized to be due to the competitive sorption between water vapor and CO2 at higher humidity levels. Therefore, the selectivity of membrane samples for water vapor over CO2 decreases with temperature and increases with relative humidity, and these trends are dominated by water vapor permeability variations. In general, the permeability results reported here suggest that ERV exchangers using polymer membranes can achieve high latent effectiveness (i.e. very high water vapor permeability) over a wide range of operating temperature and relative humidity while maintaining very low CO2 permeability and crossover rates (<1%).  INTRODUCTION Energy Recovery Ventilators (ERVs), also known as enthalpy exchangers, are used in building ventilation systems as a method of transferring heat and humidity between outgoing and incoming air streams. The use of ERVs reduces energy demands of building HVAC systems by effectively recycling energy from exhaust air and using it to pre-condition the fresh incoming air. Polymer membranes are used in these systems to separate the two air streams but to still allow transfer of both heat and water vapour (Zhang 2012). It is shown that using membrane-based ERVs can result in significant energy savings, as well as improvement of indoor air quality and comfort in buildings (Zhang & Niu 2001), (Wang et al. 2014). The basic task of a ventilation system is to remove the stale indoor air and replace it with fresh supplied air. Ideally, membranes used for ERVs’ should be highly permeable and selective for water vapor over other gases and contaminants that may be present in the building outgoing air streams (Huizing et al. 2015). Such membranes are able to maintain a high water vapour transfer rate without allowing high levels of crossover of pollutants from outgoing to incoming air. Polymeric membranes have broad ranges of permeability for various chemical species, such as water vapor, gases, Volatile Organic Compounds (VOCs’), etc. Permeation properties of polymeric membranes depend on a range of various factors such as the nature of the polymer material (chain packing and flexibility, degree of crystallinity, etc.), state of polymeric material based on glass transition temperature (rubbery vs. glassy), polymer-gas, -vapor interactions, operating conditions, etc. (Huizing et al. 2015). ERVs are used in different climates spanning a wide range of temperatures and humidities. It is therefore important to understand the effects of operating conditions (temperature and humidity) on the performance of membranes used in these applications. This experimental study systematically examines the effects of relative humidity and temperature of working air streams on the transport of water vapor and CO2 (as a major indoor air contaminant) through a number of polymeric materials suitable for use in ERVs. The results reported suggest some guidelines for material selection in ERVs used in different environments.  MATERIALS AND EXPERIMENTAL METHODS Materials Four different polymers were used as the selective coating layer of composite membranes in this study: cellulose acetate (CA), PEBAX®1074 (a polyether block amide copolymer), and a proprietary glassy polymer (G1) and a proprietary rubbery polymer (R1) from dPoint Technologies. Thin films of these polymers with a thickness ranging from 1.85 to 2.63µm were prepared by solution casting onto a substrate supplied by dPoint Technologies. The properties of the four different polymer film materials tested in this study are summarized in Table 1. Table 1.   Properties of membrane coating materials Polymer Type Tg (ºC) State at experimental temperature range Thickness (µm) CA 105 Glassy 2.348±0.704 G1 >140 Glassy 2.632±0.748 R1 -48 Rubbery 2.524±0.727 PEBAX®1074 (10) -55 Rubbery 1.849±0.559 Measurement of Permeation Properties Figure 1 shows the experimental test apparatus designed for simultaneous measurement of mixed water vapor and CO2 permeation rates. This apparatus enables the permeation testing of membrane samples in temperature and relative humidity ranges of 25-70°C and 0-95%RH, respectively.  