Chlorine Chemistry

1. Chlorine Chemistry

2. Chlorine Chemistry Chlorine Source Initial Reaction Cl2 + H2O -> HOCl + H+ + Cl- Chlorine Gas NaOCl + H2O ->HOCl + Na+ + OH- Sodium Hypochlorite Ca(OCl)2 + 2H2O ->2HOCl+ Ca++ + (OH)= Calcium Hypochlorite Secondary (Dissociation) Reaction HOCl<--> H+ +OCl- Hypochlorous Acid (HOCl) and Hypochlorite Ion (OCl-) exist in equilibrium depending on water pH, according to the Chlorine Dissociation Curve shown on the next slide. Free Chlorine is the sum concentration of both the Hypochlorous Acid and the Hypochlorite Ion in solution.

3. Chlorine Dissociation Curve 0% 10% 20% 30% OCl- 40% HOCl Percent of Chlorine as Hypochlorous Acid (HOCl) Percent of Chlorine as Hypochlorite Ion (OCl-) 50% 60% 70% 80% 90% 100% pH

4. Chlorine as a Biocide The hypochlorous acid component of free chlorine is up to 80 times more effective at inactivation of E Coli than the same concentration of hypochlorite ion over the same contact time. (Fair, et. al.) Adjusting pH from 8.0 to 7.0 results in a change from 25% HOCl to 80% HOCl, effectively increasing the disinfection power of 1 ppm of free chlorine by 40 times. Automatic pH control is recommended. Sodium hypochlorite (bleach), which is caustic, as more bleach is added, the pH increases,and can actually decrease disinfection. Chlorine gas is acidic and drives the pH down, possible conflict with corrosion control

5. Total Chlorine Chlorine combines with ammonia-nitrogen in water to form chloramines. The chloramine species present are a function of the ratio of chlorine to ammonia, and the pH. Monochloramine: NH2ClDichloramine: NHCl2Trichloramine: NCl3 As more chlorine is added to water with ammonia, the quantity of chlorine measured in the water actually decreases until a certain chlorine-to-ammonia ratio, the breakpoint, occurs. Then as more chlorine is added, it becomes available as free chlorine. Total chlorine is the sum of combined chlorine (chloramines) plus free chlorine.

6. Chlorine Analyzers • Colorimetric – DPD • Addition of reagents and pH buffer cause a color to develop based on chlorine residual • Equipment is inexpensive • Subject to color and turbidity interferences • Discrete sampling method

7. Chlorine Analyzers • Amperometric – On-line, real time • Measurement of electrical current proportional to the chlorine residual • Oxidation – Reduction reaction between the anode and cathode measured by current flow • 2 Ag 2Ag+ + 2e- (oxidation - anode) • HOCl + 2e-Cl- + OH- (reduction - cathode) • Free residual sensors look at “HOCl” • pH compensation may be required

8. Chlorine Analyzers • Membrane covered system • Electrodes have a cap with membrane placed into a holder with constant water flow • Reagent less design • Membrane cap holds electrolyte for current flow • Electrolyte contains KI for Total residual

9. Process Control

10. Process Control • Use the signal from plant analyzer(s) to control the chemistry of the treatment process • P, PID and Compound loop controllers • Use of a control element (i.e.metering pump) to maintain a desired condition (i.e. pH, chlorine residual).

11. Types of Control • Modulating • Proportional • Proportional, Integral, Derivative (PID) • Compound Loop

12. Modulating • Definition: On/Off using high and low set points bracketing a desired set point. • Oscillates above and below desired set point • Tuning is manual control of pump output • Use only when desired set point is not critical

13. ExamplepH Neutralization Desired set point 7.0 Flow in at pH 9.8-10.2 Flow out at pH 6.5 – 7.5 Note: Wide effluent pH “range”

14. Proportional • Definition: Xp changes the control element (i.e. metering pump) dependent on the deviation from the desired set point. • Sometimes call “Gain”. • Oscillates about the desired set point – no deviation from set point, no control action.

15. ExamplepH Neutralization Desired set point 7.0 Flow out at pH 6.8 – 7.3 Note: Close effluent pH “range” Flow in at pH 9.8-10.2

16. Integral • Definition: Ti is a time based function that acts to “reset” the “gain” dependent on how long system is away from desired set point. • Sometimes call “reset” • When used with Xp, oscillations can be negligible is all but the fastest moving systems.

17. ExamplepH Neutralization Desired set point 7.0 Flow in at pH 9.8-10.2 Flow out at pH 6.9 – 7.1 Note: Closer effluent pH “range”

18. Derivative • Definition: Td is a rate of change based function that changes the control element (i.e. metering pump) based on the rate of change from the desired set point. • When used with Xp and Ti can eliminate oscillations on all systems.

19. ExamplepH Neutralization Desired set point 7.0 Flow out at pH 6.95 – 7.05 Note: Small effluent pH “range” Flow in at pH 2.5-10.2

20. Compound Loop • Use the correction variable to “recalculate” the P,I and D parameters based on a flow input. • As the flow changes the time variables will change and settings must change to compensate. • When used with the PID setting off: • Flow Proportional Control.

21. ExamplepH Neutralization Desired set point 7.0 Flow out at pH 6.95 – 7.05 Note: Small effluent pH “range” Process flow varies & Flow in at pH 2.5-10.2

22. Applications Potentials • pH Neutralization – acid/base addition • Disinfection, Sanitization, Biocides: • Chlorine feed (free or total) • Chlorine Dioxide feed (ClO2 and Chlorite) • Ozone feed • ORP • Fluoride – flow proportional only, no residual control • Conductivity – boiler feed • Others – combinations of the above.