Stressed Cooling Water System Deposit Control Management

Transcription

1 Paper No Stressed Cooling Water System Deposit Control Management Libardo A. Perez, Zahid Amjad, and Robert W. Zuhl The Lubrizol Corporation, Lakeland Blvd., Wickliffe, Ohio USA ABSTRACT Water scarcity, water reuse, and environmental concerns present challenges for cooling water systems (CWSs). Operating CWSs at high cycles of concentration (COC) increases the deposition potential on heat exchangers, tower fill, and piping. These conditions necessitate robust treatment programs to prevent deposition (both fouling and scaling) and corrosion. This paper presents approaches to manage CWSs operating under high stressed conditions attributable to make up water contaminants (e.g., high calcium, temperature, aluminum). Technologies for preventing deposition in cooling systems operating under stressed conditions are discussed. Keywords: Deposit control, phosphate scaling, corrosion control, polymer technologies, suspended solids, high cycles, stressed cooling systems. INTRODUCTION CWS treatment programs typically incorporate orthophosphate to control low carbon steel metal surface corrosion which has a potential downside or causing metal-phosphate scale formation. Industrial water conservation efforts have focused on operating cooling systems at higher COC, using alternative makeup water sources (vs. fresh water), and in some cases operating systems at higher temperatures. 1 These water conservation efforts and CWS treatment programs have increased the potential for metalphosphate scale formation. Calcium phosphate (Ca/P) is the most common scale formed. Operating CWSs under stressed conditions substantially increases potential Ca/P scale formation, fouling, corrosion, and microbial activity. Developing deposit control polymers (DCPs) that both minimize Ca/P scale formation and support corrosion control has been a focus of water treatment industry research. Monomer ratios, molecular weight (MW), and manufacturing (polymerization) process are among the factors that affect a DCP s ability to prevent scale formation and disperse particulates. The Ca/P inhibiting DCPs that have been developed are predominantly acrylate copolymers containing sulfonated, hydroxyl, and/or N-acrylamide groups. 2,3,4 Optimizing DCPs continues as stressed CWS treatment programs become more prevalent. These DPCs must effectively inhibit Ca/P and function as suspended particle dispersants for water contaminants including aluminum and iron ions that increase DCP demand and/or reduce Ca/P inhibition efficacy. Understanding a DCPs ability to perform under stress conditions is essential when implementing a CWS treatment program. This paper presents the performance of several DCPs under stressed conditions including high temperature, high calcium, and aluminum (simulating water clarifier aluminum salt carryover). 1

