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What
Is Ion Exchange Resin and How Does It Work?
SAMCO
While many of us
have heard of ion exchange (IX) resins, few of us have a grasp on
how the technology actually works.
Whether you’re
weighing potential treatment strategies, looking for ways to get the most out
of your existing IX resins, or simply curious about IX chemistry, you may be
asking “What is ion exchange resin and how does it work?”
No matter your
goals, this article will help you to make better decisions on the right water
treatment strategies for your facility by helping you to better understand IX
resin technology, and how it serves a variety of water treatment and separation
needs.
What are ion exchange resins?
Ion exchange is a
reversible chemical reaction where dissolved ions are removed from solution and
replaced with other ions of the same or similar electrical charge.
Not a chemical
reactant in and of itself, IX resin is instead a physical medium that
facilitates ion exchange reactions.
Throughout the
polymer matrix are ion exchange sites, where so-called “functional groups” of
either positively-charged ions (cations) or negatively-charged ions (anions)
are affixed to the polymer network.
These functional
groups readily attract ions of an opposing charge.
What are the physical properties of IX resins?
The geometric
shape, size, and structure of IX resins can vary from one type to the next.
Most IX exchange
systems employ a resin bed consisting of tiny, porous microbeads, though some
systems, such as those used for electrodialysis, use a sheet-like mesh resin.
IX resin beads are
usually small and spherical, with a radius measuring just 0.25 to 1.25
millimeters in size.
Depending upon the
application and system design, the resin beads may have a uniform particle size
or a Gaussian size distribution.
Most applications
use gel resin beads, which have a translucent
appearance, and offer high capacity and chemical efficiency.
Macroporous resins,
which are recognizable due to their opaque white or yellow appearance, are
typically reserved for demanding conditions, since they have comparatively
greater stability and chemical resistance.
What are IX resins made of?
The IX resin matrix
is formed by cross-linking hydrocarbon chains with one another in a process
called polymerization.
The cross-linking
gives the resin polymer a stronger, more resilient structure and a greater
capacity (by volume).
While the
chemical composition of most IX resins is polystyrene, certain types
are manufactured from acrylic (either acrylonitrile or methyl acrylate).
The resin polymer
then undergoes one or more chemical treatments to bind functional groups to the
ion exchange sites located throughout the matrix.
These functional
groups are what give the IX resin its separation capabilities, and will vary
significantly from one type of resin to the next.
The most common
compositions include:
· Strong acid cation (SAC) exchange resins. SAC resins
are composed of a polystyrene matrix with a sulphonate (SO3–)
functional group that is either charged with sodium ions (Na2+)
for softening applications, or hydrogen ions
(H+) for demineralization
· Weak acid cation (WAC) exchange resins. WAC resins
are composed of an acrylic polymer that has been hydrolyzed with either
sulphuric acid or caustic soda to produce carboxylic acid functional groups.
Due to their high affinity for hydrogen ions (H+), WAC resins are
typically used to selectively remove cations associated with alkalinity.
· Strong base anion (SBA) exchange resins. SBA resins
are typically composed of a polystyrene matrix that has undergone
chloromethylation and amination to fix anions to exchange sites. Type 1 SBA
resins are produced by the application of trimethylamine, which yields chloride
ions (Cl–), while Type 2 SBA resins are produced by the application
of dimethylethanolamine, which yields hydroxide ions (OH–).
· Weak base anion (WBA) exchange resins. WBA resins
are typically composed of a polystyrene matrix that has undergone
chloromethylation, followed by amination with dimethylamine. WBA resins are
unique in that they do not have exchangeable ions, and are therefore used as
acid absorbers to remove anions associated with strong mineral acids.
· Chelating resins. Chelating resins are the most
common type of specialty resin, and are used for selective
removal of certain metals and other substances. In most cases the resin matrix
is composed of polystyrene, though a variety of substances are used for
functional groups, including thiol, triethylammonium, and aminophosphonic, among
many others.
How does ion exchange resin work?
To fully understand
how IX resins work, it is important to first understand the principles of the
ion exchange reaction.
Put simply, ion
exchange is a reversible interchange of charged particles — or ions — with
those of like charge.
