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INTRODUCTION
Pervaporation, whose term derives from the two major integral operations
involved in the separation process, namely,permeation and evaporation.
PV is defined as a separation process in which a liquid feed mixture is separated
by means of selective diffusionvaporization through a non-porous membrane.
Basically, the PV system consists of a non-porous membrane in a particular module.
PV depends upon a membrane technology which is a separation process.
PV did not attract much attention from researchers in early 1980s but from last
two decades it has emerged as a promising technique for removal of VOCs from
ground water or aqueous effluents.
PV process is less expensive in comparison with other water treatment
processes i.e. Reverse Osmosis and Ultrafilteration and possible recycling of
recovered VOC solvents.
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PV MEMBRANES
The PV membrane can be considered as a dense homogenous medium inwhich diffusion of species takes place in the free volume.
The first criteria for judging a membrane is whether it separates the target
solute from water.
The membranes used for separating VOCs from dilute solutions are mostly
hydrophobic materials.
Hydrophobic materials are preferred in this application because separating
a small amount of organic compounds from the mixture is easier and
consumes much less energy than separating water from aqueous solutions.
The most commonly used membrane in PV process is PDMS i.e.
Polydimethylsiloxane .
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PV MEMBRANE MATERIALS USED FOR VOCS REMOVAL
Polymer Structure
Polydimethylsiloxane (PDMS) (Si(CH3)2O)n
Nitrilebutadiene copolymer (NBR) (CH2CH=CHCH2CH2CH(CN))n
Polyvinylidene fluoride (PVDF) (CH2CF2)n
Styrenebutadiene (SBS) (CH2CH=CHCH2CH2CH(C6H5))n
Polyolefin (PO) (CH2CH2)n
Polyoctenamer ((CH2)3CH=CH(CH2)3)n
Poly[bis(phenoxy)phosphazene] (BPOP) (N=P((OC6H5)2))n
Polyether-block-polyamides (PEBA) (
C(=O)
PA
C(=O)
C
PE
O
)nPolyurethane (PUR) (CONHRNHCOORO)n
Block copolymerstyrene (PS) butadiene (CH(C6H5)CH2)m(CH2CH=CHCH)n
(CH(C6H5)CH2)m
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PLATE AND FRAME
Plate-and frame modules, which consist of stacks of alternating permeate
and feed layers were an early favourite because of the relatively simple design.
The most basic form of the plate-and-frame module, a single feed chamber
and a single permeate chamber separated by a flat sheet membrane, can be
used to test different membranes by simply swapping out the flat sheetmembrane.
It allows for the use of membrane materials which cannot be conveniently
produced as hollow fibres or spiral-wound elements.
The disadvantages of the plate-and-frame design are that the ratio ofmembrane area to module volume is low compared to spiral-wound or hollow
fibre modules, dismounting is time-consuming and labour-intensive, and higher
capital costs are associated with the frame structures.
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Hollow fibre modules can be configured for liquid flow on the shell
side (outside of the fibres) or on the lumen side (bore feed, inside the
hollow fibres) of the hollow fibres.
These tubes have diameters on the order of 100 m.
They have a very high surface area to module volume ratio. This makes
it possible to construct compact modules with high surface areas.
Problem with a hollow fiber module is that a whole unit has to bereplaced if failure occurs unless the defective fibre(s) can be plugged.
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A spiral-wound module is like a plate-and-frame module rolled into acylinder with liquid flow entering along the end of the cylinder and
leaving at the other end.
Depending upon the path length, permeate spacer design, andpermeate flux, significant permeate-side pressure drops can be
encountered, thereby reducing the mass transfer driving force.
Another limitation of spiral-wound modules is that the feed spacer
acts as a sieve for particles and may become blocked with particulatematter.
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Physical properties of VOCs are important because when PV is used to
remove VOCs from water knowledge about VOC is utilised for proper
system design.
VOC DIFFUSIVITY: It is a function of VOC molecular size, solution viscousity and
temperature.
Di = 7.4 * 10-8 (jMj)
1/2 T/Vi0.6
VOC PERMEABILITY : Through a non-porous membrane it is defined as product
of solubility and diffusivity.
AFFINITY FOR SPECIFIC MEMBRANE MATERIAL.
SATURATION VAPOR PRESSURE.
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The contaminated water when flows into a pervaporation
module containing the organophilic membranes. The
organics sorb into the membranes. A vacuum applied to
the system causes the organics to diffuse through the
membrane as vapor. Components of the liquid stream thatdo not move through the membrane are discharged from
the system, The vapor (permeate) is then passed through a
condenser generating a highly concentrated liquid. The
condensate separates into aqueous and organic phases. Theaqueous phase permeate can be sent back to the
pervaporation module for further treatment, while the
organic phase permeate is discharged to a receiving vessel.
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TEMPERATURE PRESSURE
FEED CONCENTRATION
FLOW VELOCITY
MEMBRANE THICKNESS
PILOT AND INFIELD APPLICATION
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