The test apparatus creates two gas streams, referred to as the “Feed” and “Sweep” streams, which counter-flow on either side of a flat sheet membrane sample held in a special permeation testing module (Figure 1(b)). The flow rates are controlled by mass flow controllers MFC-1 and MFC-2 (Alicat Scientific MCS). The outlets of the two streams are open to ambient to minimize pressure forcing crossover through the membrane samples. The feed stream contains 10% CO2 with the balance of N2 (PRAXAIR Canada). The RH of this stream is adjusted by splitting it up into two branches; one of which passes through a humidifier HUMR (dPoint Px1-32) and gets saturated before recombining with the other branch to create the desired humidity level. The RH of the feed stream is controlled by adjusting the proportion of these two branches using two manual needle valves (Figure 1). The sweep stream flowing on the opposite side of the membrane is N2 from a separate gas cylinder (PRAXAIR Canada, Grade 4.8 High purity>99.998%, H2O<3ppmv, CO2(-)) that removes the water vapor and CO2 permeate through the membrane material.    Figure 1 (a) Schematic presentation of the mixed gas permeation test apparatus, (b) membrane module  The temperature and relative humidity of all inlets and outlets of the membrane module are recorded using sensors S1 to S4 (HTM2500LF). These sensors are calibrated against a VAISALA HM70 (leveled with a HMK15 Humidity Calibrator). The flow rates of the working streams leaving the membrane test module are measured with a soap bubble flowmeter FL (Gilibrator-2) at the outlet of each side. A CO2 sensor (VAISALA GMP222) is also placed at the outlet of sweep stream to measure the concentration of CO2 permeate. In addition, pressure difference between the two streams inlets (ΔP1) is monitored using a differential pressure gauge (Dwyer 2000 Magnehelic®). The membrane permeation module along with all of the gas lines are placed inside a temperature controlled forced convection oven to conduct tests isothermally. The permeation module has an active membrane area of 1.59×10-3 m2. Data Analysis The transport of gases and vapors through dense polymer films is generally described using the solution-diffusion model (Wijmans & Baker 1995).   (1) where, Ji is the permeate flux of component i through the membrane, Pi is the membrane permeability for component i, pi,1 and pi,3 indicate the permeate partial pressure at the feed and sweep sides of membrane, respectively, and tmembrane is the membrane thickness. Using the Fick’s first law of diffusion, one can define the permeability of component i through a dense polymer membrane as   (2) where, Si is the solubility coefficient, a thermodynamic parameter accounting for the sorbed amount of permeating component i in the polymer, and Di is the diffusivity, a kinetic parameter determining the magnitude of permeability for the component (Wijmans & Baker 1995). Since the sweep gas on the permeate side is dry, pure nitrogen at near atmospheric pressure, the flux of different penetrants (water vapor and CO2) can be calculated assuming ideal gas law from the sweep stream outlet by   (3) where, pi,4 is the partial pressure of penetrant i, Q4 is the flowrate at the sweep outlet, Vm is the molar gas volume, R is the gas constant, T is absolute temperature, and A is the membrane effective area. For membranes with low resistance to water vapor transport (comparable to those of boundary layers), the actual permeability of the membrane should be corrected for the boundary layer resistances at various operating conditions. The mixed gas water vapor permeability values reported in this work are corrected for boundary layer effects of the membrane test module, according to the method described by Metz et. al. (Metz, Van De Ven, Potreck, et al. 2005) and Huizing et. al. (Huizing et al. 2014). The boundary layer resistance of the membrane module at test conditions is determined using a CFD model developed in a commercial CFD package (ANSYS FLUENT 16.1). The selectivity of a membrane for a water vapor over CO2 is consequently defined as the ratio of their permeabilities.   (4) The crossover rate of CO2 is also calculated for equal flow rates on both sides of membrane as:   (5) RESULTS AND DISCUSSION Water vapor permeability The water vapor permeability data at various relative humidities and temperatures for membrane samples PEBAX, G1, R1, and CA are shown in Figure 2 (a)-(d), respectively. Each data point represents the average of measurements for three different samples of the same membrane material. Error bars indicate the standard deviation of those three measurements.  