Best available ClO2
technologies for the pulp
industry
Andrés Quintero, Nouryon
Japan TAPPIMay 27, 2021

eka.nouryon.com

パルプ漂白における二酸化塩素発生最適化技術（仮題）Best available ClO2 technologies for the pulp industry

Andrés Quintero
Senior Process Engineer, Eka Engineering, Technology Solutions, Nouryon Pulp and
Performance Chemicals AB
Abstract
Eka Engineering is an engineering and contracting division within Nouryon Pulp and Performance Chemicals
that specializes in the design and supply of chlorine dioxide (ClO2) plants and equipment to the pulp andchemical industries.
Eka Engineering has developed a number of different process technologies, for safe and efficient ClO2
generation that makes the division a world-leading supplier of chlorine dioxide plants.
ClO2 is an oxidizer commercially used for bleaching pulp since the mid-1940s. ClO2 is the most cost-effective
bleaching agent of pulp, achieving high brightness while maintaining high strength.
ClO2 is produced on site by reducing sodium chlorate (NaClO3) in an acidic solution, either under vacuum(SVP processes) or atmospheric conditions (HP-A process). Different by-products are generated by varying
the type of reducing agent and acid.
In this paper we will focus on the HP-A, the SVP-LITE and the SVP-SCW processes which are the mostcommon ClO2 processes used nowadays.
Also, we will briefly discuss about ClO2 decompositions and other application for ClO2.

(Abstract in Japanese)1. Introduction
Eka Engineering is an engineering and contracting division within Nouryon Pulp and Performance Chemicals
that specializes in the design and supply of chlorine dioxide (ClO2) plants and equipment to the pulp andchemical industries.

Eka Engineering has designed, built, re-built, handled spare parts and operated

chlorine dioxide systems for pulp bleaching for over 40 years.
As an international leader in ClO2 technology, Eka Engineering can supply all the ClO2 processes to matchthe client requirements, from the design and procurement of the process equipment to a complete turnkey
installation or rebuild of existing plants.
Eka Engineering also offers service, support, spare parts and equipment for the large-scale production andapplication of both vacuum and atmospheric processes.

A list with the orders during the last years is shown below:
Year

Pulp millCapacity

Process

Note

2019

Bracell SP, Bracell mill, Brazil

2 x 60 ton ClO2/day

SVP-LITE/SCW

New plants in parallel (ongoing)

2019

Klabin, Ortigueira mill, Brazil

75 ton ClO2/day

SVP-SCW

Debottlenecking (ongoing)2019

Arauco, Horcones mill, Chile

51 ton ClO2/day

SVP-SCW

New plant (ongoing)

2018

Sappi, Stanger mill, South Africa

4 ton ClO2/day

HP-A

New plant

2017

Sappi, Ngodwana mill, South Africa

8 ton ClO2/day

HP-A

Capacity increase

2017

Sappi, Saiccor mill, South Africa

30 ton ClO2/day

HP-A

Conversion of Mathieson process

2016

SCA, Östrand mill, Sweden

26 ton ClO2/day

SVP-LITE

New plant

2016

Stora Enso, Skoghall mill, Sweden

15 ton ClO2/day

HP-A

New plant

2015

Fibria, Tres Lagoas mill, Brazil

60 ton ClO2/day

SVP-SCW

New plant

2015

Södra Cell, Värö mill, Sweden

35 ton ClO2/day

SVP-LITE

New plant

2015

Metsä Fibre, Äänekoski mill, Finland

65 ton ClO2/day

SVP-SCW

New plant

2. ClO2 generation process
ClO2 is an oxidizer commercially used for bleaching pulp since the mid-1940s and it can also be used toreduce NOx compounds in flue gas.
ClO2 is produced on site by reducing sodium chlorate (NaClO3) in an acidic solution, either under vacuum(SVP processes) or atmospheric conditions (HP-A process). Different by-products are produced by varying
the type of reducing agent and acid.
The most common ClO2 processes used nowadays are the HP-A, the SVP-LITE and the SVP-SCW.

2.1 HP-A Process
HP-A process
The HP-A process benefits are: (a) low investment, (b) ease of operation and (c) gives the possibility to
upgrade atmospheric installations (Mathieson process).
In the HP-A process, sodium chlorate (NaClO3) reacts with hydrogen peroxide (H2O2) in the presence ofsulphuric acid (H2SO4) to form chlorine dioxide (ClO2) and a spent acid stream containing sulphuric acid andsodium sulphate. Oxygen (O2) and water are generated as by-products of the reaction.
2 NaClO3 + H2O2 + H2SO4 → 2 ClO2 + Na2SO4 + O2 + 2 H2OTypical chemical feed consumption and products for the HP-A process are presented in the tables below:
Feed chemical

