Water Treatment Membrane Processes - Introduction -
CEEN 572 Environmental Pilot Lab Water Treatment Membrane Processes - Introduction - Membrane Separation Membrane Feed Permeate Particle or Solute Molecule
Membrane processes are used in water treatment for solid/liquid separation. RO and NF are pressure-driven membrane processes. In this figure, the orange circles represent the particles or solute molecules and the blue circles represents the solvent (water). The feed solution is forced under pressure through the membrane (animate) and the result is a relatively clean permeate solution. Separation occurs because the membrane transports one component (the water) more readily than any other components. Particle or Solute Molecule Solvent Definition of Membrane Process
In a membrane separation process, a feed consistingof a mixture of two or more components is partiallyseparated by means of a semipermeable barrierthrough which one or more species move faster thanthe other species In water and wastewater treatment applications,membrane processes are used as a solid/liquidseparation process. In this case, water is morereadily transported through the membrane thansolids (both suspended and dissolved) Classification of Membrane Operations
Driving force Mechanism of separation Membrane structure Phases in contact Classification of Membrane Operations
Pressure-driven membrane operations Permeation operations Dialysis operations Pressure-driven Operations
Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF) Reverse Osmosis (RO) Pressure-driven Membrane Processes Pressure-driven Membrane Processes
The solvent is transferred through a densemembrane tailored to retain salts and low- molecular-weight solutes To produce pure water from saline solution, feedpressure must exceed the osmotic pressure of thefeed solution In order to obtain economically viable flows, at leasttwice the osmotic pressure must be exerted ashydraulic pressure (e.g., bars (700-1,100 psi)for seawater) organic molecules MW nm inorganic ions MW nm water MW nm Pressure-driven Membrane Processes
NF Sometimes referred to as low-pressure RO or membrane softening process Lies between RO and UF in terms of selectivity of the membrane Designed to remove multivalent ions but can remove sodium and chloride fairly well Looser NF membranes are more like UF and tighter NF membranes more closely resemble RO Recently has been employed for organic control Typical operating pressure: 5-14 bar ( psi) organic molecules MW nm inorganic ions MW nm water MW nm Pressure-driven Membrane Processes
UF Considered as a clarification and disinfection operation Membrane is porous and rejects most macromolecules, microorganisms, and all types of particles Osmotic pressure effects are negligible Typical operating pressure: bar (7-70 psi) MF Major difference between MF and UF is pore size micron for MF Primary application is particulate removal (clarification) Typical pressures like UF organic molecules MW nm inorganic ions MW nm water MW nm Selection of Membrane Processes
loose and tight Membranes in Treatment of Drinking Water
The application of specific pressure-drivenmembrane process is highly dependent on thecharacteristics and quality of the source water Surface water: MF, UF, NF Groundwater (fresh): MF, UF Groundwater (brackish): MF/UF pretreatment, NF, RO Seawater: MF/UF pretreatment, NF, RO Membranes in Treatment of Wastewater
The application of specific pressure-drivenmembrane process in wastewater treatment ishighly dependent on the characteristics/quality ofthe source water and the pretreatment process/esused Raw wastewater: MF/UF, MBR, FO (not mainstreamyet) Effluent: MF/UF pretreatment, NF, RO Membrane Technologies and their Traditional Counterparts
Membrane Separation Technology Constituents Removed Comparable traditional Water Treatment Method MF Bacteria and large colloids; precipitates and coagulates Ozonation-UV, chlorination, sand filtration, bioreactors, coagulation-sedimentation UF All of the above + viruses, high MW proteins, organics Sand filter, bioreactor, activated carbon NF All of the above + divalent ions, large monovalent ions, color, odor Lime-soda softening, ion exchange RO All of the above + monovalent ions Distillation, evaporation, ion exchange ED/EDR Dissolved ionic salts Ion exchange Target Solutes MF: Microbes (protozoa and bacteria)