Infinite dilution permeability (extrapolated to 0%RH) from the measurements for the PEBAX®1074 is comparable to the values found in the literature (Sijbesma et al. 2008). It can be observed that water vapor permeability in all membranes investigated increases at higher RH values of the feed stream, for each temperature considered. This is in agreement with the reported data in many other works in the literature (Metz, et al. 2005), (Sijbesma et al. 2008), (Potreck et al. 2009), (Reijerkerk et al. 2011), and (Tsvigu et al. 2015). The observed increasing trend of water vapor permeability, as discussed in these studies, could be due to the plasticizing effect of water vapor on the polymer matrix and the increased sorption of water vapor at higher RH values. The strongest increase in water vapor permeability occurs for G1 (up to an order of magnitude at 80%RH at 50C), while PEBAX, R1, and CA show progressively lower variations at the same RH value. High water vapor permeability at high RH is very beneficial for energy recovery, especially for cooling in humid climates, where the latent heat load constitutes a large fraction of the total cooling load of the HVAC system.      Figure 2 Water vapor permeability of a) PEBAX®1074, b) G1, c) R1, d) CA at 30°C and 50°C  The water vapor permeability in all polymers investigated decreases with increasing temperature except for polymer R1 which is independent from test temperature to within the experimental uncertainty. The water vapor permeability of glassy membrane samples (CA and G1) shows a strong dependence on the test temperature, whereas the effect of temperature on the permeability of rubbery samples (PEBAX and R1) is limited. This can be explained by considering the occurrence of two opposing effects: at higher temperatures, water vapor solubility decreases due to the negative enthalpy of water sorption in polymer matrix, whereas the water vapor diffusivity increases as a result of positive activation energy for diffusion (Sijbesma et al. 2008) and (Metz, Van De Ven, Mulder, et al. 2005).  For glassy polymers where permeability is controlled by diffusion rather than by solubility, the relative decrease in solubility of water vapor is more pronounced than the relative increase in diffusivity, as often observed for diffusion-controlled polymers (Sijbesma et al. 2008).  Carbon dioxide permeability Results of the CO2 permeation measurements as a function of relative humidity for membrane samples at two test temperatures of 30°C and 50°C are summarized in Table 2. The CO2 permeability of G1 polymer is close to the detection limit of the analytical method which makes it difficult to conclude a general trend for this particular membrane. The extremely low CO2 permeability of G1 membrane, combined with its relatively high water vapor permeability, makes this membrane an attractive candidate for ERV applications. The CO2 permeability of the rest of polymers inspected, is found to moderately decrease with increasing relative humidity. The same behavior is also observed in other studies of rubbery polymers: PEBAX1074 (Potreck et al. 2009), PDMS (RAHARJO et al. 2007), as well as glassy polymers: Matrimid (Ansaloni et al. 2014), polyimide6FDA–6FpDA (Tsvigu et al. 2015). The CO2 permeability in CA membrane is the most affected on average, with a decrease of about 27% and 11% relative to the dry value at 80%RH for 30ºC and 50ºC, respectively.  Table 2.   CO2 permeability (crossover) (Barrer (%)) values in different membranes RH (%) PEBAX 1074  G1 R1 CA  30°C 50°C 30°C 50°C 30°C 50°C 30°C 50°C 20 110.5 (0.47) 181.0 (0.73) 6.0 (<0.03) 7.6 (<0.03) 50.1 (0.15) 101.9 (0.31) 11.1 (0.04) 12.7 (0.04) 30 104.8 (0.44) 174.2 (0.72) 5.9 (<0.03) 7.6 (<0.03) 50.8 (0.15) 102.7 (0.30) 11.1 (0.04) 12.8 (0.04) 40 104.1 (0.44) 174.9 (0.71) 5.9 (<0.03) 7.5 (<0.03) 47.4 (0.15) 103.4 (0.30) 11.1 (0.04) 12.8 (0.04) 50 102.9 (0.43) 174.6 (0.71) 5.9 (<0.03) 7.6 (<0.03) 48.2 (0.15) 101.1 (0.30) 10.0 (0.03) 12.0 (0.04) 60 101.8 (0.42) 171.0 (0.71) 5.9 (<0.03) 7.6 (<0.03) 47.0 (0.15) 95.3 (0.29) 10.0 (0.03) 11.3 (0.04) 70 100.0 (0.41) 167.9 (0.70) 5.9 (<0.03) 7.7 (<0.03) 48.1 (0.14) 96.3 (0.29) 8.6 (0.03) 11.3 (0.04) 80 98.0 (0.41) 169.7 (0.69) 5.9 (<0.03) 7.7 (<0.03) 44.8 (0.14) 93.3 (0.28) 8.5 (0.03) 11.3 (0.04)  The decrease in CO2 permeability with RH is hypothesized to be due to the competitive sorption between water vapor and CO2; as