CHLORINE DIOXIDE PROCESS

The Basics

Tom PoweSr. Technology

Services Engineer

CHLORINE DIOXIDE PROCESSThe Basics

• ClO2 Properties/Safety

• General Introduction to ClO2 Processes

• Vacuum Processes

• Control Strategy

CHLORINE DIOXIDE PHYSICAL PROPERTIES

• General – Formula ClO2

– Molecular Weight 67.45

• Appearance

– Gaseous Greenish Yellow– Aqueous Solution Pale Yellow to Deep Green

(Color proportional to strength)

CHLORINE DIOXIDE PHYSICAL PROPERTIES

• Exposure Limits– TLV 0.1 ppm per 8 hr shift– Exposure 5 ppm

• Odor– Detectable 1 - 2 ppm – Noticeable 2 - 5 ppm– Irritating 5 - 10 ppm – Dangerous 10 - 18 ppm – Fatal 19 ppm

CHLORINE DIOXIDE SOLUBILITY in Water

0

5

10

15

20

0 20 40 60 80 100 120 140

ClO 2 gas Partial Pressure [mmHg]

ClO

2 So

ln C

once

ntra

tion

[gpl

]

0 5 10 15

Temperature °C

DECOMPOSITIONS

• Initiators– Heavy Metals– Rust– Organic Materials– Sunlight – High Temperature– High

Concentration – Electrical Sparks

Reaction

2 ClO2 ⇒ Cl2 + 2 O2 + Heat

ClO2 DECOMPOSITIONSEVERITY

• > ClO2 gas concentration, > decomposition.

• > initial operating pressure, > decomposition.

• > the volume of gas, > decomposition.

CHLORINE DIOXIDE PRODUCTION

ClO3- + H+ + Reducing Agent ⇒ ClO2 + H2O

Chlorate Supply • Any Alkali Metal Chlorate Will Work

– (NaClO3, KClO3, etc) • Industry Practice uses NaClO3

– 99.9+% of All ClO2 Processes use NaClO3

CHEMISTRY FUNDAMENTALS

REDUCING AGENT

Choice of Reducing Agent Determines the By-

Product GasR Agent Gas

• CH3OH CH2O2

• H2O2 O2

ACID

Acid Saltcake• H2SO4 Na2SO4

PROCESS TYPES

• ATMOSPHERIC

• VACUUM

• OTHER

VACUUM PROCESS

• React the Chlorate, Acid & Reducing Agent in a Vacuum Crystallizer

• The Generator/Crystallizer operates in the range from 150 - 250 mmHg absolute.

• Acidity Varies by Process Chemistry

• The By-Product Salt is Discharged as a Dry Crystal

WHY A VACUUM PROCESS?

• Advantages

– Sulphate By-Product may be used by the Mill

– Very Efficient NaClO3 use

– Safer to operateLess violent decomposition

• Disadvantages

– Higher Capital Cost Versus Atmospheric Systems

– More complex operation

VACUUM PROCESS FLOW DIAGRAM

ClO2 GeneratorClO2 Generator

Heat AdditionHeat Addition

Gas CondensingGas Condensing

ClO2 AbsorptionClO2 AbsorptionClO2 StorageClO2 Storage Vacuum SystemVacuum System

Vent ScrubbingVent Scrubbing Chilled Water

Saltcake FiltrationSaltcake

Filtration

Saltcake DissolvingSaltcake

Dissolving

Gas CondensingGas CondensingCooling WaterClO2 ProductCooling Water

HP Steam or vacuum pump

Cooling WaterCausticCooling Water Water

Sulfuric Acid

Sodium Chlorate

Reducing Agent

LP Steam Saltcake Solution To MillAirWaterCondensate

VACUUM PROCESS FLOW DIAGRAMHP Steam

Reboiler

Vent

Filter Receiver

Filter Ejector System EjectorVentFan

GeneratorCirc. Pump

LP Steam

CondensatePump

CondensateTank

Generator

GeneratorCondenser

Absorber

To BleachPlant

AbsorberPump

ClO2Storage

HotWater

SaltcakeFilter

Air

VentScrubber

Misc.Vents

ScrubberPump

Chilled Water

Chilled Water

SystemCondenser

NaClO3Feed

SaltcakeFilter Pump

SaltcakeSol. Pump

DissolvingTank

AcidFeed

Deluge/SprayQuench

Reducing Agent Feed

Cooling Water

Saltcake Soln.

