18.3 Pulp Bleaching,
ClO2 Technology & Related
Challenges
J. Barry Billett, Nouryon
TAPPI PEERS November 3, 2020
Session18: Effective Bleaching
Strategies

eka.nouryon.com

TAPPI PEERS November 3, 2020
Session18: Effective Bleaching Strategies

Pulp Bleaching, ClO2 Technology and Related Challenges
Preface
In the highly competitive global Pulp and Paper industry the safe, reliable and efficient generation, storage
and application of chlorine dioxide (ClO2) is one of several processes critical to sustainable management of
bleached pulp production.
Introduction
Chlorine dioxide (ClO2) is a strong and selective gas phase oxidizer discovered in the early 1800’s andfirst used as a bleaching agent over a century later in the textile industry during the 1920’s. [26] Early
applications for pulp bleaching dates back to the 1940’s. Due to ClO2’s oxidizing performance
characteristics for producing brighter, stronger pulp, industry use continued to increase into the 1960’s
and 70’s. [1-5, 10, 30-46]
During the 1990’s local and federal environmental regulations further pushed the Canadian and US
industries towards ECF pulp production, 100% ClO2 substitution. Over the past century driven byenvironmental, quality, cost and competitive considerations ClO2 has emerged as the globally dominant
pulp bleaching agent. The benefits of ClO2 bleaching outweigh the associated challenges. [12, 30-46]
However, to remain competitive the challenges associated with ClO2 demand significant industry
investment and attention, particularly within aging North American (NA) facilities.
In pulp mills ClO2 must be produced on site at the point of application which introduces a host of
considerations. Safety is paramount within any modern industry and Under the US Code of Federal
Regulations (29 CFR 1910.119), ClO2 is designated as a highly hazardous chemical, thus demanding siteand organizational focus on Process Safety Management (PSM6) programs. [6, 8, 9, 13, 62] Furthermore,production of ClO2 also represents the highest annual chemical cost within operating bleach plants.
Ongoing Industry Challenges Specific to ClO2:






Regulatory compliance: Safety, environmental stewardship and process reliability
Feed chemical acquisition, storage and delivery (NaClO3, Acid and Reducing Agent)
Utility requirements: chilled water, process waters, HP/ LP steam, power
Process technology: ClO2 plant, equipment, operations and area maintenanceClO2 solution storage, delivery and efficient bleach plant application

For Canadian and US facilities, successfully addressing these challenges remain prerequisites tosustainable bleached pulp production and global competitiveness.
ClO2 Background
ClO2 is always generated as a gas and requires the reduction of sodium chlorate (NaClO3) or sodium
chlorite (NaClO2) in the presence of a proton donor. All ClO2 generating processes, regardless of scale,involve reactions with sodium chlorate. Global production of sodium chlorate is estimated at over 3
million metric tons annually with a flat to modest growth projection. Over 90% of this total global volume
is consumed by pulp bleaching for ClO2 production. The balance is used in manufacture ofchlorates/perchlorates, intermediates in reaction synthesis, fireworks, solid rocket fuel, oxygen candles,
agriculture, mining and sodium chlorite production for small scale ClO2 applications. [11, 47-64]
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Annual global production of ClO2 for pulp bleaching is estimated at 1.65 to 1.7 million metric tons
requiring approximately 2.8 million metric tons of sodium chlorate. Thus, bleached pulp consumes
over 98% of all ClO2 produced on the planet. However, like other oxidizers, ClO2 is also an excellent
biocidal agent, enjoying expanding applications as a primary water treatment disinfectant and broadspectrum micro-biocide. In fact, ClO2 increasingly may be recognized as the “Ideal Biocide” withexpanding and valuable use patterns within food preservation, processing, health care, and industrial
sectors applied directly as a gas or in solution. [26, 47-61, 64]
ClO2 Utilization
Within the Pulp and Paper industry early production of ClO2 relied on atmospheric process such as
Mathieson, Hooker R-2, and Solvay which utilized sulfur dioxide (SO2), methanol (CH3OH) and salt
(NaCl) respectively as reaction reducing agents. [10] Most atmospheric processes operating around theworld today remain limited to less than 20 metric tons per day (mTPD) of ClO2 production and rely on
H2O2 as the reducing agent. A typical atmospheric process flow diagram is presented in Figure 3.
During the late 1960’s and early 1970’s proprietary R13 and SVP14 series vacuum technology was
introduced for ClO2 production. [8, 10] Figures 2 and 4 provide basic flow diagrams of a vacuum process.
Vacuum technology soon began displacing atmospheric processes, offering expanded production capacities,
higher product concentrations and lower operating costs with improved safety and environmental emissions
features. Further driven by 1998 promulgation of the US EPA5 technology based ELG12 regulations for thepulp, paper, and paperboard category (referred to as the “Cluster Rule”) and Canadian Environmental
Protection Act, ClO2 vacuum technology quickly became the dominant process applied in the industry. [13,62]
Currently over 80 vacuum plants are operating under evergreen technology license agreements in NA
producing more than 2,500 mTPD ClO2. All but one of these NA plants have been in operation for 20 to
50 years.
Over recent decades ClO2 has emerged globally as the most cost-effective and environmentally compatible
bleaching agent for producing the highest quality pulp. [1-5, 30-46] Since the mid-1990’s design
improvements to the ClO2 vacuum process have been ongoing in the areas of safety, control and operating
efficiencies. However, the fundamental process equipment, materials of construction and reaction
chemistry implemented in NA have remained the dominant and “Best Available Technique” for ClO2
production within the global bleached pulp sector. In the last decade numerous ClO2 vacuum plants withcapacities ranging from 45 to 100 mTPD have either been commissioned or are currently in various stagesof construction in South America, Europe and Asia.
Reliable availability, efficient chemical conversion and safe operational practices specific to ClO2
production, storage and application, more than ever, are key enablers to achieving a sustainable competitive
advantage in the global bleached pulp market.
ClO2 Vacuum Technology
By far the dominant chemistry for ClO2 production involves the reduction of sodium chlorate [NaClO3]
with methanol [CH3OH] in a strong sulfuric acid [H2SO4] aqueous solution. This chemistry generates ClO2,
byproduct sodium sesquisulfate [Na3H(SO4)2] (saltcake) and formic acid [HCOOH] per the following
reaction:
12 NaClO3 + 8 H2SO4 + 3 CH3OH → 12 ClO2 + 3 HCOOH + 9 H2O + 4 Na3H(SO4)2

