Chemical Engineering September 2023 - 37

Engineering Practice
Advances in Chlor-Alkali Technologies
The products of the chlor-alkali process continue to be essential for many purposes
across the global value chain. Modern chlor-alkali technologies can vastly improve
upon the safety and environmental performance of legacy systems
Andy Liu
Chemours
T
he chlor-alkali
production
process is one of the most
common of all industrial
chemical operations. The
output of a chlor-alkali system includes
chlorine (Cl2), sodium hydroxide
(NaOH) and hydrogen gas
(H2) - all of which have broad application
and growing economic
and societal importance. For example,
the free chlorine produced
by chlor-alkali systems is the most
affordable and widely used drinking-water
disinfectant in the world.
It is also highly effective against
nearly all waterborne pathogens.
Thus, chlorine serves an essential
role as global demand for potable
water continues to rise, alongside
climate challenges related to rising
temperatures and extreme
weather events, which can lead
to excess algae growth and increased
sediment in natural water
sources (Figure 1). However, despite
its ubiquity, all chlorine is not
created equally.
Available technologies
Today, most free Cl2 is produced
electrochemically by water electrolysis
in equipment known as
chlor-alkali cells. In standard
chlor-alkali cell processes, a
clean saltwater solution (brine)
and water are converted into Cl2,
H2 and NaOH. Of the three common
cell configurations, one type
of
legacy system uses mercury
cells, while the second depends
primarily on asbestos diaphragm
cells. The third, more modern
type uses polymer electrolyte
membranes (PEMs).
The Minamata Convention, an
international treaty signed by 128
nations in 2013, set a phaseout
date of 2025 for using mercury in
the chlor-alkali manufacturing process.
After that date, chlor-alkali
Shutterstock
FIGURE 1. As environmental stresses are challenging the availability of potable drinking water, chlorine is
increasingly essential as an affordable and easily applicable treatment solution
electrolysis cells will be effectively
limited to using diaphragm cells
and PEMs.
In addition to the elmination of
mercury and asbestos in the process,
the advanced membrane
chlor-alkali process using PEMs
can enable substantial energy savings
compared to the two older
processes. In fact, the U.S. Department
of Energy's Office of Energy
Efficiency and Renewable Energy
(EERE; www.energy.gov/eere) has
stated that, of the three available
chlorine-producing technologies,
the mercury cell is the most energy-intensive,
consuming about
3,700 Kwh of electricity per metric
ton (kWh/m.t.) of Cl2 produced,
while the diaphragm cell consumes
about 2,900 kWh/m.t. PEM cells
are the most energy-efficient, at
2,500 kWh/m.t. [1].
How membrane cells work
A chlor-alkali membrane cell is
comprised of two half-cells, one
containing an anode (positively
charged) and the other a cathode
(negatively charged), separated by
a PEM (Figure 2).
A direct current is applied to the
cell while a concentrated salt (NaCl)
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SEPTEMBER 2023
solution, or concentrated brine, is
fed into the anode side of the cell,
and water is introduced into the
cathode side. The PEM that separates
the anode from the cathode
is specially formulated so that it
will conduct only positive sodium
(Na+) ions through it, while preventing
passage of negative hydroxyl
(OH-) ions into the cathode side of
the cell.
The negatively charged chloride
ions are attracted to the positively
charged anode, where they give up
an electron to the anode and coalesce
to form chlorine, which exits
the anode half-cell as a gas:
2Cl- → Cl2 + 2e-
(1)
At the same time, the negatively
charged cathode attracts the Na+
ions from the brine (anode) side of
the cell, through the membrane and
into the cathode half-cell. On the
cathode side, the water is electrolyzed
to produce negatively charged
OH- ions:
2H2O + 2e- → 2OH- + H2
(2)
The Na+ ions that have been
transported from the anode side of
37

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Chemical Engineering September 2023

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