Membrane separationConfiguration.Modules.Transport.Fouling

Configuration

FLAT

- the active layer is a flat

- synthesised as a continuous layer

- low surface area per volume

- used in flate-and-plate module 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)

Membrane module – the unit into which the membrane’s 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 module’s volume,- Facility of cleaning and sterilization,- Low costs by itself- High resistance membrane on agressive chemical, physical & biological factors.

SIMPLE MODULEThe module is the central part of membrane

instalation.

Feed composition and a flow rate inside the module will change as a function of distance.

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.

MEMBRANE MODULES

Plate-and-frame module

Spiral-wound module

Tubular module

Capillary module

Hollow-fiber module

The choice of module configuration

Based 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

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

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.

Plate-and-frame module

Advantages- High allowable work

pressure

(high viscosity liquids)- Easy to clean- Easy to replace

membranes

Disadvantages- Low membrane area

per volume

(100-400 m2/m3)

Electrodialysis, pervaporation, membrane destillation

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.

Spiral-wound module

Advantages

- High packing density

(300-1000m2/m3)

- Easy and inexpensive to adjust hydronomics by changing feed spacer thickness to overcome conc. polarization and fouling

- Low relative costs

Disadvantages- Difficult to cleaning and

sterilization- High pressure drop

(100-150kPa)

- Use only for pure medium

TUBULAR MODULES

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 .

Tubular module

Advantages- Resistance for fouling- Easy to cleaning

Disadvantages- Low packing density

(300m2/m3)- Expensive

Reverse osmosis, ultrafiltration

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.

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

HOLLOW-FIBER MODULE

The difference – dimmensions of the tubes, but module concepts are the same.

The hollow-fiber module – highest packing density 30000m2/m3.

A perforated central pipe is located in the center of the module through which the feed solution enters.

Hollow-fiber module

Advantageous to use the ‘inside-out’ type to avoid increase in permeate pressure within the fibers and it’s thin selective top-layer is better protected, whereas a higher membrane area can be achieved with the ‘outside-in’ concept.

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.

Comparison of module configurations

Membrane fouling

Polarisation phenomena are reversible processes, but in practise, a continuous decline in flux decline can often be observed.

FOULING

CONCENTRATIONPOLARISATION

TIME

FLUX

Flux as a function of time. Both concentration polarization and fouling can be distinguished

Membrane foulingThe (ir)reversible deposition of retained particles, colloids, emulsions, suspensions, macromolecules, salts etc. on or in the membrane.

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

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

Depends:-concentration,-temperature,-pH,-ionic strenght,-specific interactions (hydrogen bonding, dipole-dipole interactions)

Membrane fouling

cc rlRc

])[(

)1(180

32

2

sc dr

])1([ A

ml

s

sc

)( RcRm

PJv

Flux:

Total cake layer resistance (Rc)

rc – specific resistance of the cake lc – cake thickness

where:

Kozany – Carman relationship:

where:

where:

ds – the ‘diameter’ of the solute particle

- porosity of cake layer

ms – the mass of the cake

s – the density of the solute

A – the 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.

Membrane fouling

Ac

VcrRc

c

bc

][

1

Ac

VcrR

P

dt

dV

AJ

c

bcm

A

V

cP

rc

JJ c

cb

w

)(11

The flux can be written:

or

R = 100%

Jw – pure water flux

Rc the cake layer resistance can be obtained from the mass balance. In case of complete solute rejection:

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

JJ c

cb

w

)(11

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

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.