Pervaporation

Lecture 14.
Membrane Separation Processes (2)
• Gas Permeation
- Operation
- Major applications
• Pervaporation
- Major applications
- Pervaporation processes
- Materials and modules
- Models for permeant transport
Gas Permeation (1)
• Low-molecular-weight species (MW < 50) are separated from small
amounts of higher-molecular-weight species in feed gas
• The membrane, often dense but sometimes microporous, is
permselective for the low-molecular-weight species
• Permselectivity depends on both membrane absorption and transport
rate
• Solution-diffusion model (partial-pressure or
fugacity driving force)
H i Di PM i
Ni = ( piF - piP ) = ( piF - piP )
lM lM
• The products are a permeate enriched in A and
a retentate enriched in B
• A near-perfect separation is generally not
achievable
• If the membrane is microporous, pore size is
extremely important because it is necessary to
block the passage of species B
Gas Permeation (2)
• Since the early 1980s, applications of gas permeation with dense
polymeric membranes have increased dramatically
• Since 1986, the most rapidly developing application for gas permeation
has been air separation, for which available membranes have separation
factors for O2 with respect to N2 of 3 to 7
• Gas permeation is also favorable for H2 recovery due to high separation
factors. The rate of permeation of H2 is more than 30 times that for N2
• Major applications
(1) Separation of H2 from CH4
(2) Adjustment of H2-to-CO ratio in synthesis gas
(3) O2 enrichment of air
(4) N2 enrichment of air
(5) Removal of CO2
(6) Drying of natural gas and air
(7) Removal of helium
(8) Removal of organic solvents from air
Gas Permeation (3)
• Advantages of gas permeation compared to other separation techniques
- low capital investment, ease of installation, ease of operation,
absence of rotating parts, high process flexibility, low weight and
space requirements, and low environmental impact
- if the feed gas is already high pressure, a gas compressor is not
needed
• Almost all large-scale applications for gas permeation use spiral-wound
or hollow-fiber modules because of their high packing density
• Feed-side pressure is typically 300 to 500 psia, but can be as high as
1,650 psia
• When the feed contains condensables, it may be necessary to preheat
the feed gas to prevent condensation as the retentate becomes richer in
the high-molecular-weight species
• For high-temperature applications where polymers cannot be used,
membranes of glass, carbon, and inorganic oxides are available, but are
limited in their selectivity
Pervaporation (1)
• Pervaporation: a membrane technical method for
the separation of mixtures of liquids by partial
vaporization through a non-porous or porous
membrane
• The phase on one side of the pervaporation is
different from that on the other
• Feed to the membrane module is a liquid
mixture at pressure P1, which is high enough
to maintain a liquid phase
• The permeate side pressure P2, which may be
a vacuum, is held at or below the dew point of
the permeate, making it vapor
• A composite membrane is used that is
selective for species A, but with some finite
permeability for species B
• The dense, thin-film side of the membrane is
in contact with the liquid side
Pervaporation (2)
• History of pervaporation
- In 1917, the term of pervaporation was first reported
- In 1961, the economical potential of pervaporation was shown
- Mid 1970s, commercial applications were possible, when adequate
membrane materials became available
• Major applications
(1) Dehydration of ethanol
(2) Dehydration of other organic alcohols,
ketones, and esters
(3) Removal of organics from water
(4) Separation of close-boiling organic mixtures
like benzene-cyclohexane
• Pervaporation is favored when the feed solution is dilute in the main
permeant because sensible heat of the feed mixture provides the permeant
enthalpy of vaporization
• If the feed is rich in the main permeant, a number of membrane stages may
be needed, with a small amount of permeant produced per stage and
reheating of the retentate between stages
Pervaporation (3)
• Pervaporation processes
Hybrid process for removal of water from ethanol Dehydration of dichloroethylene
95 wt% 0.2 wt%

99.5 wt% ethanol
ethanol water < 10 ppm water
50 wt% DCE
25 wt% ethanol
60 wt%
ethanol
Comparison of Removal of volatile organic

ethanol-water compounds (VOCs) from wastewater
separabilities
Pervaporation (4)
• For water permeation, hydophilic membrane materials are preferred
• A three-layer composite membrane (porous polyester support layer,
microporous poly-acrylonitrile or polysulfone membrane, dense PVA) is
used for the dehydration of ethanol: chemical and thermal stability with
adequate permeability
• Hydrophobic membranes, such as silicone rubber and Teflon, are
preferred when organics are the permeating species
• Commercial membrane modules for pervaporation
- Plate-and-frame type (almost exclusively): because of the ease of
using gasketing materials that are resistant to organic solvents and the
ease of providing heat exchange for vaporization and high-temperature
operation
- Hollow-fiber modules: used for removal of VOCs from wastewater;
because feeds are generally clean and operation is at low pressure,
membrane fouling and damage is minimal, resulting in a useful
membrane life of 2-4 years
Pervaporation (5)
• A pervaporation module may operate with heat transfer or adiabatically,
with the enthalpy of vaporization supplied by feed enthalpy
• Heat balance in pervaporation
(m AF C PA + m BF C PB )(TF - T0 )
= [(m AF - m AP )C PA + (m BF - m BP )C PB ](TR - T0 )
+ (m AP C PA + m BP C PB )(TP - T0 )
+ m AP DH Avap + m BP DH Bvap
The enthalpies of vaporization are evaluated at Tp (dew point
at the permeate vacuum upstream of the condenser)
(m AF C PA + m BF C PB )(TF - TR )
= (m AP C PA + m BP C PB )(TP - TR )
+ m AP DH Avap + m BP DH Bvap
Pervaporation (6)
• Models for permeant transport based on partial-vapor-pressure
Because pressures on both sides of the membrane are low, the gas
phase follows the ideal-gas law
ai(1) : permeant activity for component i at the
ai(1) = f i (1) f i ( 0 ) = pi(1) Pi s (1) upstream membrane surface (1)
Pi : vapor pressure at the feed temperature
s
Liquid on the upstream side of the membrane is generally nonideal

ai(1) = g i(1) xi(1) pi(1) = g i(1) xi(1) Pi s (1)
On the vapor side of the membrane (2)
pi( 2 ) = yi( 2 ) PP( 2 )
Driving force: (g (1)
i xi(1) Pi s (1) - yi( 2 ) PP( 2 ) )
Permeant flux
PM i
Ni =
lM
(g i xi Pi s - yi PP ) N i = PM i ( g i xi Pi s - yi PP )

Pervaporation

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Lecture 14.

Membrane Separation Processes (2)

95 wt% 0.2 wt%

Comparison of Removal of volatile organic

Liquid on the upstream side of the membrane is generally nonideal

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