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ClO2 Reactor/ Generator
The reaction components are fed to a classic “Crystallizer” loop that incorporates a cone bottom vessel,
axial flow pump and a shell and tube steam heat exchanger. [14] This classic design has been specifically
adapted to continuously generate ClO2 gas under vacuum (110 to 150 mmHg absolute) at boilingtemperatures (150 to 175° F) while crystallizing and removing saltcake. [14] The loop serves as a CSTR1
yet functions as PFR2 simulating a “Batch Reactor” thus simultaneously facilitating the ClO2 gas
producing reaction, vaporization of excess water (H2O), and the crystallization and subsequent removal of
by-product Na3H(SO4)2. [14, 17] The reaction liquor or “generator liquor” in this loop, at any given time,
is a boiling transient slurry of product ClO2 gas, reactants NaClO3, CH3OH, and H2SO4, by-products H2O,
Na3H(SO4)2 and HCOOH. The composition of this generator liquor is an important process control
variable for the system. Routine manual liquor samples (recommended every 2 hours) are required for
titrimetric determination of H2SO4 and NaClO3 concentrations and visual determination of solid salt cakecontent. These data are critical to operational feedback control of feed chemical flow rates and optimalbatch reaction simulation. Generator liquor is an extremely corrosive, erosive and abrasive, near boiling,solution which emits ClO2 gas when removed from the process. As a result, personnel must exercise
extreme caution and use appropriate personal protective equipment (PPE26) when conducting routine
sample collection and analysis. Chemistry in the reaction loop, for several reasons, is critical toestablishment of a “steady-state” reaction; typical generator liquor concentration control parameters are:
 NaClO3

200gpl ± 20gpl

 H2SO4

400gpl ± 20gpl

 Na3H(SO4)2

25 to 30 %ASV

Titanium, grade 2 and/ or grade 7, is the exclusive material of construction used in all areas of process that
contact generator liquor and through the gas phase area of the Generator to the exit of the gas cooler/
generator condenser. Given generator liquor composition, cost and availability, titanium remains the mostpractical material of specification for this area of the ClO2 vacuum process.
However, operational caution is required because titanium is quite incompatible with strong acids. [22,23] Sufficient NaClO3 solution must first be added to the reaction loop to protect the titanium from acidattack prior to addition of H2SO4 required for ClO2 generation. Sodium chlorate is also a very strongoxidizer and once introduced to the reaction loop it forms an oxidative surface layer thus passivating andprotecting the titanium alloy from subsequent acid corrosion. Further precautions regarding H2SO4 arealways required to prevent loss of containment which may result in personnel injury and/ or extensive
equipment damage.
ClO2 Absorber and Scrubbers
Once the product ClO2 gas exits the generator condenser this high velocity stream enters the lower portionof the absorber. The vessel is a traditional mass transfer, counter-current, packed column that employs
chilled water as the gas absorption medium flowing from top to bottom. [14, 18] The ClO2 gas flows
upward as the chilled water flows downward thus forming the ClO2 solution, which is then transferredfrom the tower discharge to product storage(s) for subsequent bleach sequence application. The process
vacuum is pulled from the top of the absorber and through the entire gas system back to the Generator.
The vacuum motive source is created by either a simple titanium steam eductor/ venturi or a liquid ringtitanium vacuum pump. [14]Page 3 of 15