Turbidity (particles and colloids) UF: Same as MF + viruses, some NOM NF: Same as UF + NOM, SOCs (e.g., Atrazine), Divalent cations (Ca2+, Mg2+, Zn2+, Cd2+, etc.), Polyvalent anions (SO42-, PO43-, AsO43-, CrO42-, etc.) RO: Same as NF + simple ions (TDS, NO3-, ClO4-) MF + Coagulant: viruses, NOM (also fouling reduction) UF + PAC: SOCs, NOM (also fouling reduction) Submerged MF and UF: Fe and Mn (aeration), NOM (with coagulant), SOCs (with PAC) Ranges of Pressure and Flux
Membrane Class Pore Size or MWCO (m or Dalton) Pressure Flux psi kPa gfd (gal/ft2day) LMH(L/m2hr) MF 0.1 0.5 mm 10-100 60 120 100 200 UF 1 100 kD 50-500 30 60 50 100 NF 100 500 D 700-2,800 15 30 20 50 RO n/a ,000 1400-7,000 MF (Immersed) 0.2 mm -1.4 -10 UF (Immersed) 0.04 mm -7.0 -50 35 85 conversions 1 atm = kPa (kN/m2) = 14.7 psi 1 kPa = psior 1 psi = 6.90 kPa 1 psi = atm gfd = LMH x 1.7 Ranges of Energy Consumption
Membrane Class Recovery Pressure Energy consumptionkWh per psi kPa 1,000 gal m3 MF 94-98 15 100 0.1 0.4 UF 70-80 75 525 0.8 3.0 NF 80-85 125 875 1.4 5.3 LPRO 70-85 225 1,575 2.7 10.2 RO 400 2,800 4.8 18.2 ED 75-85 2.5 9.5 Permeation Operations
Gas Permeation (GP) Gas Diffusion Pervaporation (PV) Membrane Stripping (MS) Membrane Distillation (MD) Engineered Osmosis (EO) Gas Separation Industrial Applications of GP:
Separation of H2 from CH4, (H2 permeation ratethrough dense membrane is very high) Adjustment of H2-to-CO ratio in synthesis of gas O2 enrichment of air Removal of CO2 Drying of natural gas and air Removal of He and organic solvents from air Gas Separation AIR N2 VENT Pervaporation (PV) Liquid/vapor separation liquid partially vaporized through a dense membrane Solvent dissolves in the polymeric membrane, diffuses, and evaporates on the permeate side Rate of transfer of a constituent depends on its solubility in the membrane Activity difference maintained by partial vacuum on the permeate side Separation of solvents PV is used to separate dissolved volatileorganic compounds (VOCs) fromaqueous solutions Pervaporation (PV) PV membrane separation systems are used in foodprocessing, gas separation, and water treatment.Examples of applications include: Recovery of flavor compounds from food industry processstreams Recovery of ethanol from fermentation and food industry process streams Removal of organic contaminants from waste streams C2H4/C3H8 Membrane Stripping and Membrane Distillation (MD)
Separation of volatile constituents from solution: Removal of volatile solutes or volatile solvent Microporous hydrophobic membrane Polypropylene, PTFE, PVDF, nylon Heated aqueous feed solution is brought intocontact with feed side of the membrane The hydrophobic nature of membrane preventspenetration of aqueous solution into the pores Lower vapor pressure on the permeate side of themembrane induces evaporation through the pores Pores remain dry throughout the process ! Membrane Stripping and Membrane Distillation Basic Configurations
Feed Solution Clean Water Feed Solution Sweep Gas Air Gap Feed Solution Direct Contact MD (DCMD) Sweeping Gas MD Air Gap MD Feed Solution Vacuum = Membrane Stripping Vacuum MD (VMD) Brine Reconcentration
Engineered Osmosis Forward osmosis (applications: wastewater treatment,pretreatment, desalination, concentration) Brine Reconcentration Draw Solution Feed DP=Dp Brine Feed Osmosis Forward Osmosis (FO) engineered osmosis UFO-MBR System Wastewater Feed Stream Diluted DS DS Tank Anoxic Tank
RO Permeate Tank Concentrated DS UF Permeate and Backwash Tank Dialysis (DIA) Donnan Dialysis Electrodialysis (ED)
DialysisOperations Dialysis (DIA) Donnan Dialysis Electrodialysis (ED) Dialysis Operations The solute is transferred through the membrane, not the solvent The driving force is activity or an electrical potential difference Dialysis (DIA) The driving force is a transmembrane concentration difference Selective passage of ions and low-molecular-weight solutes and rejection of larger colloidal and high-molecular-weight solutes Main application is hemodialysis Electrodialysis (ED) Operation by which ions diffuse through ion- exchange membrane under the influence of electricpotential Ion exchange membranes AEM: quaternary amines CEM: carboxylates, sulfonates Plate and frame stacks Efficient for desalination of brackish water Electrodialysis Reversal (EDR)
