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Characterization of Fouling with Hygroscopic and Non-hygroscopic Aerosols in Composite Polymer Membranes… Engarnevis, Amin; Huizing, Ryan; Green, Sheldon; Rogak, Steven 2016-10

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Amin Engarnevis a*, Ryan Huizing b, Sheldon I. Green a, Steven N. Rogak aa Department of Mechanical Engineering, University of British Columbiab dPoint Technologies Inc.Characterization of Fouling with Hygroscopic and Non-hygroscopic Aerosols in Composite Polymer Membranes for Water Vapor Transport Applicationsa place of mindFigure 3: Particle mobility size distribution00. 100 1000Normalized dN/dlogDpParticle Size (nm)NaClGraphite• Particle deposition fraction measurement• Water Vapor flux (pre- and post-loading)• Pressurized air crossover leak rate (at 1 PSI)o Performance Testing of Membranes includes [5]:Table 1: Properties of membrane samplesOverviewCross-flow ERV coreGases Water vaporOnly transfer heat and vapor through the membraneNo transfer of gases, contaminants, and odors through the membraneMembraneCross-SectionContaminantsFigure 1: Ideal Membranes for building ERV applicationComposite membranes using a thin vapor-permeable polymerlayer over a structural substrate are used in gas dehydration,food-packaging, and humidity control of indoor spaces. Oneapplication of such membranes is in enthalpy exchanger coresused in Energy Recovery Ventilators (ERV) of building HVACsystems.There are many studies of membrane fouling from liquids (e.g.reverse osmosis [1]) and gases (e.g. micro- and ultra-filtration[2]) but we have found only one study for compositemembranes in HVAC-relevant conditions. Charles & Johnson [3]characterized the air-side particulate fouling of a hollow-fibermembrane during a membrane evaporative cooling process.They found significant biological growth, but only minor impactson water vapor transfer – not surprising because the fouling wasfrom clean air (~3000 particles/cc) over only 120 hours.In our work, the impact of accelerated exposure to air pollutionon the water vapor flux through commercial membrane media isinvestigated to develop an understating of potential air-sideparticulate fouling mechanisms and resulted performancedegradation during membrane lifetime in the field.1) Dry loading with RH<20% for the aerosol airstream2) Wet loading cycles in which dry loaded samples areexposed to intermittent humid conditions (RH~75%)leading to surface condensationExperimental MethodologyFigure 2: Experimental schematicDense selective layer (no pores)Membrane Cross Section [4]Coating layer: 1-5 µm thickMicroporous substrate: ~100 µm thickCoatingSubstrateH2Oo Composite membranes are composed of a densepolymer film coated on the surface of a porous polymersubstrate.o The dense coating layer (<10µm) provides a water-vapor-selective barrier and the porous substrate layerprovides the mechanical strength of the membrane.o The two sides of the membrane are referred to as‘coated’ and ‘uncoated’ sides.o Two aerosol types:o Upstream/downstream size distributions by TSI SMPS3080 (used in deposition calculations)o Membrane surface charge removed by immersion for 30minutes in isopropyl alcoholo Membrane samples are placed inside a counter-flow testmodule (active area of 456 mm2) that passes two airstreams on opposing sides of the membrane:o Cumulative exposure is approximately that of one year ofexposure in a heavily polluted environment.Figure 4: Composite Polymer membranes• Hygroscopic salt (NaCl, Dg=88nm) from TSI 3076 atomizer• Non-hygroscopic soot-like spark-generated graphite (SGG)(Dg=82nm) from a PALAS GFG 1000o Loading conditions:• A-to-B: particle-laden airstream flowing in a circuit thatallows for control of the size and concentration ofaerosol, the RH and the flowrate• C-to-D: sweep dry, HEPA-filtered airstream that allowscontrol of the flowrate and temperature.*Contact: amin.engarnevis@mech.ubc.cao Neutralization of surface charge byIPA reduces deposition of thesmallest particles.o Aerosol particles neutralized witha soft x-Ray neutralizer (TSI 3088)showed a tendency to formuniform, compact deposit layersleading to cake layer formation onmembrane surfaces and fluxreductions of up to 5% of theclean membrane value.o Although the state of the aerosoland surface charges influence thedeposition fraction and thedeposit morphology, it is shownthrough vapor flux measurementsthat they have little directinfluence on the degradation ofmembrane permeability.Uniform, compact deposit formed from neutralized particlesSparse aggregates formed from non-neutralized particlesFigure 6: Effect of particle charge on deposit formation patterns on membrane surfaces00. 