chapter 6. membrane process (concentration driving...
TRANSCRIPT
Chang-Han Yun / Ph.D.
National Chungbuk University
November 25, 2015 (Wed)
Chapter 6. Membrane Process
(Concentration Driving Force)
2 Chapter 6. Membrane Process(Concentration) Chungbuk University
Contents
Contents Contents
6.5 Other Driving Force
6.4 Concentration Driving Force
6.3 Pressure Driven Force
6.2 Osmosis
6.1 Introduction
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6.5 Thermally Driven Membrane Process
Thermally driven membrane process
Heat flow + mass flow thermo-osmosis or thermo-diffusion
No phase transition
Conductive heat flux by Fourier's law, (6-107)
where λ = constant, thermal conductivity or heat conductivity
Integration of Eq(6-107) across the membrane at steady-state flow and constant λ
(6-108)
Membrane distillation
Use porous membrane
Separate two liquids by no wetting the membrane
ΔT between two liquids vapor pressure difference
Vapor permeation flow direction :
High T(high vapor P) side → Low T(low vapor P) side
6.5.1 Introduction
Medium λ(W/m)
Gases
Organic liquids
Water
Polymers
Metals
0.02
0.2
0.6
2.0
20 ∼ 200
<Figure 6-47> Temperature profile across a homogeneous membrane
[Table 6-21] λ of various media
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6.5 Thermally Driven Membrane Process
Membrane distillation(<Figure 6-48>)
Two liquids at different temperatures are separated by a porous membrane.
Membrane : not directly involved in separation(barrier between the two phases)
No wetting membrane by liquids
(otherwise the pores will be filled immediately as a result of capillary forces)
Non-wettable porous hydrophobic membranes must be used.
Sequence of transport
① Evaporation on the high-temperature side
② Transport of vapor molecules through pores
of hydrophobic porous membrane
③ Condensation on the low-temperature side
VLE determine selectivity
6.5.2 Membrane
Distillation
<Figure 6-48> Schematic representation of membrane distillation.
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6.5 Thermally Driven Membrane Process
<Example>
Ethanol/water mixture
• Hydrophobic membrane : not wetted at low ethanol concentrations
• Permeation rate of ethanol will always be relatively higher than water
NaCl in water
• Vaporize water only only water permeate very high selectivity
Transport of volatile components through the membrane
Described by phenomenological equations(Flux ∝ driving force, ΔT)
ΔT vapor pressure difference(Δp) ※ Antoine equation : T ↔ p
Flux, Ji = B∙Δpi (6-109)
where B = membrane-based parameter, proportionality factor
Δp = system-based parameter
6.5.2 Membrane
Distillation
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6.5 Thermally Driven Membrane Process
Membrane parameters(B)
Material (hydrophobic/hydrophilic)
Pore structure(pore size, distribution) : must be small and narrow distribution
Porosity : major parameter, as high as possible
Thickness : major parameter
Conductivity
System-based parameter(Δp) is mainly determined by ΔT.
6.5.2 Membrane
Distillation
Wettability
Determined by the interaction between the liquid and the polymeric material
Low affinity no wetting
From contact angle(θ) wettability
Low affinity(no wetting) : contact angle(θ) > 90°
High affinity(wetting) : contact angle(θ) < 90°
6.5.2.1 Process parameters
<Figure 6-49> Contact angles of liquid droplets
on a solid (nonporous) material.
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6.5 Thermally Driven Membrane Process
Wetting pressure(Δp) from Laplace equation, (6-110)
θ > 90° cosθ < 0 Δp > 0
Wettability depends on three factors:
• Pore size (r) r↓ Δp↑
• Surface tension of the liquid (γ1)
• Surface energy of the membrane material (θ or cosθ)
The 1st parameter, pore size(r)↓ wetting pressure↑
The 2nd parameter, surface tension of the liquid(γ1)
Related to intermolecular forces
(dispersion forces, polar forces, H-bonding)
Water(H-bonding)
intermolecular force = very strong
surface tension = high
6.5.2 Membrane
Distillation
<Figure 6-50> Wetting pressure (liquid entry pressure)
for a porous polytetrafluoroethylene(PTFE) membrane.