EDR is a variant of ED The same membranes are used to provide a continuous self-cleaning electrodialysis process which uses periodic reversal of the DC polarity to allow systems to run at higher recovery rates Polarity reversal causes the concentrating and diluting flow streams to switch after every cycle Any fouling or scaling constituents are removed when the process reverses, sending fresh product water through compartments previously filled with concentrated waste streams EDR systems operate with higher concentrations in the brine or concentrate streams with less flow to waste Electrodialysis Reversal (EDR)
Applications for EDR technology include municipaldrinking water, industrial process water, andwastewater reuse projects Municipalities find EDR successful for the removal ofradium, arsenic, and perchlorate as well asdesalination of well and surface waters Other Classifications Separating Mechanisms
Separation based on difference in size (sieving) MF, UF, DIA Separation based on difference in solubility anddiffusivity of material in the membrane (solution- diffusion mechanism) GP, PV, RO, FO Separation based on difference in charges of thespecies to be separated (electrochemical effects) ED, EDR Rejection Capabilities (pressure-driven processes)
RO membranes are typically characterized by manufacturersin terms of NaCl rejection, e.g., 96% or 99.9% NaCl rejection NF membranes may be characterized in terms of NaCl orMgSO4 rejection or they may be characterized in terms ofmolecular weight cut-off (MWCO)*, e.g., 98% MgSO4 and 80%NaCl rejection UF membranes are typically characterized using MWCO, e.g., 13,000 or 80,000 MWCO MF membranes are typically characterized by pore size, e.g.,0.1 or 1 m * MWCO is determined by fitting rejection data of acromolecules (e.g.,dextrans or proteins) * MWCO determined by fitting rejection data of non-polar organics Porosity Porous membranes (MF, UF, NF, DIA) Nanoporous membranes
Macroporous: > 50 nm Mesoporous: 2 50 nm Microporous: < 2 nm Nanoporous membranes Dense media Diffusion of species takes place in the free volume presentbetween the macromolecules chains of the membranematerial IX membranes Specific type of nanoporous membranes Morphology Symmetric Asymmetric Single Material
Cylindrical Porous Porous Homogeneous (resistance to mass transfer is determined by total membrane thickness) Asymmetric Single Material top layer Porous Porous with Top Layer Asymmetric Composite dense skin layer (0.1 to 0.5 m) porous membrane (50 to 150 m) (resistance to mass transfer determined by skin layer thickness) Structure of Membranes Geometry / Packaging Flat-sheet membranes (spiral wound, plate-and- frame) Tubular membranes (shell-and-tube, immersed) Tubes Capillaries Hollow fibers Geometry / Packaging Spiral Wound Membrane Manufacturing (video + audio) Spiral Wound Module Single Element vs. Bank or Array
Roga Module 1 ca. 1964 Spiral Wound Module Installation Hollow Fiber Membrane Single Fiber (left) vs. Module (right) Hollow Fiber Membrane Module (left) vs. bank (right) Submerged Membrane MF/UF
Uses Surface water treatment Pretreatment for RO Membrane bio reactors (MBR) Filtration for non-potable reuse (add MF after secondary WW treatment and produce water for irrigation) Operation Membranes are immersed in basin of feed water Operate under suction Advantages Operate at lower pressures than pressurized systems Less fouling potential - good for wastewater treatment Membrane cleaning and fixing Submerged Membranes Filtrate Feed Water To RO Air Bubble Scouring of
Basin Filtrate Feed Water To RO Air Bubble Scouring of Membrane Surface MF/UF Hollow Fiber Membranes System Configuration Mix Fill and React (1 hr) BR1 MT BR2
WELCOME TO MINES PARK BR2 React Draw (1 hr) SEPTIC TANK Permeate Tank Immersed Membrane (GE / Zenon) Cassette vs. System Immersed Membrane Backwash (USFilter) Submerged MBR - Zenon Submerged Membrane Designs Flow Configuration Cross Flow vs. Dead End Filtration
Cross Flow Operation Feed Concentrate Permeate Dead End Operation Feed concentrate/retentate/reject/brine permeate/product Permeate Cross Flow Operation Feed flow is parallel to membrane surface
Concentrate vt vd Permeate Feed flow is parallel to membrane surface Retained particles are scoured Have concentrate stream Preferable for concentrated solutions to controlthickness of deposit on membrane (fouling) Dead End Operation Feed flow is perpendicular to membrane surface
