Ion Exchange Resins and Processes for Industrial Water Treatment

The industrial water treatment marketplace is extremely
broad in both size and scope and may mean different things to different people.
For some, the word "industrial" may bring to mind large paper mills,
refineries, electrical utility stations, petrochemical plants or steel mills.
To others, the image of textile mills, automotive assembly or food processing
plants may come to mind. Pharmaceuticals, semiconductors and absolutely a
myriad of light manufacturing plants all fit under the umbrella of the
industrial marketplace.

Each of the businesses falling under the industrial umbrella
is unique when it comes to such things as the economic drivers of the business
or the treated water quantity and quality required for plant production and
operation. The location of the plant and the raw water source will determine
the raw water quality and its variability. Location also affects the wastewater
treatment requirements and other regulations. All of these factors must be
considered when choosing an ion exchange process for a given industrial water
treatment application.

Other factors also are important in the industrial water
treatment marketplace. Most industrial facilities have cut back the number of
employees they have. This means that many operators having the responsibility
of looking after water treatment equipment also have multiple tasks and
responsibilities in other areas of the plant. This cuts into the time they have
to maintain and operate the water treatment system. Also, managers and supervisors
often can be young and less experienced in today's business environment. These
factors have led to the demand for higher levels of automation controlling
water treatment equipment and an increased demand for services surrounding an
ion exchange system. Economic and accounting factors have increased the demand
for total outsourcing (build/own/operate contracts). These factors have
increased business opportunities for suppliers of water treatment systems and
services.

While the opportunities will vary in any given location, one
thing is certain: The successful suppliers to industrial sites will be forced
to be very knowledgeable in all aspects of industrial water treatment and water
treatment as it relates to the specific industrial sites and applications within
their operating region. They will require a well-trained, reliable staff. They
also will need to have a good general knowledge, both technically and
economically, of the industries they intend to serve. Service should be one of
their product offerings and needs to be defined in the terms of the specific
customer. Often, regional suppliers can be very competitive to large global
suppliers especially in the area of offering continued and ongoing service for
the water treatment system. This service may include inspection, repair, system
upgrades, resin cleaning, resin replacement, disposal of spent resins or
complete regeneration services. With this in mind, let's take a closer look at
the technology and operation of ion exchange resins and processes used today in
industrial water treatment systems.

Ion Exchange Resins

There are four major classes of resins used in industrial
water treatment: strong acid cation, weak acid cation, strong base anion and
weak base anion. Each of these major resin classes has several physical or
chemical variations within the class. The variations impart different operating
properties to the resin. A good ion exchange system designer not only will
design the system to meet all design specifications but also will utilize resins
that will allow the system to operate at peak efficiency and maximum cost
effectiveness.

Resin Structure

Modern synthetic ion exchange resins that are now used in
water treatment applications were developed and perfected around the time of
World War II. The majority of resins in use today have a styrene-divinylbenzene
copolymer bead structure. This structure gives the ion exchange resin bead
certain physical properties. Another important resin bead structure for water
treatment resins is the acrylic resin structure. The operating properties for
acrylic resins are different from those of an equivalent styrene-divinylbenzene
resin. One cannot say which resin structure is "better" without
knowing the site-specific operating conditions. The "better" resin will
be the one that has operating properties that match up best with the site's
operating parameters, thus maximizing operating efficiency and cost
effectiveness.

Another difference in resin structure is the difference
between gel and macroporous resins. Gel resins are the most broadly used resins
in the United States. In an analysis of ion exchange resin samples sent in for
analysis to a major laboratory1, only approximately 2 percent were samples of
macroporous resins. Gel resins generally can be characterized as having smaller
pores in the resin structure, higher initial exchange capacity and a lower
purchase price than macroporous resins of the same type. Macroporous resins
usually are considered for their ability to elute foulants easier due to the larger
pore structure and they often may stand up better in harsher operating
environments.

A recent development in the structure of resins is the
availability of the uniform particle sized (UPS) resins. Resin manufacturing processes
previously made resin beads from small to large in a Gaussian distribution
pattern (bell-shaped curve). Today, several manufacturers have perfected
manufacturing processes that allow the resin beads to form in essentially one
particle size. This produces several unique operating characteristics for a
resin and allows manufacturers to provide resins better suited to a given
application. In general, regarding demineralization and softening applications,
UPS resins will rinse down using less water, backwash

with less water use and have lower pressure drops for a
given bed depth. However, unless the system is closely monitored and
controlled, these advantages can go unnoticed and, therefore, may not be
properly utilized. Where systems designed for the home or commercial
marketplace may not have the controls or monitoring devices needed to take full
advantage of UPS resins, industrial systems should be constructed with the
controls and monitoring systems that allow users to take advantage of such resins.

Strong Acid Cation Resins

Strong acid cation (SAC) resins probably are the most common
resins in use today. They are used in softening and demineralization
applications. In softening applications, the resin is used in the sodium form
(regenerated with salt) and in demineralization applications the resin is used
in the hydrogen form (regenerated with acid). SAC resins also can be used in a
split-stream dealkalization process. SAC resins can be purchased with different
percentages of crosslinkage. The common SAC resin is 8 percent crosslinked.
However, SAC resins are available in both higher and lower levels of
crosslinkage. Often a 10 percent crosslinked resin will be used in applications
where the influent water has a higher level of chlorine or an elevated water
temperature. Chlorine as well as oxygen at elevated temperatures will attack a
resin's crosslinkage. Having the higher initial level of crosslinkage often
will provide for a longer useful resin life. SAC resins with less than 8
percent crosslinkage may be used in electric utility condensate polishing
applications where they are reported to do an excellent job of removing
corrosion products (crud) from the utility's condensate.