the water vapor activity increases, its sorption coefficient increases and restricts the sorption of CO2 resulting in a net lower permeability (Sijbesma et al. 2008). This hypothesis is also supported by the fact that CO2 permeability decreases less at 50°C compared to 30°C, potentially due to a lower water vapor sorption at higher temperatures. In contrast with the water vapor permeability, the CO2 permeability increases with increasing test temperature. This may be a consequence of the greater increase in the diffusion coefficient of CO2 than the decrease in its solubility, in response to temperature increases (Reijerkerk et al. 2011). It is also noticeable that for the rubbery membrane samples, CO2 permeability is much higher than those obtained for the glassy polymers. This is attributed to the general behavior of rubbery polymers, showing much larger diffusivity for non-condensable gases due to their high chain flexibility. Moreover, the PEBAX polymer studied here is a block copolymer consisting of soft, rubbery, and hydrophilic PEO blocks. The ether linkage present in the soft PEO blocks makes this polymer selective for CO2 (Sijbesma et al. 2008). As a result, PEBAX is less selective for water vapor over CO2, thus less attractive for the ERV application. However, in applications with lower levels of indoor contaminants such as class 1 and class 2 air set by ASHRAE standard 62.1 (ASHRAE 2016), PEBAX is still a viable candidate due to its much higher water vapor permeability. Membrane selectivity Figure 3 shows the mixed H2O/CO2 selectivity of the membrane samples as a function of relative humidity in the feed stream at two test temperatures.     Figure 3 Water vapor/CO2 selectivity for four tested membranes: a) at 30°C and b) at 50°C In general, most dense polymer membranes show significantly higher permeability for water vapor compared to CO2, leading to a high selectivity. This is attributed to the high solubility (high critical temperature) and high diffusivity (small molecular size) of water vapor molecules (Metz, Van De Ven, Potreck, et al. 2005). Among the polymers investigated, the H2O/CO2 selectivity is higher for the glassy samples due to the much lower permeability of CO2 through the rigid polymer chains of these membranes. As can be observed in Figure 3, the selectivity of all of membranes increases with RH because the water vapor permeability increases significantly with RH, while the CO2 permeability slightly decreases or remains constant. G1 polymer shows the strongest increase in permeability with RH (up to about an order of magnitude at 80%RH at 50ºC) which is in line with its water vapor permeability behavior. In general, increasing the temperature from 30ºC to 50ºC reduces the H2O/CO2 selectivity, owing to the increased CO2 permeability at higher temperatures. For the G1 polymer, however, this trend is reversed for RH values over 70% due to the dominance of the water vapor permeability increase. CONCLUSIONS The effects of relative humidity and temperature on the permeation properties of a series of standard polymer membranes, suitable for ERV applications, has been investigated. The intrinsic water vapor permeability and selectivity for water vapor relative to CO2 (as a major indoor air contaminant) was measured for four different polymer samples via binary water vapor-Carbon dioxide permeation measurements. Although the results suggest that membrane permeation properties are strongly material-dependent, a number of general conclusions regarding the ERV application may be made:  The combination of very high water vapor permeability with high selectivity for water vapor over gaseous contaminants such as CO2 shows the great potential of polymeric membranes for ERV applications.  The permeation behavior of polymer membranes greatly depends on whether the polymer is below or above its glass transition temperature. In general, rubbery polymers show higher water vapor permeability rates, but less selectivity for water vapor over gaseous indoor contaminants (CO2 in this case). Glassy polymer membranes, on the other hand, show extremely low crossover rates for contaminants (<0.03%) over a wide range of operating conditions.  Depending on the membrane material, the operating temperature, and the relative humidity, the permeability and selectivity of membranes can vary up to about an order of magnitude. This variability would lead to significant changes in the actual performance of an ERV and the whole HVAC system, impacting building energy efficiency.   