Experts in Chem-Feed and Water Treatment Chlorine Chemistry

Experts in Chem-Feed and Water Treatment Chlorine Source Chlorine Gas Sodium Hypochlorite Calcium Hypochlorite Initial Reaction Cl 2 + H 2 O -> HOCl + H + + Cl - NaOCl + H 2 O -> HOCl + Na + + OH - Ca(OCl) 2 + 2H 2 O -> 2HOCl + Ca ++ + (OH) = 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.

Experts in Chem-Feed and Water Treatment 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% pH Percent of Chlorine as Hypochlorous Acid (HOCl) Percent of Chlorine as Hypochlorite Ion (OCl - ) HOCl OCl - Chlorine Dissociation Curve

Experts in Chem-Feed and Water Treatment 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.) 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 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.

Experts in Chem-Feed and Water Treatment Monochloramine: NH 2 Cl Dichloramine: NHCl 2 Trichloramine: NCl 3 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. 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.

Experts in Chem-Feed and Water Treatment 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

Experts in Chem-Feed and Water Treatment 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

Experts in Chem-Feed and Water Treatment 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

Experts in Chem-Feed and Water Treatment Process Control

Experts in Chem-Feed and Water Treatment  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).

Experts in Chem-Feed and Water Treatment Types of Control  Modulating  Proportional  Proportional, Integral, Derivative (PID)  Compound Loop

Experts in Chem-Feed and Water Treatment 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

Experts in Chem-Feed and Water Treatment Example pH Neutralization Flow in at pH Flow out at pH 6.5 – 7.5 Note: Wide effluent pH “range” Desired set point 7.0

Experts in Chem-Feed and Water Treatment 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.

Experts in Chem-Feed and Water Treatment Example pH Neutralization Flow in at pH Flow out at pH 6.8 – 7.3 Note: Close effluent pH “range” Desired set point 7.0

Experts in Chem-Feed and Water Treatment 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.

Experts in Chem-Feed and Water Treatment Example pH Neutralization Flow in at pH Flow out at pH 6.9 – 7.1 Note: Closer effluent pH “range” Desired set point 7.0

Experts in Chem-Feed and Water Treatment 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.

Experts in Chem-Feed and Water Treatment Example pH Neutralization Flow in at pH Flow out at pH 6.95 – 7.05 Note: Small effluent pH “range” Desired set point 7.0

Experts in Chem-Feed and Water Treatment 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.

Experts in Chem-Feed and Water Treatment Example pH Neutralization Process flow varies & Flow in at pH Flow out at pH 6.95 – 7.05 Note: Small effluent pH “range” Desired set point 7.0

Experts in Chem-Feed and Water Treatment 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.