2 EXPERIMENTAL Solutions were prepared by using double distilled water, and reagent grade calcium chloride dihydrate, sodium bicarbonate, magnesium chloride hexa-hydrate, sodium phosphate dodeca-hydrate, and tetrapotassium pyrophosphate. These reagent grade chemicals were obtained from Alfa Aesar. Aluminum sulfate hexa-deca-hydrate was obtained from Macron Fine Chemicals. HEDP (1-Hydroxythylidene-1,1- diphosphonic acid) and TTA (tolyltriazole) were commercial materials obtained from water treatment (WT) industry suppliers. Calcium and magnesium concentrations were determined by standard EDTA colorimetric titrations. Phosphate concentrations were determined by spectrophotometric methods (Hach DR3900) and turbidity measurements were conducted by using a Hach 2100N turbidimeter. Table 1 list the commercial DCPs evaluated in this study including three acrylic acid/sulfonic acid (AA/SA) copolymers and three AA-based copolymers containing three or more monomers at least one of which is a sulfonate. All DCP stock solutions were prepared in distilled water on an active solids (total solids less counter ions from post-polymerization neutralization with NaOH) basis as were treatment dosages. Table 1: DCPs Evaluated Additive Composition MW* CK775 Poly(acrylic acid: [2-acrylamido-2-methylpropane sulfonic acid]**) or AA/SA with <15k 74/26 monomer weight ratio CPD20 Poly(AA/SA) with 80/20 monomer weight ratio 4.5k CPN23 Poly(AA/SA) with 60/40 ratio >10k CK798 Poly(AA : SA : acid: sulfonated styrene) or AA/SA/SS <15k CPD31 Poly(AA : SA : non-ionic) or AA/SA/NI 4.5k CPA54 Poly(AA : meth-methacrylate : 2-propene-1-sulfonic acid, 2-methyl- : benzene 15k sulfonic acid, 4-[(2-methyl-2-propenyl)oxy]-) * MW = Weight average molecular weight. ** AMPS monomer. A dynamic simulation test rig (DSTR) was used to evaluate DCP efficacy (as scale and fouling control agents) for CWSs. The DSTR units were designed to evaluate both scale and corrosion control under heat transfer, dynamic flow, and simulated typical CWS conditions. Bulk water and heat transfer surface temperatures, ph, and flow velocity were all controlled. Treated synthetic water was prepared by adding component ions to deionized water. Makeup and blowdown adjustments were made by continuously feeding synthetic cooling water containing the appropriate treatment to the unit s sump. A sump overflow facilitated blowdown. CWS treatment program efficacy was assessed by the deposit amount formed at the heat transfer and other surfaces present in the DSTR unit as well as considering any bulk precipitation (turbidity) and test solution ion concentrations. Table 2 shows the normal or low stress water chemistry, operating conditions, and the baseline treatment (BLT1) used for typical condition DSTR experiments. The BLT1 used in all tests included ortho- and pyro-phosphate to prevent low carbon steel corrosion, tolyltriazole (TTA) to prevent yellow metal corrosion, and HEDP to prevent potential calcium carbonate deposition. Collectively, the water chemistry, operating conditions, and BLT1 reflect typical industrial cooling systems. To evaluate DCPs performance under stressed hardness (>800 mg/l as CaCO 3), the baseline treatment was modified by eliminating the 2.4 mg/l pyro-phosphate (BLT2). This is possible because 14 mg/l PO 4 at 800 mg/l calcium as CaCO 3 provides acceptable cathodic and anodic corrosion protection. Trade name 2

3 Table 2: DSTR Testing Water Chemistry, Operating Conditions and BLT1 Parameter Value Parameter Value [Ca] as CaCO mg/l Aluminum 0 mg/l [Mg] as CaCO mg/l Bulk water temperature 120 F (48.8 C) [HCO 3] as CaCO 3 50 mg/l Skin temperature 140 F (60 C) ph 7.2 Holding-time index 1.75 days Ortho-phosphate* as PO 4 14 mg/l Hydroxyapatite saturation index**** 14 Pyro-phosphate* as PO mg/l Heat exchanger metallurgy SS** and AB*** Tolyltriazole* (TTA) 4 mg/l Coupon metallurgy AB and LCS HEDP* 0.6 mg/l Flow velocity 2.75 fps * BLT1 component. ** SS = Stainless steel. *** Admiralty brass **** HAP-SI, calculated using French Creek Software s WaterCycle Rx program. Figures 1 and 2 are the DSTR schematic diagram and unit photograph, respectively. Figure 1: Dynamic System Testing Rig (DSTR) Schematic Diagram Trade name 3

4 Figure 2: DSTR Photograph RESULTS AND DISCUSION The DSTR tests were designed to simulate both typical and high stress cooling water conditions; the high stress conditions include higher temperature, higher calcium levels, and the presence of aluminum ions. The DSTR experiments were conducted using different DCP dosages based on experience with the goal of achieving acceptable performance with at least two DCPs at as low dosage as possible. All DCPs were evaluated at one or more dosages under each set of DSTR conditions. A DCP s primary role in phosphate-based CWT programs, is maintaining PO 4 soluble levels necessary to facilitate corrosion protection. Therefore, a common operating practice is to maintain the recirculating or bulk water delta (Δ) PO 4 (or difference between non-filtered and filtered PO 4) at less than 1 mg/l to prevent phosphate scaling. Other DSTR performance criteria include low carbon steel (LCS) and admiralty brass (AB) corrosion rates (CRs) below one (1) mil per year and 0.2 mpy, respectively as well as clean heat transfer surfaces (HTS) for both stainless steel (SS) and AB. A very slight deposit is referred to as an almost clean surface. Photographs taken during the experiments illustrate the HTS deposit observations. 4

