Separation processesDr. Hassan Sawalha

Chemical Engineering DepartmentAn-Najah National University

Pervaporation

Two phase system

Liquid feed and vapor permeate.

Asymmetric composite membrane

Selective for species A

Species B usually has some finite

permeability.

The dense film is in contact with

the liquid side

• Vacuum• vaporization

Two words: permselective and evaporation

Water selective: Hydrophilic membrane

Organic selective: hydrophobic membrane

Applications

(1) dehydration of ethanol

(2) dehydration of other organic alcohols, ketones, and esters

(3) removal of organics from water.

(4) separation of organic mixtures, e.g., benzene-cyclohexane, is

receiving much attention.

Pervaporation is best applied when the feed solution is dilute in the main permeant

Hybrid process distillation-pervaporation for removal of water from ethanol.

The distillate purity is limited because of the 95.6 wt% ethanol in water azeotrope.

95.6 % ethanol

60 % ethanol40% water

Pervaporation unit

99.5 % ethanol (retentate)

25 % ethanol (permeate)

Transport equations in pervaporation Because phase change and nonideal-solution effects in the liquid

feed

Simple equations i.e. for dialys is do not apply to pervaporation.

A particularly convenient PV model is that of Wijmans and Baker

They express the driving force for permeation in terms of a

partial-vapor-pressure difference.

Because pressures on the both sides of the membrane are low, the

gas phase follows the ideal-gas law.

Therefore, at the upstream membrane surface (I), permeant activity for component i is expressed as:

Flux

Gas Permeation

the feed gas at high pressure PI,

contains some low-molecular-weight

species (MW < 50)

higher-molecular-weight species.

Usually a sweep gas is not used,

permeate side of the membrane is

maintained at a much lower pressure, P2,

often near-ambient pressure.

The membrane is often dense

permselective for certain of the low-

molecular-weight species.

Applications

(1) separation of hydrogen from methane;

(2) adjustment of H2-to-CO ratio in synthesis gas

(3) O2 enrichment of air

(4) N2 enrichment of air;

(5) removal of C02;

(6) drying of natural gas and air

Transport equation

In dense membranes species absorbed at the surface

then transported through the membrane by one or more

mechanisms.

Permselectivity depends on both membrane absorption

and the membrane transport rate.

Usually all mechanisms are formulated in terms of a

partial-pressure

Flux

solution

Ultrafiltration

Ultrafiltration and microfiltration are more commonly used for recovering the solutes

Rejection

Concentration factor

Process Configurations

An ultrafiltration process is commonly conducted

in one of four configurations or combinations:

(1) batch ultrafiltration,

(2) continuous bleed-and-feed ultrafiltration,

(3) batch diafiltration

(4) continuous bleed-and-feeddiafiltration.

Batch Ultrafiltration

Separation with ultrafiltration

HW

Continuous Feed-and-Bleed Ultrafiltration A large fraction of the retentate is recycled at steady state

Bleed is that portion of the retentate that is not recycled, but is

withdrawn as product retentate

At startup the entire retentate is recycled

Until the desired retentate concentration is achieved,

At which time bleed is initiated

The advantages and disadvantages of feed-and-bleed operation

The single-pass mode is usually unsuitable for ultrafiltration

because the main product is the concentrate rather than the

permeate (as in reverse osmosis)

High yields of permeate are required in order to adequately

concentrate solutes in the retentate

a single-pass ultrafiltration requires a very long membrane

path or a very large membrane area

The advantages and disadvantages of feed-and-bleed operation

with the high recycle ratio,

the concentration of solutes

on the retentate side is high

resulting in the lowest flux,

Larger membrane area

Solution: Multistage continuous feed-and-bleed ultrafiltration

where the retentate (bleed) from each stage is sent to the next stage,

while the permeates from the stages are collected into a final composite permeate

the final and highest concentration is only present in the final stage.

Diafiltration Involves the addition of solvent (usually water) to the retentate,

followed by filtration.

Additional solvent dilutes the retentate so as to increase the flux.

Thus ultrafiltration is employed to a certain limiting concentration of

solutes,

followed by diafiltration to further enhance solute separation.

The final retentate may not be very concentrated in retained solutes,

but it contains a smaller fraction of permeable solutes

MICROFILTRATION

microfiltration is a pressure-driven, microporous membrane process

used to retain matter commonly of 0.1-10 microns.

the matter may include large colloids, small and solid particles, blood cells, yeast,

bacteria and other microbial cells, and very large and soluble macromolecules

Membrane structures for microfiltration

screen filters that collect retained matter on the surface

depth filters that trap particles at constrictions within the membrane

depth filters include:

1. relatively thick, high-porosity (80-85%) castcellulose-ester

membranes having an open, tortuous, sponge-like structure;

2. thin, low-porosity (nominal 10%) polyester or polycarbonate

track-etch membranes of a sieve-like structure with narrow

distribution of straight through,cylindrical pores.

The latter have a much sharper cutoff, resulting in enhanced

separation factors

Common modes of microfiltration.

Transport equations

Equations for computing TFF microfiltration are those developed for

ultrafiltration. This includes batch, continuous feed-and-bleed, and

diafiltration operation modes.

Equations for DEF microfiltration are those for conventional, batch,

solid-liquid, slurry filtration, frequently referred to as cake filtration.

Transport equations DEF microfiltration

improvements in yield by a combined operation in which:

(1) Constant-flux operation is employed in Stage 1 up to a limiting pressure

drop, followed by

(2) Constant-pressure operation in Stage 2 until a minimum flux is reached

Constant-Flux Operation

Constant-Pressure Operation