voc removal

7/28/2019 Voc Removal

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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

voc removal

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