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6.5 Thermally Driven Membrane Process
The 3rd parameter : contact angle(θ) between the liquid and the polymer
Affinity between liquid and polymer = very small θ > 90° no wetting
Surface tension of the polymer : the 3rd important factor
High surface energy polymer Easily wetting
6.5.2 Membrane
Distillation
[Table 6-22] Surface tension of some liquids at 20°C
Liquid Surface tension(γ1), 103 N/m
Water
Methanol
Ethanol
Glycerol
Formamide
n-hexane
72.8
22.6
22.8
63.4
58.2
18.4
Polymer Surface energy(γS)
(103 N/m)
Polytetrafluoroethylene
(PTFE)
polytrifluoroethylene
Polyvinylidenefluoride
(PVDF)
Polyvinylchloride(PVC)
Polyethylene(PE)
Polypropylene(PP)
Polystyrene
19.1
23.9
30.3
36.7
33.2
30.0
42.0
[Table 6-23] Surface energies of some polymers
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6.5 Thermally Driven Membrane Process 6.5.2 Membrane
Distillation
Membranes for membrane distillation
Low surface energy polymer : hydrophobic(PTFE, PVDF, PE, PP)
High surface tension liquids : water
Small pore size and narrow pore size distribution
0.2 ∼ 0.3 μm of pore sizes(same with MF)
High porosity (70 to 80%)
Thin membranes
※ Selectivity determined by VLE membrane cannot be optimized further.
6.5.2.2 Membrane
avoid
wetting
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6.5 Thermally Driven Membrane Process 6.5.2 Membrane
Distillation
Wettability of the membrane determine applications
Apply mainly to aqueous solutions containing inorganic
Classification of applications ① Permeate = desired product major in most application
② Retentate i = desired product
1. Production of pure water
Possible to get a high quality permeate
• water for the semiconductor industry
• boiler feed water for power plants
• desalination of seawater
Salt concentration in feed↑(<Figure 6-52>)
• No change of salt conc. in permeate
• Vapor pressure↓ flux↓ weakly
※ RO : Salt concentration in feed↑
osmotic pressure↑ flux↓ strongly
6.5.2.3 Applications
<Figure 6-52> Flux and selectivity as a function of
the NaCI conc. for a porous PP membrane (Accurel)
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6.5 Thermally Driven Membrane Process 6.5.2 Membrane
Distillation
2. Removal of volatile organic components (VOC's) from aqueous phase
3. Concentration of solutions
waste water treatment
concentration of salts, acids, etc.
4. Removal of volatile bioproducts
Remove volatile bioproducts(ethanol, butanol, acetone, aroma compounds)
from fermentation product
Brief consideration for process design
Feed side temperature : high → low
Permeate side temperature : low → high
Counter-current flow constant temperature difference
(vapor pressure difference is not constant!)
Use heat exchanger to recover thermal energy
Use vacuum on permeate side to remove VOC from feed aqueous phase
<Figure 6-53> Schematic drawing of
a counter-current set-up.
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6.5 Thermally Driven Membrane Process 6.5.2 Membrane
Distillation
Advantage over normal distillation
Hollow fiber application large contact area per volume small-scale
<Figure 6-54> Schematic drawing of a membrane distillation unit combined
with a heat-exchanger in order to recover a part of the energy.
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6.5 Thermally Driven Membrane Process 6.5.2 Membrane
Distillation
6.5.2.4 Summary of membrane distillation
Items Characteristics
Membranes Symmetric or Asymmetric porous
Thickness 20 ∼ 100 μm
Pore size ≈ 0.2 ∼ 1.0 μm
Driving force Vapour pressure difference
Separation principle Vapour-Liquid equilibrium
Membrane material Hydrophobic (PTFE, PP)
Applications
Production of pure water
• laboratories
• semiconductor industry
• desalination of seawater
• production of boiler feed water
• concentration of aqueous solutions
Removal of VOC's
• contaminated surface water (benzene, TCE)
• fermentation products (ethanol, butanol)
• aroma compounds
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6.6 Membrane Contactor
Membrane contactor
Role of membrane : Barrier to form interface between two different phases
Distribution coefficient determine separation performance
Supply huge contact area per volume scale down conventional dispersed-phase contactor
(not enhanced mass transfer) more attractive than conventional
<Example>
Advantage over conventional dispersed-phase equipment
Large contact area per volume
Elimination of flooding and entrainment of the dispersed phase
6.6.1 Introduction
Equipment Effective contact surface areas per volume
Typical packed and trayed columns 30 ∼300 (m2/m3)
Membrane contactor 1,600 ∼ 6,000 (m2/m3)
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6.6 Membrane Contactor
Disadvantages over conventional dispersed-phase equipment
Add additional phase(membrane)
• Dependent on the type of membrane and the system applied
• Membrane phase : increase overall mass transfer resistance
Wetting the membrane pore instability of the system ※ Major problem
Flux of component i
Ji = kov,i Δci (6-111) with (6-112)
where kov,i = overall mass transfer coefficient
<Assume> Resistance of boundary layer = negligible not true for liquid phase
Eq(6-112) → (6-113)
where Ki = distribution coefficient of component i from feed into membrane
Di = diffusion coefficient of component i in the membrane
Δci = bulk concentration difference
6.6.1 Introduction
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6.6 Membrane Contactor
Blood oxygenation : Most widely used application
Hollow fiber type membrane
By gradient in partial pressure O2 : Gas → Blood; CO2 : Blood → gas phase
Membrane
Hydrophobic membrane(<Figure 6-56>, left)
Membrane : PTFE, PP, PE
Liquid : aqueous solution
no wetting the pore
pore filled by gas phase
Hydrophilic membrane
(<Figure 6-56>, right)
Pore filled by aqueous phase
Not desirable in general
6.6.1 Gas-Liquid(G-L)
Membrane Contactor
6.6.1.1 Introduction
In general, porous membranes are used.
