Chapter 7. Polarization Phenomena & Membrane Fouling (Part II)

Transcription

1 National November 18, 2015 (Wed) (Part II) Chang-Han Yun / Ph.D.

2 Contents 7.9 Concentration Polarization in Diffusive Membrane Separations 7.10 Concentration Polarization in Electrodialysis Contents Contents 7.11 Temperature Polarization 7.12 Membrane Fouling 7.13 Methods to Reduce Fouling 7.14 Compaction 2

3 7.9 Concentration Polarization in Diffusive Membrane Separations Processes characterized by a solution-diffusion mechanism Assumptions for existence of resistance(<figure 7-20>) (dependent on the hydrodynamics, resistance of the membrane for specific permeating solute) 1. Resistance only in membrane (boundary layer resistances = negligible) 2. Resistance in boundary layer and membrane or Resistance only in boundary layer <Figure 7-20> Concentration profiles for diffusive membrane processes: (a) without boundary layer resistances, and (b) with boundary layer resistances. where C sm i,1 = feed concentration of component i at feed side membrane surface C sm i,2 = permeate concentration of component i at permeate side membrane surface 3

4 7.9 Concentration Polarization in Diffusive Membrane Separations Distribution coefficient (K) = (7-45) Flux expression At steady state, flux of i = same in each phase Flux of i in feed-side boundary layer, J i = k 1 (c sm i,1 - c s i,1) (7-46) Flux of i in permeate-side boundary layer, J i = k 2 (c sm i,2 - c s i,2) (7-47) Flux through membrane, (7-48) Eq(7-45) Eq(7-48) : (7-49) Overall mass transfer coefficient (k ov ) Eq(7-46) + Eq(7-47) + Eq(7-49) J i = k ov (c s i,1 - c s i,2) (7-50) Overall mass transfer coefficient (k ov ) : (7-51) 4

5 7.10 Concentration Polarization in Electrodialysis Difference of Electrodialysis(ED) with pressure-driven membrane processes Driving forces & Separation principle Polarization phenomena = severely affect the separation efficiency Example for illustration for phenomenon of concentration polarization System Cation-exchange membrane between cathode anode Solution : NaCl in water Flow of Na + : left to right(to cathode) through membrane Membrane resistance = negligible Concentration on the left-hand side of membrane Concentration on the right-hand side of membrane Generate diffusive flow <Figure 7-21> Concentration polarization in electrodialysis in the presence of a cation-selective membrane. 5

6 7.10 Concentration Polarization in Electrodialysis Flux expression Flux of Na + through membrane by electrical potential difference : (7-52) Transport of Na + in boundary layer by electrical potential difference : (7-53) Diffusive flow in the boundary layer : (7-54) where J m = electrically driven fluxes in membrane J bl = electrically driven fluxes in boundary layer J bl D = diffusive flux in the boundary layer t m = transport numbers of the cation in membrane t bl = transport numbers of the cation in boundary layer z = valence of the cation F = Faraday constant i = electrical current dc/dx = concentration gradient in boundary layer 6

7 7.10 Concentration Polarization in Electrodialysis At steady state, flux of Na + through membrane = electrical & diffusive flux in boundary layer (7-55) Assuming constant diffusion coefficient (linear concentration profile) and integration BC 1 : c = c m at x = 0 BC 2 : c = c b at x = δ Reduced cation concentration : (7-56) Increased cation concentration : (7-57) Ohmic resistance is located mainly in boundary layer if the concentration becomes too low. Occur ion depletion resistance dissipate electrical energy as heat (electrolysis of water) From Eq(7-56) current density (i) in boundary layer : (7-58) Electrical potential difference i & J of Na + Na + concentration [see Eq(7-58] Na + concentration at membrane surface (c m ) 0 obtain a limiting current density( i lim ) : (7-59) 7

8 7.10 Concentration Polarization in Electrodialysis To minimize the effect of polarization Minimize thickness of boundary layer hydrodynamics and cell design = very important Use feed spacers and special module designs For same valence in the boundary layer(equal thickness of boundary layer, same cell construction) Mobility of anions > cation i lim at cation-exchange membrane < at an anion-exchange membrane 8

9 7.11 Temperature Polarization Temperature polarization for non-isothermal process like membrane distillation Temperature polarization : temperature difference between liquid in bulk membrane System Feed : high saline water with high temperature high vapor pressure Strip : low saline water with low temperature Membrane : hydrophobic and pore filled with air Heat flux Evaporation Conduction through membrane wall & pore Diffusion of water vapor Heat transfer resistance Membrane Boundary layer <Figure 7-22> Temperature polarization in membrane distillation. 9