Permeate Feed flow is perpendicular to membrane surface Retained particles form a cake layer on surface No concentrate stream Preferable for dilute solutions due to lower energyrequirements (pumping) Comparison of Cross-flow Membrane Configurations
Cost Packing Density Operating Pressure Capacity Membrane Types Fouling Resistance Cleanability Traditional Spiral-Wound Low High Many Fair Hollow Fiber UF-High RO-Very High UF-Low RO-High Few UF-Good RO-Poor Tubular UF-Moderate Very Good Plate & Frame Moderate Adapted from "Select Engineering Principles of Crossflow Membrane Technology" Osmonics Inc. Technical Paper, P/N 56821 Membrane Materials Polymeric membranes: Polysulfone Polyethersulfone
Polyphenylsulfone Polyvinylidene Fluoride (PVDF) Polypropylene (PP) Polyethylene (PE) Cellulose and Cellulose acetates (CA) Polyamide (PA) Polyacrylonitrile (PAN) Polytetrafluoroethylene (PTFE) Polycarbonate (PC) Polymethylmethacrylate (PMMA) Ceramic membranes: Aalumina Titania Zirconia ATZ mix chemical, mechanical and thermal stability ability of steam sterilization and back flushing high abrasion resistance high fluxes durable bacteria resistance possibility of regeneration dry storage after cleaning Membrane Properties Pure water permeability (PWP) Pore size
Molecular Weight Cut-Off (MWCO) Hydrophobicity/hydrophilicity Surface/pore charge Surface roughness Chemical stability / chlorine tolerance Principles of Mass Transport and Rejection in Pressure-Driven Membrane Processes
particles macromolecules ions microbes Overview RO, NF, UF, and MF have many similarities(geometry, flow configuration, material) Principals of rejection differ substantially In RO, function of relative affinity of solute and solvent tothe membrane In MF, mainly due to physical sieving Membrane Performance The performance of a membrane is determined bymainly two parameters, flux and rejection: Flux (J), or permeation rate, is the volume flowing throughthe membrane per unit area per time (Q/A) Rejection (R), refers to a local relationship betweenupstream and downstream concentrations Another important parameter is recovery (r), whichis defined as the amount of material collected as auseful product divided by the total amount of thematerial entering the process: in membraneseparations, the useful product is most often thepermeate water Water and Solute Flux Water flux (Jw), or permeation rate, is the volume flowing through the membrane per unit area per time (Q/A) In membrane processes it is a function of driving force, membrane properties, and feed quality Specific permeate flux is the water flux calculated above normalized to the applied driving force Note: in MF/UF, DP = net applied pressure (NAP) Water and Solute Flux Solute Flux is the mass of solute flowing through themembrane per unit of area per time In membrane processes it is a function of drivingforce (concentration), membrane properties, andsolute/particle properties Water Recovery Rate (r)
The ratio of the useful product (permeate) flow rate and theflow rate of feed to the process Global recovery rate: Where Qp is the product (permeate) flow rate and Qf is the feed flow rate In membrane processes, because of the modularity andvarious configurations, it is important to distinguish betweenmembrane/module recovery and system recovery Material Balance in Membrane Separation
A, B Concentrate Qc, Cc, Pc Feed Qf, Cf, Pf Permeate Qp, Cp, Pp Material Balance in Membrane Separation
Mass balance for water flow Qf = Qc + Qp Mass balance for solute flux QfCf = QcCc + QpCp Product recovery r = (Qp/Qf)100% Example of Process Recovery
Assuming each membrane (or each stage) operatedat 20% recovery, what is the total system recovery R1 gal R2 gal 100 gal R3 gal P1 gal P2 gal P3 gal Example of Process Recovery
Repeat the exercise with 50% recovery per stage,what is the production rate of the third stage? R1 gal R2 gal 100 gal R3 gal P1 gal P2 gal P3 gal Staging In RO and NF operations, membranes are often staged
tapered design compensates for loss of feed volume throughsystem Recirculation In MF and UF, some concentrate is often recirculatedto the inlet Flexible (can control degree of recirculation) Economics (tradeoff between power for recirculationpump and additional recovery) Material Balance in Membrane Separation
