ABSTRACT. Key words: high silica, silica polymerization, scale formation, water scarcity, water reuse INTRODUCTION

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1 Paper No Managing a Cooling System to Prevent Silica Deposition Libardo Perez, Zahid Amjad, and Robert W. Zuhl The Lubrizol Corporation, Energy and Water Lakeland Blvd., Wickliffe, Ohio ABSTRACT Water scarcity has increased the necessity for water re-use, utilization of poor quality water, and operating systems at higher cycles of concentration (COC). High silica content make-up water is a major obstacle to cost-effective treatment programs for system operating at higher COC. The approach most often used for silica limited cooling systems involves using a base treatment program with an overlay or supplemental treatment such as high concentrations of polymer(s) in combination with phosphonate(s) to control silica at concentrations approaching 300 mg/l. However, this approach leads to very expensive treatments. This study presents a novel treatment program that incorporates a very efficient silica polymerization inhibitor in combination with current corrosion control technology. This study compares the performance of a novel treatment program with two alternatives and discusses their relative cost effectiveness. Key words: high silica, silica polymerization, scale formation, water scarcity, water reuse INTRODUCTION Silica scaling remains as one of the most difficult challenges in the cooling water treatment. Deposition of silica on cooling systems surfaces, such as heat exchangers, pipes, and tower fill film, significantly reduces system performance and cause unscheduled costly shutdowns and chemical or mechanical cleaning operations. Current chemical silica control treatment programs have showed limited efficacy and/or are not economically attractive to take the risk of implementing such technologies. As a consequence, common practices to control silica deposition are focused on running systems at low cycles of concentration (COC) or removing silica from make-up water. 1,2 Operating a cooling system at low COC to prevent silica deposition is perhaps the most common practice. However, this implies the use of large quantities of water which is challenging in regions with water scarcity. The rule of thumb is to keep the reactive silica concentration below 200 mg/l and ph low enough to avoid any possibility of magnesium silicate formation. 3 1

2 Silica removal from make-up water involves the use of precipitation softeners for reacting silica with a metal hydroxide such as magnesium hydroxide. Efficient operating systems require both capital investment and well trained personnel. EXPERIMENTAL All solutions were prepared by using reagent grade calcium chloride dihydrate, sodium bicarbonate, magnesium chloride hexahydrate, sodium silicate, tri-sodium phosphate dodecahydrate, potassium pyrophosphate, iron (III) chloride, and double distilled water. Calcium and magnesium concentrations were determined by standard ethylenediamine tetraacetic acid (EDTA) colorimetric titrations and silica and phosphate were determined by colorimetric methods. Dynamic silica inhibition tests were performed by using a 3L double-walled 304 stainless steel jacketed beaker equipped with two heat exchanger tubes. The two heat exchanger tubes, one made from stainless steel and the other from admiralty brass, were suspended from the cell lid and immersed in supersaturated silica solutions. The supersaturated silica solutions was kept at 37.8 C by recirculating water at 40 C through the double-wall of the cell and differential temperatures were established with the heat exchangers by circulating water through the tubes. Hot water (50 C) was circulated through the stainless steel heat exchanger tube and cold 15 C water circulated through the admiralty heat exchanger. Low carbon steel coupon and admiralty brass coupons were installed for corrosion monitoring. Figure 1 illustrates the experimental set-up. Phosphate-pyrophosphate was used for mild steel corrosion inhibition and tolyltriazole (TTA) for copper corrosion inhibition. Cold Water 15 C Hot Water 50 C Copper or Copper Alloy Heat Exchanger Sample Port 316 Stainless Steel Heat Exchanger Water 3L Jacketed Beaker Copper or Copper Alloy Coupon Mild Steel Coupon Stirring bar Water Figure 1: Experimental set-up for silica polymerization inhibition testing 2