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Process gases exiting the ejector or vacuum pump are then directed through a secondary cooler/ condenser
which discharges to a chilled water Vent Scrubber. Any weak ClO2 solution captured by the Vent Scrubber
is then directed back to the top of the absorber and becomes part of product solution to storage. The
discharge from the Vent Scrubber is typically directed to either a dedicated or a general bleach plantchemical scrubber for ClO2 destruction. One or more centrifugal vent fans typically provide the motive
force necessary to pull the process gases and local vent emissions through both the water scrubber andchemical scrubber.
By design the process takes full advantage of ClO2’s physical properties and extreme hydrophilic nature.
ClO2 gas is unstable; as concentration increases stability decreases. ClO2 will violently decompose/
explode when the gas concentration nears or reaches approximately 10%, by volume, under STP3
conditions. [7, 8, 9, 13, 24] However, once dissolved in aqueous solution ClO2 gas becomes relatively
stable at STP3 conditions. Furthermore, ClO2’s solubility characteristics are inverse; as solution
temperature decreases, ClO2 solubility increases (see Figure 1). Therefore, chilled water is the ideal
absorption, transport, storage and application medium for the extremely high ClO2 concentrations
required for efficient pulp bleaching (10 to 12 gpl).
The material of construction for the absorber and all downstream columns is typically hand laid-up fiber
reinforced plastic (FRP) with a specified chemical corrosion barrier. Internal trays (distributors, packing
supports) are typically made of grade 2 titanium. Packing may be either ceramic or plastic, depending on
specific conditions of service. Tower packing type, size and materials are specified based upon packed
bed volume relative to anticipated ClO2 demand and specific mass transfer parameters. [14, 19, 21, 22]
Figure 1. ClO2 Temperature Solubility:

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Sesquisulphate Filter
Stoichiometrically the reaction in the generator loop produces approximately 1.3 units of sesquisulfate[Na3H(SO4)2] (saltcake) for every unit of ClO2 generated. This by-product must then be crystallized andcontinuously removed from the generator liquor (slurry) in proportion to the ClO2 produced. Maintaining astable and predictable percentage of this saltcake in the generator liquor is a critical control variable androutinely measured as apparent settled volume (%ASV). Panel operators can also infer relative %ASV bymonitoring field instrumentation reporting generator liquor specific gravity and circulation pump motor
amps. Obviously, a rise in both remote parameters are indicative of a rise in %ASV.
Focal variables for maintaining consistent saltcake volume in the reaction slurry (25 to 30 % ASV) are
generator liquor level, ClO2 production rate and efficient filter operation. Consistent %ASV in the slurry isimportant to facilitating a stable “steady-state” reaction and efficient ClO2 production. However, continuous
elevated operation or excursions in %ASV (>40 to 60+%) will result in operational upsets, blockage inslurry transfer lines between the reaction loop and filter, and/ or titanium equipment damage internal to thereaction loop. Failure to control solids in the process will produce extremely negative and undesirable
results.
By design the filter system involves pumping a slip-stream slurry from the reaction loop to the surface of arotating drum with an internal vacuum. The system functions much like a common rotating drum pulp
washer. Yet, in functional contrast, the salt cake filter captures, washes and discards solid saltcake from thedrum surface while recovering the valuable chemical effluent pulled into the drum interior and returning itto the reaction loop. Therefore, efficient system operation, cake removal and chemical recovery is vital tosustainable reaction conversion efficiencies in the process. Once feed chemicals enter the reaction loop,
there are only five ways of possible escape, four of which are extremely undesirable:






Loss of containment
System decomposition
Generator liquor carry-over to the gas absorption train
Reaction conversion to ClO2 gas
Inefficient filter operation/ washing and chemical loss with discarded saltcake

