Chemical Engineering February 2012 - 18

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Air Liquide
Air Liquide
FIGURE 3. The air-separation modules shown here are for a
nitrogen-production unit
to membranes in CO2 removal applications,
Schott warns that there is
a downside in that membranes are
separators and not purifiers. " For
every bit of CO2 you remove, you also
remove a small amount of methane so
you end up with a permeate stream,
which may require multiple stages to
increase methane recovery. "
And depending upon the allowable
specifications for percentage of CO2
in the feed, membranes can't always
reach the lower levels achieved by
solvent technologies. So the necessary
product specification for the gas
must be taken into account when
considering membrane technologies
in these applications.
However, as membrane technology
providers continue to improve their
products, this may not be an issue for
much longer. " We continue to improve
the membrane productivity of our Separex
line of membranes for CO2 removal
so that the capacity of gas separation
you're able to get out of a single
membrane element increases while at
the same time we are working toward
reducing the cost of putting that membrane
inside a system, " says Schott.
In addition, Air Liquide has developed
a hollow-fiber membrane product
that purifies biogas, landfill and digester
gas, where it removes CO2 from
methane associated with off-gasses
from these applications. " This allows
us to upgrade biogas into pipeline
quality or natural gas used for CNG or
LNG, " says Marchese. " This is a very
novel application for membranes. "
FIGURE 4. A complete membrane system for H2 is shown here
Hydrogen purification
In the oil and gas industry, the biggest
challenge right now is being able to
meet tougher fuel specifications, which
requires the recovery of hydrogen from
off-gas streams. Currently membrane
systems and pressure swing adsorption
(PSA) units are the existing options.
PSAs will provide higher purity,
but lower pressure products while
membranes give higher pressures, but
lower purity levels, so the technology
of choice depends upon the source of
the stream and whether pressure or
purity is the main concern.
However, improvements to hydrogen
purification membranes may also help
boost the use of membranes in these
applications. " With regard to hydrogen
purification from hydrocarbons,
Air Liquide introduced a polymer that
allows for greater contaminant resistance,
so that when we put this polymer
in a hydrogen refinery or chemical
processing facility, we expect longer operating
life from that particular membrane
module, " says Marchese.
And with a growing demand for
new hydrogen purification technologies
in the refining sector, membrane
technology proves to be a good, incremental
way to extract hydrogen from
existing hydrocarbon streams as opposed
to building a new hydrogen
plant or stream methane reformer, he
continues. Air Liquide's hollow-fiber
hydrogen purification membrane can
be employed to recover the hydrogen
in that off-gas so it is not flared or
wasted (Figure 4).
18 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012
In both hydrogen purification and
CO2 removal, membrane providers
are working on improvements in materials
that would provide even more
benefits. UOP, for example, is focusing
on new materials, new formats and
hybrid or mixed matrix membranes.
" This means putting molecular sieves
or other inorganic materials inside a
polymer membrane to improve the
performance of that polymer membrane, "
says Schott. " You can make inorganic
membranes, but they tend to
be very brittle and expensive, so this
hybrid approach of polymer plus inorganic
materials could be a bridge to
better performance at lower cost than
a straight inorganic membrane. "
Air Liquide, too, is making improvements
in the way it manufactures
membrane modules and is exploring
different materials to create the
" next generation membrane. " These
improvements, says Marchese, are
geared toward improving the separation
efficiency and selectivity to make
processes even more efficient.
The future of membranes
In addition to improving upon current
membrane technology for existing
applications, some companies are
working on research and development
aimed at creating entirely new
membranes for applications in the
foreseeable future.
Two of the areas driving this developmental
trend are large-scale alternative-energy
projects and the need
for hydrogen in making cleaner fuels
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Chemical Engineering February 2012

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