Theory questions

• Selectitvity

• Flux

• Stability

• Inexpensive to manufacture and to replace

Brekathrough pressure through pores

• Reverse osmosis

• Nanofiltration

• Ultrafiltration

• Microfiltration

• Particle filtration

1. Membrane element

2. Membrane module

3. Membrane plant

4. The overall integrated process

• Electrodialysis

• Capacitive deionisation (CDI)

• Pervaporation (PV)

• Gas permeation (GP) and vapor permeation (VP)

• “Membrane contactors“ e.g. oxygenators

• An applied pressure is used to overcome the osmotic pressure

• Osmosis would cause water to permeate the the semi-permeable membrane to dilute the concentrated solution

• Through reverse osmosis, this process is inverted and water is diffusing from the highly concentrated solution into the slightly concentrated solution

• Pervaporation: processing method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane

A thin film composite membrane contains of a porous support and a dense top layer

Fabrication via:

• Interfacial polymerization

• Dip coating

  
  

1. Porous support is soaked with an aqueous medium

2. Non-aqueous medium is applied on top of the support

3. Forms thin dense layer on top

• Dense membranes: Gas separation & pervaporation

• Porous membranes: Ultrafiltration & Microfiltration

• Structure related properties:

  • Pore size (distribution)

  • Surface porosity

  • Thickness

  • Pore shape

• Permeation related:

  • Clean water flux

  • MWCO

  • Retention

  • Selectivity

• Bubble point method

• Mercury intrusion porometry

• Permeation measurement

• Scanning electron microscopy

The solution diffusion model describes the permeation through a dense membrane with regard to the Solubility of the solution and the diffusion coeeficient of the membrane

P = D * S

P: Permeability

D: Diffusivity

S: Solubility

• The first bubble forms at the largest pore: information is generated about largest pore’s pore size

  
  

• Measured flow rate gives pore size according to Hagen-Poiseuille

  
  

• Molecular weight cut off (MWCO) is defined as the lowest molecularweight of a solute of which 90% are retained by the membrane

  
  

• Permeability

• Physical properties

• surface analysis

• Solution: Henry’s law

  • c = S * p

• Diffusion: Fick’s 1st law

  • J = -D * (dc/dx)

• Modulation of the Permeate flux J and the retention R by

  • Operating parameters ( delta_p, c_f, T, v)

  • Membrane properties (such as solubility, diffusivity, membrane thickness, structure)

  • Properties of the fluid system (such as dynamic viscosity, condensation, temperature of components)

• In general: Linear increase of permeate flux with increasing pressure

• Flow through capillaries: Hagen-Poiseuille-Equation assuming laminar flow

• Flow through pores: Carman-Kozeny equation

Nominal pore size diameter: d_n= 0,1 micrometer

Combination of Fick’s first law and Henry’s law

In case of ideal gas behaviour (permanent gases) the flux of aspecies k is proportional to the partial pressure differencebetween feed and permeate

Diffusion type where the gas-wall collisions are dominant (particles collide way more often with walls than with other particles)

• Serial

• Parallel

  
  

• Membrane controlled if

  • Membrane with relatively small flux

  • Small concentration polarisation

• Laminar layer controlled if

  • High transmembrane flux

  • High concentration polarisation

  • low diffusivity of less permeable component

• Feed Concentration polarisation

• Pressure loss in support layer

• Concetration polarisation in porous support and in the permeate

• Fouling, scaleing, pore blocking

• Temperature polarisation

• co-current flow

• counter flow

• cross flow

• free permeate

• perfect mixture

  
  

main goal: Reduction of the thickness of the boundary layer

• Definition of channels

• Increased mass transfer

• Greater flux

• But also increased pressure loss

• Minimal production costs

  • Low-cost fabrication

  • Low-cost materials

• Minimal cost of operation

  • Low pressure drop

  • High mass transport properties

  • good cleaning properties

• Minimal susceptibility to blocking

  • Steady flow

  • Prevention of dead zones

  • Prevention of channeling

• 2end module (dead end)

• 3end module

• 4end module (extraction)

  
  