The typical saltcake removal system incorporates a rotary drum vacuum filter, motive vacuum source, slip
stream transfer lines between the reaction loop and filter, a static gas liquid separator and, ultimately
continuous saltcake discharge. [14, 17-21] Material composition of system components contacting thegenerator liquor and associated gases are minimally grade 2 titanium.
Efficient filter system operation requires routine manual inspection and local adjustment of fluid transfer,
drum vacuum, cake pick-up and washing parameters by operators. Safely conducting such routine activities
demands knowledgeable and experienced attention including, but not limited to, use of appropriate PPE26
and functional verification of all system incorporated safety features. Sesquisulphate solution is a hot (150
to 170 °F) highly corrosive and abrasive slurry having a dry cake acid composition between 20 and 30% by
weight. Therefore, saltcake solution, like generator liquor, when contacted has the potential to result inchemical and/ or thermal dermal degradation.
Most vacuum plants placed into service over the past decade or those currently under construction further
incorporate a secondary sodium sulfate (Na2SO4) washing and filtration system to enhance overall process
efficiency, figure 5. Such secondary systems use the solubility characteristics of the H2SO4 / Na2SO4 / H2O
system to separate H2SO4 from the discharged sesquisulphate stream in a metathesis tank while recrystallizing a neutral Na2SO4 saltcake from this weak acid slurry. [14, 17-21] The neutral saltcake is thenremoved from this slurry by a second rotary drum vacuum filter for sodium/ sulfur makeup in the recovery
cycle. System advantages include, recovery of weak acid for pH control and/ or supplemental generator
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acid feed, reduction in caustic (NaOH) use associated with sesquisulphate neutralization and improved
NaClO3 recovery and ultimately optimum chemical conversion efficiency in the ClO2 reaction loop.
Stoichiometric representation of a secondary sodium sulfate filter system:
2Na3H(SO4)2 → H2SO4 + 3Na2SO4

Overall process stoichiometry, with secondary filter washing:
12 NaClO3 + 6 H2SO4 + 3 CH3OH → 12 ClO2 + 3 HCOOH + 6 Na2SO4 + 9 H2O

Chemicals and Safety:
When working with chemicals, particularly oxidizers, the risk to personal safety increases significantly as
concentrations and volumes increase. All bleach plant personnel should remain fully aware of thehazardous risks associated with all the ClO2 process feed, product and intermediate chemicals. Personalsafety begins with sound knowledge of all chemical SDS’s10, specifically as they may relate to exposure,reactivity, chemical compatibility, physical properties, temperatures, pressures, volumes, condition ofstate (gas, liquid and/or solid) and situational process circumstances. Hydrogen peroxide (H2O2) and
chlorine dioxide (ClO2) are excellent oxidizers used in various stages of pulp bleaching. [4, 5, 30-46]
However, both are stored and applied at concentrations and volumes that pose risk to health and safety.
A simple comparison between pulp bleaching and broad-spectrum disinfection applications for both H2O2
and ClO2 serves to illustrate the significance of concentration to personal exposure, health and safety. In
pulp bleaching H2O2 solution is typically received and stored at 50% concentration in volumes exceeding
10 metric tons thus avoiding threshold limit values (TLV’s) which would define it as a highly hazardous
chemical under 29 CFR 1910.119. While ClO2 is produced on site in concentrations of 8 to 12 grams perliter (gpl), or 8 to 12 thousand parts per million (ppm) and stored in volumes typically exceeding 20
metric tons as a dissolved gas in solution. Obviously, given the constraints associated with ClO2 for pulp
bleaching use classification as a “Highly Hazardous Chemical” is unavoidable within the industry. [3, 13]
In contrast, as a disinfectant/ anti-microbial H2O2 is sold commercially for topical application in one-liter
volumes or less at 3% concentration. While per EPA5 guidelines, ClO2 volumes applied for disinfection of
potable water range from hundreds to thousands of pounds per day at concentrations up to 0.8 ppm or
0.0008 gpl. [12, 64] Depending on concentration, volume, temperature, and pressure all chemicals stored
and used in a pulp mill present a risk to health and personal safety.

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Figure 2. Simple ClO2 Vacuum Process Block Flow Diagram

Salt
Filter

Process
VacuumFilterVacuumVent
Fan
Chilled
Water

Reaction
Loop

Absorber

Scrubbing
Media

Vent
Scrubber

Process
Vents

ChemicalScrubberFeed
Chemicals

ClO2
Storage

BleachPlant

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Figure 3. Typical Atmospheric Process Diagram
Vent gas

