Membrane separation Configuration.Modules.Transport.Fouling.
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Transcript of Membrane separation Configuration.Modules.Transport.Fouling.
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.