In general, increasing the water vapor permeability of a membrane leads to an increase in the latent effectiveness of ERV exchanger cores and higher energy saving potentials for high humidity conditions. However, the extent of increase in latent effectiveness of an ERV core is influenced by multiple factors (e.g. flow passage geometry and operating flow rates) beyond just the membrane properties.  Further work should investigate the effects of operating conditions in a similar range on the permeation of other indoor contaminant species, in particular VOCs with low threshold concentrations such as formaldehyde, toluene, etc. ACKNOWLEDGMENTS This work was financially supported by Natural Sciences and Engineering Research Council of Canada (NSERC) through a Collaborative Research and Development (CRD) grant sponsored by dPoint Technologies Inc.    NOMENCLATURE Pi  =  permeability, Barrer1 Ji  =  water vapor and CO2 flux, m3 (STP).m-2.s-1 (mL3 (STP).in-2.d-1) Q  =  gas flow rate, m3.s-1 (ft3.min-1)  pi  =  partial pressure, Pa (Psi) Vm  =  Molar volume of gas at standard temperature and pressure, 22.414 dm3.mol-1 (379.48 ft3.lbmol-1) R =  gas constant, 8.314 J.mol-1.K-1 (1545.349 lbf.ft.lbmol-1.oR-1) T =  temperature, K (°R) A =  membrane active area, m2 (in2) χ =  crossover (%) C =  concentration, mol.m3 (mol.in3) Subscripts 1 =  Feed inlet 2 =  Feed outlet 3 =  Sweep inlet 4 =  Sweep outlet REFERENCES Ansaloni, L. et al., 2014. Effect of relative humidity and temperature on gas transport in Matrimid??: Experimental study and modeling. Journal of Membrane Science, 471, pp.392–401. ASHRAE, 2016. ANSI/ASHRAE Standard 62.1-2016 Ventilation for Acceptable Indoor Air Quality. Huizing, R., Chen, H. & Wong, F., 2015. Contaminant Transport in Membrane Based Energy Recovery Ventilators. Science and Technology for the Built Environment, 21(January), pp.54–66. Huizing, R., Mérida, W. & Ko, F., 2014. Impregnated electrospun nanofibrous membranes for water vapour transport applications. Journal of Membrane Science, 461, pp.146–160. Metz, S.J., Van De Ven, W.J.C., Mulder, M.H. V, et al., 2005. Mixed gas water vapor/N2 transport in poly(ethylene oxide) poly(butylene terephthalate) block copolymers. Journal of Membrane Science, 266(1–2), pp.51–61. Metz, S.J., Van De Ven, W.J.C., Potreck, J., et al., 2005. Transport of water vapor and inert gas mixtures through highly selective and highly permeable polymer membranes. Journal of Membrane Science, 251, pp.29–41. Potreck, J. et al., 2009. Mixed water vapor/gas transport through the rubbery polymer PEBAX® 1074. Journal of Membrane Science, 338(1–2), pp.11–16. RAHARJO, R., FREEMAN, B. & SANDERS, E., 2007. Pure and mixed gas CH4 and n-C4H10 sorption and dilation in poly(dimethylsiloxane). Journal of Membrane Science, 292(1–2), pp.45–61. Reijerkerk, S.R. et al., 2011. Highly hydrophilic, rubbery membranes for CO2 capture and dehydration of flue gas. International Journal of Greenhouse Gas Control, 5(1), pp.26–36. Sijbesma, H. et al., 2008. Flue gas dehydration using polymer membranes. Journal of Membrane Science, 313(1–2), pp.263–276. Tsvigu, C. et al., 2015. Effect of relative humidity and temperature on the gas transport properties of 6FDA-6FpDA polyimide: Experimental study and modelling. Journal of Membrane Science, 485, pp.60–68. Wang, L., Haves, P. & Breshears, J., 2014. The Energy Saving Potential of Membrane-Based Enthalpy Recovery in Vav Systems for Commercial Office Buildings. Wijmans, J.G. & Baker, R.W., 1995. The solution-diffusion model: A review. Journal of Membrane Science, 107, pp.1–21. Zhang, L.Z., 2012. Progress on heat and moisture recovery with membranes: From fundamentals to engineering applications. Energy Conversion and Management, 63, pp.173–195. Zhang, L.Z. & Niu, J.L., 2001. Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong. Energy, 26, pp.119–135.                                                            1 For consistency with gas permeability unit commonly used in membrane technology literature, Barrer units are used for reporting permeability values of membrane samples in this study. 1Barrer = 7.5×10-18 m3 (STP).m.m-2.s-1.Pa-1 = 1.6691 mL (STP).mil.in-2.d-1.atm-1 

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