Chilled water

H2O2
ClO2 gas

NaClO3

Absorption
tower

Ventilation

H2SO4

Primary
reactor

Secondaryreactor

StripperAir

Spent acid

ClO2 water
to storage

Figure 4. Typical Vacuum Process Diagram

Figure 5. Typical Vacuum Process with Secondary Washing Diagram

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Management of Highly Hazardous Chemicals
Based on TLV’s11 the two bleach plant chemicals designated under PSM6 as “Highly Hazardous”
are methanol (extremely flammable, potentially explosive, toxic) and chlorine dioxide gas (toxic,
potentially explosive). However, given the scale of operations, all chemicals and intermediates
applied in a pulp mill and ClO2 process are potentially hazardous to health and safety, including
H2O, depending on state, temperature and/ or pressure. [6, 13]
Health, safety and environmental considerations, including regulatory compliance, arecornerstones of industry and organizational sustainability. ClO2 is designated as a highly hazardous
chemical under PSM6 because it is stored in high concentrations and volumes. [6] ClO2 has thepotential to spontaneously decompose should gas concentrations reach approximately 10% by
volume. Therefore, great attention is lent to designing continuous negative pressure vent sweeps
across storages, which pull fugitive ClO2 gas off the tanks and into the Vent Scrubber. Furthermore,
all ClO2 storage tanks are required to have appropriate positive pressure relief vents in the eventof sweep air failure. Design of such storage relief venting was historically based on a peak pressureof 6 psig at approximately 10% ClO2 gas by volume with a relief area/volume ratio of 0.02. [8]However, more recent models for relief lid sizing have been developed based on vented
deflagrations. [24] Experimental results have indicated that the lower concentration limit for the
explosive decomposition of chlorine dioxide gas is 9.5% ([ClO2]/[air]), whereas there is nocorresponding upper concentration limit. [7, 8, 24, 63]
Site specific PSM6 programs must incorporate the following fourteen elements:
1. Employee Participation
8. Mechanical Integrity Program
2. Process Safety Information
9. Hot Work Permits
3. Process Hazard Analysis
10. Management of Change (MOC8)
4. Operating Procedures and Protocols
11. Incident “Root Cause” Investigations
5. Operator Training
12. Emergency Planning and Response
6. Contractor Training
13. Compliance Audits
7. Pre-Startup Safety Review
14. Protection of Process Trade Secrets
Given the potential for catastrophic release of ClO2 gas in the unlikely event of storage tank failure
there may be further PSM6 requirements based on population densities surrounding the mill site.
Examples include RMP9 and coordination with local first responder organizations and site
ERT21.[63] In addition to explosion potential ClO2 gas is also toxic to the human respiratory
system. Either acute exposure to elevated levels or extended chronic exposure to low levels canresult in mild to extreme mucosal irritation and ultimately cause chemical pneumonia. Therefore,
extreme caution and appropriate PPE26 are advised when entering any PSM6 designated or processarea that may have ClO2 gas present.
Emergency Response Planning Guidelines (ERPG) published by the American Industrial Hygiene
Association provides the basis for established human exposure limits to ClO2 gas. The ERPG
findings for chlorine dioxide are based on collective acute and subacute lethality data in animals.
NIOSH18 REL-STEL15 (Recommended Exposure Limits - Short Term Exposure Limit) (NIOSH18
1997) is defined analogous to the ACGIH19 TLV11-TWA17. and IDLH16. The NIOSH18/ OSHA4“Standard Completions Program” only defines exposures for respirator selection representing amaximum concentration from which, one could escape exposure without experiencing any escapeimpairing or irreversible health effects. [6, 9, 28, 29, 63]
Established Human ClO2 Gas Exposure Limits
NIOSH18 IDLH16 5 ppm - exposure
NIOSH18 STEL15 0.3 ppm - 15 minutes, 3 times per 8-hr day, with 20-minute minimum interval
between exposures
OSHA4 PEL-TWA17 0.1 ppm - 8 hr. exposure
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Further safety precautions routinely include installation of gas detection monitors on each levelof the ClO2 plant, and at strategic locations in the process storage areas and throughout thebleach plant. Depending on site specific safety protocols these monitors are usually set to alarm
audibly at either 0.1 or 0.3 ppm ClO2. Some mill sites also require use of personal gas monitors.
Fortunately, ClO2 gas is self-alerting presenting a faint odor of bleach or chlorine at 0.1 ppm
levels or less, thus necessitating area evacuation or use of respiratory protection. Based on ERPG
