Chemical Engineering September 2020 - 30

Frames Group
FIGURE 5. A molecular sieve unit may be installed
downstream of an electrolyzer for dehydration
purposes
storage more convenient. Hoeller's
proprietary PEM technology was demonstrated
in a proof-of-concept at the
Fraunhofer Institute for Solar Energy
Systems (ISE; Freiburg im Breisgau,
Germany; www.ise.fraunhofer.de),
and Kramer says that the company
expects to install a prototype unit by
the end of 2020. Discussions are also
in place regarding a pilot project for
the new PEM stacks at a wind farm in
Schleswig-Holstein, Germany.
AEM electrolysis
One emerging technology for H2 production
is anion-exchange membrane
(AEM) electrolysis (Figure 4). AEM is
somewhat of a hybrid solution combining
the benefits of PEM and traditional
diaphragm-based alkaline electrolysis,
explains Oliver Conradi, who
specializes in membrane research at
Evonik Industries AG (Essen, Germany;
www.evonik.com). " Alkaline
electrolysis obviously involves very
basic conditions, while PEM involves
an acidic environment. These respective
pH values require certain materials.
In alkaline conditions, you can use
cheaper materials, such as stainless
steel and nickel, whereas with PEM,
you must use platinum or other precious
metals for the catalyst, and the
electrochemical cell must be based
on titanium, so the investment cost for
PEM is much higher, " explains Conradi.
However, PEM systems overcome
some of the fundamental limitations
of traditional alkaline electrolysis
- due to the specific cell design in
alkaline systems, current density and
efficiency are limited, and it is more difficult
to pressurize an alkaline system,
meaning that an additional compression
step is typically required. " In PEM
units, the dense membrane makes
it easier to pressurize the whole sys30
tem.
With AEM, you can fundamentally
combine the advantages of both
state-of-the-art technologies, while
you compensate for their drawbacks, "
says Conradi, noting that the primary
hurdle in developing an effective AEM
system is developing a suitable polymeric
membrane material that can
withstand alkaline conditions.
A particular area of focus is on the
cationic moiety, which is responsible
for transporting the hydroxide ions
from the cathode to the anode. In
addition to stability in an alkaline environment,
the polymer must also provide
high ion conductivity and stability
under pressurized electrolyzer conditions.
Inspired by an existing membrane
technology for gas separation,
Evonik has developed a new polymer
chemistry featuring a proprietary ionconducting
cationic moiety. As part
of the AEM-focused Channel consortium,
Evonik is expanding production
of the polymer and also scaling up
membrane fabrication on a pilot coating
line. " The consortium is building
an AEM electrolyzer to demonstrate
that the membrane and other components
work under challenging conditions, "
explains Conradi. The group's
first AEM demonstrator unit is at the
laboratory scale, where test protocols
are being run to reflect real-world
conditions. " The next milestones will
be proving system reliability and scaling
up the stack sizes, while also
scaling up membrane processing, "
he continues.
Downstream H2 processing
Although electrolyzers have made
strides in efficiency and cost, the produced
H2 still often requires post-processing
steps, such as compression,
dehydration or purification. " Electrolysis
stacks usually do not produce hydrogen
that is directly suitable for use.
If you want to store, distribute or utilize
the produced hydrogen, contaminants
need to be removed, " says Jordi
Zonneveld, manager of the hydrogen
portfolio at Frames Group (Alphen
aan den Rijn, the Netherlands; www.
frames-group.com). " Since PEM technology
uses only ultra-pure water, the
only contaminant is water, and potentially
a very small amount of oxygen.
Alkaline electrolysis uses a KOH solution
as the process fluid, and therefore
traces of KOH in the produced hydrogen
need to be removed, as well. "
Depending on the gas flow and
purity requirements, there are several
steps that may be required to prepare
H2 for its end-use applications. For
instance, says Zonneveld, knock-out
drums with demisting internals and
optional gas-cooling equipment are
generally used as a first step to bring
hydrogen purity up to 99.9%. Then, if
higher purity is required, a molecularsieve
unit (Figure 5) may be required.
He also mentions that dehydration
using triethylene glycol - a common
technology for natural-gas processing
- has shown potential for H2 purification,
but there have not yet been
any large-scale H2 applications.
Compression of H2 also introduces
unique challenges. " H2 has a very
high energy density per mass, but a
very low density, so compressors are
needed downstream of electrolyzers
to compress the H2 for efficient storage
and transportation, " says Stefanie
Peters, managing partner at Neuman
& Esser Group (NEA; Übach-Palenberg,
Germany; www.neuman-esser.
de). The low molecular weight of H2
also poses issues. " Turbomachinery
faces significant problems in capturing
the H2 in the compression chamber,
and only positive-displacement
machinery like piston and diaphragm
compressors are suitable for efficient
compression to required H2 discharge
pressures, " add Peters. For instance,
dry-running piston compressors can
achieve discharge pressures up to
300 bar. When equipped with lubricated
cylinders, the discharge pressures
are potentially as high as 700
bar, but this option introduces trace
amounts of oil contamination, so in
cases where no contamination is
acceptable, oil-free diaphragm compressors
are the preferred high-pressure
option, as they can achieve more
than 5,000 bar discharge pressure.
As the demand for electrolyzers
and green H2 continues to grow,
technological
improvements,
not
just in the electrolyzers themselves,
but also in post-processing, will continue
to be vital areas of research and
development work.
■
Mary Page Bailey
ChemiCal engineering www.Chemengonline.Com September 2020
http://www.ise.fraunhofer.de http://www.neuman-esser http://www.evonik.com http://www.frames-group.com http://www.Chemengonline.Com
Chemical Engineering September 2020

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