Cooling water treatment

1. COOLING WATER TREATMENT By Prem Baboo National Fertilizers Ltd.India. Fellow of Institution of Engineers, India

2. INTRODUCTION • Location- At Vijaipur, Dist-Guna Around 850km from Mumbai. • Plant details Unit Vijipur I Vijaypur II Ammonia Plant MTPD 1520 1520 Urea Plant MTPD 2620 2620 CPP MW 3 x 17.5 3 x 17.5 Raw Material NG NG/Naphtha Commisioning 1987 1998

3. Details of Cooling Tower . Ammonia I Urea I Ammonia II Urea II Type Induced draft Cross flow Induced draft Cross flow Induced draft Cross flow Induced draft Cross flow No. Of Cells 6 5 6 5 Delta T 10 10 10 10 CR m3/hr 17000 16000 18000 17000 System Hold Up 7500 7000 7500 7000

4. Make up Water Quality Parameter UNITS Typical Range pH 7.5 – 8.2 Total Hardness Ppm 75 – 120 Ca-Hardness Ppm 50 – 80 Mg-Hardness Ppm 25 – 40 Silica Ppm 10 – 25 Chlorides Ppm 10 – 25 M-Alkalinity Ppm 60 – 150 Sulphates Ppm 10 – 50 TDS Ppm 150 – 200

5. Re-circulating Water Parameters . Parameter Units Normal Operating Range pH NTU 6.8 – 7.5 Turbidity Ppm 5 Total Hardness Ppm 700 – 800 TDS Ppm 2200max Silica Ppm 100max Chlorides Ppm 150 – 250 Iron Ppm 1.5max Zinc Ppm 1.0max Total PO4 Ppm 4 – 8 COC 6-8 TBC Counts/ml 1 x 105 SRB Counts/100ml 100

6. Monitoring Tools 1. Corrosion Coupons 2. Deposit monitor 3. Bio-fouling monitors 4. Test heat exchanger 5. Microbial counts 6. ORP meter 7. Inspection of cooler during shut down

7. Cooling Systems • Once-through systems • Closed recirculating systems • Open evaporative recirculating

8. Simple Cooling Water Diagram Simple cooling water diagram. Cooling tower Blow down [evaporation] Return water Coolers Heat exchangers

9. Cooling Water Terminology • Cooling water - water used to cool process fluid, condense steam, cool oil, air, etc • Make-up water - fresh water added to make-up for loss water • Evaporation - droplets of hot return water that evaporate taking heat with them cooling the remaining water • Blow-down or bleed-off - water that is being drained or loss beyond control

10. Terminology • Cycle of concentration - how many times concentrated the cooling water is compared to the make up water (dissolved solid concentration) • Drift loss - loss of water through windage • Circulation Rate - total circulation pump flow rate

11. Terminology • Supply temperature - temperature of the supply water • Return temperature - temperature of the return water • Delta T (temperature different) - the difference between return and supply temperature (T return - T supply)

12. Relationship of various parameters • Cycles Of Concentration C = Concentration in Recirculation Concentration In Make-up • Evaporation Loss E = 0.0018 x deltaT x R x TF (Tower Factor)* ( T expressed in oC) * Tower factor is based on humidity/% contribution of evaporation to delta T

13. Relationship of various parameters • Windage Loss W = 0.05 to 0.2 x R / 100 • Blow Down B = E / ( C – 1 ) • Make-up M = E + W + B

14. Cooling Water Chemistry • pH • Conductivity or Total Dissolved Solid • Turbidity or Total Suspended Solid • Total hardness • Calcium hardness • Alkalinity (p and m) • Chloride • Sulfate • Silica

15. More Parameters • Total Iron • Inhibitor residuals (i.e. o-phosphate, phosphonate, zinc, molybdate, etc.) • Bacteria counts (TBC , SRB, Nitrifying ) • Chlorine i.e FRC • ORP • CLO2 levels

16. pH • Low pH means more hydrogen ions • Hydrogen ions depolarizes corrosion cells accelerating corrosion • High pH means more hydroxyl ions • Environment for scales formation • Environment for microbiological activities

17. Conductivity or TDS • High values mean more dissolved minerals • Higher ions movement improves electrical conduction • Increase the rate of electrochemical corrosion.