3 Table 1 lists the polymeric (e.g., Carbosperse K-700 polymers, polymers containing AMPS monomer [SA]) and non-polymeric scale inhibitors used during the dynamic silica inhibition tests. This includes a proprietary patent pending copolymer (CP9) for silica / silicate control. In order to prevent calcium and/or iron phosphate/pyrophosphate precipitation during the experiments, CP2 (a commercially available metal phosphate scale inhibitor and dispersant) was added as a component of the baseline treatment (BLT) for all experiments. All inhibitor dosages discussed herein are expressed as active. Table 1: Polymeric and Non-Polymeric Additives Used During Experiments Additive Composition Mol. Wt. Acronym HEDP 1-Hydroxythylidene-1,1-diphosphonic acid 206 HEDP K-798 Poly(acrylic acid: 2-acrylamido-2-methylpropane sulfonic acid: <15k CP2 sulfonated styrene) or AA/SA/SS CCPD Poly(AA : SA : non-ionic) or AA/SA/NI 5k CP3 CCPA Proprietary acrylic copolymer N.R.* CP5 K-XP228 Proprietary copolymer N.R.* CP9 Notes: * N.R. = Not reported. Table 2 summarizes the system operational conditions and Table 3 provides the water chemistry parameters including the BLT components. Table 2: System Operational Conditions Parameter Value and Units Bulk water temperature 37.8 C Skin temperature (stainless steel heat exchanger) 50 C Skin temperature (admiralty brass heat exchanger) 15 C Bulk water ph 7.0 to 7.2 Experiment duration 100 hr Stirring rate 300 rpm Table 3: Water Chemistry Parameter Calcium as calcium carbonate Magnesium as calcium carbonate Silica as SiO 2 Ortho-phosphate as PO 4 * Alkalinity as calcium carbonate Phosphate scale inhibitor (CP2) as active * Tolyltriazole (TTA)* *Baseline treatment (BLT) component. Value and Units 600 mg/l 300 mg/l 350 mg/l 12.0 mg/l 30.0 mg/l 7.5 mg/l 5.0 mg/l All inhibitors, silica, phosphate, and corrosion inhibitors were added to the working water solution before adding calcium and magnesium chloride solutions. Monomeric silica concentration as well as phosphate and calcium were monitored by taking aliquots and filtering these by using a 0.2-µm membrane filter. Silica Inhibition Testing RESULTS AND DISCUSSION In order to assess the performance of inhibitors, a control test (Run 0) incorporating the BLT was conducted in the absence of silica inhibitor agents. As mentioned in Table 3, the test duration was Trade name 3

4 100 hr after which all surfaces were visually inspected and coupons corrosion rates were obtained. At the end of the control (Run 0), the stainless steel heat transfer surface showed slight deposition while the admiralty brass heat exchangers showed moderate deposition. Coupons showed slight deposition with acceptable corrosion rates of mm/y (2.3 mpy) for the low carbon steel coupon and mm/y (0.2 mpy) for the admiralty brass. The mild steel coupon showed some edge corrosion at the bottom end. The bulk water reactive silica concentration decreased from an original value of 353 mg/l to a value of 208 mg/l all expressed as SiO 2. Solution turbidity went from an initial value of 0.24 NTU to 5.69 NTU. Collectively these data indicate that heavy bulk precipitation has occurred. To establish a reference performance, the efficacy of CP3 (a commercially available copolymer promoted as silica/silicate inhibitor) was evaluated. Initially (Run 1), 30 mg/l CP3 was added to the BLT used in the control (Run 0) for corrosion and metal phosphate scale control. At the end of Run 1, both heat transfer surfaces showed slight deposition and acceptable corrosion rates, which were similar to that obtained in the control (Run 0). The silica level decreased from 348 mg/l initially to a low value of 238 mg/l at the end of Run 1, indicating that CP3 did not prevent either silica inhibition or deposition. As in the control (Run 0), bulk silica precipitation took place in the water during Run 1 with the turbidity increasing from NTU initially to 3.67 NTU at the end of the test. Figure 2 compares reactive silica levels measured at different times during the Runs 0 and 1. Reactive Silica (mg/l) Figure 2: Dynamic Silica Inhibition Testing for Run 0 (Control) and Run 1 (30 mg /L CP3 + BLT) Control 30 mg/l CP3 + BLT Time (hr) It has been reported that adding phosphonates increases the performance of polymers that display efficacy as silica polymerization inhibitors. 4-6 Therefore, we evaluated the performance of CP3 supplemented by HEDP in two tests and compared the data to Run 1; the three tests are summarized in Table 4 below. Table 4: Silica Control Treatments Tested using CP3 without and with HEDP plus BLT Run CP3 HEDP BLT 1 30 mg/l 0 mg/l See Table mg/l 2.5 mg/l See Table mg/l 3.5 mg/l See Table 3 Figure 3 compares the results for Runs 1, 3, 6 that incorporate CP3 as a silica polymerization inhibitor. The data indicate that HEDP addition modestly improved performance, but the treatment used in both Runs 3 and 6 failed to keep silica above 300 mg/l. All surfaces in Runs 3 and 6 showed almost no deposition on stainless steel surface as well on both coupons. However, the cold water admiralty brass heat exchanger showed slight deposition during Run 3 and very slight deposition during Run 6. Silica 4