5 Typical Water Chemistry and Operating Conditions Table 3 shows DSTR results without and with the DCPs listed in Table 1. Duplicate runs with BLT1 and without DCP present had moderate to heavy deposits on the AB HTS, slight granular deposition on the SS HTS, 1.5 to 1.9 NTU turbidity, 4.4 mg/l Δ PO 4, and negligible metal surface corrosion was observed. Acceptable tests results for all performance parameters are shown in bold font. Table 3 shows CK798 at 3 mg/l was the lowest DCP dosage providing acceptable results: clean HTSs, as well as Δ PO4 and corrosion rates (both LCS & AB) meeting target performance criteria. Figures 3 (a) and (b) show the clean AB and SS HTSs using 3 mg/l CK798. Table 3 data indicate that 1.7x higher dosages (5 mg/l) were required for both CPD31 and CPN23 to achieve comparable results. Figure 4 shows very slight deposition on the AB HTS and clean SS HTS using 4 mg/l CPN23. Table 3: DSTR Results - Typical Conditions (e.g., BLT1, 600 mg/l Ca as CaCO 3, 49 ⁰C bulk water, 60 ⁰C skin, HAP-SI = 14) Treatment Final Δ PO4 (mg/l) Final Turb.* (NTU) LCS AB SS HTS Deposit AB HTS Deposit Target performance <1 --- <1 <0.2 Clean Clean BLT1 w/o DCP Slight - some granular Moderate to heavy BLT1 w/o DCP Slight non-uniform Moderate to heavy 2 mg/l CK798 + BLT Very light spot Very slight 3 mg/l CK798 + BLT Clean Clean 4 mg/l CPD31 + BLT Clean Very slight non-uniform 5 mg/l CPD31 + BLT Clean Clean 4 mg/l CPA54 + BLT Very slight non-uniform Very slight few spots 4 mg/l CPD20 + BLT Slight - non-uniform Slight uniform 5 mg/l CPD20 + BLT Clean Very slight few spots 4 mg/l CK775 + BLT Very slight non-uniform Very slight - non-uniform 4 mg/l CPN23 + BLT Clean Very slight uniform 5 mg/l CPN23 + BLT Clean Clean * Turb. = Turbidity, ** CR= Corrosion Rate The DSTR typical water chemistry and operating conditions results in Table 3 suggest the DCP performance ranking (best to worst) shown below. CK798 > CPD31 CPN23 > CPA54 CK775 > CPD20 5

6 Figure 3: DSTR Typical Conditions Using 3 mg/l CK798 at EOE* (a) AB HTS (b) SS HTS *EOE = End of experiment Figure 4: DSTR Typical Conditions Using 4 mg/l CPN23 at EOE AB HTS (Very Slight Uniform Deposition) SS HTS (Clean) Temperature Effect The effect of temperature on DCP efficacy was evaluated by operating the DSTR at higher bulk water (52 ºC) and skin (70 ºC) temperatures than the typical conditions (49 ºC bulk water and 60 ºC skin, respectively). Table 4 presents results for the high temperature condition 51) DSTR evaluations. Acceptable tests results for all performance parameters are shown in bold font. The data indicate that 5 mg/l CK798 outperformed other DCPs providing clean HTSs and acceptable CRs. Higher (1.4x to 1.6x) CPN23 and CPD31 dosages (7 and 8 mg/l, respectively) were required to achieve comparable performance. Higher CK775, CPA54, CPD20 dosages (9 mg/l, 9 mg/l, and 11 mg/l, respectively) were needed to achieve performance goals. Clearly, the results of higher temperatures operating conditions vary depending on the DCP. 6