18. Turbidity and Total Suspended Solid • The content of suspended solid in the water - silt, debris, air-borne materials • Higher values indicate potential fouling due to deposition of the solid • The deposition might be combined with microbiological activities - microbiological sludge and MIC

19. Total Hardness • The contents of permanent hardness - calcium, magnesium, barium, strontium • Generally indicates the total content of calcium and magnesium as CaCO3 • Read as CaCO3 due to the molecular weight - 100 • High values indicate potential scales formation when there is a presence of complexing anions

20. Calcium Hardness • The content of calcium in water read as CaCO3 • The most common component of scales in water system • May form calcium carbonate, calcium phosphate and calcium sulfate scales • High values may also indicate less corrosive (electrochemically) water

21. Alkalinity • Acid neutralizing ability • Free mineral acidity - CO2 at pH < 4.2 • M-alkalinity consisting of HCO3 - and CO3 2- beginning from methyl-orange point pH >4.2 • P-alkalinity consisting of CO3 2- and OH- beginning from phenolpthalien point pH >8.2

22. Chloride and Sulfate • Corrosive ions - form metal chloride and sulfate then mineral acids • Cause pitting corrosion • Chloride - environment for SCC - stainless steel • Sulfate - required element for SRB

23. Total Iron • High values may indicate corrosion activities • Potential deposition of corrosion products - fouling and under-deposit corrosion

24. Inhibitor Residuals • Depending of inhibitors used and control ranges • Inhibitors - phosphate (ortho or total), phosphonate, zinc, molybdate, toly-triazole • Low level - insufficient protection • High level - potential scales formation (precipitative chemistry) and non economical

25. Why do we treat cooling water? • Corrosion of ferrous and non-ferrous metals - electrochemical • Precipitation and deposition of mineral scales • Deposition of suspended solid • Microbiological sludge deposit • Biofilm or microbiological slime • Microbiologically influenced corrosion

26. Corrosion Scaling PROBLEMS Fouling General Fouling Microbial Fouling Algae, Fungi,

27. CORROSION • Corrosion is an electrochemical process by which metals return to their native state • Mild Steel reverts back to Iron Oxide • This is also true for copper alloys, Zinc, Aluminum etc.

28. Localised corrosion • c + Cathode + Fe(OH)3 Fe2O3 Fe(OH)2 Fe++ - Anode - Fe Metal e- e- e- e- + Cathode +OH- 02+H2O

29. Corrosion Cell 2Fe + O2 + 2H2O ---> 2Fe (OH )2 Ferrous Hydroxide 2Fe(OH)2+H2O+1/2O2 ->2Fe(OH )3 Hydrated Ferric Oxide

30. Prevention Of Corrosion • Condition the metal – coating (Zinc,Epoxy Resin) . – Alloy the metal (Stainless Steel) • Condition the environment Remove Oxygen • Use corrosion inhibitors

31. Corrosion Inhibition Mechanisms • Oxidation • Oxidation with film strengthening • Cathodic polarization • Cathodic precipitants

32. Anodic Inhibitors An anodic Inhibitor interferes with the Anodic process • Chromate • Molybdate • Phosphate • Nitrite • Phosphonates

33. Oxidation (With complexation) • . Fe++ 2e-- PO4PO4 Fe3(PO4)2 FePO4

34. Cathodic Inhibitors A Cathodic inhibitor interferes with the cathodic process by precipitating an insoluble species onto the cathodic site. • Zinc • Calcium Carbonate • Polyphosphate • Phosphate • Phosphonates

35. Cathodic Precipitants • . Zn e- e- e- e- e- O2 O2 O2 O2 O2 OH- OH- OH- Zn Ca HCO3 HCO3 HCO3 HCO3 HCO3 HCO3Ca Ca Ca Zn Zn Zn Zn Zn(OH)2 HCO3 + OH H2O +CO3 CO3 CO3Zn CaCO3

36. Zinc • Forms zinc hydroxide and zinc carbonate complexes at cathode • Good for soft water • Above pH 8 will begin to precipitate in bulk water • Zn is stabilised by phosphonates and polymers • Levels from 0.25-3.0 ppm used

37. Poly and Ortho-phosphate • Form complexes with Ca at cathode • Need to formulate stabilizing polymer with package • Also reacts at anode to form iron phosphate complex • Levels of 2-15ppm typically used depending on program