Weak Acid Cation Resins

Weak acid cation (WAC) resins can be used in
demineralization and dealkalization systems. They are very efficient when
matched up with the proper influent water chemistry. In the study referenced
previously, only 2.6 percent of the samples submitted for analysis were WAC
resins and of these, 63 percent of them came from locations outside of the
United States. With many water supplies in the United States being high in
hardness and alkalinity, these figures may indicate that system designers in
the United States largely ignore the use of WAC resins. Designers of industrial
water treatment systems may want to look at WAC resins more often in an effort
to improve system performance and decrease operating costs.

SAC resins remove all cations that are held tighter to the
resin than the regenerant ion being used. WAC resins remove only cations
associated with alkalinity. While WAC resins can remove monovalent ions such as
sodium associated with hydroxide alkalinity, in most water treatment
applications they are used to remove divalent ions such as calcium associated
with carbonate alkalinity. When the water has hardness to alkalinity ratios of
1:1 or higher, they work very efficiently with very high operating capacities
up to 50 kg/ft3 and will regenerate efficiently as well using regenerant at
only approximately 110 percent of stoichiometry. They are so efficient that
they often are regenerated with the spent regenerant from the SAC vessel in
demineralization applications. Compare this to SAC operating capacities of
around 20-25 kg/ft3 and regenerant usage of about 250-300 percent of
stoichiometry.

Strong Base Anion Resins

Strong base anion (SBA) resins are used in ion exchange
demineralization processes. They also are used in dealkalization,
desilicization and organic trap applications. There are two types of SBA
resins. Type I SBA resins are used where low levels of silica leakage is an
important operating criteria or in warmer climates where source water
temperatures may be quite warm for a significant part of the year. They operate
at improved efficiency when warm caustic (@120º F) is used to regenerate
the resin bed.

Type II SBA resins have an exchange site that is chemically
weaker than Type I resins. Therefore, they must be regenerated at lower
temperatures (@95º F.) and normally are not used in climates where warm
water temperatures are experienced for a good part of the year. However, Type
II SBA resins have the advantage of a higher initial exchange capacity. They
can be the resins of choice in applications that do not have heated caustic
regenerant or where a low silica level is not a critical operating
specification. Type II SBA resins also are the resins of choice in
salt-regenerated dealkalization applications.

Weak Base Anion Resins

Weak base anion (WBA) resins actually are acid absorbers as
much as they are ion exchange resins. They remove only the anions of the strong
mineral acids (sulfate, chloride and nitrate). They allow the
carbonate/bicarbonate and silica ions to pass through. Therefore, they cannot
be used to make demineralized water without a SBA resin bed following in the
train to remove the carbonate/bicarbonate and silica. The advantage of using
the WBA resin is its efficiency. It is fully regenerated using only about 120
percent of stoichiometry. Like their WAC counterparts, WBA resins can be
regenerated using the spent caustic from the SBA resin bed making their use
very efficient especially when used on water having a high percentage of anion
loading from sulfate, chloride or nitrate.

WBA resins can vary in operating characteristics. Many have
some initial strong base functionality. There also are several hybrid resins
that have a combination of strong and weak base functionality by design. Many
WBA resins also can be used for removing organic substances from the water
before they have a chance to reach and foul the SBA resin. Organics elute off
the WBA resins better during the regeneration process. When selecting a WBA
resin for a specific application, it often is best to consult with the resin
manufacturers for selection advice.

As you can tell, there is a wide variety of different ion
exchange resins available for industrial ion exchange applications. The primary
input for design decisions will be based on raw water analyses and
specifications for finished water quantity and quality. As systems become
larger, considerations for operating cost efficiency, waste minimization,
instrumentation and automation, operating life expectancy of the ion exchange
system and even the corporate culture of the end user, become more important factors
in the design. Most resin manufacturers offer computerized design programs that
help design systems using their resins. The programs help select system designs
that are both technically effective and cost efficient. However, computer
programs are not always perfect. Somebody with operating knowledge and
experience always should review the results obtained from computerized design
programs.

With the wide number of resin types available in the
marketplace, it is probable that there is more than one technically effective
solution that will meet all the system's design specifications. This is where
experience and knowledge is required to help select the system design that will
do the job expected by the customer under the conditions existing at the customer
site. This experience will include a thorough knowledge of all available resin
types along with their various advantages and disadvantages so they can be
applied in ion exchange systems that are both technically sound and cost
effective. Sound equipment design, effective use of available resins and
readily available services that meet all the expectations of the industrial
customer will lead to a win/win long-term customer relationship that benefits
both the supplier and the customer.              

Wayne Bernahl is president of W. Bernahl Enterprises, Ltd. He has worked in the industrial water treatment marketplace for 37 years. Most of this time was in technical marketing and consulting positions dealing with ion exchange and reverse osmosis applications.

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