10 7.10 Temperature Polarization At steady state, heat flux(φ) through the boundary layers = flux though membrane Heat balance over the membrane from feed to permeate (7-60) where α 1 = heat transfer coefficients on warm side of the membrane α 2 = heat transfer coefficients on cold side of the membrane φ ΔH v and φ ΔH c = heat fluxes caused by convective transport through the pores l = membrane thickness λ m = overall heat conductivity of the membrane <Assume> φ ΔH v = -φ ΔH c T b,1 T m,1 = T m,2 - T b,2 = ΔT bl (ΔT in boundary layer) T m,1 - T m,2 = ΔT m (ΔT across membrane) T b,1 - T b,2 = ΔT b (ΔT between bulk feed bulk permeate) α 1 = α 2 = α 10

11 7.10 Temperature Polarization From Eq(60) (7-61) Where λ m = overall heat conductivity = sum of two parallel resistances = ε λg + (1 - ε) λp (7-62) λ p = heat conductivity through the solid (polymer) λ g = heat conductivity through the pores filled with gas and vapor ε = surface porosity <Assume> Shape of pores = cylindrical In general, λ p > 10 to 100 λ g Convective heat flow through the membrane pores, φ ΔH c = ρ ΔH v J (7-63) Combination of Eq(7-63) and Eq(7-61) (7-64) Meaning ΔT m Volume flux (J ) Heat conductivity for polymer (λ p ) temperature polarization Heat transfer coefficient(α) & membrane thickness(l) temperature polarization 11

12 7.11 Temperature Polarization Thermo-osmosis Dense homogeneous membrane no pores No phase transitions occur at the liquid/membrane interfaces Heat transferred by only conduction through the solid membrane matrix Temperature polarization in thermo-osmosis membrane similar to Eq(7-61), except no enthalpy of vaporization and condensation (7-65) λ m in thermo-osmosis[eq(7-65)] > λ m in membrane distillation[eq(7-64)] stronger effect on temperature polarization Convective term(δh v ) = depends on volume flux effect of temperature polarization : membrane distillation > thertmo-osmosis (on the basis of same ΔT and same polymer) 12

13 7.12 Membrane Fouling Concentration polarization and fouling <Figure 7-23> Flux as a function of time. Types of foulant Organic precipitates (macromolecules, biological substances, etc.) Inorganic precipitates (metal hydroxides, calcium salts, etc.) Particulate 13

14 7.12 Membrane Fouling Phenomenon of fouling Very complex and difficult to describe theoretically Dependent on physical and chemical parameters concentration, temperature, ph, ionic strength specific interactions (H bonding, dipole-dipole interactions) For process design, need reliable values of flux decline Flux description by a resistances-in-series model Resistance = membrane (R m ) + cake layer (R c ) (7-66) R c = l c r c (7-67) R c = cake layer resistance l c = cake thickness r c = specific resistance of the cake <Figure 7-24> Schematic of the cake filtration model. 14

15 7.12 Membrane Fouling Specific resistance of the cake (r c ) assumed to be constant over cake layer expressed by the Kozeny-Carman relationship (7-68) where m s = diameter of the solute particle ε = porosity of the cake layer Thickness lc of the cake, (7-69) where m s = mass of the cake(difficult to estimate) P s = density of the solute A = membrane area Effective thickness of the cake layer Several micrometers many mono-layers ( ) of macromolecules Dependent on the type of solutes and especially on operating conditions and time 15

16 7.12 Membrane Fouling Estimation of cake layer resistance (R c ) from mass balance In case of a complete solute rejection (R = 100%) (7-70) where J w = pure-water flux (7-71) V = permeate volume c b = bulk concentrations (7-72) 1/J V ΔP = applied pressure <Figure 7-25> Reciprocal flux as a function of the permeate volume. 16

17 7.12 Membrane Fouling If membrane resistance = negligible By integration of Eq(7-71) from t=0 to t=t (7-73) Typical relationship for unstirred dead-end filtration Permeate volume (V) t -0.5 Rewriting Eq(7-73) in terms of the flux (J), (7-74) There are many sophisticated theories. Fouling = very complex Fouling can not analyzed by a single equation based on a certain theory. A simple empirical equation J = J o t n, n < 0 (7-75) where J = actual flux J o = initial flux n = f(cross-flow velocity) <Figure 7-26> Flux versus time according to Eq(7-74). 17

