B. Buecker, Buecker & Associates, Lawrence, Kansas (U.S.)
The refinery, petrochemical, chemical and other manufacturing industries have seen and continue to see many retirements of senior technical employees, some of whom have been with their plants since startup. New personnel are often confronted with challenges that require knowledgeable and quick action, which they may have difficulty providing without thorough training. A very important—and sometimes overlooked—unit operation within these facilities is makeup water treatment for steam generators and other processes. Many cases have been documented in which makeup system upsets have led to boiler tube failures that partially or completely shut down unit processes. Note: Cooling water systems are also often neglected to the detriment of plant operations. The three parts of this article (August, September and October 2024) serve as a guide for those employees who may be on a steep learning curve when it comes to makeup system understanding.
Ion exchange. Membrane technology was unavailable in the early 20th century when boilers and other processes requiring high-purity water evolved—ion exchange has provided the answer for many applications.
The roots of industrial ion exchange can be traced back to the early 1900s when the German scientist Gans experimented with synthetic zeolites (sodium aluminum silicates) to exchange calcium and magnesium ions (hardness) for sodium.4 Sodium salts are very soluble: by exchanging dissolved hardness for sodium, the scaling potential of water can be greatly reduced.
In the 1930s and 1940s, scientists developed synthetic, polymer-based ion exchange (IX) resins, and these revolutionized the industry (FIG. 26).
The most common backbone material for IX beads is polystyrene-divinylbenzene. During the bead manufacturing process, active sites are placed on the beads: each bead may have up to 4 × 1019 sites capable of ion exchange. The common active molecule is the sulfonate group (SO3–). Then and now, sodium softening is acceptable for low-pressure boilers (< 600 psi), with ion exchange sites in the form of SO3–Na+ (FIG. 27).
A modern sodium softener design is shown in FIG. 28.
Similar to a multi-media filter, the inlet water is distributed above the resin bed via a symmetrical pattern of spray nozzles arranged to provide uniform flow. As the influent passes through the resin, the following exchange reactions take place, as shown in FIG. 29.
The sulfonate groups have a stronger affinity for calcium, magnesium and cations other than sodium, and will exchange hardness for Na+ as the water flows through the resin (FIG. 30).
FIG. 31 is an extract taken from the recent revision of the American Society of Mechanical Engineers (ASME) industrial boiler water guidelines.
As the guidelines suggest, lower-pressure drum boilers can tolerate moderate concentrations of many impurities with the exception of hardness. Too often, however, resources and attention are allocated to other plant equipment and operations, resulting in inadequate softener performance. The result is deposition of hardness compounds in boiler tubes (FIG. 32) that causes overheating, corrosion and eventually tube failures.
Over time, resin becomes exhausted. Grab sample and on-line hardness monitoring are good tools to detect hardness leakage. At that point, regeneration is required. A close inspection of the reactions listed in FIG. 29 shows that the equilibrium reaction of the normal exchange process is shifted well to the right. Regeneration must force the reactions in the opposite direction; accordingly, a strong brine solution is needed. The standard before each regeneration is a backwash step to rinse debris and resin fines from the bed.
Two regeneration designs—co-current and counter-current—are possible. In co-current regeneration, brine is introduced at the top of the resin bed and flows along the same path as the normal process stream. While this is the simplest arrangement mechanically, one major difficulty is that the extracted cations must pass through the entire bed during the elution process. This increases the required regenerant volume, and some of the ions may remain as a “heel” at the bottom of the resin and then leak into the effluent during process runs. With counter-current regeneration, the brine is pumped from the bottom to the top of the resin bed. This allows the exchanged ions to escape the resin much more easily. This concept is explored further in the next section.
The maximum solubility of NaCl at 68°F (20°C) is approximately 26% by weight. Raw brine is diluted to around 10% for injection into the softener vessel. The typical flowrate design for brine injection is 0.5 gpm/ft³–1.5 gpm/ft³ of resin, with an overall treatment of 10 lb./ft3 of salt. Regenerations of 30 min are common for these conditions.
Demineralization. As power boiler technology advanced in the 20th century, the need for high-purity water became acute. Advanced ion exchange served as the solution. The following resins became standard for many applications:
Strong acid cation (SAC) resins that release hydrogen ions (H+) during ion exchange.
