experts in chem-feed and water treatment chlorine chemistry

Experts in Chem-Feed and Water Treatment

Chlorine Chemistry


Chlorine Chemistry

Chlorine Source

Chlorine Gas

Sodium Hypochlorite

Calcium Hypochlorite

Initial Reaction

Cl2 + H2O -> HOCl + H+ + Cl-

NaOCl + H2O -> HOCl + Na+ + OH-

Ca(OCl)2 + 2H2O -> 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.


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5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

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pH

Per

cen

t o

f C

hlo

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e as

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och

loro

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d (

HO

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Per

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lori

te I

on

(O

Cl- )

HOCl

OCl-

Chlorine Dissociation Curve


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.


Monochloramine: NH2Cl Dichloramine: NHCl2

Trichloramine: NCl3

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.


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


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


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


Process Control


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).


Types of Control

Modulating Proportional Proportional, Integral, Derivative (PID) Compound Loop


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


ExamplepH Neutralization

Flow in at pH 9.8-10.2 Flow out at pH 6.5 – 7.5

Note: Wide effluent pH “range”

Desired set point 7.0


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.




Note: Close effluent pH “range”



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.




Note: Closer effluent pH “range”



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.




Note: Small effluent pH “range”



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.



Process flow varies &Flow in at pH 2.5-10.2

Flow out at pH 6.95 – 7.05

Note: Small effluent pH “range”



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.

experts in chem-feed and water treatment chlorine chemistry

Documents

total chlorine chlorine

ratio of chlorine

certain chlorine

quantity of chlorine

ph percent of chlorine

ppm of free chlorine

water ph

chlorine residual equipment