chapter 7. polarization phenomena & membrane fouling (part...
TRANSCRIPT
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Chang-Han Yun / Ph.D.
National Chungbuk University
November 18, 2015 (Wed)
Chapter 7. Polarization Phenomena & Membrane Fouling
(Part II)
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2 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
Contents
Contents Contents
7.13 Methods to Reduce Fouling
7.12 Membrane Fouling
7.11 Temperature Polarization
7.10 Concentration Polarization in Electrodialysis
7.9 Concentration Polarization in Diffusive Membrane Separations
7.14 Compaction
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3 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
7.9 Concentration Polarization in Diffusive Membrane Separations
Processes characterized by a solution-diffusion mechanism
Assumptions for existence of resistance()
(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
Concentration profiles for diffusive membrane processes:
(a) without boundary layer resistances, and (b) with boundary layer resistances.
where Csmi,1 = feed concentration of component i at feed side membrane surface
Csmi,2 = permeate concentration of component i at permeate side membrane surface
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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, Ji = k1 (csm
i,1 - csi,1) (7-46)
Flux of i in permeate-side boundary layer, Ji = k2 (csm
i,2 - csi,2) (7-47)
Flux through membrane, (7-48)
Eq(7-45) → Eq(7-48) : (7-49)
Overall mass transfer coefficient (kov)
Eq(7-46) + Eq(7-47) + Eq(7-49) ⇨ Ji = kov (csi,1 - c
si,2) (7-50)
Overall mass transfer coefficient (kov) : (7-51)
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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
Concentration on the left-hand side of membrane↓
Concentration on the right-hand side of membrane↑
Flow of Na+ : left to right(to cathode) through membrane
Membrane resistance = negligible
Generate diffusive flow
Concentration polarization in electrodialysis
in the presence of a cation-selective membrane.
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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 Jm = electrically driven fluxes in membrane
Jbl = electrically driven fluxes in boundary layer
JDbl = diffusive flux in the boundary layer
tm = transport numbers of the cation in membrane
tbl = 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
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7 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
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 = cm at x = 0
BC 2 : c = cb 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 (cm) → 0
⇨ obtain a limiting current density( ilim) : (7-59)
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8 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
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
⇨ ilim at cation-exchange membrane < at an anion-exchange membrane
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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
Temperature polarization
in membrane distillation.
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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 α1 = heat transfer coefficients on warm side of the membrane
α2 = heat transfer coefficients on cold side of the membrane
φ ΔHv and φ ΔHc = heat fluxes caused by convective transport through the pores
ℓ = membrane thickness
λm = overall heat conductivity of the membrane
φ ΔHv = -φ ΔHc
Tb,1 – Tm,1 = Tm,2 - Tb,2 = ΔTbl (ΔT in boundary layer)
Tm,1 - Tm,2 = ΔTm (ΔT across membrane)
Tb,1 - Tb,2 = ΔTb (ΔT between bulk feed ↔ bulk permeate)
α1 = α2 = α
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7.10 Temperature Polarization
From Eq(60) → (7-61)
Where λm = overall heat conductivity = sum of two parallel resistances = ε∙λg + (1 - ε)∙λp (7-62)
λp = heat conductivity through the solid (polymer)
λg = heat conductivity through the pores filled with gas and vapor
ε = surface porosity Shape of pores = cylindrical
In general, λp > 10 to 100 × λg
Convective heat flow through the membrane pores, φ∙ΔHc = ρ∙ΔHv∙J (7-63)
Combination of Eq(7-63) and Eq(7-61) → (7-64)
『Meaning』
Heat transfer coefficient(α)↑ & membrane thickness(ℓ)↑⇨ temperature polarization↓
temperature polarization↑ ΔTm↑ ⇨ Volume flux↑ (J↑)
Heat conductivity for polymer (λp)↑
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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)
λm in thermo-osmosis[Eq(7-65)] > λm in membrane distillation[Eq(7-64)]
⇨ stronger effect on temperature polarization
Convective term(ΔHv) = depends on volume flux
⇨ effect of temperature polarization : membrane distillation > thertmo-osmosis
(on the basis of same ΔT and same polymer)
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13 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
7.12 Membrane Fouling
Concentration polarization and fouling
Types of foulant
Organic precipitates (macromolecules, biological substances, etc.)
Inorganic precipitates (metal hydroxides, calcium salts, etc.)
Particulate
Flux as a function of time.
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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 (Rm) + cake layer (Rc)
(7-66)
Rc = ℓc rc (7-67)
Rc = cake layer resistance
rc = specific resistance of the cake
Schematic of the cake filtration model.
ℓc = cake thickness
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7.12 Membrane Fouling
Specific resistance of the cake (rc)
assumed to be constant over cake layer
expressed by the Kozeny-Carman relationship
(7-68)
where ms = diameter of the solute particle
ε = porosity of the cake layer
Thickness ℓc of the cake, (7-69)
where ms = mass of the cake(difficult to estimate)
Ps = density of the solute
A = membrane area
Effective thickness of the cake layer
Several micrometers ⇨ many mono-layers (≈ 100 ∼ 1000) of macromolecules
Dependent on the type of solutes and especially on operating conditions and time
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7.12 Membrane Fouling
Estimation of cake layer resistance (Rc) from mass balance
In case of a complete solute rejection (R = 100%)
(7-70)
(7-71)
(7-72) ⇨ 1/J ∝ V
Reciprocal flux as a function of the permeate volume.
where Jw = pure-water flux
V = permeate volume
cb = bulk concentrations
ΔP = applied pressure
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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 = Jo∙tn , n < 0 (7-75)
where J = actual flux
Jo = initial flux
n = f(cross-flow velocity)
Flux versus time according to Eq(7-74).
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18 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
7.12 Membrane Fouling
Measurement of fouling index
Silting index (SI)
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 : Rc + Rm
• 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)
7.12.1
Fouling Tests in RO
Schematic drawing
of MFI apparatus.
MFI experimental results
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Fouling potential↑ ⇨ MFI↑()
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)
MFI values as a function of the concentration
of the fouling solute in the bulk solution
7.12.1
Fouling Tests in RO 7.12 Membrane Fouling
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7.13 Methods to Reduce Fouling
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
7.12.1
Fouling Tests in RO
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21 Chapter 7. Polarization Phenomena & Membrane Fouling Chungbuk University
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
The principle of back-flushing.
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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.
Schematic of flux versus time behavior
In a given MF process with and without back-flushing
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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 H3PO4 , or weak such as citric acid)
• alkali (NaOH)
• detergents (alkaline, non-ionic)
• enzymes (proteases, amylases, glucanases)
• complexing agents (EDTA, polyacrylates, sodium hexametaphosphate)
• disinfectants (H2O2 and NaOCI)
Steam and gas (ethylene oxide) sterilization
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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
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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