Strong base anion (SBA) resins that release hydroxyl ions (OH-) during ion exchange.
FIG. 33 is an enhanced illustration of FIG. 27: it shows the active sites for hydrogen ion exchange and illustrates in greater detail the crosslinking of the polymers in the resin beads. This crosslinking can be modified to increase bead strength or to increase the number of active sites within each bead.
The resins detailed in TABLE 4 can remove virtually all dissolved mineral ions.
With a cation and anion vessel placed in series, the H+ produced from the cation resin reacts with the OH- from the anion resin to form water, resulting in a pristine effluent.
To meet the high-purity requirements of utility boilers, the arrangement in FIG. 34 emerged as the leading technology in the middle of the 20th century.
The flow path is SAC, SBA and then a mixed-bed (MB) polishing unit to produce the effluent required for high-purity applications. Many systems of this type were placed in industrial facilities, although alternative designs that utilized weak acid cation (WAC) and weak base anion (WBA) resins also emerged.4 WAC and WBA operations are beyond the scope of this article.
As the name implies, a mixed-bed unit contains both SAC and SBA resins, which are thoroughly mixed. The MB polishes the SAC/SBA effluent—when operated properly, this will reduce impurity concentrations to low part-per-billion levels, equivalent to a specific conductivity of less than 0.1 µS/cm. (Absolutely pure water has a conductivity of 0.055 µS/cm.)
Like sodium softener resin, demineralizer resins have a finite capacity. When SAC resins reach exhaustion, sodium ions begin to leak. Silica (SiO2) typically emerges first when SBA resins exhaust. The initial step upon resin exhaustion is backwash. This process removes filtered solids and fines generated from bead fracturing. The next step is regeneration. The process is similar to sodium softener regeneration, but with different reagents. Sulfuric acid is the common SAC regenerant in the U.S, while many European systems have been designed for hydrochloric acid. Caustic is the SBA regenerant. IX systems are usually designed with dilution systems to lower the concentrations of these chemicals to 4%–6% concentration, or thereabouts. In some cases, particularly with SAC regenerations using sulfuric acid, a staged process may be needed to prevent precipitation of calcium sulfate.
Many systems, and especially modern units, were designed for counter-current regeneration rather than co-current. This configuration reduces the needed regenerant volume and improves effluent quality, as illustrated in FIG. 35.
Mixed-bed resins require resin separation for regeneration (FIG. 36). Anion resin is lighter than cation resin, and when the mixed resins are backwashed, the two resins settle into distinct layers. The vessel has a mid-level interfacial collector that sits at the cation-anion interface.
FIG. 37 illustrates mixed-bed internals, with the resin in place for a regeneration.
During regeneration, the caustic solution is introduced above the anion resin and acid solution is injected upward through the cation resin. To prevent cross contamination of the resins, the spent regenerant from each process is collected by an interface lateral.
As explained in the next section, the days of stand-alone SAC-SBA-MB installations are mostly in the past. However, listed below are some (but certainly not all) of the important troubleshooting issues for those systems still in service.
Cation units:
Sodium leakage into the effluent
Regeneration upsets and poor regenerations
Increased ion loading from changes in raw water chemistry
Resin damage
Attack by oxidizing biocides
Performance upsets or failure of reducing agent feed or activated carbon filtration upstream of the demineralizer
Shortened throughput
Resin fouling
Excessive suspended solids from pretreatment system upsets
Coagulant/flocculant fouling from overfeed of upstream clarifier chemicals
High iron and manganese concentrations in the raw water
Calcium sulfate deposits that form during regenerations.
Poor regenerations
Loss of resin from excessively high backwash rates. Water temperature is a definite influence for backwash flow settings.
Anion units:
High organic concentrations
SBA exchange sites gradually degrade to WBA moieties that still have some exchange functionality, but much less than before
Loss of resin from excessively high backwash rates
Anion resin is lighter than cation resin and will exit first
Extended resin rinse times following regeneration
Organic fouling.
Additional troubleshooting items and corrective measures are available in the literature.4
Ion exchange evolution. A few decades ago, alternative technologies such as packed-bed demineralizers gained some popularity. These systems utilize very fine resin—essentially in a powdered form—that is placed within compact vessels. Packed-bed units have short run times but also short regeneration times. Pretreatment for suspended solids removal is extremely critical, as these units cannot be backwashed. No further details are offered here, because another configuration is becoming very common, with its roots in the power industry.