38. Scaling Caused by crystalline growth of salts on the system surfaces • CaCO3 (Calcite) : Major Scale • Silica :Amorphous silica precipitates,when *SiO2 > 150 ppm at pH < 8.0 . > 180 ppm at pH > 8.0 • MgSiO2 :Adsorption of silica on precipitated Mg(OH)2 (Brucite) • Ca5(PO4)3OH (Hydroxyapatite) • CaF2 (Fluorite) : Potential foulant

39. Solubilities (20 / 100 deg C) • Sodium Chloride • Sodium Sulphate • Sodium Carbonate • Sodium Bicarbonate • Sodium Phosphate • Calcium Chloride • Calcium Sulphate • Calcium Carbonate • Calcium Bicarbonate • Calcium phosphate 36% / 39% 16% / 30% 32% / 31% 8% / Dec 11% / Dec 43% / 61% 0.3% / 0.06% 0.005% /0.002% 0.08% / Dec 0.0003%/ 0.0002%

40. Solubilities ( 20 deg C ) • Silica • Ferrous Hydroxide • Ferric Hydroxide • 0.005% • 0.0007% • 0.0001%

41. Formation of Hardness scale • Calcium Carbonate has inverse solubility • Ca(HCO3)2-------> CaCO3 + H2O + CO2 • Mg(HCO3 )2------> MgCO3 + H2O + CO2 • MgCO3+ H2O-----> Mg(OH)2 + CO2

42. Scale Formation • CaCO3 precipitates at Saturation pHs and depends primarily upon: – Level Of calcium hardness – Level Of Bicarbonate alkalinity – pH – TDS – Temperature – Water velocity

43. Scale Inhibition • Remove hardness salts. • pH control with Acid • Dose scale inhibitor

44. Acid Dosing • Used to limit pH in hard water systems. • Helps in inhibitor selection • Potential for water and treatment savings by allowing an increase in COC • Each 1ppm M Alkalinity will require: • 1.0 ppm sulphuric acid • 2.0 ppm Hydrochloric acid • 1.8 ppm Nitric acid

45. Scale Inhibitors • Added to extend Induction time beyond Retention time • Induction time decreases with increase in Saturation level ( Driving force) • Effectiveness of Inhibitor depends on the extent to which it increases Induction time at lesser dosage • Inhibitor dosage is increased with increase in Induction time • If retention time is less than induction time there is very little need of scale inhibitors

46. Dispersion • A process by which charged particles are prevented from agglomerating into larger particles rendering them less settleable. • Most cooling water particulates have a net negative charge. Acrylate dispersants also have a net negative charge. Addition of dispersant increases charge inhibiting agglomeration.

47. Dispersants • Polyacrylate • Acrylate /Acrylamide • Acrylate terpolymers • Sulfonated styrene • Maleic Acid Homopolymer • Maleic acid co and terpolymers

48. Bio Fouling • Caused by the excessive growth of microorganisms. • Cooling water system-ideal incubator for growth

49. Problems • Pitting corrosion-depolarising action of O2 released during their metabolic process. • Shield metal surfaces from the action of inhibitors • cause legionella pneumophila disease

50. Chemical Control • Microbiocides e.g. Bacteriacides,Fungicides,Algaecides • Microbiostats e.g. Bacteriastats,Fungistats,Algaestats • Surfactants

51. Microorganisms • Viruses – Consists of protein & DNA/RNA (Nucleic acids) – Survive by multiplying in other host cells - plant or Animals

52. Cooling Water as a Medium Ideal temperature (200C-600C) pH 6 to 8 Often exposed to sunlight Some made of wood Nitrogen and phosphorous based inhibitors Suspended and airborne debris Good aeration Presence of process fluids like ammonia, urea, other organics and sunlight

53. . • Algae – Photosynthesis – Uni/Multicellular – Diverse Forms  Filamentous  Colonial  Plantlike • Diatoms – A Group of Algae – Organic walls impregnated with silica

54. . • Blue Green Algae – Photosynthetic bacteria • Fungi – Aerobic growth above the waterline – Do not contain chlorophyll