Nonporous membranes(rubbery polymer) are also possible.
<Figure 6-56> Gas-liquid contactor with
A non-wetted membrane (left side) and
a wetted membrane (right side).
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6.6 Membrane Contactor
Possible application in industry except for blood oxygenator
O2 transfer in fermentation processes and aerobic waste water treatment without bubble
CO2 transfer to beverages (water, lemonades or beer)
Removal of O2 from aqueous phase(L-G contactor)
Separation of saturated/unsaturated hydrocarbons (paraffin/olefin separation)
such as ethane/ethylene and propane/propylene
• Apply hydrophobic porous membrane(organic vapor ↔ aqueous)
• Olefin complex with silver ions
• Adsorption stage : use aqueous phase AgNO3 solution remove olefin
• Desorption stage : use sweep stream
Removal of acid gases(CO2, H2S, CO, SO2,
NOx) from flue gas, biogas and natural gas
and the removal of NH3
6.6.1 Gas-Liquid(G-L)
Membrane Contactor
<Figure 6-57> Separation of ethane/ethylene in an
absorption/desorption stage membrane contactor
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6.6 Membrane Contactor
<Example>
Feed phase : organic solvent wetting the pore, not miscible with aqueous
Permeate phase : aqueous no wetting the membrane pore
Membrane : hydrophobic pore filled by feed phase(organic)
Aqueous-Organic interface will formed at the permeate side (<Figure 6-58a>)
6.6.2 Liquid-Liquid(L-L)
Membrane Contactor
<Figure 6-58> L-L membrane contactor
with a wettable liquid feed phase(left) and
a non-wettable liquid feed phase(right).
Porous membrane
Non-porous membrane L-L membrane contactor
Feed phase : wet membrane
Feed phase : not wet membrane
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Resistance : Boundary layer of feed-side + Membrane + Boundary layer of strip-side
In case of hydrophobic membrane : Resistance at aqueous phase ≫ organic phase
Pressure : aqueous phase > organic phase (∵ to protect organic flow to aqueous)
Application : alternative for the conventional extraction process
heavy metals
Phenol
Bioproducts
microsolutes like herbicides
Insecticides
pesticides
6.6.2 Liquid-Liquid(L-L)
Membrane Contactor 6.6 Membrane Contactor
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Disadvantage of porous membrane contactor : Shear stress, pressure gradients
To solve problems, nonporous membrane contactor or coating onto porous membrane
<Example> Nonporous membrane (silicone rubber) applied in blood oxygenation
Big advantage of these nonporous systems : no meniscus and stable system
Disadvantage : additional resistance need swelling or reducing coating thickness
In real, membrane resistance reduced dramatically by swelling effect
Boundary layer resistance in aqueous phase = highest among resistance
6.6.3 Non-porous
Membrane Contactor 6.6 Membrane Contactor
<Figure 6-59> Gas-liquid membrane contactor
with a porous membrane, left)
and a dense membrane (right).
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6.6.4 Summary of
Membrane Contactor 6.6 Membrane Contactor
Items Characteristics
Membranes Porous (hydrophobic or hydrophilic), Nonporous, or Composites
Thickness 20 ∼ 100 μm
Pore size Nonporous or 0.05 ∼ 1.0 μm
Driving force Concentration or Vapour pressure difference
Separation principle Distribution coefficient
Membrane material Hydrophobic (PTFE, PP, Silicone rubber)
Applications
G-L contactor
• SO2, CO2, CO, NOX from flue gases
• VOC from off gas
• NH3 from air (intensive farmery)
• O2 transfer (blood oxygenation, aerobic fermentation)
• Saturated/unsaturated (ethane/ethylene)
L-G contactor
• Volatile bioproducts (alcohols, aroma compounds)
• O2 removal from water
L-L contactor
• Heavy metals
• Fermentation products (citric acid, acetic acid, lactic acid, penicillin)
• Phenolics
• CO2 and H2S from natural gas
• CO2 from biogas
• CO2 transfer (beverages)
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6.6.5 Thermo-osmosis 6.6 Membrane Contactor
Thermo-osmosis (or thermo diffusion)
Porous or nonporous membrane separates two phases different in temperature.