Cooling Water

Condensate

VACUUM PROCESS FLOW DIAGRAMwith Saltcake Washing

Vent

ClO2 GeneratorClO2 Generator

Heat AdditionHeat Addition

Gas CondensingGas Condensing

ClO2 AbsorptionClO2 AbsorptionClO2 StorageClO2 Storage Vacuum SystemVacuum System

Vent ScrubbingVent Scrubbing Chilled Water

Saltcake Filtration No.1Saltcake

Filtration No.1Saltcake

RecrystallizerSaltcake Recrystallizer

Water

Saltcake Solution To Mill

Saltcake Filtration No.2Saltcake

Filtration No.2Filtrate StorageFiltrate Storage

Saltcake DissolvingSaltcake

Dissolving

AirWater

Gas CondensingGas CondensingCooling WaterClO2 ProductCooling Water

HP Steam

Cooling WaterCooling Water Air

Sulfuric Acid

CausticSodium Chlorate

Methanol

LP SteamCondensate

YIELD

95

96

97

98

99

100

Yie

ld (%

)

Lite

/R-8

SC

W/R

-10

HP

Yield Components

• Feeds– Chlorate Feed

• Losses – Chlorate for side reactions– Chlorate in Saltcake– Chlorate lost as ClO2 in the

vents• Calculation

Yield = (Feeds-Losses)/Feeds

SALTCAKE FILTRATION REQUIREMENTS

0

0.5

1

1.5

2

2.5

3

T Sa

ltcak

e/T

ClO

2

Lite SCW HP

PROCESS TRADE NAMES

Reducing Agent Acid Acidity Salt Form Name By

NaCl H2SO4 Low Crystal SVP-Conventional EkaH2SO4 Low Crystal R-3 Process SterlingH2SO4 High Solution R-2 Process Hooker

CH3OH H2SO4 Moderate Crystal SVP-Lite EkaH2SO4 Moderate Crystal SVP-SCW EkaH2SO4 High Crystal R-8 Process SterlingH2SO4 High Solution Solvay SolvayH2SO4 High Crystal R-10 Sterling

H2O2 H2SO4 Low Crystal SVP-HP EkaH2SO4 High Solution HP-A Eka

SO2 H2SO4 High Solution Mathieson Olin-Mathieson

PROCESS COMPARISONS - Chemicals

ITEM MethanolBased

MethanolBased withWashing

PeroxideBased

Chlorate,Ton/TonClO2

1.64 1.64 1.64

ReducingAgent,Ton/TonClO2

0.16 0.16 0.33

Sulfuric Acid,Ton/TonClO2

1.0 0.75 0.75

Salt Cake,Ton/TonClO2

1.44 1.06 1.06

PROCESS COMPARISONS - Utilities

ITEM MethanolBased

MethanolBased withWashing

PeroxideBased

LP Steam,Ton/Ton ClO2

4.3 5.1 4.6

HP Steam,Ton/Ton ClO2

0.92 1.26 1.76

Chilled Water,Ton/Ton ClO2

91.0 89.1 91.2

Cooling &Hot Water,Ton/Ton ClO2

298 341 292

KEY PROCESS CONTROL VARIABLES

• Generator Level

• Generator Chemistry

• Generator Solids Level

• Chemical Feed Flows

• Generator Pressure

• Generator Temperature

• ClO2 Solution Concentration

Do They correlate?

RULE OF 10 GUIDELINES

SODIUM CHLORATE SULFURIC ACIDGPL MOLARITY GPL NORMALITY100 0.94 250 5.10110 1.03 260 5.30120 1.13 270 5.51130 1.22 280 5.71140 1.32 290 5.91150 1.41 300 6.12160 1.50 310 6.32170 1.60 320 6.53180 1.69 330 6.73190 1.79 340 6.93200 1.88 350 7.14210 1.97 360 7.34220 2.07 370 7.54230 2.16 380 7.75240 2.25 390 7.95250 2.35 400 8.16260 2.44 410 8.36270 2.54 420 8.56280 2.63 430 8.77290 2.72 440 8.97300 2.82 450 9.18

TARGET: 10.0 RANGE: 9.0 - 10.5

To calculate the "Rule of 10" number: Find the chlorate molarity and the acid normality on the above chart that corresponds to the test performed on the generator solution. Add these two numbers together. Make the appropriate chemical feed flow adjustment

GENERATOR LIQUOR TARGET ZONE

“Rule of 10”

01234567

5 6 7 8 9 10 11Acidity (N)

Chl

orat

e (M

)

A + C = 9 A + C = 10 A + C = 11

White-Out Zone

Sluggish Reaction

GENERATOR LEVEL CONTROL

• Maintaining a constant level in the generator is critical for controlling generator concentrations.

• If an excess of water is boiled away in the generator, the level will decrease and generator concentrations will increase. An inverse effect occurs when not enough water is boiled away.

• Also small adjustments, generally 500#/hr or less, of low-pressure steam to the reboiler should be made to minimize level swings.

PROCESS ADJUSTMENTS

• When making adjustments to the generator operation keep in mind that the generator holds a large inventory of chemicals.

• To avoid overshooting your target adjustments should be made slowly.

• Change the steam flow at the same time you change the feed flows.

• The changes in production rate must be done slowly.

SUMMARY

• The chlorine dioxide manufacturing process has evolved from salt-based atmospheric systems to the methanol and peroxide based vacuum systems that dominate the industry today.

• The operation of a chlorine dioxide vacuum process is mill specific.

• The key to smooth operation of a chlorine dioxide vacuum process is generator level control.

Questions?