Ion Exchange Resins and Processes for Industrial Water Treatment
Let's take a closer look at the technology and operation of ion exchange resins and processes used today in industrial water treatment systems.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.
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.