Mass balance for water flow Qf = Qc + Qp Mass balance for solute flux QfCf = QcCc + QpCp Product recovery r = (Qp/Qf)100% Global Rejection Rejection (R) Location-specific ratio of product concentration andfeed concentration Global rejection where cp is the solute concentration in the permeate and cf is thesolute concentration in the feed Global system rejection May yield different value as function of time Variability of feed, permeate {Cp=f(R)}, membrane condition Rejection (R) Local rejection due to change in bulk feed concentration in theflow channel cwall cbulk cfeed If we know the permeate flux and mass transfer coefficient(we will talk about it later), the concentration at themembrane surface can be predicted by calculating apolarization factor (PF) cwall = PF cbulk Apparent rejection calculated based on bulk concentration: Temperature Effects Higher transmembrane pressure in the winter, or
More membranes in the winter to preventfouling/cleaning Water demand difference between summer andwinter may offset loss in membrane productivity Membrane fouling Temperature Effects Change in temperature may result in a wide range ofeffects that go beyond the viscosity of the permeatealone Different ways to model effects of temperature: Arrhenius equation: JT = J20 exp (s/T) J20 = permeate flux at reference temperature of 20 C s = empirical constant, membrane specific T = temperature For MF and UF: Flux20 = FluxT (T/20); or Temperature Corrected Flux (TCF) Temperature Effects Primary effect due to influence on viscosity
For temperatures in the range of 0-35 C (centipoise) = T x10-4 T2 centipoises = Pa-sec x 1,000 (Pa-sec) = 3.797x10-11 T4 9.963*10-9 T x10-6 T2 5.589x10-5 T x10-3 Membrane Fouling Deposition Scaling Biofouling Organic fouling
Silt and suspended solids Scaling Inorganic deposits formed due to concentration of sparingly soluble salts beyond the chemical solubility limit Biofouling Microbiological growth entering or within element Organic fouling Interactions of natural or synthetic organics Organic compounds can have a much greater effect on permeate flux than clays or other inorganic colloids, even at lower mass concentration. Scaling SEM Scaling SEM Silt Density Index (SDI)
Empirical test of filterability Measures the tendency of a raw water to foul amembrane Use 0.45 m filter in a dead-end filtration cell ti time required to filter a fixed volume of raw waterthrough a clean membrane (~500 ml) tf time required to filter the same volume after themembrane has been used for a defined length of time Standard conditions: 47 mm filter, 2 bar (30 psi)transmembrane pressure, total time (tt) of 900 sec Water Treatment Membrane Processes Microfiltration and Ultrafiltration
CEEN 470 Water and Wastewater Unit Operations Water Treatment Membrane ProcessesMicrofiltration and Ultrafiltration Overview Initial use of deep filtration microfiltersdisposable,not sustainable MF membranes provide removal by retention ofcontaminants on the membrane surface Lowest pressure membrane process Pore size of 0.05 5 micron Cake filtration provides additional removalcapabilities smaller particles than pore size can beremoved Current Status MF and UF generally accepted as being capable ofmeeting filtration requirements for drinking waterproduction Turbidity removal / disinfection MF can resolve the conflict between need to provideprimary disinfection and DBP formation LT2ESWTR identified membranes as treatmenttechnique for higher level removal ofcryptosporidium Substantial diversification of membrane processesand configurations Filtration Spectrum Treatment Capabilities
Removal of particulate matter Turbidity Particles Microbial control Removal of organic and inorganic species when feedwater is pretreated (coagulation, adsorption) DOC/DBP precursors color / taste / odor Pesticides Iron / manganese (aeration / chemical oxidation) Arsenic Treatment Capabilities
Parameter Pretreatment needed for substantial removal MF UF Particulate/microbial Turbidity None Protozoa Bacteria Viruses Coagulation Organic TOC Coagulation / PAC DBP precursor Color T&O Pesticides PAC Inorganic Iron & manganese Oxidation Arsenic Hydrogen sulfide Modes of Application Turbidity Removal Particle Removal Water Permeation Across Clean MF/UF Membranes
Pure water transport through clean porousmembrane is: Directly proportional to transmembrane pressure (P) Inversely proportional to viscosity () Modeled using modified form of Darcys Law: Rm hydraulic resistance of the clean membrane towater permeation (units?) (mu (Pasec)) Rm = 1/m Example: Membrane resistance