• Flat sheet membranes

  • Plate and frame modules

  • Cushion module

  • Spiral-wound module

• Tubular membranes

  • Tube and shell module

  • Capillary module

  • Hollow-fiber module

• Advantages

  • Single membranes can be replaced

  • Low tendency for blocking/fouling

  • Application of flat sheet membranes possible

  • Potting not necessary

• Disadvantages

  • Numerous sealings

  • High pressure loss due to deflections

  • Rather low packing density (< 400 m²/m³)

• Advantages

  • Less sealings

  • Low permeate-sided pressure drop

  • High pressure operation possible

• Disadvantages

  • Weldable membranes needed

  • Rather low packing density (< 500 m²/m³)

• Advantages

  • Easy and cheap production

  • Rather high packing density (< 1000 m²/m³)

  • Good exchange of substances by feedspacer

• Disadvantages

  • Membrane has to be weldable

  • Partial long flow path on permeate

  • Difficult to clean

• Characteristics

  • d = 5-25 mm

  • Feed on inside of tubes

  • Support tube

• Advantages

  • Turbulent flow

  • Low tendecy for blocking/fouling

  • Easy to clean

  • Morderate pressure drop of module

• Disadvantages

  • Low packing density

  • High ratio of feed flow rate to membrane area

• Characteristics

  • d <5 mm

  • Lumen-side or shell-side feed

  • Self supported membranes

  • Operated in cross flow or dead end

• Adavantages

  • Higher packing density than tube moduls

  • Reduced spec. production costs

• Disadvantages

  • Often laminar flow patterns resulting in limited mass transfer

Main application: waste water treatment, retention of activated sludge

• Blocking at the top: Caused by hairs and fibers through upwards flow

• Blocking at the bottom: Blocking by side-aeration

Consequence: Reduced filtration area

• Goal: redurce polarization effects

• Passive measures:

  • Additional components (spacers etc.)

  • Active measures:

    • Multiphase flow (aeration)

    • Dynamic cross-flow filtration

    • High frequency back flushing

• Modules in parallel

• Modules in series

• One step with recycle

• Two step

• Two stages

Low flow velocity:

• Reduced driving force (Concentration)

• Increased concentration polarization

• Increased fouling

• Back mixing may occur

  High flow velocity:

• Reduced driving force (pressure)

• High pressure loss

• Damage to the module

Modules in parallel

Modulesin series

One step with recycle

Two stages

Two step

Christmas tree structure

Feed & bleed strcuture

GP: the separation of gases with membranes

GP through a dense membrane is described by the solution diffusion model

• Permeance Q

  • Specific value depending on the membrane thickness

    
    

• Permeability P

  • Independent of membrane thickness

  • P = D * S

For an ideal membrane alpha should be as large as possible

Selectivity over permeability/ permeance

Condensable gases = vapor

• T_Process < T_G

• high influence of molecular size on diffusion coefficient

• diffusivity determines selectivity

• typical applications: N2/O2-separation, CH4/CO2 –separation

• D_rubbery/D_glassy = 100 - 100.000

• solubility determines the selectivity

• typical applications: solvent recovery from off-gas

• High packing density

• small pressure loss

• stability

• economic production

• N2/O2 - seperation

• CO2/CH4 - seperation

• Solvent recovery

• GP economical for small/moderate product streams and moderate purities (90...99 %)

• Product = retentate → any purity within a single stage (BUT: η↓ with low α)

• High selective membranes

• Investment cost 70 % for the compressor

• Integral asymmetric

• Thin film composite (TFC)

the minimum applied pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane

• Dissociation factor: in wie vielen Ionen teilt sich das Molekül auf

Limiting factors are:

• Membrane deterioration: Decomposition by

  • Acids, Bases, Chlorine, Oxygen….

• Blocking by Fouling

  • Layer on membrane from suspended materials, metal oxids etc.

