Membrane separation. Configuration.Modules.Transport.Fouling. FLAT - the active layer is a flat - synthesised as a continuous layer - low surface area per volume
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- 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
- 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)Configuration
Membrane module – the unit into which the membrane’s area is packed.
Requirements for membrane:
M.M. have to be keep:
high ratio of membrane surface to module’s volume,
physical & biological factors.
The 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.SIMPLE MODULE
Hollow-fiber moduleMEMBRANE MODULES
Based on economic considerations
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
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’
Is used to improve mass transfer,
to reduce concentration polarisation by applying a proper spacer material.
High allowable work pressure
(high viscosity liquids)
Easy to clean
Easy to replace membranes
Low membrane area per volume
(100-400 m2/m3)Plate-and-frame module
Electrodialysis, pervaporation, membrane destillation
Membrane and permeate-side
spacer material are glued
along three edges build a membrane envelope.
The feed flows axial through the cylindrical module
parallel along the central pipe whereas the permeate
flows radially toward the central pipe.
Pressure vessel containig 3 spiral-wound modules arranged in series
-High packing density
- Easy and inexpensive to adjust hydronomics by changing feed spacer thickness to overcome conc. polarization and fouling
- Low relative costs
Difficult to cleaning and sterilization
High pressure drop
- Use only for pure mediumSpiral-wound module
Cross section of monolithic ceramic module
Schematic drawing of tubular module
The feed solution always flows through the centre of the tubes while the permeate flows through supporting tube into the module housing .
Resistance for fouling
Easy to cleaning
Low packing density (300m2/m3)
Reverse osmosis, ultrafiltration
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.
Two types of module arrangements can bedistinguised
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
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.
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.
High packing density
Low relative costs
Poor resistance of fouling
Difficult to clean
Difficult to change the membraneHollow-fiber module
Microfiltration, ultrafiltration, reverse osmosis, pervaporation,
liquid membranes and the membrane cofactors where the boundary layer resistance
may become very important as well.
Polarisation phenomena are reversible processes, but in practise, a continuous decline in flux decline can often be observed.
Flux as a function of time. Both concentration polarization
and fouling can be distinguished
The (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.
specific interactions (hydrogen bonding, dipole-dipole interactions)Membrane fouling
Kozany – Carman relationship:
ds– the ‘diameter’ of
the solute particle
Total cake layer resistance (Rc)
- porosity of cake layer
ms – the mass of the cake
s – the density of the solute
rc – specific resistance of the cake
lc – cake thickness
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.
Rc the cake layer resistance can be obtained from the mass balance.
In case of complete solute rejection:
R = 100%
The flux can be written:
Jw – pure water flux
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)
- heat treatment
- pH adjustment
- addition of complexing agents (EDTA etc.)
- adsorption onto active carbon
- chemical clarification
- hydraulic cleaning ( back-flushing )
- mechanical cleaning
- chemical cleaning
- electric cleaning
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.
Flux versus time behaviour in a given microfiltration
process with and without back-flushing