GAS PERMEATION AND LIQUID PERMEATION

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ASHWINI KUMAR RISHABH RAJ

PERMEATION

Permeation is the penetration of a permeate (such as

liquid,gas or vapour) through a solid.

It is directly related to the concentration gradient of the

permeate, a material's intrinsic permeability, and the

materials' Mass diffusivity.

Permeation is modeled by equations such as Fick's laws

of diffusion, and can be measured using tools such as a

minipermeameter.

Permeation works through diffusion; the permeant will

move from high concentration to low concentration

across the interface.

THEORY

Gas permeation is the term used to describe a membrane

separation process using a non-porous semi-permeable

membrane. In this, a gaseous feed stream is fractionated

into per-meate and non-permeate streams. Transport

occurs by a solution-diffusion mechanism and

membrane selectivity is based upon the relative

permeation rates of the components through the

membrane. Each gaseous component transporting

through the membrane has a characteristic permeation

rate that is a function of the ability to dissolve and

diffuse

Separation mechanism:

different velocity of gas permeation (sorption, diffusion,

sieving effect, desorption)

Driving force: partial pressure gradient.

Working pressure: up to 100 bar.

Non-porous polymeric membranes:

PDMS(polydimethylsulphoxan),PS(polysulphone),PES.

Ceramic Membranes (small pores for Knudsen).

Metallic membranes (Pd and Ag alloys).

process

In the process as shown in fig. The feed gas at high

pressure P1 contains some low molecular weight species

(molecular weight <50) to be separated from small

amounts of high molecular weight species.

The other side of the membrane is maintained at much

lower pressure P2 often near ambient pressure the

membrane used is micro pores perm selectivity for

certain low molecular weight species in the feed

gas(species A)

If the membrane is dense these species are absorbed at

the surface and then transported though the membrane

by one or mechanism.

Thus perm selectivity depends on both membrane

adsorption and membrane transport rate.

In this process perfect separation is generally not

achievable, If the difference of molecular weight of

substance is near by.

Calculation:

The gas permeability was calculated according to the

following equation.

where l is the film thickness (m), Q is the permeate

volumetric flow rate (mol/s), p2 is the feed absolute

pressure, p1 is the downstream absolute pressure (Pa)

and A is the membrane area available for transport (m2).

Permeability is reported in units of (mol. m /(m2.s.Pa)).

Application:

Separation of hydrogen from methane,

Removal of carbon di oxide,

Removal of organic solvent from air,

Nitrogen enrichment from air.

Advantages:

Low capital investment,

Ease of installation,

Ease of operation,

High pressure flexibility,

Low weight and space requirement,

Low environmental impact.

Residuals gases

Gas permeation

to fuel-gas

Hydrogen

Recycle of n-C4

Unitat de isomerització

n-Butane

Isobutane

Recycle

H2 (96%)

Hydrogen recovery in a butane isomeration plant. A typical PRISM® Separator

(Airproducts)

LIQUID PERMEATION

DEFINITION-When a volatile liquid mixture is

imposed on the feed side of a nonporous membrane and

the other side of the membrane has a liquid phase ,the

process is called liquid permeation.

The molecular structure of the permeating gas or liquid

is an important factor in permeation.Liquid permeation

depends upon the molecular size of the liquid,that is to

say,molecules such as pentane permeate more rapidly

than larger molecules such as decane.

The polarity of the liquid also plays an important role in

permeation.For example,non polar liquids such as

toluene permeate more rapidly than polar aniline in

membranes such as polyethylene.