Comparison of thermo-osmosis with membrane distillation
Comparison Items Thermo-osmosis Membrane distillation
Driving force Temperature difference Temperature difference
Act of membrane Determines selectivity Barrier between two non-wettable liquids
Selectivity is determined by the VLE
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6.7.1 Introduction 6.7 Electrically Driven Membrane Processes
Electrically driven membrane processes
Driving force : electrical potential difference
Solutes to be separated : charged ions or molecules
Membrane : ionic exchange(electrically charged) membrane in general
In electrical field
Positive ions (cations) migrate to the negative electrode (cathode).
Negative ions (anions) migrate to the positive electrode (anode).
Uncharged molecules are not affected by this driving force
Ion exchange membrane
Cation-exchange membranes : allow the passage of positively charged cations
Anion exchange membranes : allow the passage of negatively charged anions
Transport of ions across an ionic membrane : based on Donnan exclusion mechanism
Various combination of electrical potential difference and electrically charged membranes
Electrodialysis
Membrane electrolysis
Role of charged membrane : selective barrier according to ionic charge of species
Bipolar membranes
Fuel cells
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Electro-dialysis
Electrically charged membranes are used to remove ions from an aqueous solution.
A number of cation- and anion-exchange membranes are placed in an alternating pattern
between a cathode and an anode.
Na+ migrate to the cathode and the Cl- migrate to the anode.
Cl- cannot pass the negatively charged membrane(cation exchange membrane)
Na+ cannot pass the positively charged membrane(anion exchange membrane)
Ionic concentration increase in alternating compartments accompanied by
a simultaneous decrease in ionic concentration in the other compartments.
Consequently alternate dilute and concentrate solutions are formed
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
<Figure 6-59> The principle
of electrodialysis.
Electrolysis occurs at the electrodes.
• Cathode : 2H2O + 2e– → H2 + 2OH– produce H2 and OH–
• Anode : 2Cl– → Cl2 + 2e–
H2O → 1/2O2 + 2H+ + 2e– produce Cl2, O2, H+
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Ion flux ∝ electrical current I (A) or current density i (A/cm2)
Requirement of electrical current required to remove a number of ions
I = z ℱ q Δci / ξ (6-114)
where z = valence, q = flow rate
ℱ = Faraday constant (1 Faraday = 96,500 coulomb/eq or ampere-sec/eq)
Δci = concentration difference of ion i between feed and permeate(eq/l)
ξ = current utilization
※ Current utilization(ξ)
Related to number of cell pairs in stack (ξ = n × electrical efficiency)
Information about the fraction of the total current applied effectively used to transfer the ions.
1 Faraday(96,500 coulombs or 26.8 A/hr)
• transfer 1 g-equivalent or equivalent of cations to the cathode (23 g of Na+)
• and 1 g-equivalent or equivalent of anions to the anode (35.5 g of Cl-).
6.7.2.1 Process parameter
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Ohm's law,
E = I∙R (6-115)
where R = resistance of the total membrane stack
Resistance(R)(<Figure 6-61>)
R = Rcp∙N (6-116)
where Rcp = resistance of one cell pair (per unit area)
N = number of cell pair in the stack
Rcp = Ram + Rpc + Rcm + Rfc (6-117)
where Rcp = resistance of one cell pair (per unit area)
Ram = resistance of the anion-exchange membrane
Rpc = resistance of the 'permeate' compartment
Rcm = resistance of the cation-exchange membrane
Rfc = resistance of the 'feed' compartment
<Figure 6-60> Resistances which apply in a cell pair.
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Current density↑ number of ions transferred↑(<Figure 6-62>)
However, the current density cannot be increased by an unlimited amount
<Figure 6-62>
Region 1 : Ohmic region(i ∝ V, electrical potential difference by Ohm's law)
Region 2 : current reaches limiting current density(𝑖lim) Ohmic resistance↑
※ 𝑖lim(mA/cm2) : current necessary to transfer all the available ions
Region 3 : over-limiting current and water splitting will occur to generate ions
※ No ions are available anymore to transfer the charge.
<Figure 6-62> Current-voltage characteristic of
an ion-exchange membrane
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Current-voltage characteristic for different ionic concentrations(<Figure 6-63>)
Ionic concentration↑ limiting current density↑
Plot as E/𝑖 versus 𝑖-1 determine 𝑖lim more accurately(<Figure 6-64>)
<Figure 6-63> Current-voltage characteristic of an
ion-exchange membrane for various ionic concentrations.
