Chapter 6. Membrane Process (Concentration Driving Force)contents.kocw.net/KOCW/document/2015/chungbuk/yoon... · where B = membrane-based parameter, ... ⇨ osmotic pressure ⇨

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


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



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.



<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



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.



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.



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


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



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)



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.



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.



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


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)



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



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).



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



<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


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


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


<Figure 6-59> Gas-liquid membrane contactor

with a porous membrane, left)

and a dense membrane (right).


6.6.4 Summary of



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)


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


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


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



<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+



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



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.



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



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.



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)



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



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



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



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



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



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



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.



6.7.2.4 Summary of electrodialysis


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


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.


6.7.3 Membrane


<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


6.7.3 Membrane


<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.


6.7.3 Membrane


<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.


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.


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


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


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


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



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



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



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



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



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



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.



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)





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)





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



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.

Chapter 6. Membrane Process (Concentration Driving Force)contents.kocw.net/KOCW/document/2015/chungbuk/yoon... · where B = membrane-based parameter, ... ⇨ osmotic pressure ⇨

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