data, such low-level exposure to ClO2 gas is not life-threatening. In fact, more recent studies
indicate that ClO2 gas at low levels, well below the TWA17 of 0.1 ppm, could be useful as anantimicrobial in the presence of humans to mitigate and/ or prevent viral infection and possibly
other related respiratory tract infections. The Journal of General Virology reported that
continuous exposure to 0.03 ppm ClO2 gas inactivated highly pathogenic aerosol H1N1 virus inexperiments using mice. Specifically, ClO2 gas could be used in places such as hospitals, clinics,
offices, hotels, schools and airport buildings without interrupting normal activities. [25]Control and Safety Interlocks
Distributive control systems (DCS) provide the interface between real-time process data andremote operational process control. In ClO2 vacuum process areas a typical DCS incorporates over
100 control loops. Each loop may include local and/ or remote indication and remote operator
manipulation of flow valves, motors and switches. In total the DCS usually facilitates remote
operational control of 15 to 20 motor starters and 25 to 30 flow valves. [20]Safety interlock functions programmed into the DCS functional loops initiate automatedemergency shutdown of the ClO2 vacuum process. Typically, more than 20 interlock functions areprogramed into the DCS logic for purposes of protecting personnel, equipment and the environment
from release of and exposure to hazardous process chemicals.
Process safety does not rely solely on DCS interlocks to protect personnel, equipment and theenvironment. Ultimately, system safety rests with the process operators while interlock emphasis isfocused on preventing serious and costly incidents from occurring. Vacuum processes additionallyincorporate dozens of safety features ranging from in-line check valves, back-flow preventors andpressure relief devices to automated block flow valves. Within the US routine PHA7 review andfunctional testing of safety systems and DCS interlocks is required under PSM6 regulation and arehighly recommended activities for all operating ClO2 processes.
To further enhance efficient, reliable and safe production of ClO2 many plants have added
advanced process control (APC) systems to interface with their DCS. APC software is configured
to monitor manipulated process variables and issue feedback control variable set-points to the
DCS. APC systems focus on four key control variables: NaClO3, CH3OH and H2SO4 chemical feedflow rates and steam flow to the Reboiler. Optional control variables may include acid dilution waterflow rate, saltcake filter motor VSD20 and/ or saltcake filter feed pump VSD20. APC systems do notinterfere with DCS interlocks or safety systems and APC systems do not replace operators or theneed to conduct routine generator liquor analysis. However, they significantly improve thesimulation of a steady state “Batch Reaction” thus optimizing overall chemical conversion
efficiencies and process reliability. [20]MaintenancePredictable availability of ClO2 is critical to the production of bleached pulp. Consequently,
reliability centered maintenance programs, safety protocols, appropriate specifications, technology
enhancements, and ready access to spare process components are keys to reliable ClO2 production.
The materials used in the vacuum process are intentionally robust and specific for the intended
purpose providing extremely long service expectations. However, these facts do not preclude theneed for routine maintenance and internal inspection of plant and equipment. With proper caretitanium equipment can last a lifetime. However, loss of containment of either sesquisulphate
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and/or generator liquor presents a significant risk of corrosion and service life reduction. Externalsurfaces of titanium equipment do not have passivating sodium chlorate present, therefore acidic
chemicals will attack the alloy from the outside. Steel components such as heat exchanger shells
and structural beams are vulnerable to acid attack as well as ClO2 corrosion, which can result frominsufficient process ventilation. Implementation of a “zero leak policy” within the entire processarea is highly recommended to ensure maximum service life and ultimately reduce costly plantand equipment maintenance, repairs and/or replacements. FRP equipment is durable but does nothave an indefinite service life like titanium. Absorbers and scrubbers typically last 15-20 years
before needing replacement or major repair. Ceramic packed beds may need replacement every
5-10 years depending on quality of ceramic, chilled water quality, and operations. Motor-driven
equipment requires appropriate routine maintenance per OEM22 specification. PTFE24 gasket
material should be replaced with every flange break or every 8 to 10 years without flange break.