<Figure 6-64> Schematic drawing of a R (= E/𝑖) versus the reciprocal current.
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
The current density, (6-118)
where tm,i = transport numbers in membrane
tb1,i = transport numbers in layer
δ = thickness of the boundary layer
Concentration polarization
Severely affects the current density
𝑖→ 𝑖lim as cm → 0
Eq(6-118) → (6-119)
Mass transfer coefficient(k) = D/δ
determine 𝑖lim strongly by hydrodynamics of the system
(cross-flow velocity, cell geometry)
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Other effects that influences the performance of the process
① Osmotic flow
• inherently part of the process and can not be avoided
• Ions transferred from one compartment to other generate osmotic pressure
• By osmotic pressure(π), osmotic flow(diluate → concentrate) system performance↓
② Less effective Donnan exclusion
• High ionic concentrations Donnan exclusion↓
※ By these effects electrodialysis : competitive at relatively low concentrations
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Electrodialysis
Applied electrical potential difference Ions transported through membranes
Ion-exchange membranes used to make the membranes selective for ions
Anion-exchange membrane(<Figure 6-65>)
Positively charged groups(ex, derived from 4°ammonium salts) attached to polymer
Positively charged cations are repelled from the anion-exchange membrane.
Cation-exchange membranes(<Figure 6-65>)
Negatively charged groups(sulfonic or carboxylic acid) attached to polymer
Negatively charged anions are repelled by the cation-exchange membrane.
Two different types of ion-exchange membranes
① Heterogeneous type ion-exchange membranes
Manufacturing
6.7.2.2 Membranes for electrodialysis
• Combining ion-exchange resins with a film-forming polymer
• Converting them into a film by dry-molding or calendering
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Electrical resistance : relatively high
Mechanical strength : relatively poor especially at high swelling values
② Homogeneous membranes
Manufacturing procedure : Introduction of an ionic group into a polymer film
Charge is distributed uniformly over the membrane
Crosslinking reduce extensive swelling
<Figure 6-65> Anion and cation exchange
Membranes based on polystyrene and
divinylbenzene.
Anion exchange membrane
Cation exchange membrane
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Requirements for an ion-exchange membrane
High electrical conductivity combined with a high ionic permeability
Ionic charge density↑ electrical conductivity↑ but swelling tendency↑
For lower swelling, need crosslinking
Diffusion coefficient of ions inside the membrane
For a highly swollen : 10–6 cm2/s
For a highly crosslinked system : 10–10 cm2/s
Basic parameters for a good membrane:
high selectivity
high electrical conductivity
moderate degree of swelling
high mechanical strength
Electrical resistance of ion-exchange membranes : 2 ∼ 10 Ω∙cm2
Charge density : 1∼2 mequiv/g dry polymer
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Potable water from brackish water
Most important application of electrodialysis
Product : diluate
Production of salt
Very special application (reverse case with potable water production)
Product : concentrate
Other industrial application
De-mineralization of whey
De-acidification of fruit juices
Production of boiler feed water
Removal of organic acids from a fermentation
6.7.2.3 Applications
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Amino acids
Contain both a basic and an acidic group
Amphoteric character positively or negatively charged depending on the pH
H2NCHRCOO- ↔ + H3NCHRCOO- ↔ + H2NCHRCOOH
(a) at high pH (b) at pH=7 (b) at low pH
At high pH
• Amino acid = negative charge(structure a)
• Migrates towards the anode when an electrical field is applied
At low pH
• Amino acid = positive charge(structure c)
• Migrates towards the cathode
If structures a and c are exactly in balance
• No net charge(structure b) and amino acid = not migrate in an electrical field
• pH under these conditions is called the isoelcctric point of the amino acid.
1) Separation of amino acids
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
Isoelectric point
Very characteristic parameter for a protein
Different proteins different isoelectric points
<Figure 6-66>
The cell employed is divided into three compartments
• Center compartment : adjusted to the isoelectric point (I.P.)
of a specific (to be separated) protein A
• One compartment : at a pH < I.P.
• Other compartment : at a pH > I.P.
pH of protein solution = pH of protein A
added to the middle compartment
Other proteins in the system will develop
either a positive or a negative charge.
(depending on their specific I.P.)
Diffuse to the electrode respectively.
<Figure 6-66> Separation of amino acids.
