Chelants for Water Treatment

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1 Dow.com

2 Chelants for Water Treatment Stephanie Potisek, Research Scientist, Dow Oil, Gas and Mining

3 Inorganic Scale Fundamentals in the Oilfield What is Scale? Scale is the deposition of inorganic minerals present in produced waters. It may consist of one or more minerals and it may co-deposit with other materials. How does scale form? Scale may form when the water becomes supersaturated in mineral components. The precipitation is influenced by Thermodynamic effects, Kinetic effects, and Fluid flow conditions Common Scales in the Oilfield Solubility in Water (25 o C) Driving Force for Precipitation Consequences of Scale Deposition Calcium Carbonate (Calcite) Barium Sulfate (Barite) Calcium Sulfate (Anhydrite) CaCO 3 BaSO 4 CaSO 4 14 ppm 2.2 ppm 2090 ppm Pressure drop Temperature increase Mixing incompatible waters Temperature drop Mixing incompatible waters Temperature increase Flow restrictions Reduced production Under-deposit corrosion Safety issues Loss of equipment efficiency Formation damage Injectivity loss (injection wells) 3

Scale in Industrial Water Treating Problems Heat exchange less efficient Narrows pipe widths and increases the energy used in pumping the water through the pipes.

4 Scale in Industrial Water Treating Problems Heat exchange less efficient Narrows pipe widths and increases the energy used in pumping the water through the pipes. System failures Reduction in system reliability Process downtime Causes Calcium, magnesium, iron, and silica, aluminum, copper CaSO 4 Mg(OH) 2 Solutions Post deposition Mechanical solutions Dissolvers - chelants Chemical treatment Polymers, phosphates 4

Massive Explosion Linked to Plant s Boiler Feb. 19,1958 massive explosion at Reynolds Metals Building in McCook, IL. 6 men were killed and 40 more injured.

5 Massive Explosion Linked to Plant s Boiler Feb. 19,1958 massive explosion at Reynolds Metals Building in McCook, IL. 6 men were killed and 40 more injured. Investigations showed that the plant s boiler was the cause of the explosion. Severe scaling inside the boiler prohibited proper heat transfer, which let to severe overheating and eventual explosion. 5

6 Boiler Systems Dissolved mineral salts in water precipitate and form solid deposits due to exceeded solubility limits. Build up of layers on the metal surfaces of the systems such as boilers and cooling towers. Scale deposition reduces circulation, but heat input is constant amount of steam increases increasing deposits 6

7 Background What is a Chelant? Used as dissolvers to remove scale Salt of weak organic acids that coordinate to vacant sites on cations (metals, i.e. Ca+2) Purpose keep metals soluble and control metal ions in aqueous solutions React with metal ions by sharing electrons to form ringed structures Form highly stable complexes--metals favor chelate complexes and block Complexes are water soluble M + 7

8 Common Chelants DPTA EDTA NTA HEDTA 8

9 How do Chelants Work? Scale Dissolver: Solid scale is dissolved due to a surface complexation reaction between the chelant and the mineral surface Chelants need to be in direct contact with the scaled-up surface They also need time to react Metal Temperature increases the dissolution rate Ion Ca, Mg, Fe 9

10 Ligands and Chelants Metal ion + Ligand M + xl Coordination Complex MLx Multi-valent (i.e. multi-charged) metal ions have 4-6 reactive sites called coordination sites. Ligands bind to multi-valent metal ions at these coordination sites. Single ligands which bind to more than one coordination site at a time are called chelating agents or chelants. EDTA and DTPA are chelants. Metal ion + Chelant Metal Chelate 10

11 Ligands and Chelants EDA chelate of Cu(II) Ammonia complex of Cu (II) Ammonia is a monodentate ligand. One ammonia molecule binds to one metal ion coordination site. Ethylenediamine is a bidentate chelant. One ethylenediamine molecule binds to two metal ion coordination sites. Because chelants form ring structures with metal ions, chelants form more stable complexes with metal ions than monodentate ligands do. EDTA chelate EDTA can behave as a hexadentate ligand. One molecule of EDTA can bind to as many as six metal ion coordination sites. 11