Well-planned preventative and reactive maintenance programs in coordination with routine andperiodic process inspection schedules are imperatives for sustaining reliable ClO2 production. [15]Troubleshooting
Given the instability of ClO2 gas, process upsets occasionally occur and typically result in
decompositions within the reaction loop. It is possible, yet quite rare, for decompositions tooccur in other areas of the process. The reaction loop is designed to contain and mitigate theimpact of decompositions. ClO2 decompositions are instantaneous, taking place in fractions of asecond to produce oxygen, chlorine and heat, resulting in the significant rise of generator gas
pressures and temperatures proportional to the reaction magnitude. In general, the root cause of
decompositions can be traced to mechanical, chemical or operational origins per the following
examples.
Mechanical:
 Vacuum leaks: intrusion of ambient air into the process between the reaction loop andvacuum source.
 Power failure: complete loss of process vacuum, resulting in an intense decomposition
 Instrumentation or transmitter failure: control valve sticking or broken stem, inaccurate
data transmitted to the DCS
Chemical: Organic or alloy (primarily iron) contamination of feed chemicals and/ or process waters
entering the reaction loop
 Excessive foam formation on the surface of the liquor in the generator, typically caused
by impurities in the chemical feeds and/ or waters entering the process
Operational:
 Low generator level resulting in circulation pump cavitation
 Exposure of the generator gas space to local UV25 light or heat sources Improper position of manual process valves Poor control of target chemical concentrations in the reaction loop
Successful troubleshooting and elimination of such process disruptions requires comprehensive
process understanding and knowledge of site-specific operations. Strict application of the KT23
system for solving such process upsets usually produces favorable results in the shortest timepossible. KT23 involves the identification and prioritization of causational changes in normal
process functions and then strict adherence to implementing one corrective action at a time. [16]In this sense KT23 methodology is like classic “factorial experimental design” involving
multivariable processes where; one and only one variable is manipulated while holding all othervariables constant. Analysis of subsequent data may then lead to conclusions regarding single
variable manipulation impact on the entire process. Review of DCS data historians, such as PI or
Parkview, can often significantly aid in the identification of causational process changes. The
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ClO2 vacuum process is a dynamic multivariable process, therefore identification and eliminationof problems require a patient, knowledgeable, systematic and disciplined approach.
Experienced and knowledgeable operations and maintenance personnel are critical assets tosuccessful ClO2 production. Over the past three decades pulp mill labor forces have aged alongwith plant and equipment. As these experienced operators retire local knowledge and experienceis lost. Less experienced operators replacing this generation would benefit from routine process,technology and safety education and training specific to ClO2 production.
Conclusions
For decades ClO2 vacuum processes have proven safe, reliable and predictable for pulp bleaching
applications. Thus, remaining the “best available global technique” for ClO2 production based onrather simple reaction chemistry, the molecule’s physical properties, and a robust multi-variable
process designed to control fundamental parameters of physics; pressure, temperature, volume,
concentration and fluid flow. [14-22, 30-46]
Commitment to “Reliability-centered Maintenance” including timely and appropriate investment
in plant and equipment in combination with the implementation of sound operational and chemicalsafety; education, training and procedural protocols are the keys to sustainable ClO2 production
and ultimately competitive advantage in bleached pulp market sectors. [15, 16]
Through evergreen license agreements NA industry retains access to ClO2 OEM22 process andsupport services for design, engineering, operation, troubleshooting, regulatory compliance,
education, training, plant maintenance and spare parts.
Acknowledgements
The author would like to thank Michael Burke, Doug Reid, Andrés Quintero, Steve Main, Nouryon
Pulp and Performance Chemicals LLC, Mark Minday and Dave Massey, retired. for their expertiseand technical contributions in the preparation of this paper.
Definition of Terms:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.