55. . • Mould – Fungus which forms visible layer on the surfaces - Wood/Walls/Foods • Yeast – Unicellular Fungi • Protozoa – Diverse group of unicellular Microorganisms

56. Bacteria • Unicellular • Cells may grow attached to each other in clusters , chains , rods or filaments • Require carbon source for growth • Different shapes – Rods Bacillus – Spherical Coccus – Spiral Spirill • Protected by slime • Multiply by cell division

57. Bacteria (Classification ) • Aerobic Requires O2 & CO2 • Anaerobic Grow in O2 free atmosphere • Facultative Grow in both conditions • Autotrophes Inorganic nutrients • Heterotrophes Organic nutrients • Psychrophiles < 22 0c • Mesophiles 22 ~ 45 0 C • Thermophiles > 45 0 C • Planktonic Free floating organisms in . Water • Sessile Surface attached growing in . Biofilm

58. Methods Of Control • Physical – Nutrient Removal - Remove food or energy source . e.g. Sunlight , Dead Leaves.Process Contamination. – Temperature Control - Increase temperature Not really practical on a Cooling System

59. Methods Of Control • Chemical – pH Adjustment • With the help of Acid / Caustic (pH’s Over 10.0 Required) – Microbiocide Control • Kill Organisms by use of toxic material e.g. Algaecides,Fungicides, Bacteriacides

60. Chemical Control • Microbiocides e.g. Bacteriacides,Fungicides,Algaecides • Microbiostats e.g. Bacteriastats,Fungistats,Algaestats • Surfactants

61. Biocide Classification • Oxidising Materials – Have the ability to oxidise organic matter – Irreversibly oxidise protein groups • Non-Oxidising Materials – Destroy or inhibit normal cell metabolism by any of the following ways:- • Altering permeability of cell wall • Destroying protein groups • Precipitating protein • Blocking metabolite reaction

62. Sulphate Reducing Bacteria • Anaerobic and convert dissolved sulphur compounds to H2S 10 H++ SO4 -2+ 4Fe --> 4Fe+2+H2S +4H2O H2S + Fe+2 --> FeS + H+ • H2S released corrodes Carbon steel and Copper based alloys. • Localised pH depressions cause further attack • Exist mainly below deposits devoid of oxygen • Corrosion rate as high as 100 mpy occurs

63. Nitrifying / Iron Bacteria • Nitrifying Bacteria : – Oxidation of Ammonia . NH3 + 2O2 ---> HNO3 + H2O – Nitrosomonas , Nitrobacter • Iron Bacteria : – Oxidation of ferrous ions . ++ . – Fe + O2 ----> Fe2O3 – Crenothrex

64. Limitations Of Chlorination • Not effective in alkaline water Cl + H2O = HOCl + HCl HOCl -> OCl- + H+ OCl- is 1/80 th time as effective as HOCl Deactivated by the reducing agents H2S ,SO2,,NH3,polyacrylamide, Monoethnolamine,etc. • Deactivates some Organo phosphonates, Does not penetrate slimes • Extremely corrosive to many metals-maintenance of chlorinator is difficult. • Environmental limitations - 0.1 ppm. Free Cl2 can kill fish • When not effective use bromine compounds,chlorine dioxide,ozone

65. Chlorine A strong smelling, greenish-yellow gas with pungent odor which is extremely irritating to mucous membranes.

66. Chlorine Gas • Hazardous • Heavier than air • Strong oxidizer • Low capital requirements • Produces chlorinated by-products • Efficacy - pH dependent

67. Chemistry Chlorination Chlorine gas dissolves in water and hydrolyses as: Cl2 + H2O  HCl + HOCl (hypoclorous acid) HOCl ↔ H+ + OCl- – (hypo chlorite ion) The percentage distribution of hypochlorite ion and undissociated hypochlorous acid can be calculated for various pH values. The amount of hypochlorite ion becomes appreciable above pH 6 while molecular chlorine is non-existent. HOCl is about 80 times more effective than OCl- as a biocide

68. Microbicidal Efficiency • HOCl – the microbicidal efficiency is due to the relative ease with which it can penetrate cell walls. The penetration is quite comparable to water. • OCl- - Poor disinfectant (about 1/80% efficiency of HOCl). It is unable to diffuse cell wall of microorganisms due to negative electrical charge.