7 Table 4: DSTR Results - High Temperature Stressed Conditions (e.g., BLT1, 600 mg/l Ca as CaCO 3, 52 ⁰C bulk water, 70 ⁰C skin, HAP-SI = 51) Treatment Final Δ PO4 (mg/l) Final Turb.* (NTU) LCS AB SS HTS Deposit AB HTS Deposit Target performance <1 --- <1 <0.2 Clean Clean 4 mg/l CK798 + BLT Very slight uniform Slight non-uniform 5 mg/l CK798 + BLT Clean Clean 7 mg/l CPD31 + BLT Very slight uniform Slight uniform 8 mg/l CPD31 + BLT Clean A spot? 8 mg/l CPD31 + BLT Clean Clean 8 mg/l CPA54 + BLT Clean Very slight uniform 9 mg/l CPA54 + BLT Clean Clean 9 mg/l CPD20 + BLT Slight uniform Slight uniform 10 mg/l CPD20 + BLT Very slight non-uniform Slight non-uniform 11 mg/l CPD20 + BLT Clean Clean 8 mg/l CK775 + BLT Clean Very slight uniform 9 mg/l CK775 + BLT Clean Clean 6 mg/l CPN23 + BLT Clean Slight non uniform 7 mg/l CPN23 + BLT Clean Clean * Turb. = Turbidity, ** CR= Corrosion Rate The DSTR evaluation typical water chemistry and operating conditions at high bulk water and skin temperature results in Table suggest the DCP performance ranking (best to worst) shown below. CK798 > CPN23 > CPD31 > CPA54 CK775 > CPD20 Figure 5 shows the clean AB HTSs for the high temperature conditions discussed above when using different dosages (5 mg/l CK798 and 7 mg/l CPN23). Figure 6 shows the slight non-uniform deposits that formed when using 10 mg/l CPD20 under the same conditions. Figure 5: DSTR at High Temperature Stress Conditions Using Different DCPs - EOE AB HTSs Top: Using 5 mg/l CK798 (Clean HTS) Bottom: Using 7 mg/l CPN23 (Clean HTS) 7

8 Figure 6: DSTR at High Temperature Stress Conditions Using 10 mg/l CPD20 EOE AB HTS (Slight Non-Uniform Deposition) Hardness Effect Operating CWSs at either high COCs or using reuse water as CWS makeup can cause high calcium concentrations in recirculating waters thereby increasing the potentials for Ca/P and/or calcium carbonate (CaCO 3) scale formation. In order to evaluate the effect of high calcium concentrations, the DSTRs were operating using modified Table 2 water chemistry and operating conditions; the pyrophosphate (not needed for corrosion control in high hardness conditions) was eliminated from the baseline treatment (to create BLT2) and the calcium and chloride levels were changed accordingly. Table 5 tabulates DSTR results conducted at 900 mg/l Ca as CaCO 3 56). Acceptable tests results for all performance parameters are shown in bold font. Six mg/l CK798 provided clean HTSs and acceptable coupon CRs. Higher dosages each of the other five DCPs were required to achieve performance goals. The DSTR results under the high stress calcium conditions, suggest the same DCP performance ranking (best to worst) as the typical and high temperature conditions. CK798 > CPD31 CPN23 > CPA54 CK775 > CPD20 8