CSTR (Continuous Stirred-Tank Reactor)
PFR (Plug Flow Reactor)
STP (Standard Temperature and Pressure)
OSHA (US Occupational Safety and Health Administration)
EPA (US Environmental Protection Agency)
PSM (Process Safety Management)
PHA (Process Hazard Analysis)
MOC (Management of Change)
RMP (Risk Management Plan)
SDS (Safety Data Sheet)
TLV (Threshold Limit Value)
ELG (Effluent Limitations Guidelines)
R (Rapson)
SVP (Single Vessel Process)
STEL (Short Term Exposure Limit)
IDLH (Immediately Dangerous to Life and Health)
TWA (Time Weighted Average)
NIOSH (US National Institute for Occupational Safety and Health)
ACGIH (American Conference of Governmental Industrial Hygienists)
VSD (Variable Speed Drive)
ERT (Emergency Response Team)OEM (Original Equipment Manufacturer)
KT (Kepner Tregoe)
PTFE (polytetrafluoroethylene, trade name Teflon)
UV (Ultraviolet)
PPE (Personal Protective Equipment)

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TAPPI PEERS November 3, 2020
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References Cited:
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TAPPI PEERS November 3, 2020
Session18: Effective Bleaching Strategies

35. Cote, D. Chemical Cost Reduction on 50F Bleaching Plant. 2017, Feb. Annual Conference of the Pulp andPaper Industry in Canada, Paper Week Canada.
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41. Jour, P., Lindgren, K. et. al., Decreased Water Usage in a Softwood ECF Bleaching Sequence – Full Mill
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42. McDonough, T.J, Brightness Development in the Final ClO2 Stage of an ECF Kraft Pulp Bleaching
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44. Reid, D., pH in the Bleach Plant – Why it’s Important and What to do About It. Part 1 – ClO2 Stages. 2018.
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45. Sezgi, U., Resende, J. et. al., Effects of D0–Stage Temperature, pH, and KAPPA Factor on Chlorine Dioxide
Decomposition and E0–(EP)-D1 Bleaching Performance for Eucalypt Pulps. 2016, April. TAPPI Journal,
Vol. 15 No. 4, pp 285-295.
46. Han, Y., Sherman, D.M., Linton, R.H., Nielsen, S.S. and Nelson, P.E., The Effects of Washing and Chlorine
Dioxide Gas on Survival and Attachment of Escherichia Coli O157: H7 to Green Pepper Surfaces. 2000.
Food Microbiology, 17(5), pp.521-533.
47. Han, Y., Linton, R.H., Nielsen, S.S. and Nelson, P.E., Inactivation of Escherichia coli O157: H7 on SurfaceUninjured and Injured Green Pepper (Capsicum annuum L.) by Chlorine Dioxide Gas as Demonstrated by
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48. Du, J., Han, Y. and Linton, R.H., Efficacy of Chlorine Dioxide Gas in Reducing Escherichia coli O157: H7
on Apple Surfaces. 2003. Food Microbiology, 20(5), pp.583-591.
49. Lee, S.Y., Costello, M. and Kang, D.H., Efficacy of Chlorine Dioxide Gas as a Sanitizer of Lettuce Leaves.
2004. Journal of Food Protection®, 67(7), pp.1371-1376.
50. Han, Y., Selby, T.L., Schultze, K.K., Nelson, P.E. and Linton, R.H., Decontamination of Strawberries Using
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51. Yuk, H.G., Bartz, J.A. and Schneider, K.R., Effectiveness of Individual or Combined Sanitizer Treatments for
Inactivating Salmonella Spp. on Smooth Surface, Stem Scar, and Wounds of Tomatoes. 2005. Journal of Food
Science, 70(9), pp.M409-M414.
52. Sy, K.V., McWatters, K.H. and Beuchat, L.R., Efficacy of Gaseous Chlorine Dioxide as a Sanitizer for Killing
Salmonella, Yeasts, and Molds on Blueberries, Strawberries, and Raspberries. 2005. Journal of Food
Protection®, 68(6), pp.1165-1175.
53. Lee, S.Y., Dancer, G.I., Chang, S.S., Rhee, M.S. and Kang, D.H., Efficacy of Chlorine Dioxide Gas Against
Alicyclobacillus acidoterrestris Spores on Apple Surfaces. 2006. International Journal of Food Microbiology,
108(3), pp.364-368.
54. Popa, I., Hanson, E.J., Todd, E.C., Schilder, A.C. and Ryser, E.T., Efficacy of Chlorine Dioxide Gas Sachets
for Enhancing the Microbiological Quality and Safety of Blueberries. 2007. Journal of Food Protection®,70(9), pp.2084-2088.
55. Mahmoud, B.S., Bhagat, A.R. and Linton, R.H., Inactivation Kinetics of Inoculated Escherichia coli O157:
H7, Listeria monocytogenes and Salmonella enterica on Strawberries by Chlorine Dioxide Gas. 2007. Food
Microbiology, 24(7), pp.736-744.
56. Jin-Hua, D.U., Mao-Run, F.U., Miao-Miao, L.I. and Wei, X.I.A., Effects of Chlorine Dioxide Gas on
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57. Mahmoud, B.S.M., Vaidya, N.A., Corvalan, C.M. and Linton, R.H., Inactivation Kinetics of Inoculated
Escherichia coli O157: H7, Listeria monocytogenes and Salmonella Poona on Whole Cantaloupe by
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58. Bhagat, A., Mahmoud, B.S. and Linton, R.H., Inactivation of Salmonella enterica and Listeria
monocytogenes Inoculated on Hydroponic Tomatoes Using Chlorine Dioxide Gas. 2010. Foodborne
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59. Vaid, R., Linton, R.H. and Morgan, M.T., Comparison of Inactivation of Listeria monocytogenes Within a
Biofilm Matrix Using Chlorine Dioxide Gas, Aqueous Chlorine Dioxide and Sodium Hypochlorite
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TAPPI PEERS November 3, 2020
Session18: Effective Bleaching Strategies

60. Trinetta, V., Vaidya, N., Linton, R. and Morgan, M., A Comparative Study on the Effectiveness of Chlorine
Dioxide Gas, Ozone Gas and E-beam Irradiation Treatments for Inactivation of Pathogens Inoculated onto
Tomato, Cantaloupe and Lettuce Seeds. 2011. International Journal of Food Microbiology, 146(2), pp.203206.
61. Trinetta, V., Vaidya, N., Linton, R. and Morgan, M., Evaluation of Chlorine Dioxide Gas Residues on Selected
Food Produce. 2011. Journal of Food Science, 76(1), pp. T11-T15.
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64. US Environmental Protection Agency. Alternative Disinfectants and Oxidants Guidance Manual. 1999,
April. Office of Water (4607) EPA 851-R-99-014.

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