By Fabio Sousa and Cliff Lebowitz
Operations and chemistry management for the Puerto Rico Electric Power Authority (PREPA) Aguirre thermoelectric plant in Salinas reported dramatic reduction of acid and caustic for boiler feedwater deionization/demineralization (DI) through the changing of their ion exchange (IX) resin and regeneration procedure. The move also reduced man-hours for DI wastewater treatment.
During the past 30 years, the water table has dropped 20 feet (6.096 m) at the plant, with deeper pumping causing magnesium and calcium to reach 420 to 450 parts per million (ppm). Chloride had reached 80 to 90 ppm and silica 37 ppm.
Due to spiraling costs for acid and caustic resin and the time to treat it, a new solution had to be found. But no new capital investment was available to solve the problem that derived from increasing hardness in their deep-well boiler feedwater source.
Solution required
To protect turbine blades and minimize deposit buildup in steam piping, the plant required boiler feedwater silica (SiO2) at < 125 parts per billion (ppb). They also targeted complete removal of chloride (Cl-1), magnesium (Mg) and calcium (Ca).
Since the installation of the new resin, SiO2 is running < 50-60 ppb. The chloride, magnesium and calcium are non-detectable.
“Solving the problem by changing the water source was not an option when the condition was detected,” said Justo L. Gonzalez, operations manager. “PREPA owns a lake, but there was no capital available at that time for the channel, pipeline, pumps and probably also ultrafiltration pretreatment that would have been required.
“Changing to higher efficiency ion exchange resin is now being recommended as the best way to improve the performance of the demineralization system at other PREPA plants. Many of them have the same problem with their deep-well sources.”
Transition results
The transition resulted in performance recognition for the plant and its chemist, Luis A. Reyes ‘Tony’ Santini. He received the Manuel A. Perez Government Employees Performance Prize in recognition of major improvements in plant operating efficiency.
“For regeneration before we changed resins, we needed 11 lbs. (five kg) of acid per cubic foot (cubic meter) of resin, and now we only need 5.5 to six lbs. (2.5 to 3.17 kg),” said Santini. “Meanwhile, 10 lbs. (4.53 kg) of caustic in the bath previously, is now down to about six to seven lbs. (2.72 to 3.17 kg). As a result, we have saved more than $750,000 (USD) in the first year and over $1,000,000 (USD) in the second year in chemical costs along.
“Due to longer service cycle time, we now need 40 percent less man-hours for demineralization and 20 percent less for DI wastewater treatment. That has allowed me to use my technicians for other analyses to help make the plant run more efficiently, while I have more time for equipment repair and maintenance. 
We have also cut water consumption for regeneration by 25 percent, or about 100,000 gallons (378,541 L) per day, helping us to avoid a crisis in water supply for the high-pressure boilers.”
System design
An IX DI system serves each of the plant’s two 450 MW boilers. Each 1,005°F (540.55°C), 2,600 psi boiler uses 1,950 gallons (7,381 L) per minute of feedwater and requires 6,000 gallons (22,712 L) per hour of makeup water. The DI systems perform eight to ten regenerations per month of 12,000 gallon (45,425 L) each, generating about six million gallons (22,712,470 L) of wastewater that derives from DI and some polishing inside the system.
Each of three treatment trains contains three cations of 500 cu.ft. (46.44 cu.m) and three anions of 250 cu.ft. (23.22 cu.m). Raw water conductivity has reached over 900 micromhos/cm. Cation conductivity is 1,400 micromhos/cm and the anion ranges from 15 to 40 micromhos/cm. Conductivity of water out of the final step mixed-bed is 0.056 micromhos.
Santini recalled that when shipments of acid and caustic to the plant had reached 16 truckloads per month, he was asked to investigate if it needed to be that much. “I started consulting with our distributor, who had experience as a plant chemist,” he said. “After review and consultation, he advised that a resin change might help.
“I then looked at a variety of different resins and found them all to be the same. But the selected resin had a shallow-shell technology that proved beneficial for our needs.”
Shallow-shell technology (SST) resins have inert centers. Only the outer shell is functionalized, which shortens the ion exchange diffusion path. This utilization allows for faster and more complete regeneration of cation resin with lower levels of regenerant (acids), which further leads to more complete ion exchange and regeneration.
More complete regeneration in turn allows for a higher, more efficient utilization of the regenerant. This permits a reduction of the caustic levels for the anion and provides for higher capacities, better handling of iron and organic fouling and more resistance to oxidation than standard grade resins. PREPA estimates that when compared to conventional softening or demineralization resins, regenerant cost reductions of 20 to 50 percent are often possible without sacrificing capacity or increasing leakages.
Testing procedures
“We took about six months to test their cation resin in one of the vessels and proved we could gain 37 to 38 percent savings on chemical costs,” adds Santini. “We provided a comprehensive presentation for plant management and then took a year and a half to changeover to ensure the new supplier could help us change regeneration times and flows”.
“We have found that the premium price for the selected resin has been more than made up for by the savings we have realized. And we have much appreciated their high level of technical support.”
It has been noted that payback periods are very short for ion exchange resin (IER) replacements like the one at PREPA’s Aguirre plant. Likewise, their potential applications are very diverse.
The return on investment (ROI) in the higher-priced IER is often realized in only a few months. After that short period, all chemical savings due to the new IER directly reduce plant operating costs, while the impact of IER cost continues to be very low.
The benefits realized at PREPA, through a combination of the unique IER technology and vendor technical support are available to most users of demineralization plants, regardless of the type of industry they are serving or their site location.
About the companies and products
Fabio Sousa is Regional Director for Latin America of Purolite Company Ion Exchange Resins. A combination of Purolite’s SST60H and PFA300 resins that was installed at PREPA, as recommended by vendor Manuel Bismarck as an innovative approach for coping with jumps in prices for acid and caustic. Purolite’s family of high-efficiency ion exchange resins, including the SST™ cation resin, were selected. For further information, contact The Purolite Company, 150 Monument Road, Bala Cynwyd, PA 19004, Tel. 800-343-1500, www.purolite.com, info@puroliteusa.com.
Cliff Lebowitz heads Indumark, an independent technical case history reporting firm with offices in New York, NY and Doylestown, PA. Prior to starting Indumark in 1980, Cliff was employed as an industrial case history specialist, biology and chemistry teacher and daily newspaper reporter. He holds a B.S. in biology from Rutgers University. Indumark case histories (usually industrial equipment applications) are based on interviews with end users and their engineering firms and are approved by them for accuracy and completeness.