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6.7.2 Electrodialysis 6.7 Electrically Driven Membrane Processes
6.7.2.4 Summary of electrodialysis
Items Characteristics
Membranes Cation-exchange and anion-exchange membranes
Thickness ≈ few hundred μm(100 ∼500 μm)
Pore size Nonporous
Driving force Electrical potential difference
Separation principle Donnan exclusion mechanism
Membrane material Crosslinked copolymers based on divinylbenzene(DVB) with PS or
Polyvinylpyridine copolymers of PTFE and poly(sulfonyl fluoride-vinyl ether)
Applications
Desalination of water
Desalination in food and pharmaceutical industry
Separation of amino acids
Production of salt
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6.7.3 Membrane
Electrolysis 6.7 Electrically Driven Membrane Processes
Membrane electrolysis
Electrolysis process is combined with a membrane separation process.
<Example>
Chlor-Alkali(CA) process : NaCl Cl2 and NaOH
Electrolytic recovery of (heavy) metals
Production of acid
Base from the corresponding salts
CA process(<Figure 6-67>)
Use cation exchange membranes
Cell containing only two compartments separated
by cation exchange membranes
6.7.3.1 The 'chlor-alkali'(CA) process
<Figure 6-67> Schematic arrangement of the 'chlor-aIkali' process.
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6.7.3 Membrane
Electrolysis 6.7 Electrically Driven Membrane Processes
<Figure 6-67>
NaCl solution is pumped through the left-hand compartment
At anode, Cl- Cl2 by electrolysis
At cathode(right-hand compartment), electrolysis of H2O produce H2 and OH–
OH– migrate towards the anode but cannot pass the cation-exchange membrane.
Left-hand compartment Cl2 gas
※ In membrane electrolysis process,
each compartment requires two
electrodes (<Figure 6-68>)
Na+ migrate towards the cathode
<Figure 6-68> Schematic arrangement of
the 'chlor-aIkali' process.
Right-hand compartment NaOH solution and H2 gas
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6.7.3 Membrane
Electrolysis 6.7 Electrically Driven Membrane Processes
<Figure 6-69> Schematic drawing of
a bipolar membrane.
Bipolar membrane(<Figure 6-69>)
Consist of a cation-exchange membrane and an anion-exchange membrane
Intermediate layer between two membranes which are laminated together
6.7.3.2 Bipolar membranes
<Figure 6-70> Production of caustic soda and
sulfuric acid using bipolar membranes.
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6.7.3 Membrane
Electrolysis 6.7 Electrically Driven Membrane Processes
<Example> Production of sulfuric acid and sodium hydroxide(<Figure 6-70>)
Bipolar membrane : placed in between cation-exchange and anion-exchange
Introduce Na2SO4 solution into cell between cation-exchange and anion exchange
SO42- pass through anion-exchange membrane towards the anode
produce H2SO4 by association with H+ provided by the bipolar membrane
Na+ pass through cation exchange membrane towards the cathode
produce NaOH with OH- provided by the bipolar membrane
Produce H2SO4 and NaOH from Na2SO4 solution
※ At membrane electrolysis process
Electrolysis of H2O supply H+ and OH- at both electrodes
Energy consumption is higher than in the case of the bipolar membrane process.
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6.7.4 Fuel cells 6.7 Electrically Driven Membrane Processes
Fuel cells(<Figure 6-71> : Derivative of an electrical driven process
Galvanic cell : chemical energy electric energy
Reductor : H2, CH4, CH3OH supply at anode compartment
• Anode reaction : 2H2 → 4H+ + 4e–
• e– : flow through the external circuit from anode to cathode
• H+ : diffuse through cation exchange membrane to the cathode compartment
Oxidator : O2 supply at cathode compartment
• Cathode reaction : 4H+ + O2 + 4e– → 2H2O
Cell reaction : 2H2 + O2 → 2H2O
with an electromotive force Eo = 1.2 V
Isothermal process and not involving pressure-volume work
Change in free enthalpy of mixing, ΔG = - nℱEo (6-120)
where n = Number of electron transferred per molecule
ℱ = Faraday constant
ΔG = - nℱEo = - (2) × (96,500) × (1.2) = - 231.6 kJ/mol
<Figure 6-71> Schematic drawing of a fuel cell.
