Dealer Dynamics
Ion Exchange Softener Design–
A Gentle Introduction
By Greg Reyneke, CWS-VI
W
hen selecting a softening resin
for your particular application, consult carefully with the
vendor to ensure the resin will actually
meet your needs, not just the one that
makes the most commission. Some dealers
purchase pre-built systems and others assemble their own. This article shares a few
important concepts about resin selection
that will hopefully assist both camps. It
is important to stress that my goal here is
not to turn you into a systems engineer,
just to look into our world to help make
things a little clearer.
When I design systems, I select resins with rapid kinetics and a structured
exchange matrix that allows for effective
cleaning in a variety of adverse conditions,
with uniform performance across a wide
variety of operating temperatures. Before
even selecting a resin, I always consider
the following:
Service flowrate:
• Higher flowrates require deeper
columns of resin to ensure adequate
exchange.
Ion
Symbol
Selectivity
factor
Radium
Ra+2
13.0
Barium
Ba+2
5.8
Lead
Pb+2
5.0
Stontium
Sr+2
4.8
Calcium
Ca+2
3.1
Copper
Cu+2
2.6
Cadmium
Ca+2
1.9
Zinc
Zn+2
1.8
Iron
Fe+2
1.7
Magnesium
Mg+2
1.67
K+
1.67
Potassium
Manganese
Mn+2
1.6
Ammonium
NH4+
1.3
Sodium
Na+
1.0
H+
0.67
Hydrogen
Water Conditioning & Purification
• Overall columnar area should be
designed to minimize pressure loss.
• Don’t oversize the bed; keep the
service flow at least > 2gpm/ft2 to
prevent channeling
Water temperature:
• Cold water increases pressure drop
through an ion exchange column.
• Cold water significantly reduces ion
exchange kinetics, requiring a deeper
column to ensure adequate ion exchange.
• Warmer water requires a faster backwash to provide adequate bed expansion.
Water pressure:
• Certain minimum water pressures are
required to ensure adequate eduction
of regenerant.
Available water flow:
• The available water flow is not merely
to satisfy the desired service flowrate,
but also cleaning/regeneration cycles
(especially on multiplex systems).
It is quite common to overlook the
foregoing, since many designers are either
uneducated, lazy, in a hurry or simply
underestimate the importance of the fundamentals. Once the fundamentals have
been determined, then the astute designer
will further ascertain the following:
Amount of hardness being addressed:
• Higher hardness levels require deeper
columns of resin to ensure adequate
exchange.
• Higher hardness levels usually
include higher levels of complicating
and interfering ions.
• Higher hardness levels can cause scaling in regeneration assemblies and
require cleaning agents that might
damage resin.
Amount of interfering and
complicating factors in the water:
• Softening resins have varying affinity
for cations in water; generally, the
higher the molecular weight of the
ion, the higher the selectivity. Inter-
fering ions will not only replace the
exchange ion on the resin (sodium/
potassium), but also any other ions
that are less attractive to the resin.
• Organics and bacteria in water can
foul ion exchange sites, significantly
reducing ion exchange capacity, as
well as potentially creating a home for
other bacteria that might have negative health effects.
• Interfering factors, such as iron and
other metals, will not only interfere
with softening ability, but also potentially foul the resin.
Water conductivity/TDS:
• Water conductivity has an effect
on ion exchange success: the more
conductive the water, the more interfering ions there are to attach onto
exchange sites and potentially induce
hardness leakage.
Amount of acceptable hardness
leakage:
• How much hardness is acceptable
Glossary
Mesh
This sometimes ambiguous term refers to
the size of a particle. In the ion exchange
industry, we use the US Standard
mesh, where 16 mesh is approximately
1,200 microns in diameter, 50 mesh is
approximately 300 microns in diameter,
and 100 mesh is 150 microns in diameter.
Regenerant
Chemical compound used to replenish ion
exchange sites throughout the resin media.
Sodium chloride or potassium chloride
salts are commonly used to regenerate
water softeners.
Regeneration
Process of inducing a liquid solution
of regenerant to the resin and forcing
exchanged contaminants off of exchange
sites through mass action. Exchange
sites are then occupied by ions from the
regenerant.
April 2013
in the finished water? Is this system
serving a home, a laundromat or
an expensive boiler system? Hardness leakage levels acceptable for a
household system are completely
unacceptable for a commercial/industrial system. The amount of acceptable hardness leakage will determine
which resin is used, how much of it is
used, how it will be regenerated and
even how much regenerant salt is to
be used for the regeneration.
Armed with more knowledge, we
can now begin the process of selecting
an appropriate ion-exchange resin. As
previously discussed, there are many
varieties of softening resin available on
the open market, as well as a number of
highly advanced proprietary softening
media. Key to choosing a softening resin
is understanding some of the terms used
to describe it:
Capacity
The amount of contaminants the
softening media can exchange under a
set of standardized parameters (these can
vary by manufacturer) in equivalent units
(usually calcium carbonate). Capacities
are also expressed at different salting
levels, allowing design flexibility when
balancing salt efficiency with permissible
hardness leakage.
Cross-linkage
Linear polymer chains bound within
Water Conditioning & Purification
the matrix of the resin bead give it structure and insolubility. Cross-linking materials can vary and have good or bad effects
on capacity. Generally speaking, higher
capacity resins tend to have lower capacities and kinetics, but there are definitely
exceptions to this rule. Higher cross-linkage resins last longer in chlorinated water
and increase in strength as the percentage
of cross-linkage increases.
Size
Resin bead sizes vary. The size and
uniformity of the bed will have a dramatic effect on net throughput, capacity
and service hardness leakage. Regular
resins are shipped within a Gaussian size
distribution of 16 to 50 mesh. Smaller diameter, fine-mesh resin beads yield more
capacity per volume, but at the expense of
increased pressure drop through the bed
and usually more sensitivity to oxidative
damage.
Structure
The structure of an ion exchange
resin describes how the resin is built. Gel
resins are standard hard polymer beads.
Macroporous resins are highly crosslinked with carefully engineered voids
to allow fast interaction between water
and exchange sites. Certain specialtystructured matrix media leverage the best
of both worlds to allow maximum ion
exchange capacity, strength and resistance
to osmotic shock during operation.
Uniformity
Size is a benefit, since it provides for
improved flow through the bed as well as
uniform kinetics. Uniform beds regenerate faster with less salt and water.
Whole-bead count
This refers to the percentage of resin
beads that are whole or completely spherical in a sample. Expressed as a percentage,
it is a good general indication of quality
control during manufacture. Broken beads
contribute to pressure drop through a
bed and inevitably lead to channeling.
Whole-bead counts less than 90 percent
are entirely unacceptable in new resins.
Now you have enough information
to begin making informed decisions when
consulting with your equipment manufacturer to ensure that you are always
providing your customers with the best
water at the very best price.
About the author
S Greg Reyneke is the
Managing Partner at Red
Fox Advisors, a multidisciplinary consultancy focused on solving complex
environmental problems.
April 2013