As suggested previously, SAC-SBA-MB demineralizer technology proved to be a significant advancement for producing the makeup required for high-pressure steam generators. Often, however, run times may only be a few hours before regeneration is required. As RO technology matured, water treatment experts realized that retrofitting a reverse osmosis unit ahead of a IX demineralizer could greatly extend run lengths and reduce chemical usage. Such retrofits became common at many plants. This idea has evolved even further. FIG. 38 illustrates a high-purity makeup water configuration that has become popular for combined-cycle power plants, but could be applied at many facilities.
Note the term “IX Bottles” in the diagram. These are portable mixed-bed exchangers. A number of water treatment companies provide a service in which a representative will come to the plant, remove an exhausted bottle and replace it with a bottle of completely regenerated resin. This eliminates the need for plant personnel to regenerate the resin and deal with issues related to regenerant chemical costs and safety concerns.
An alternative technology to mixed-bed polishing is electrodeionization (EDI)—shown in FIG. 39—also referred to as continuous electrodeiononization (CEDI). EDI employs a combination of ion exchange membranes, ion exchange resins and electrical potential.
Through an induced electrical potential, cations and anions in the influent are drawn to the cathodes and anodes, respectfully, of each cell. Ion-specific membranes either allow or block ion passage and concentrate the ions within specific compartments while producing purified water in others. As ion concentration decreases, the electrical resistance increases: to combat this effect, ion exchange resin is placed between the membranes to improve ion flow. Multiple compartments make up a cell, which are then combined in multiples to make up a complete system. Purified water goes to the plant process, with the concentrate discharged to waste. A key aspect of EDI is that the electrical potential splits some of the water molecules into H+ and OH- ions. These continually regenerate the ion exchange resins, obliviating the need for off-line regeneration. A typical minimum requirement for EDI feed is two-pass RO permeate, as anything of lower quality can cause performance difficulties.
Takeaway. This three-part series outlined some of the most modern makeup water production methods for steam generators and other applications that require purified influent. Much work continues to improve these technologies and others, with considerable focus on the treatment of difficult sources—such as wastewater plant effluent—that increasingly are replacing fresh water supplies. This work is part of the sustainability efforts underway at refineries, petrochemical plants and other industries. HP
DISCLAIMER
The author has included much information in this article from consultation with Ed Sylvester, Director; Filtration, Ion Exchange & Membrane Technologies, ChemTreat Inc., Glen Allen, Virginia (U.S.), who focuses heavily on the refinery industry. However, the views represent those of the author. None of the programs or methods outlined in this series should be implemented without first contacting reputable manufacturers and consultants.
NOTES
Veolia Water Technologies’ Acti-Flo®
Evoqua Water Technologies’ CoMag®
LITERATURE CITED
Buecker, E, et al., “Water essentials handbook,” ChemTreat Inc., 2023, online: https://www.chemtreat.com/water-essentials-handbook/
Byrne, W., Reverse Osmosis: A Practical Guide for Industrial Users, 2nd Ed., Tall Oaks Publishing Inc., Littleton, Colorado, 2002.
Sylvester, E., “Makeup water treatment processes: Ignore at your peril,” presented at the 42nd Annual Electric Utility Chemistry Workshop, Champaign, Illinois, June 4–6, 2024.
Owens, D., Practical principles of ion exchange water treatment, Tall Oaks Publishing Inc., Littleton, Colorado, 1995.
The American Society of Mechanical Engineers (ASME), “Consensus on operating practices for the control of feedwater and boiler water chemistry in modern industrial boilers,” New York, New York, 2021.
Brad Buecker is President of Buecker & Associates LLC, and specializes in consulting and technical writing/marketing. Buecker has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois, U.S.) and at the Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas (U.S.) station. His work has also included 11 years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting Water/wastewater Supervisor at a chemical plant. Buecker earned a BS degree in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored > 250 articles for various technical trade magazines, and he has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, CTI, the Electric Utility Chemistry Workshop planning committee, and is active with the International Water Conference and Power-Gen International.