44 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.7.4 Fuel cells 6.7 Electrically Driven Membrane Processes
Efficiency under standard conditions(298 K) with water in liquid state
Enthalpy of formation ΔHfo = - 285.83 kJ/mol theoretical efficiency = 81 %
Reaction at higher temperature(water : vapor phase) efficiency↑
∵ ΔHfo ↓(also ΔHo ↓ but not that much)
Variation of fuel cells dependent on type of
electrolyte / electrodes / temperature
Solid Polymer Fuel Cell(SPFC)
• Use an cation exchange membrane(Nafion) for H+ transfer
• Operate only at relatively low temperature (below 100 )
Molten Carbonate Fuel Cell(MCFC) and Solid Oxygen Fuel Cell(SOFC)
• Use inorganic materials for ion transfer
• Operate much higher temperature(500 ∼ 1000 )
Using hydrocarbon(reductor) instead of H2 and H2O2(oxidator) instead of O2
Advantage of fuel cells
High efficiency ※ Conventional engine to generate electricity : max 60%
No generation or less generation of pollution
45 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.7.5 Electrolytic Regeneration
of Mixed-bed IX Resin 6.7 Electrically Driven Membrane Processes
Hybrid process(<Figure 6-72>) : Ion-exchange(IX) + Electrodialysis(ED)
Combination of an electropotential difference and ionic membranes
Ion-exchange(IX) resin
Use frequently to produce ultrapure water(> 18 MΩ∙cm)
• Cation exchange resin : R-H + Na+ → R-Na + H+
• Anion exchange resin : R-OH + Cl- → R-Cl + OH-
Disadvantage : periodical regeneration by HCl and NaOH
Combination with electrodialysis(ED) continuous regeneration without chemicals
Compartments
2 electrode compartments
2 compartments filled with IX resin(Mixed)
1 compartment for the concentrated feed
46 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.7.5 Electrolytic Regeneration
of Mixed-bed IX Resin 6.7 Electrically Driven Membrane Processes
Operation principle
① Feed water enters the system and is deionized by the ion-exchange resins.
② ΔE free ions either diffuse to electrode compartments or concentrate compartment.
③ In concentrate compartment, IX membrane prevent ions to diffuse into the IX compartments.
<Figure 6-72> Principle of a continuous deionization Process in which ED and IX are combined
47 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.8 Membrane Reactors and Membrane Bio-reactors
Membrane reactor or Membrane bioreactor to improve the productivity
Coupled to a chemical or biochemical reaction to shift the chemical equilibrium
• Remove one of end-products to shift the reaction to the right side
Conversion rate↑ (Production yield↑)
Reaction and purification simultaneously in single equipment
more favorable than conventional processes on energy
Basic concept
① Reaction and separation in one unit (<Figure 6-73a>)
• Catalyst is coupled to the membrane.
② Combining reaction and separation units
and recycling reactants (<Figure 6-73b>).
<Figure 6-73> Two concepts of a membrane reactor
Reaction & Separation unit
Reaction unit Separation unit
(a) catalytic membrane (bio)reactor
(b) membrane recycle reactor
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6.8 Membrane Reactors and Membrane Bio-reactors
Catalyst inside the bore of the tube (<Figure 6-74a>)
Most simple and straightforward system
Advantage
• Simplicity in preparation and operation
• Easy change of catalyst
Permeate product across membrane need permselective membranes
<Figure 6-74> Schematic drawing of various membrane reactor concepts for a tubular configuration
6.8.1 Membrane
Reactor
(a) bore of the tube filled with catalyst
49 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.8 Membrane Reactors and Membrane Bio-reactors
Immobilized catalyst onto the membrane (<Figure 6-74b>, <Figure 6-74c>)
Removing one of end-products shift reaction to right hand side conversion↑
Controlled addition of reactants productivity↑
6.8.1 Membrane
Reactor
<Figure 6-74> Schematic drawing of various membrane reactor concepts for a tubular configuration
(b) top layer with catalyst
(c) membrane wall with catalyst
50 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.8 Membrane Reactors and Membrane Bio-reactors
Membrane
Separation to remove either a gaseous or liquid compound
Catalytically active
Employed at increased temperatures by using inorganic materials
Typical examples for inorganic membrane reactors
Dehydrogenation : remove H2
Oxidation and hydrogenation :
add O2 and H2
Problems to apply commercial purpose
Low separation factor
Leakage at higher temperatures
Poisoning of catalyst
Mass transfer limitations
6.8.1 Membrane
Reactor
Reaction Reactants to Products
Dehydrogenation
Hydrogenation
Oxidation
ethane → ethylene
propane → propene
cyclohexane → benzene
cyclohexane → cyclohexene
ethylbenzene → styrene
butene → butadiene
isopropylalcohol → acetone
• propene → propane
• butene → butane
• ethylene → ethane
CO → CO2
ethylene → ethylene oxide
propylene → propylene oxide
[Table 6-24] Reactions in catalytic membrane reactors
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6.8 Membrane Reactors and Membrane Bio-reactors
Use non-selective membranes to control the stoichiometry of the reaction
<Example> Desulfurization reaction of flue gas
By the Claus reaction, SO2 + 2H2S ↔ 3/8S8 + 2H2O
Fluctuation in SO2 concentration very difficult to control in conventional reactor
Maintaining stoichiometry by carry out the reaction
within the wall of a porous ceramic membrane(<Figure 6-75>)
Membrane
Macroporous with pores in μm range
without any ability to separate gases
Use porous α-Al2O3 as membrane
coated with γ-Al2O3 as catalyst
6.8.2 Non-selective
Membrane Reactor
<Figure 6-75> Schematic drawing of the concentration profiles of the various components
in a non-selective membrane reactor for the Claus reaction
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6.8 Membrane Reactors and Membrane Bio-reactors
Reaction(<Figure 6-75>)
Reaction temperatures(T) > 150°C
Products(water and sulfur) are removed as vapor.