69. Chlorine Effectiveness At Various pH 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 4 5 6 7 8 9 10 Percentage HOCl pH Value Percentage OCl -

70. Microbiological Action • Diffusion of active agent through cell wall and attack the enzymes group whose destruction results in death to the organism. Hence microorganisms are not immune to chlorine Factors affecting chlorine efficiency: 1.Concentration of Free Chlorine 2.Contact time 3.Temperature 4.Types and concentration of organisms 5.pH 6.Contaminants

71. Chlorine Di-Oxide • Draw backs of chlorine can be over come with help of Clo2 mainly in NH3 contaminated water. • It can be produced on site as • 2NaClO2 + Cl2  2 ClO2 + 2NaCl • ClO2 does not react with ammonia thus gets effective in controlling microorganisms.

72. Limitation of Chlorine • Chlorine reacts with organics, hence exerts a chlorine demand leading to higher chlorine consumption and non- maintenance of residual • Difficult to handle and dose • Efficacy of chlorine is pH dependent • Chlorine is highly corrosive

73. Chlorine Reactivity 1. With Ammonia - HOCl + NH3  NH2Cl (mono chloramines) + H2O - NH2Cl + HOCl  NHCl2 (dichloramine) + H2O - NHCl2 + HOCl  NCl3 (trichloramine) + H2O It means one ppm of ammonia can react with 3 ppm of chlorine, hence will increase chlorine demand

74. Chlorine Reactivity 2. With Organic Nitrogen • Proteins hydrolyzes to amino acids. • Chlorination chemistry of these are extremely complex • Because of various hydrolysis products. • Finally the products are mono/di- chloramines.

75. Chlorine Reactivity 3. With Urea • Urea hydrolyzes with nitrogen breaking down to ammonia in presence of urease enzyme. • If the hydrolysis lacks this enzyme, the formation of NH3 is greatly inhibited. • If significant quantity of urea-N is present and hydrolysis proceeds at slow rate, unstable residue could result. • Urea-N would then be reservoir for the production of ammonia.

76. Chlorine Reactivity 4. Inorganic Carbon: C + Cl2 + 2H2O  4 HCl + CO2 This takes place in dechlorination with granular activated carbon. 5. Cyanide: At alkaline pH 8.5 or higher, 2Cl2 + 4NaOH + 2NaCN  2NaCNO + 4NaCl + 2H2O

77. Chlorine Reactivity 6. Hydrogen sulphide: H2S + 4Cl2 + 4H2O  H2SO4 + 8HCl Here 8.3 ppm of chlorine is required to oxidize 1 ppm of H2S.

78. Chlorine Reactivity 9. Hydrocarbons: Hydrocarbons create high chlorine demand due to high oxidisable organics.

79. Chlorine Dioxide Application Technology

80. Chlorine Dioxide Introduction • Strong oxidizer • Not a halogen • Selective reactivity • Generated on site • pH independent • Low capital requirements

81. Chlorine Dioxide (Contd.) • Rapid acting. Lower contact time for micobiological kill compared to chlorine • Less corrosive compared to chlorine • Does not hydrolyse to form acid • Does not react to form chloramines • Does not form trihalomethanes with organic matter like chlorine • Does not produce any chlorinated compounds

82. Chlorine Dioxide Mechanism of Kill  Disruption of protein synthesis or lysing of cell  No resistivity by organisms

83. Chlorine Dioxide Effectiveness at Various pHs 0 10 20 30 40 50 60 70 80 90 100 4 5 6 7 8 9 10 % Active pHLb./Equal Performance 1

84. Chlorine Dioxide Safety Considerations  Not handled as a gas  Typical use is < 0.3% solutions  On-site generator is required

85. Selection criteria of suitable oxidant • Efficacy • Safety handling • Regulatory reporting • Process contamination • pH dependence

86. Safety Sodium Hypochlorite Sodium Bromide Chlorine Dioxide Ozone BCDMH Chlorine Gas Best Worst

87. Reporting Requirements Chlorine Dioxide Hypochlorite Bromine Compound Ozone Chlorine Gas Least Greatest

88. Performance in Contaminated Systems Chlorine Dioxide Ozone Bromine Compounds Chlorine Gas Sodium Hypochlorite Best Worst

