separation processes dr. hassan sawalha chemical engineering department an-najah national university
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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