12 Metal Complexation/Chelation Reaction An equilibrium reaction... M + xl MLx metal ion ligand (chelant) metal complex (metal chelate) Governed by an equilibrium constant, Koverall Koverall = [MLx] [M] [L]x As used here, K is an overall stability constant. 12

13 Metal Complexation/Chelation Reaction Stability constants are usually reported as log K values As the value of log K increases: The metal is more strongly bound to the chelant. The chelant ties up more of the available metal ion. There is less free (nonchelated) metal ion present. Effectiveness also limited by concentration of competing ions log K = 2 90% complexation of metal ion log K = % complexation log K = % complexation log K = % complexation log K = % complexation Metal Ion EDTA NTA Ca Mg Fe Fe

14 ph Effects Chelant effectiveness is ph dependent As ph increases (alkalinity increases), hydroxide ions compete with the chelant for metal ions. Fe3+ + EDTA-4 [FeEDTA]Fe3+ + OH- + EDTA-4 [Fe(OH)EDTA]-2 Fe3+ + 3OHFe(OH)3 As ph decreases (acidity increases), protons compete with metal ions for chelant. Fe3+ + EDTA4[FeEDTA]Fe3+ + H+ + EDTA4[HFeEDTA] 4H+ + EDTA4H4EDTA 14

15 Conditional Stability Constants Vary with ph Chelation of Ferric Ion with EDTA FeEDTA Fe(H)EDTA Fe(OH)EDTA Fe(OH)2EDTA Fe(OH)2EDTA2 FeOH Fe(OH)2 Fe(OH)3(S) log K Fe(OH)4 Fe2(OH)2 EDTA(H) EDTA(H)2 EDTA(H)3 EDTA(H)4 EDTA(H)5 HOH log K Must solve many simultaneous equations to understand just Fe + EDTA Consider multiple metal ions, cations, chelants 15

16 The Real World In even more complex (realistic) systems where multiple ligands and metal ions are present... Ligands compete with one another for metal ions. Metal ions compete with one another for ligands. Ligands Metal Ions EDTA Fe3+ DTPA Cu2+ Citrate Mn2+ Phosphate Ca2+ Phosphonate Mg2+ We can calculate/model these interactions 16

software Sophisticated computer models help predict and optimize chelant behavior Behavior of more than 40 metals, over

17 Metal Ion Control Chart and PIMIC Metal Ion Control Chart Used as an initial guide in choosing the chelant suitable for the desired ph and problem ion PIMIC (Predictive and Interpretative Metal Ion Control) Dow internal computer modeling software Sophisticated computer models help predict and optimize chelant behavior Behavior of more than 40 metals, over 90 chelants and 26 anions can be modeled physical variables ph, ionic strength, and temperature are included in models 17

Stephanie Potisek SLPotisek@dow.com Associate Scientist Dow Oil, Gas & Mining Notice: No freedom from infringement of any patent owned by Dow or others is to be inferred.

18 Stephanie Potisek Associate Scientist Dow Oil, Gas & Mining Notice: No freedom from infringement of any patent owned by Dow or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer's use and for ensuring that Customer's workplace and disposal practices are in compliance with applicable laws and other government enactments. The product shown in this literature may not be available for sale and/or available in all geographies where Dow is represented. The claims made may not have been approved for use in all countries. Dow assumes no obligation or liability for the information in this document. References to Dow or the Company mean the Dow legal entity selling the products to Customer unless otherwise expressly noted. NO WARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY EXCLUDED. Form No

Home, Institutional and Personal Care Solutions. ACUSOL Dispersant Polymers for Household Products and Industrial & Institutional Cleaners

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