5 precipitation in the bulk water was relatively moderate with final turbidity values of 2.26 and 1.98 NTU for Runs 3 and 6, respectively. 360 Figure 3: Dynamic Silica Inhibition Testing for Runs 1, 3, & 6 (CP3 w/o & w/ HEDP plus BLT) Reactive Silica (mg/l) mg/l CP3 + BLT 30 mg/l CP mg/l HEDP + BLT 75 mg/l CP mg/l HEDP + BLT Time (hr) Similar results were obtained when tests were conducted with another commercial polymer (CP5), whose composition was already described by Amjad and Zuhl. 7 Two tests were performed used this commercial polymer. These are summarized in Table 5 below. Table 5: Silica Control Treatments Tested using CP5 without and with HEDP plus BLT Run CP5 HEDP BLT 9 30 mg/l 0 mg/l See Table mg/l 2.5 mg/l See Table 3 Figure 4 compares the results for Runs 9 and 10 incorporating CP5 which showed behavior similar to the runs incorporating CP3. However, the CP5 containing program performance was slightly lower than that of CP3 program performance. In Run 9, 30 mg/l CP5 maintained 227 mg/l silica compared to 238 mg/l silica using 30 mg/l CP3. Furthermore, the final turbidity values were 4.24 and 3.67 NTU for the CP5 and CP3 containing programs, respectively. When 2.5 mg/l HEDP supplemented the silica inhibitor, CP5 was able to maintain 276 mg/l silica in solution which is slightly lower that the 282 mg/l silica using CP3 under same conditions. Final turbidity values for the polymer plus silica inhibitor programs were 2.94 and 2.26 NTU for the CP5 and CP3 plus HEDP containing programs, respectively. 5

6 360 Figure 4: Dynamic Silica Inhibition Testing for Runs 9 & 10 (CP5 w/o & w/ HEDP plus BLT) Reactive Silica (mg/l) mg/l CP5 30 mg/l CP mg/l HEDP + BLT Time (hr) A patent pending proprietary silica inhibitor (CP9) was tested using the same procedure previously described. Figure 5 shows silica polymerization inhibition as a function of CP9 dosage supplementing the BLT. In Run 7, 10 mg/l CP9 maintained silica levels as high as 345 mg/l. This is remarkable because the initial silica level was 353 mg/l, heat exchanger and coupons surfaces were basically clean. Similar performance was obtained with an 8.0 mg/l CP9 dosage (Run 2). When tested at 6.0 mg/l CP9 (Run 4), the water s reactive silica level was 326 mg/l but the admiralty heat exchanger surface showed very slight deposition. In all three tests using CP9 plus the BLT (Runs 7, 2, and 4), corrosion was under control with corrosion below mm/y (1.0 mpy) for mild steel and below mm/y (0.2 mpy) for admiralty brass. Silica precipitation in the bulk water was also controlled for all three runs with the final turbidity values below 1.0 NTU. 360 Figure 5: Dynamic Silica Inhibition Testing for Runs 2, 4, & 7 (Variable CP9 plus BLT) Reactive Silica (mg/l) mg/l CP9 + BLT 8.0 mg/l CP9 + BLT 10.0 mg/l CP9 + BLT Time (hr) 6

7 It is well known that the presence of iron increases silica polymerization. This presents a concern when corrosion is not controlled and/or iron is present in the feed water (e.g., iron in well water). Figure 6 shows the effect of iron (III) on silica polymerization inhibition as a function of CP9 dosage in combination with the BLT. Runs 5 and 8 evaluated the impact of 2.5 mg/l iron (III) on CP9 performance at 8 and 10 mg/l dosages, respectively. The heat transfer surfaces and coupons showed no evidence of deposition at the end of both experiments. However, in Run 5 with 8.0 mg/l CP9 plus the BLT, the bulk water reactive silica decreased to 320 mg/l and turbidity increased to 1.12 NTU. In Run 8 using 10.0 mg/l CP9 plus the BLT, the final monomeric silica level slightly decreased to 339 mg/l and the final turbidity was acceptable at NTU. In addition, the corrosion rates were under control in both Runs 5 and 8. Reactive Silica (mg/l) Figure 6: Dynamic Silica Inhibition Testing for Runs 5 & 8 (Variable CP9 + Iron (III) + BLT) 8.0 mg/l CP mg/l Fe + BLT 10.0 mg/l CP mg/l Fe + BLT Time (hr) These test results indicates that relatively low CP9 dosages can control silica polymerization and deposition in cooling systems when used as an overlay or top-off of normal treatment programs implemented to control corrosion and non-silica related scale. The question is how this approach impacts system treatment costs? Economic Considerations As previous discussed, operating cooling systems at low COC is one way to control silica fouling. With water becoming increasing scarce and industries forced to use poor quality water or reuse water, high silica content makeup water is more frequently encountered due to increasing COC as a means to conserve water. Significant water use reductions and cost savings can be achieved by incrementally increasing COC. The higher the COC, the lower the water usage costs as well as the costs for chemical treatment used to treat the cooling system and remediate the blowdown discharged to the environment. Cooling systems limited by makeup (MU) water silica levels are normally run at low cycles. An effective silica control agent facilitates increasing COC and reducing water consumption as well as decreasing the baseline (current) treatment program cost. The following example will be used to illustrate the economic issues to be considered to assess the benefits of using a treatment program that allow higher COC. Tables 6 and 7 provide the system parameters, water cycles, and water consumption used to determine economic benefits. These tables provide system operating conditions as well as system water consumption at different COC. A 50 mg/l silica concentration was assumed for the MU water. 7

