chapter 7. polarization phenomena & membrane fouling (part...

Chang-Han Yun / Ph.D.

National Chungbuk University

November 18, 2015 (Wed)

Chapter 7. Polarization Phenomena & Membrane Fouling

(Part II)

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



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



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)



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.



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



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)



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



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.



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



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)↑



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)



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.



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



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



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



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).



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


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



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



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.



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



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



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


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

chapter 7. polarization phenomena & membrane fouling (part...

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