An MF membrane is tested in the lab by filteringclean, deionized water and the flux is found to be2,000 LMH (L/m2-hr) at 20 C and 0.7 bar. Calculate the membrane resistance coefficient. Water Permeation Across Clean MF/UF Membranes
Absolute transmembrane pressure vs. pressuregradient Typical units of water flux gfd (gal/ft2-day) LMH (l/m2-hr) LMH x 1.7 = gfd Flow Through a Cylindrical Pore (Poiseuilles Law)
P/z is pressure gradient In real membranes pores are not perfectly cylindrical Dimensionless tortuosity factor (t) is often introduced Flow Through Membrane Pores
rpore = #pores/A Significant Parameters
Pore size has the highest effect on resistance towater flow Pore size distribution Specific flux for membrane comparison Calculated based on area on feed side Reduction in Membrane Productivity
Flux Decline Mechanisms Fouling Concentration polarization Resistance in Series Reductions in Permeate Flux
Crossflow Filtration Pressure Feed Concentrate Membrane Permeate Reduction in Permeate Flux over Time
Reversible vs. irreversible fouling Increase in Transmembrane Pressure Over Time
Reversible vs. irreversible fouling Classwork: UF Recovery
Assuming the following operating scenario: UF treatment plant with 48 HydraCap60 membraneelements 20 min operation with average productivity of 55 gfd backwash with product water for 45 sec uses 1500 gal ofpermeate What additional information is needed? What is the water recovery rate? What is the backwashing flux? 500 ft2 per element total 24,000 ft2 120 gfd backwash 120gfd x 24,000 ft2 x d/1440 min x min/60 sec x 45 sec = 1500 gal High-PressureMembrane Technologies: Nanofiltration and Reverse Osmosis Membrane Separation Nanofiltration (NF) Membrane Softening
Reverse Osmosis (RO) Hyperfiltration RO and NF Membrane Applications
Desalting/total dissolved solids 1967 first spiral-wound configurations (General Atomics) for brackish water desalination Disinfection by-product precursors Leading to TTHMs and HAA formation Hardness, color, and turbidity After introducing NF membranes in 1986 (Filmtec Corp.) Inorganic chemicals For heavy metal, nitrate and fluoride removal RO and NF Membrane Applications
Synthetic organic chemicals Pesticide removal since the 1990s Pathogens Very effective, but usually no driver for implementation Indirect Potable Reuse since the 1980s using reclaimed water for direct injection Colligative Properties of Ionic Solutions
Properties of solution that depend on the number ofsolute molecules present, but not on the nature ofthe solute (number of particles in a given volume ofsolvent and not the mass of the particles) Osmotic pressure, vapor pressure, freezing pointdepression, and boiling point elevation are examplesof colligative properties Osmotic pressure and vapor pressure are twoproperties of solution that play a major role invarious membrane processes Ionic vs. Covalent Electrolyte vs. Nonelectrolyte
Substance when dissolve can break up to ions or stayintact (i.e., NaCl vs. sugar) Type % Ionization Solubility Electrolyte: conduct electricity Strong electrolyte 100% Very soluble Weak electrolyte < 100% Slight-to-very soluble Nonelectrolyte: does not conduct electricity No conduction 0% Insoluble or soluble Semipermeable membrane
Solute molecule Solvent molecule Osmosis Semipermeable membrane: permits passage of somecomponents of a solution, example: cell membranesand cellophane Osmosis: the movement of a solvent from low soluteconcentration to high solute concentration There is movement in both directions across asemipermeable membrane (!) As solvent moves across the membrane, the fluidlevels in the arms of a U-tube becomes uneven Eventually the pressure difference between the armsstops osmosis Experiment in Osmosis Examples of Osmosis Cucumber placed in NaCl solution loses water toshrivel up and become a pickle Limp carrot placed in water becomes firm becausewater enters via osmosis Salty food causes retention of water and swelling oftissues (edema) Water moves into plants through osmosis Salt added to meat or sugar to fruit preventsbacterial infection (a bacterium placed on the saltwill lose water through osmosis and die) Osmotic Pressure Osmotic pressure, , isthe pressure requiredto stop osmosis = osmotic pressure M = molarity (mol/L) R = Ideal Gas Constant = Latm/molK T = temperature (K) Osmotic Pressure Applied pressure needed to stop osmosis STEP 1