9 Table 5: DSTR Results - High Calcium Stressed Conditions (e.g., BLT2, 900 mg/l Ca as CaCO 3, 49 ⁰C bulk water, 60 ⁰C skin, HAP-SI = 56) Treatment Final Δ PO4 (mg/l) Final Turb.* (NTU) LCS AB SS HTS Deposit AB HTS Deposit Target performance <1 --- <1 <0.2 Clean Clean 5 mg/l CK798 + BTL Clean Very slight uniform 6 mg/l CK798 + BTL Clean Clean 7 mg/l CPD31 + BTL Very slight spots Very slight uniform 8 mg/l CPD31 + BTL Clean Clean 9 mg/l CPA54 + BTL Clean Very slight non- uniform 10 mg/l CPA54 + BTL Clean Clean 9 mg/l CPD20 + BTL Slight non-uniform Slight uniform 11 mg/l CPD20 + BTL Clean Clean 9 mg/l CK775 + BTL Clean Very slight uniform 10 mg/l CK775 + BTL Clean Clean 7 mg/l CPN23 + BTL Clean Very slight non-uniform 8 mg/l CPN23 + BTL Clean Clean * Turb. = Turbidity, ** CR= Corrosion Rate Table 6 shows DSTR evaluation results under very high hardness conditions (1,200 mg/l Ca as CaCO 3, 114). Acceptable tests results for all performance parameters are shown in bold font. As expected, these stressed conditions require higher DCP dosages ranging from 8 mg/l CK798 to 14 mg/l CPD20 to achieve performance goals. The overall DCP ranking (best to worst) is shown below. CK798 > CPD31 CPN23 > CPA54 CK775 > CPD20 Table 6: DSTR Results Very High Calcium Stressed Conditions (e.g., BLT2, 1200 mg/l Ca as CaCO 3, 49 ⁰C bulk water, 60 ⁰C skin, HAP-SI = 114) Treatment Final Δ PO4 (mg/l) Final Turb.* (NTU) LCS AB SS HTS Deposit AB HTS Deposit Target performance <1 --- <1 <0.2 Clean Clean 7 mg/l CK798 + BTL Very slight non-uniform Very slight uniform 8 mg/l CK798 + BTL Clean Clean 9 mg/l CPD31 + BTL Very slight non-uniform Slight uniform 11 mg/l CPD31 + BTL Clean Clean 12 mg/l CPA54 + BTL Clean Very slight uniform 13 mg/l CPA54 + BTL Clean Clean 12 mg/l CPD20 + BTL Slight uniform Slight uniform 14 mg/l CPD20 + BTL Clean Clean 12 mg/l CK775 + BTL Very slight non-uniform Very slight uniform 13 mg/l CK775 + BTL Clean Clean 10 mg/l CPN23 + BTL Clean Very slight non-uniform 11 mg/l CPN23 + BTL Clean Clean * Turb. = Turbidity, ** CR= Corrosion Rate 9

10 Aluminum Contamination Effect Aluminum carryover from water clarification operations is a common problem. The presence of aluminum and/or iron in cooling system recirculating waters can adversely impact treatment program performance and/or increase the DCP demand due to DCP adsorption on hydrolyzed metal ions. To simulate aluminum contamination, the DSTRs were run using the Table 2 typical water and operation conditions while continuously adding 2 mg/l aluminum. Table 7 presents the test data including the acceptable results for all performance parameters that are shown in bold font. Table 7: DSTR Results - Typical Conditions and Aluminum Contamination (e.g., BLT1, 600 mg/l Ca as CaCO 3, 49 ⁰C bulk water, 60 ⁰C skin, HAP-SI = 14) Treatment Final Δ PO4 (mg/l) Final Turb.* (NTU) LCS AB SS HTS Deposit AB HTS Deposit Target performance <1 --- <1 <0.2 Clean Clean 4 mg/l CK798 + BLT Very slight uniform Very slight uniform 5 mg/l CK798 + BLT Clean Clean 6 mg/l CPD31 + BLT Slight uniform Slight uniform 7 mg/l CPD31 + BLT Very slight uniform Very slight uniform 8 mg/l CPD31 + BLT Clean Clean 10 mg/l CPA54 + BLT Clean Very slight uniform 10 mg/l CPD20 + BLT Very slight non-uniform Slight non-uniform 10 mg/l CK775 + BLT Clean Very slight uniform 7 mg/l CPN23 + BLT Clean Very slight uniform 8 mg/l CPN23 + BLT Clean Clean * Turb. = Turbidity, ** CR= Corrosion Rate As expected, aluminum contamination increases DCP dosages required to maintain clean HTSs and achieve target coupon CRs, e.g., 5 mg/l CK798 dosage (vs. 3 mg/l dosage in the absence of aluminum). Similarly results were obtained for CPD31 and CPN23 at 8 mg/l dosages vs. the 5 mg/l required without aluminum. The other three DCPs (i.e., CPA54, CK775, and CPD20) were tested at 10 mg/l dosages ( 2x required in absence of aluminum) but none maintained clean HTSs and/or delta PO 4 values below 1.0 mg/l. CPD20 provided the poorest performance with unacceptable HTSs and coupon CRs with some de-zincification on the AB HTS (see Figure 7). The DCP performance ranking (best to worst) for the DSTR evaluations in the presence of aluminum appears below. CK798 > CPD31 CPN23 > CPA54 CK775 > CPD20 10