• Blocking by scaling

  • Salt deposit on the membrane

• Decreasing performance due to reduction of the permeate flux

  • Osmotic pressure, viscotity

• Physical foulants, related to deposition of particles

  • Particulate fouling: Larger particles can be removed by various mean

  • Coagulation

• Organic foulants, interact with the membrane

  • Adsorption of organics on membrane surface → irreversible fouling

• Anti-fouling measures (examples):

  • Feed water desinfection

  • continuous chlorination (HOCl)

• Crystallization of inorganic substances on the membrane surface

• increase in salt conc. towards the membrane surface due to concentration polarization

• Precipitation occurs if solution is supersaturated

• Common examples: CaSO4, CaCO3

CaSO4:

Restriction and reduction of the maximum recovery rate

CaCO3:

Acid addition

• Desalination of sea and brackish water

• Water reclamation e.g. concentration of food juice and sugars (food industry),concentration of milk (dairy industry)

• Industrial wastewater treatment

• Landfill leachate treatment (Deponiesickerwasser)

• Production of ultra pure water (electronic industry)

• Water recovery is limited by osmotic pressure and scaling

• The pressurized retentate contains energy

Spiral wound module

• Particle/ Molecular size

• Molecular/ colloidal strcuture

• Interaction membrane-substance

• Porous membrane: Convective mass transport

• Small specific flow resistance

• Complete rejection of bacteria

• Partial rejection of viruses

• Materials

  • Organic: PP, PE, PTFE;…

  • Inorganic: Sintering materials, ceramic, ….

• Concentration of suspensions

• Biological and pharmaceutical industry

• food industry

• wastewater treatment

• Separation, fractionation of dissolvedsubstances

• Removal of viruses

• food industry

• water treatment

• Porous membrane: Convective mass transport

• Flow resistance of the membrane is NOT negligible

• Asymmetric structure

• Complete rejection of bacteria and viruses (difference to MF)

• Phase inversion membrane, composite membrane

• Membrane materials:

  • Organic: PE, PES,PAN,….

  • Inorganic: Oxidic zirconium, Al2O3/TiO2

• Bacterial challenge tests for measuring pore size and membrane integrity

• Log-Reduction value

TMP = Transmembrane pressure

Inverse to the permeance divided by the viscosity

• Molecular weight cut off

• Molecular weight of a particle that is retained by 90% by the membrane

MWCO = 60 kDa

Atomare Masseneinheit, 1 Da = 1,66 * 10^(-27) kg

Hydrophilic: PVDF, PA, PES, PS, …

Hydrophobic: PP

If charged particles are in suspension, their potential is compensated by the accumulation of ions in the suspension medium. Firmly bound ions accumulate on the particle surface in the so-called Stern layer. Other ions are more loosely bound in a diffuse, i.e. disordered, layer. Thus, the particle appears electrically neutral from a great distance, because all particle charges are compensated by ions of the suspension medium.

If the particle is then moving, its losing some of the lose bound ions and is therefore not charged neutral again, but has a potential.

by moving the charged particle through an applied electric field.

The resulting velocity is then a measure of the zeta potential.

• Zeta-Potential is f(pH, ionic strength)

• isoelectric point = pH value where the zeta-potential is zero

• Irreversible cake layer

• Pore blocking

• Inner pore adsorption

• Biofouling

• One exit

• Discontinous operation

• Two exit

• Continuous operation is possible

• Hydrodynamic conditions in the membrane

• Membrane material (hydrophilicity, surface charge)

• Backwashing (water chemicals, air)

• Environmental engineering

• Metal processing industry

• Pharmaceutical indsutry

• food industry

• 
  

• Separation of complex mixture with close-boiling substances

• Separation of multiple azeotropes

• Feed: liquid

• Driving force: partial pressure difference

  • Either applied by vacuum on permeate side

  • Or by sweep gas stream

• Liquid component from the feed evaporates and passes the membranes, vaporous permeate

• Feed: Saturated vapor stream

• Advantage: feed is already vaporous

• Driving force: partial pressure difference

  • Either applied by vacuum on permeate side

  • Or by sweep gas stream

modules in series

interstage reheating

vacuum due to condensing

• hydrophobic membranes are highly permeablefor bigger, condensable molecules

• Sorption-selective controlled

• Best used to treat diluted organic stream→ enrich organic compound in permeate

• highlypermeable for water molecules

• Useful for alcohol water separation bc. of the high selectivity

• Diffusion selectivity favors water → smaller molecule

• Best used to treat concentrated organic stream→ concentrate water in small permeate volume