Pure Solvent Solution Semipermeable Membrane Osmotic Pressure Osmotic Pressure is a VERY sensitive measure ofMolarity Seawater contains 34 g NaCl per literM = 34 g/L / 58.5 g/mol = M 25 C) = (0.582 mol/L)2( Latm/molK )(298 K)= 28.4 atm 1 atm supports column of water m high (28.4 atm)(10.34 m/atm) = 294 m (294 m)(3.28 ft/m) ~ 963 feet Hoover Dam (726.4ft (221.4m)) Vant Hoff Factor (i) Factor which accounts for deviation of colligativeproperties due to electrolytic nature of solutes Vant Hoff Factor (i) can be measured by Typical nonelectrolyte (i.e., urea, sucrose, glucose): i = 1 Electrolytic solute: Electrolyte Ideal i Measured i NaCl 2 1.9 HIO3 1.7 MgCl2 3 2.7 AlCl3 4 3.2 Discrepancy between Real and Ideal Vant Hoff Factor i
Consider: AlCl3 Al Cl Ideally i = 4 because four particles in solution, butactually Real i = 3.2 The reason measured value (3.2) is lower than ideal:strong electrolytic attraction forces betweenoppositely charged ions causes some of the ions tobe held together through ion-pair Vant Hoff factor follows ideal value best when theconcentration is very low. Large deviation from VantHoff factor with very high concentrations Exercise A mixture of solid NaCl and solid sucrose, C12H22O11has an unknown composition.When 15.0 g of themixture is dissolved in enough water to make 500 mLof solution, the solution exhibits an osmotic pressureof 6.41 atm at 25 C. Determine the mass percentage of NaCl in the mixture. What mass of NaCl and sucrose must be added so that theosmotic pressure is 10 atm at 25 C ? Reverse Osmosis / Nanofiltration
Pressure greater than soln STEP 2 Pure Solvent Solution Semipermeable Membrane Mass Transfer and Permeate Flux
Water (permeate) Flux Jw = Qp/A = Aw(P ) [gfd, LMH] Js = BDCs Kw = water mass transfer coefficient P = transmembrane pressure differential = reflection coefficient (unitless) = transmembrane osmotic pressure differential A = effective membrane area B = salt permeability Specific Permeate Flux Jw = Qp/A*P [gfd/psi, LMH/kPa] Note: P-p = net applied pressure (NAP) Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute The Reflection Coefficient,
1 < < 0 = 0, non-selective = 1, semipermeable solute water pore solute water pore Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute The Reflection Coefficient,
1 < < 0, partial selective solute water pore Kw: also pure water permeability Explain reflection coefficient for one pore, 0 or 1. S is than the percent of pores small enough to reject The solute Mass Transfer and Permeate Flux
Mass transfer limited Permeate flux independent of transmembrane pressure Film Layer Model Permeate flux k: mass transfer coefficient Linton-Sherwood Correlation: Film layer model is adaquate to simulate permeate flux when true solutes accumulate near the membrane surface. Clean Water Flux Transmembrane pressure Concentration Polarization and Membrane Scaling
Cp Product Cb Bulk Boundary Layer Cm Cs Membrane Scale J J = Qp/Ah K = 1.62(UD2/2bL), U=Qt/Av J is convective flux toward membrane Ddc/dx is diffusive flux away from membrane cb is concentration of bulk solution cs is saturation concentration cm is concentration at membrane surface cp is concentration of permeate is concentration boundary layer in this layer cb increases to cm if cm>cs then precipitation/scaling occurs Mass Transfer and Permeate Flux
Temperature Effects JT = J20 exp (s/T) J20 = permeate flux at reference temperature of 20 C s = empirical constant, membrane specific T = Temperature Temperature Corrected Flux (TCF) Temperature Effects Water Flux (L/m2-hr) NaCl Conc. (g/L) 2 4 6 8 10
12 14 16 18 20 22 25 35 45 55 65 75 NaCl Conc. (g/L) Water Flux (L/m2-hr) Feed 20 degC - Permeate 20 degC Feed 25 degC - Permeate 20 degC Feed 30 degC - Permeate 20 degC No Temp Control Feed 30 degC - Permeate deg30C Influence of Operating Parameters on RO
Important implication of SDmodel is that rejectionincreases with feed pressure Water flux (DP - Dp) Salt flux dcs/dz Increasing feed pressure doesntchange salt flux! Flux and recovery inverselyrelated due to reduction indriving force from feed toretentate in the module Impact on Permeate Flux
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 50 100 150 200 250 Runtime (hours) Normailzed Permeate Flux (J/Jo) NF-90 NF-270 NF-4040 TFC-S The importance of fouling is further underscored by its effect on the permeate flux of four different membranes with very different initial permeabilities.