18 7.12 Membrane Fouling Fouling Tests in RO Measurement of fouling index Silting index (SI) <Figure 7-27> Schematic drawing of MFI apparatus. Plugging index (PI) Fouling index (FI) or silt density index (SDI) Modified fouling index or the membrane filtration index (MFI) Membrane Filtration Index (MFI) Based on cake filtration (blocking filtration) Concept of cake filtration Flux through 2 resistances in series : R c + R m Integration od Eq(7-71) over a time t (7-76) Plot of t/n vs. V straight line Slope of this line = MFI (7-77) <Figure 7-28> MFI experimental results 18

19 7.12 Membrane Fouling Fouling Tests in RO Fouling potential MFI (<Figure 7-29>) Advantage of use of MFI values By comparing various solutions, different fouling behavior can be observed. A maximum allowable MFI value can be given for a specific plant. Flux decline can be predicted to some extent. Drawback of use of MFI values MFI values = qualitative MFI experiment = Dead-end experiments (RO in practice : cross-flow mode) Assumed that cake resistance f(pressure) : not true MFI method = based on cake filtration only (other factors contribute to fouling too) <Figure 7-29> MFI values as a function of the concentration of the fouling solute in the bulk solution 19

20 7.13 Methods to Reduce Fouling Fouling Tests in RO Pretreatment of the feed solution Heat treatment, ph adjustment, addition of complexing agents (EDTA etc.), chlorination, adsorption onto active carbon, chemical clarification, pretreatment with MF/UF ph adjustment = very important with proteins Minimize fouling at ph value corresponding to the isoelectric point of the protein ( i.e. at the point at which the protein is electrically neutral) Membrane properties A change of membrane properties can reduce fouling. Narrow pore size distribution can reduce fouling (this effect should not be overestimated). Use hydrophilic rather than hydrophobic membranes Generally proteins adsorb more strongly at hydrophobic surfaces and are less readily removed than at hydrophilic surfaces. Use negatively charged membrane for feed containing negatively charged colloids Pre-adsorption of the membrane by a component which can be easily removed 20

21 7.13 Methods to Reduce Fouling Module and process conditions Mass transfer coefficient concentration polarization Applying high flow velocities in cross flow filtration Adapting low(er) flux membranes Use of various kinds of turbulence promoters Use fluidized bed systems and rotary module systems for small scale application Cleaning 1. hydraulic cleaning 2. mechanical cleaning 3. chemical cleaning 4. electric cleaning <Figure 7-31> The principle of back-flushing. 21

22 7.13 Methods to Reduce Fouling 1. Hydraulic cleaning Methods include back-flushing (only applicable to MF and open UF membranes) Back-flushing procedure 1 After a given period of time, release feed pressure 2 Change flow direction of the permeate from the permeate side to the feed side (to remove fouling layer within membrane or at membrane surface) Back-Shock method Reduce time interval of back-flushing to seconds No time cake to build up layer resistance remains low maintain the flux at quite high. <Figure 7-30> Schematic of flux versus time behavior In a given MF process with and without back-flushing 22

23 7.13 Methods to Reduce Fouling 2. Mechanical cleaning only applicable in tubular systems using oversized sponge balls 3. Chemical cleaning Most important method for reducing fouling Use chemicals separately or in combination to remove foulant by oxidation and/or desorbing Concentration of chemicals and cleaning time = very important according to membrane Some important (classes of) chemicals are: acids (strong such as H 3 PO 4, or weak such as citric acid) alkali (NaOH) detergents (alkaline, non-ionic) enzymes (proteases, amylases, glucanases) complexing agents (EDTA, polyacrylates, sodium hexametaphosphate) disinfectants (H 2 O 2 and NaOCI) Steam and gas (ethylene oxide) sterilization 23

24 7.13 Methods to Reduce Fouling 4. Electric cleaning Very special method for cleaning Applying an electric field across a membrane migrate charged particles or molecules to the electric field Apply to remove particles or molecules from interphase without interrupting process Apply electric field at certain time intervals A drawback Use electric conducting membranes Use a special module arrangement with electrodes 24

25 7.14 Compaction Compaction Mechanical deformation of a polymeric membrane matrix Occurs in pressure-driven membrane operations mostly(especially occur in RO) However, in NF and UF compaction may occur as well and Extent depends on the pressure employed and membrane morphology. Possible in sub-layer of gas separation membrane by applying high pressure Desifing porous structure flux Deformation = irreversible in general no recovering flux after relaxing pressure 25