This occurs when
ions present on an insoluble IX resin matrix effectively swap places with ions
of a similar charge that are present in a surrounding solution.
The IX resin
functions this way because of its functional groups, which are essentially
fixed ions that are permanently bound within the polymer matrix of the resin.
These charged ions
will readily bond with ions of an opposing charge, which are delivered through
the application of a counterion solution.
These counterions
will continue to bond with the functional groups until equilibrium is reached.
During an IX cycle,
the solution to be treated would be added to the IX resin bed and allowed to
flow through the beads.
As the solution
moves through the IX resin, the functional groups of the resin attract any
counterions present in the solution.
If the functional
groups have a greater affinity for the new counterions than those already
present, then the ions in solution will dislodge the existing ions and take
their place, bonding with the functional groups through shared electrostatic
attraction.
In general, the
greater the size and/or valency of an ion, the greater affinity it will have
with ions of an opposite charge.
Let’s apply these
concepts to a typical IX water softening system.
In this example,
the softening mechanism consists of a cation exchange resin where sulphonate
anion (SO3–) functional groups are fixed to the IX resin
matrix.
A counterion
solution containing sodium cations (Na+) is then applied to the
resin.
The Na+ are
held to the fixed SO3– anions by electrostatic
attraction, resulting in a net neutral charge in the resin.
During an active IX
cycle, a stream containing hardness ions (Ca2+ or Mg2+)
is added to the cation exchange resin.
Since the SO3– functional
groups have a greater affinity for the hardness cations than for the Na+ ions,
the hardness ions displace the Na+ ions,
which then flow out of the IX unit as part of the treated stream.
The hardness ions
(Ca2+ or Mg2+), on the other hand, are retained by
the IX resin.
What is resin regeneration?
Over time,
contaminant ions bind with all available exchange sites in the IX resin.
Once the resin is
exhausted, it must be restored for further use through what is known as a
regeneration cycle.
During a
regeneration cycle, the IX reaction is essentially reversed through the
application of a concentrated regenerant solution.
Depending upon the
type of resin and the application at hand, the regenerant may be a salt, acid,
or caustic solution.
As the regeneration
cycle proceeds, the IX resin releases contaminant ions, swapping them for ions
present in the regenerant solution.
The contaminant
ions will exit the IX system as part of the regenerant effluent stream, and
will need to be properly discharged.
In most cases, the
resin is rinsed to remove any residual regenerant prior to the next active IX
cycle.
How SAMCO can help
SAMCO has over 40
years’ experience in identifying appropriate IX resin technologies to help
lower costs and waste volumes while increasing product quality.
For more
information or to get in touch, contact us here to set up a consultation
with an engineer or request a quote.
We can walk you
through the steps for developing the proper solution and realistic cost for
your IX treatment system needs.
To learn more about
SAMCO’s innovative IX resin solutions, visit our page on ion exchange
resin technologies here.
SAMCO promises complete
partner and customer satisfaction through our comprehensive, project-based
approach that delivers customized industrial solutions.
Since
its 1998 founding, SAMCO Technologies has provided custom water, wastewater,
process separation, and filtration solutions to a diverse range of industries.
Our individualized, project-based approach continues to define our niche in
modern industry, motivating us to deliver comprehensive, efficient solutions to
meet our customers’ unique needs.
At
SAMCO, we anticipate the needs of industry, and respond with forward-thinking
solutions. Our focus on industrial applications began in 1987 with the founding
of Northeast equipment supplier and systems servicer CS Kimeric. Acquired from
a Western New York soft water provider with over 30 years in the business, CS
Kimeric was established to provide specialized service for industrial
applications. Over the course of the next decade, it became clear that
industrial clients would benefit from working with a partner capable of
delivering comprehensive, concept-to-completion solutions. In 1998, founder and
CEO Richard Posa established SAMCO as an integrated provider of design,
fabrication, startup, and maintenance services.
Today,
SAMCO serves the process water needs of clients across the nation and globe
from its headquarters in Buffalo, New York. Leveraging the collective skills of
experienced chemical, civil, environmental, electrical, mechanical and process
engineers, chemists, and skilled tradesmen, SAMCO blends a culture of teamwork,
commitment and passion to help solve your unique industrial water treatment
needs.
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