• Separation of water: Dewatering of solvents andsolvent mixtures

• Separation of organicsubstances: separation of ethanol

• 
  

Separation of alcohol:

• Separation of methanol from hydrocarbons (MTBEsynthesis, dimethylcarbonate production)

• Separation of ethanol from hydrocarbons (ETBEsynthesis)

• membrane area

• number of modules/ reheaters

• minimal permeate pressure drop/ temperature drop

• minimal polarization

PV / VP with distillation in hybrid processes for breaking azerotropes and removing single-component with a high-purity side stream from multi-component mixtures

Device for non-dispersive interface generatiomn with a membrane

-> Alternative to conventional contacting apparatuses (extraction, adsorption,stripping)

• Non-dispersing contact

• Large packing density

• Well defined flow conditions in fiber lumen

• Well-defined and constant surface area

Membrane is an additional transport resistance

• Effiency loss due to

  • Blocked fiber by suspended solids

  • Fouling

• Limited lifetime of membranes (replacement costs)

• Possibilty of unstable phase separation

  • Pressure changes

  • Change of wetting properties

• Linear scale-up of costs

The trans-membrane pressure

• must be kept within ∆pmin < ∆p < ∆pmax

• Otherwise leakage of the wetting phase or breakthrough of the non wetting phase

• Breakthrough pressure is described by the Young-LaPlace equation

• Because of the pressure drop in counter current mode

• Membrane wetting:

  • Keep the phase with the better mass transfer within the membrane pores

• Flow in Lumen

  • Advantage: no backmixing

  • Disadvantage: high pressure drop

• Flow on shell-side:

  • Advantage: High transfer coeff.

  • Disadvantage: Back-mixing and dead zones

• membrane oxygenators e.g. articifial lung

• Pertraction (Permeation + Extraction) e.g. the Removal of phenol fromprocess water

• Diffusion Dialysis (Acid- andBasedialysis)

Faraday constant: electric charge per mole of electrons

F = e * N_A = 96485,33 C/mol

-> Used for calculating the electrochemical potential

AEM: anion exchange membrane

CEM: Cathion exchange membrane

• Respektive to the fixed ion in the membrane, the equally charged counter ion can pass the membrane, while the oppossitly charged coion is rejected

• Donnan equilibrium: uneven distribution of charged solute particles that occurs when a semipermeable membrane is permeable to the solvent and some, but not all, of the ions present in the solution.

s = solution

m = membrane

fix = Fixions

Co = Coins ( which can’t pass)

• higher concentrations of fixed ions in membrane

• lower concentration of electrolyte solution

• (decreasing chemical valence of coions)

• (affinity of ion exchange material with respect to counterions

• limiting current density is reached, when no ions exist to transfer the charge

i_lim depends on:

• thickness of boundary layer, which depends on Re

• the dilute concentrate c+_DBulk

• Batch

  • Small feed amounts

  • High dilution

  • Non-Productive times

• Feed & bleed

  • medium-to-large feed amounts

  • high recirculation rates

• continuous

  • low energy consupmtion

• Seeding operation mode

Inverse or reverse electrodialysis (RED) works by using the same mechanism as electrodialysis, except that in RED the polarity of the electrodes is periodically reversed (approximately 3 to 4 times per hour); and the output of the concentrated and diluted solutions are exchanged automatically by means of valves.

• Softening of hard water

• Citrate removal from fruit juices

• Tartrate Elimination from wine

• Water dissociation

Ion exchange membranes:

• High inner charge

• Semipermeable

• Block co-ions

• Water for cooling towers

• Wastewater reusal

• Water softening

• Brackish water desalination

Systems my be:

• Explosive, toxic, carcinogenic

• multi component mixtures

• complex, recycles, hybrid

Simplifications and smart assumptions

• Mass/ mole balances

• Momentum balances

• Energy balances

• Non-ideal effects e.g.

