configuration.modules.transport.fouling1

8/3/2019 Configuration.modules.transport.fouling1

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Membrane separation

Configuration.Modules.Transport.Fouling


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Configuration FLAT

- the active layer is a flat

- synthesised as acontinuous layer

- low surface area per

volume

- used in flate-and-platemodule and spiral-wound

module

TUBULAR

- usually active layer is

inside- the permeate crosses the

membrane layer to the

outside (feed inside)

- high surface per volume- several lenghts and

diameters (>10mm)


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Membrane modulethe unit into which the membranes area is packed.

- Protects membranes against mechanical damage

- Permits get high area in small volume

Requirements for membrane:

- High selectivity separation components- High permeability with respects to solvent

M.M. have to be keep:

- High productivity of process,

- Leaktighness between stream of permeate and retentate in the

high ratio of membrane surface to modules volume,

- Facility of cleaning and sterilization,

- Low costs by itself

- High resistance membrane on agressive chemical,

physical & biological factors.


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SIMPLE MODULEThe module is the central part of

membrane instalation.

Feed composition and a flow rate inside

the module will change as a function ofdistance.

Permeate stream is the fraction of the feed

stream of the feed stream which passes

through the membrane.

Retentate stream is the fraction retained on

the membrane.


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MEMBRANE MODULES Plate-and-frame

module

Spiral-wound module

Tubular module

Capillary module

Hollow-fiber module


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The choice of module configurationBased on economic considerations

Type of separation problem

Ease of cleaning

Ease of maintenance

Ease of operations

Compactness of the system

Scale

Possibility of membrane replacement


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PLATE-AND-FRAME MODULE

The number of sets needed for a given membrane area furnished with sealing

ring and two end plates then builds up to a plate-and-framestack


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Plate-and-frame module

Schematic flow path in plate-and-frame

module

In order to reduce channeling- a tendency a flow

along a fixed pathway and to establish as uniform flow

distribution so-called stop-discs

Tortous-path plate

Is used to improve mass transfer,

to reduce concentration polarisation

by applying a proper spacer material.


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Plate-and-frame moduleAdvantages

- High allowable work

pressure(high viscosity liquids)

- Easy to clean

- Easy to replacemembranes

Disadvantages

- Low membrane area

per volume(100-400 m2/m3)

Electrodialysis, pervaporation, membrane destillation


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SPIRAL-WOUND MODULE

Pressure vessel containig 3 spiral-wound modules arranged in series

The feed flows axial through the cylindrical module

parallel along the central pipe whereas the permeate

flows radially toward the central pipe.

Membrane and permeate-side

spacer material are glued

along three edges build a membrane

envelope.


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Spiral-wound module

Advantages

-High packing density

(300-1000m2/m3)

- Easy and inexpensive to

adjust hydronomics by

changing feed spacer

thickness to overcomeconc. polarization and

fouling

- Low relative costs

Disadvantages

- Difficult to cleaning and

sterilization- High pressure drop

(100-150kPa)

- Use only for pure medium


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TUBULAR MODULES


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Tubular module

Schematic drawing of tubular module

Cross section of monolithic ceramic module

The feed solution always flows through the centre of the tubes while the permeate flows

through supporting tube into the module housing .


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Tubular module

Advantages

- Resistance for fouling

- Easy to cleaning

Disadvantages

- Low packing density

(300m2/m3)

- Expensive

Reverse osmosis, ultrafiltration


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Capillary module

Capillary module consists of a large numbers of capillaries assembled together in a module.

The free ends of the capillaries are potted agents such as epoxy resins, polyurethans.


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CAPILLARY MODULE

The choice between the two concepts is mainly based on the application where the parameters such a pressure,

pressure drop, type of membrane available etc. are important.

Depending on the concept chosen, asymmetric capillaries are used with their skin on the outside or inside

Two types of module arrangements can be distinguised


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HOLLOW-FIBER MODULE

The differencedimmensions of the tubes, but module concepts are the same.

The hollow-fiber modulehighest packing density 30000m2/m3.

A perforated central pipe is located in the center of the module through which the

feed solution enters.


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Hollow-fiber module

Advantageous to use the inside-out type to avoid increase in permeate

pressure within the fibers and its thin selective top-layer is better protected,

whereas a higher membrane area can be achieved with the outside-in concept.


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Hollow-fiber module

Advantages

- High packing density

500-9000 m2/m3

- Low relative costs

Disadvantages

- Poor resistance of

fouling- Difficult to clean

- Difficult to change the

membrane

Microfiltration, ultrafiltration, reverse osmosis, pervaporation,

liquid membranes and the membrane cofactors where the boundary layer resistance

may become very important as well.


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Comparison of module

configurations


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Membrane fouling

Polarisation phenomena are reversible processes, but in practise, a

continuous decline in flux decline can often be observed.

FOULING

CONCENTRATION

POLARISATION

TIME

FLUX

Flux as a function of time. Both concentration polarization

and fouling can be distinguished


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Membrane fouling

The (ir)reversible deposition of retained particles, colloids,emulsions, suspensions, macromolecules, salts etc. on or in themembrane.

The includes adsorption, pore blocking, precipitation and cake formation. Occurs inmicrofiltration and ultrafiltration.

Pressure driven processes, type of separation and the type of membrane used todetermine the extent of fouling.

Depends:

-concentration,

-temperature,

-pH,

-ionic strenght,

-specific interactions (hydrogen bonding, dipole-dipole interactions)


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Membrane fouling

cc rlRc

])[(

)1(180

32

2

s

c

dr

])1([ A

ml

s

s

c

)( RcRm

PJv

Flux:

Total cake layer resistance (Rc)

rcspecific resistance of the cake

lccake thickness

where:

KozanyCarman relationship:

where:

where:

dsthe diameter of

the solute particle

- porosity of cake layer

msthe mass of the cake

sthe density of the soluteAthe membrane area

The thickness of the layer depends on the type of solute

and especially on operating conditions and time.

The growing layer of accumulates results in a continuous flux decline.


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Membrane fouling

Ac

VcrRc

c

bc

][

1

Ac

VcrR

P

dt

dV

AJ

c

bc

m

A

V

cP

rc

JJc

cb

w

)(11

The flux can be written:

or

R = 100%

Jwpure water flux

Rc the cake layer resistance can be obtained from the mass balance.

In case of complete solute rejection:


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Membrane fouling

1/J

V/A

1/Jw

increases decreases1/J

V/A

1/Jw

PCb

Reciprocal flux is indeed linearly related to the permeate volume V for various concentrations (Cb)

and applied pressures (P) in an unstirred dead-end filtration experiment with BSA as solute.

Reciprocal flux as a function of the permeate volume for different concentrations (1) and applied pressures (2)

A

V

cP

rc

JJc

cb

w

)(11


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Methods to reduce fouling

Pretreatment of the feed solution

- heat treatment

- pH adjustment

- addition of complexing agents (EDTA etc.)

- chlorination

- adsorption onto active carbon

- chemical clarification

- premicrofiltration

- preultrafiltration

Membrane properties Module & process conditions

Cleaning

- hydraulic cleaning ( back-flushing )

- mechanical cleaning

- chemical cleaning

- electric cleaning


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Membrane fouling

Flux versus time behaviour in a given microfiltration

process with and without back-flushing

Alternate pressuring and depressuring and

by changing the flow direction at a given

frequency.

After a given period of time, the feed

pressure is released and the direction of the

permeate reversed from the permeate side

to the feed side in order to remove the

fouling layer within the membrane or at

the membrane surface.

configuration.modules.transport.fouling1

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