100Deposition FractionParticle Diameter (nm)IPA-neutralized PP substrate & X-Ray ONIPA-neutralized PP substrate & X-Ray OFF00. 100Deposition FractionParticle Diameter (nm)Non-neutralized PP substrate & X-Ray OFFNon-neutralized PP substrate & X-Ray ONFigure 5: Impact of particle and surface charges on particle depositiono The effects of particle charge distribution, number concentration, temperature-gradient(Thermophoresis), and membrane surface on the rate of particle deposition were investigatedusing a TSI SMPS 3080.[1] Goosen, M. F. A., et al. "Fouling of reverse osmosis and ultrafiltrationmembranes: a critical review." Separation Science and Technology 39.10 (2005):2261-2297.[2] Guo, Wenshan, Huu-Hao Ngo, and Jianxin Li. "A mini-review on membranefouling." Bioresource technology 122 (2012): 27-34.[3] Charles, N. & Johnson, D., 2008. The occurrence and characterization offouling during membrane evaporative cooling. Journal of Membrane Science,319(1-2), pp.44–53.[4] Huizing, 2010. Coated membranes for enthalpy exchange and otherapplications, US20120061045.[5] Sylvester, A., et. Al. “Impact of Environmental Tobacco Smoke onMembrane-Based Energy Recovery Ventilators”, AAAR 35th Annual Conference,October 17 - 21, 2016, Portland, Oregon, USA.ReferencesMembrane Fouling Mechanisms Conclusionso Moderate membrane fouling by both hygroscopic andnon hygroscopic particles in dry conditions (i.e. nocondensation occurs) has minimal impact on watervapor flux of membrane samples.o Heavy loadings of such particles on both ‘coated’ and‘uncoated’ sides that form a thick cake layer(≥membrane thickness) can result in a slight flux decline(<5%) due to the added resistance of the cake layer.o Deposition of hygroscopic nanoparticles on the‘uncoated’ side of microporous substrate of membrane,in the presence of condensation, could significantlydecrease water vapor transport through membranesamples (up to 15%)o Air-side particulate fouling of composite membranes canbe controlled and minimized by measures such as: (1)Exposing coated side of membrane to the stream withmore nanoparticles; (2) Membrane module installationssuch that any potential condensation occurs on thecoated side; (3) Periodic membrane cleaning (e.g.washing with distilled water)Figure 7: Changes in membrane flux resulted from particle loading(a) Cake fouling layer of salt particles formedat low RH (<20%) does not affect the pores ofmembrane substrate.(b) Individual sub-micron particles grow andform super-micron crystals and aggregates atelevated RH (50%), but do not affect thesubstrate pores.(c) Salt particles dissolve in water droplets,nucleated on the membrane surface, and reachpores in aqueous form. These ions re-crystalizein dry condition resulting in pore-narrowing.o SEM images, as well as the fact that the loaded membrane flux can be restored to its pre-exposure value by a simple wash, imply that re-crystallization of salt ions dissolved in condensedwater onto the pores of membrane substrate is a potential explanation for the changes.Figure 8: Structural Changes of hygroscopic salt particles deposited at different RH.0.700.750.800.850.900.951.001.050 2 4 6 8 10Normalized Water Vapor FluxLoading Area Density (gr.m-2)SGG-Dry-Uncoated SubstrateSGG-Dry-Uncoated side of membraneSalt-Dry-Uncoated substrateSalt-Dry-Uncoated side of membraneSalt-Wet-Uncoated SubstrateSalt-Wet-Uncoated side of membraneSGG-Wet-Uncoated side of memnraneTypical Repeatabilityo Membranes wet-loaded with hygroscopicparticles on the ‘uncoated’ side (Fig. 7)showed vapor flux decline up to 15%, whilstmembranes loaded on the ‘coated’ side didnot show a significant flux decline undersimilar wet loading conditions.o Uncoated substrate samples show aconsistently higher flux decline comparedto membranes under similar wet loading.This supports the hypothesis that observedflux decline is caused by increasedresistance of the microporous membranesubstrate due to a pore-narrowing process.Pore-narrowingParticle Deposition on Membrane Surfaces


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