89. Performance at Elevated pH Ozone Chlorine Dioxide Bromine Compound Sodium Hypochlorite Chlorine Gas Best Worst

90. Best Alternative • CHLORINE is still a widely used oxidant * Inexpensive, historically established, being phased out • HYPOCHLORITE is cheapest alternative * Similar performance to chlorine, degradation is problem • BROMINE CHEMISTRY, halogen alternative * Better performance, can be costly, pH dependent • CHLORINE DIOXIDE, non-halogen alternative Cost-effective broad spectrum, safely fed, pH independent, non-chlorinating agent • OZONE, New Approach * Capital intensive, strong oxidant, no chemicals

91. Chlorine Dioxide Generation

92. Chlorine Dioxide Advantages/Benefits • Gas dissolved in water • Strong oxidizer • Not a halogen • Selective reactivity

93. Chlorine Dioxide Advantages/Benefits • Generated on-site • Rapid acting • pH independent • Low capital requirements

94. Physical Characteristics Color : Yellow-green State : Gas Odor : Similar to chlorine Solubility: 2.9 gr/L

95. ClO2 Generator Generation Methods Chlorine Gas Method Three Pump Method

96. Precursor Source Water Inlet Chlorine 1 2 3 4 56 7 8 9 10 11 12 13 48"H x 42"W x 17"D 14 ClO2 15 1. Electric Control Box 2. Flow Indicator (GPM) 3. Hand/Off/Auto Switch 4. Ball Valve 5. Solenoid Valve 6. Pressure Gauge 7. In-line Flowmeter 8. Ball Check Valve 9. Chlorine Eduction 10. Chlorine Solenoid Valve 11. Chlorine Rotameter 12. Precursor Pump 13. ClO2 Generator 14. Emergency Shutdown Switch 15. Loss Of Chlorine Switch ClO2 Generator Chlorine Gas Method

97. ClO2 Generation Gaseous Chlorine Method 2NaClO2 + Cl2 2ClO2 + 2NaCl Sodium Chlorine Chlorine Sodium Chlorite Dioxide Chloride

98. ClO2 Generation Three Pump Method 2NaClO2 + NaOCl + 2HCl  Sodium Sodium Hydrochloric Chlorite Hypochlorite Acid 2ClO2 + 3NaCl + H2O Chlorine Sodium Water Dioxide Chloride

99. Hydrochloric Acid Source Sodium Hypochlorite Source Precursor Source 1 2 3 456 7 8 9 10 48"H x 42"W x 17"D 11 ClO2 9 9 Water Inlet 1. Electric Control Box 2. Flow Indicator (GPM) 3. Hand/Off/Auto Switch 4. Ball Valve 5. Solenoid Valve 6. Pressure Gauge 7. In-line Flowmeter 8. Ball Check Valve 9. Chemical Pumps 10. ClO2 Generator 11. Emergency Shutdown Switch ClO2 Generator Three Pump Method

100. ClO2 Generator Generation Method Three Pump Method Advantages • Higher Capacity • High Back- Pressure Capacities • Higher Turndown • No Chlorine Gas Necessary Disadvantages • Slightly Higher Cost • Additional Chemical Storage • Incompatible Chemicals

101. Typical ClO2 Dosages Rendering Odor Control :2-10 ppm Cooling Water Treatment: 0.1-0.5 ppm Food Processing : 2-10 ppm Paper Mill Slime Control :0.25-0.45 lb ClO2/ton paper

102. Sodium Chlorite Precautions DO NOT allow solution to dry. DO NOT mix with any other chemicals. DO NOT use wooden pallets or paddles. DO NOT wear leather or cloth external clothing.

103. Normal Shutdown Procedure • Turn operating switch to “Off” • Water flush occurs briefly • Drain unit • If chlorine used, close valve • Drain and flush all chemical systems

104. Equipment Site Survey Location • Well-ventilated area • Eyewash/shower near generator • Eyewash/shower near bulk storage • Washdown water source available • Approved drain • Well lighted

105. Monitoring Tools • Corrosion coupons • Deposit monitor - visual indication of deposit formation • Biofouling monitor - indicate loss of pressure due to biofilm • TBC and SRB dip slides • Test heat exchangers • ORP meters • Site Management • Daily reporting

106. Corrosion Monitoring by Coupons Method • Corrosion Rate (MP

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