11 Figure 7: DSTR Typical Conditions with Aluminum Contamination Using 10 mg/l CPD20 EOE AB HTS (Slight Non-Uniform Deposition and Corrosion) CONCLUSIONS Our observations and conclusions based on industry trends and laboratory work discussed here are as follows: 1. The results herein are consistent with those presented in our previous evaluations The need for high performance DCPs as components of phosphate-based CWT programs is growing due to water scarcity and water conservation efforts. 3. Laboratory dynamic simulation test rigs (DSTRs) were used to evaluate the influence of stressed operating conditions (i.e., high temperature, high hardness, and aluminum contamination) on the performance of six commercially available DCPs as CWT program components. The DCPs evaluated include three AA/SA copolymers and three AA-based copolymers containing three or more monomers (at least one which is a sulfonate). The results indicate: a. DCPs vary in their ability to tolerate the stressed operating conditions evaluated herein; much higher dosages of less efficient DCPs are required. b. DCP performance strongly depends on several factors including DCP molecular weight and composition (i.e., co-monomer type and relative amounts). c. CK798 (an acrylic acid / sulfonic acid / sulfonate styrene terpolymer) provided the best performance under all test conditions (stressed and unstressed). DCPs that perform at low dosages and handle stressed conditions (e.g., high phosphate, high suspended solids loadings [e.g., clay, iron oxide] high temperatures, high calcium levels, aluminum contamination) can significantly impact how effectively phosphate-based CWT programs handle upset conditions (e.g., DCP dosage disruptions) and/or feedwater quality variations. ACKNOWLEDGEMENTS The authors thank John Zibrida for the WaterCycle Rx program calculations and The Lubrizol Corporation for permission and support to conduct this study and present the results to NACE International Corrosion

12 REFERENCES 1. L.A. Perez and D.T. Freese, Scale Prevention at High LSI, High Cycles, and High ph without the Need for Acid Feed, CORROSION 1997, Paper No. 174 (New Orleans, LA: NACE 1997). 2. T. Imai, T. Uchida, S. Ano, and T. Tsuneki, A Newly Developed Polymer to Inhibit Scale in Cooling Water Systems, Materials Performance 28 (1989) pp: Z. Amjad and W.F. Masler, Scale Control with Terpolymers Containing Styrene Sulfonic Acid, US Patent 4,952,327 (1990). 4. F. Chen, N-(hydroxyalkyl) Acrylamide Copolymers for Corrosion Control, Proceedings of the ACS Division of Polymeric Materials: Science and Engineering 53 (1985) pp: L.A. Perez, Z. Amjad, and R.W. Zuhl, Deposit Control Polymers for Stressed Phosphate-Based Cooling Water Systems, 2015 Annual Convention of the Association of Water Technologies, Nashville, TN (2015). 12