SO2 is introduced to one side of the membrane and H2S at the other side.
Both gases diffuse into porous membrane : rate-limiting step
React instantaneously to sulfur and water
※ Reaction plane : somewhere inside the membrane
Products diffuse to either side : rate-limiting step
Automatic control of stoichiometry in reaction
Chang in concentration of one of reactants Changing concentration profile
Shifting reaction plane automatically
SO2 concentrations↓ shift reaction plane towards SO2
introduction of variable diffusion resistances maintain stoichiometry
※ This concept can be applied as well for removal NOx (de-NOx).
6.8.2 Non-selective
Membrane Reactor
Condensation of products
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6.8 Membrane Reactors and Membrane Bio-reactors
Water
Easily removed by pervaporation
Condensation or poly-condensation reaction(water = one of products)
Use pervaporation if the reaction temperature is not too high.
<Example> Esterification reaction(<Figure 6-76>)
Carried out in a batch reactor coupled with a pervaporation unit
Remove water constantly
General esterification reaction : acid + alcohol ↔ ester + water
Equilibrium constant, (6-121) &(6-122)
where k1 = rate constant of the forward reaction
k–1 = rate constant of the reverse reaction
K = f(T) strongly
(for liquid phase reaction ≠ f(p) or negligible)
6.8.3 Membrane Reactor
in Liquid Phase Reactions
<Figure 6-76> Combination of pervaporation
and reactor in an esterification process.
54 Chapter 6. Membrane Process(Concentration) Chungbuk University
6.8 Membrane Reactors and Membrane Bio-reactors
Rate equation for the ester formation
(6-123)
Rate equation for the formation of water
(6-124)
Removal rate of water through the pervaporation unit rate (qw), m3/s
(6-125)
where A (m2) = membrane area
Pw (m3∙rn/m2∙s∙Pa) = permeability coefficient of water in membrane
ℓ(m) = membrane thickness
pw,f (Pa) = partial pressure of water in the feed
(The partial pressure of water at the feed side is assumed to be negligible)
6.8.3 Membrane Reactor
in Liquid Phase Reactions
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6.8 Membrane Reactors and Membrane Bio-reactors
By using molar unit rather than volume and assuming that
qw ∝ molar concentrations at low water concentrations
Eq(6-125) → qw = Bcwater (6-126)
dc/dt = generation – elimination, from Eq(6-126) & (6-124)
(6-127)
use to calculate the conversion rate when Pw is known.
<Figure 6-77> Conversion of an esterification without
Pervaporation (B = 0) and with pervaporation (B > 10)
6.8.3 Membrane Reactor
in Liquid Phase Reactions
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6.8 Membrane Reactors and Membrane Bio-reactors
Membrane bio-reactor
Remove inhibitory component by membrane in fermentation process
improve the bio-conversion
4 different species contained in typical fermentation process
Substrate
Biocatalyst(microorganism)
Nutrients (salts and co-enzymes required for the bioconversion)
Product(s)
Continuous cell recycle set-up in <Figure 6-78>
Remove products and retain the microorganism or enzymes
from fermentation broth through the membrane unit
Add substrate and nutrients and remove products continuously
bio-catalyst(microorganism) concentration↑
6.8.4 Membrane
Bioreactor
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6.8 Membrane Reactors and Membrane Bio-reactors
Product ? choice of the membrane system
<Example> Low MW products of fermentation
Application of pervaporation
• alcohols (ethanol, butanol)
Application of electrodialysis
• organic acids (citric acid, acetic acid, lactic acid)
• vitamins (vitamin B12)
Advantage of membrane bioreactors
Continuous fermentation
High microorganism densities
Selective removal of product with
retaining nutrients and substrate
6.8.4 Membrane
Bioreactor
• ketones (acetone)
• amino acids (lysine)
• antibiotics (penicillin)
<Figure 6-78> Schematic drawing of a membrane recycle (bio)
Reactor in which a reactor is combined with a membrane unit.