  • Concentration polarization

  • real gas behavior

Often only 10-20%

-> Costs are dominated by driving force

• Optimize existinbg plants

• Design new plants

• Integrate new technologies

• Process debottlenecking

• Module

  • Permeability/ selectivity

  • geometry

  • heat transfer

• Process

  • Pressure

  • Temperature

  • streams

RO: 150 Da

NF: 200-100 Da

UF: > 1000 Da

• Flat sheet or hollow fiber/ tubular

• Polymeric membranes for aqueous or organophilic systems

• Ceramic/ inorganic membranes for harsh filtration conditions

• Retention of multivalent ions

  • Removal of hardness from process or drinking water

• Retention of organic substances

  • Treatment of drinking water

  • Removement of color molecules from effluent of textile industry

• Fractionation of organic substances

  • Removal of malic acid from wine

• Nanofiltration of organic solvents

• Immersion

  • Microporous support is immersed to a low concentrated polymer casting solution

• Plasma polymerization

  • Radicals are embedded in the microporous support and activated plasma iniates the polymerization reaction

• Interfacial polymerization

  • Microporous support is soaked one by one into two immiscible liquids that contain reactants

  • Polymerization reaction at a phase interface of the two immiscible liquids

The retention R gets negative if a higher permeate concentration of the component ci,P is reached, compared to the feed concentration of this component ci,F .

Retention increases with permeate flux (TMP) but decreases at high fluxes

• Increase: convection solvent flux increases – diffusive ion flux changes little

• Decrease: Concentration polarization increases ion concentration at the membrane

Depends on:

• Total flux (pressure dependend)

• Concentration gradient across the membrane

• Electrical charge of the membrane material/ valency of the ions

Solubility limit: Describes the maximum amount ofsalt which can be dissolved in water

Precipation occurs in supersaturated solution

-> Increase in salt concentration towards the membrane surface due to concentration polarization

• Organic systems

  • Organic Solvent Nanofiltration (OSN)

• Aqueous systems

  • Acid Purification and Metal Recovery

  • Nanofiltration of Preatreated Sewage Sludge

  • Phosphorus recovery

• Development of organic solvent resistant nanofiltration modules

• Separation of complex organic compounds from organic solvents

• Tailoring the MWCO by chemical degree of polymerization

Upstream:

• First step in a bioprocess where biomass (organisms) are grown in bioreactors

Downstream:

• Treatment of any culture broth after bioreactor cultivation

• Separation of products from micoorganisms, by-products, nutrients, othercomponents

• 60 - 80% of total production costs

Usually pressure driven and depend on size, diffusivity, charge, shape

• Selective product removal from the reaction mixture (extractor)

• Retention of biocatalyst (extractor)

  • Membrane retains only catalyst allowing all other

    components to permeate

• Controlled addition of substrate to the reaction mixture (distributor)

  • Addition of gaseous or organic substrates to aqueous

    mixture via membranes

• Processing of biotherapeutics by sterile filtration, since thermal stability is low

• Dead-end filtration (good for low-concentration solutions)

• Applications:

  • Removal of bacteria and particles from feedstock solution (upstream)

  • Protection of downstream units from fouling

  • Sterile filtration of buffers, products and gases

Requirement: <1 virus particle/ million doses

Removal options:

• Chemical or physical inactivation

• Size exclusion via membrane

Applications:

• protein purification

• concentration of macromolekules

• desalting

• Initial harvest of products from cell cultures

• Cross flow operation

• Fed batch

• Batch

• Diafiltration process

• Separation of two solutes (e.g. salt from a protein)

• Solvent lost with permeate is renplenished using fresh solevnt

• Two phase system (stationary binding phase and mobile carrierphase)

• Pricinple:

  • Mobile phase is pumped through column

  • Detector shows peaks of solutes after leaving the column

  • Different substances have different retention times

• Ion exchange

• Reverse phase/ hydrophobic interaction

• Size exclusion

• Affinity

• Low pressure drop

• Convection-driven rather than diffusion-driven

• Absence of compaction of packed beds

• Reduced diffusional paths lenghts

• Ease of scale-up

• Binding capacity remains low in contrast to conventional chromatography

• Downstream process, e.g. protein purification

• Separation of proteins, nucleic acids, Iipids, antibiotics, sugars, etc.

• Very strict purification requirements > 99.99%

• Loading

• Washing

• Elution

• Regeneration