8 Table 6: System Operating Parameters for Economic Considerations Parameter Value Parameter Value System volume (m 3 ) 4,500 Recirculation rate 31,800 Temperature ( C) 60 Delta temperature ( C) 5.5 Relative humidity factor 0.85 Alkalinity factor 0.98 Turbidity (NTU) 1.0 Conductivity (µmhos) 1,200 Evaporation rate 270 Controlled ph 7.0 Table 7: Water Cycles, Blowdown, and Makeup (MU) Water * Cycles Ca (mg/l as CaCO3) Mg (mg/l as CaCO3) Cl (mg/l) Fe (mg/l) SiO 2 (mg/l) Blowdown MU Water Make-up ** *Based on system operating conditions described in Table 6. ** Rule of thumb limit. 3 Given the conditions described above, cooling systems must be operated below 4.0 COC in order to maintain silica control. Using the information provided in Tables 6 and 7, the potential water savings by increasing COC can be calculated. Table 8 illustrates water savings as a function of makeup water silica concentration (at 50, 75, and 100 mg/l) assuming all other parameters remain constant. The COC limit is based on a target maximum 300 mg/l total silica concentration in the bulk water. MU Water Silica (mg/l) Table 8: Makeup (MU) and Blowdown (BD) Water Savings MU BD Maximum MU BD Water Cycles Water with Maximum Cycles without Inhibitor MU Water Savings BD Reduction Inhibitor Makeup (MU) water cost savings can be calculated. For illustration purposes, let s assume that the MU water cost is US $1.00 per m 3. Table 9 shows the annual water cost saving on makeup water. Table 9: Annual Water Cost Savings Initial Target MU Water Reduction Cycles Cycles , , ,180,000 MU Water Silica (mg/l) Annual Water Cost Saving USD 8

9 Savings from reducing blowdown water discharges to the environment can also be calculated. Treatment program cost reductions can also be calculated based on reduction in MU water and/or BD. Using treatment cost per m 3 of water and cost per kg of product, the saving can be calculated using the equation below. Annual treatment cost savings = (Annual m 3 BD water savings) x (Product cost per kg) x (Product dosage [kg/m 3 ] at COC) After calculating the saving, an assessment of the cost of the new program (normal treatment program plus silica inhibitor top-off) can be determined. RISK ASSESSMENT Operating systems at higher COC increases the potential for scale and corrosion formation. It is necessary to evaluate the efficacy of baseline treatment program. Successful silica inhibition control, as with any scale control program, depends on achieving acceptable corrosion control. Is the current treatment program performing as intended (meeting expectations), or should it be changed and/or a more robust treatment program implemented? Beyond acceptable corrosion control, good control of operating parameters (e.g., ph, blowdown rate, COC, product feed rate) is a must as well as good monitoring tools. Because silica solubility increases with temperature and ph, the installation and use of scale monitoring tools at cold surfaces (e.g., tower film fill) is essential to prevent any system upset due to silica scale formation. Measuring silica concentrations in aqueous solutions requires an understanding of silica chemistry and attention to analytical detail. Inability to account for all the silica in an aqueous system may be attributable to a combination of limitations with the measurement and/or sampling method(s) as well as silica precipitation in the system. 8 Silica characterizations should include quantification of total, soluble, precipitated, and colloidal silica. The risk of silica scale formation can be minimized by addressing the consideration above.. CONCLUSIONS Tests performed to assess the efficacy of polymer CP9 as silica polymerization inhibitor showed that this polymer is able to keep silica under control up to 350 mg/l as SiO 2 under the conditions studied and at a dosage much lower than two other commercially available silica inhibitors. Neither one of the competitor s silica inhibitor were able to preventing silica polymerization at 350 mg/l SiO 2. The impact of iron (III) was studied and found to have an antagonistic effect on inhibitor performance. However, system performance in the presence of iron (III) can be maintained by increasing CP9 dosage. The proprietary silica inhibitor (CP9) performs well at a relatively low dosages and therefore provides an alternative to other technologies. An economic assessment comparing current total costs to potential total costs using a modified water treatment program incorporating CP9 is the best way to evaluate the feasibility of increasing COC above the 200 mg/l SiO 2 rule of thumb operating limit. As water scarcity increases, new technologies such as CP9 should provide an excellent water conservation tool. 9

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