Expected consumption

Sodium chlorate

1,65 ton NaClO3 (100%) / ton ClO2

Sulphuric acid

2,20 ton H2SO4 (100%) / ton ClO2

Hydrogen peroxide

0,30 ton H2O2 (100%) / ton ClO2

Product / Main by-productExpected production

Chlorine dioxide solution

8 g ClO2/lSpent acid

5 ton spent acid / ton ClO2

The figure below shows a simplified HP-A process flow diagram:
To atmosphere

Chilled water

H2O2

Vent fan

ClO2 gas

NaClO3

Absorption
tower

Ventilation

H2SO4

Primary
reactor

Secondaryreactor

StripperAir

Spent acid

ClO2 water
to storage(1) Primary reactor
Sodium chlorate, sulphuric acid, and hydrogen peroxide are all fed into the primary reactor. Air nozzles
diffuse an air flow through the reactor solution. The reactor solution temperature is controlled via a coolingcoil and the reactor pressure is kept slightly negative. The ClO2 gas generated in this reactor is collected in agas header and sent to the absorption tower. Approximately 90-95% of the chlorine dioxide is produced in
the primary reactor. The reactor solution overflows by level difference into the secondary reactor.
(2) Secondary reactor
Hydrogen peroxide is fed into the secondary reactor to increase the overall chlorate yield in the process. Air
is introduced in the same manner as in the primary reactor. Temperature and pressure are likewise controlled.
The ClO2 gas generated in this reactor is also collected in the gas header to the absorption tower. The reactor
solution overflows by level difference into the Stripper.
(3) Air blower system
The purpose of the air blower is to (a) mix the reactor solution in both reactors, (b) strip the chlorine dioxide
out of the reactor solution and (c) dilute the ClO2 in the gas stream to the absorption tower for a safe operation.
(4) Stripper and spent acid tank
Spent acid overflows from secondary reactor into the stripper. Air from the blower flows counter-current tothe spent acid to strip out any residual chlorine dioxide. The off-gas from the stripper is combined with theoff-gas from the secondary reactor. The stripped spent acid flows by gravity to the spent acid tank.
In the spent acid tank additional ClO2 stripping takes place. ClO2 gas collected from this tank is directed tothe middle of the absorption tower through the ventilation system.
(5)Absorption tower
The purpose of the ClO2 absorption tower is to absorb chlorine dioxide gas into chilled water to form a ClO2
solution that can be safely stored and then used in the bleaching plant. The chlorine dioxide, oxygen, air,and water vapor collected in the gas header enters the absorption tower and flows up the column. Chilledwater is added to the top of the absorption tower and flows down the column. As the water and gas pass eachother, the ClO2 is absorbed into the water to form the ClO2 water. The ClO2 water is pumped to the storage
tanks.
(6)Ventilation
A vent fan creates a slight negative pressure improving the stability of the ClO2 gas in the process. The fan
provides ventilation to the ClO2 water storage tanks and the spent acid tank preventing ClO2 gas emissions
to the building. The vent gas from the process and tanks ventilation is then released to the atmosphere.

2.2 SVP-LITE process
The SVP-LITE process benefits are: (a) ease of operation, (b) minimal amount of by-product chlorine isformed and (c) the salt cake product is less than with atmospheric process.

In the SVP-LITE process, sodium chlorate (NaClO3) reacts with methanol (CH3OH) in sulphuric acidsolution (H2SO4) to produce chlorine dioxide (ClO2) and a sodium sesquisulphate solution (Na3H(SO4)2).
12 NaClO3 + 8 H2SO4 + 3 CH3OH → 12 ClO2 + 3 HCOOH + 4 Na3H(SO4)2 + 9 H2O
Typical chemical feed consumption and products for the SVP-LITE process are presented in the tables below:
Feed chemical

Expected consumption

Sodium chlorate

1,65 ton NaClO3 (100%) / ton ClO2

Sulphuric acid

1 ton H2SO4 (100%) / ton ClO2

Methanol

0,18 ton CH3OH (100%) / ton ClO2

Product / Main by-productExpected production

Chlorine dioxide solution

10 g ClO2/l

Sodium sesquisulphate

1,35 ton (100%) / ton ClO2

The figure below shows a simplified SVP-LITE process flow diagram:
Vacuum pump /EjectorVentilation

Chilledwater

Vacuum pump /Ejector
CWR

SesquisulphatefilterCW
SVP
generator

Gas
condenser

Absorption
tower

H2SO4
LP
steam

Chilledwater(weak ClO2)

Reboiler

Hot water
Cond.

NaClO3
CH3OH
Circulation
pump

Dissolvingtank

Sesquisulphate
solution

ClO2 water
to storage(1) SVP generator system
The heart of the process is an all-titanium generator, a vessel where the reaction takes place. The generator
is designed a) to provide the necessary volume to carry out the reaction of the feed chemicals to generate
chlorine dioxide, b) to evaporate the water present in the feed chemicals to maintain the level and c) to
crystallize the sodium sesquisulphate produced by the reaction. A titanium pump circulates the generator
solution through the reboiler. Low pressure steam is fed to the reboiler shell. The vacuum in the system iscreated by a steam ejector or a vacuum pump.
(2) ClO2 absorption systema) The chlorine dioxide, b) formic acid, and c) evaporated water along with d) unreacted methanol are drawn
into the gas condenser. In the gas condenser, the water vapor is condensed. The process gases are then drawn
into the absorption tower by the vacuum pump or steam ejector.
Chilled water is added to the top of the absorption tower and flows down through the packing. The ClO2 gasfrom the gas condenser enters at the bottom of the absorption tower. As the water and gas pass each other,
the ClO2, methanol, and formic acid present in the gas are absorbed into the water to form ClO2 water. The
ClO2 water is pumped to the storage tanks.
Part of the chilled water used for the absorption is first fed to the back-up scrubber (ventilation system) prior
to being used in the absorption tower.
(3) Sesquisulphate filter system
The generator solution is removed from the bottom of the generator circulation pipe and pumped by the filter
feed pump to the sesquisulphate filter.
The generator solution flows into the sesquisulphate filter vat and overflows (by gravity) back to the generator
circulation pipe. The sesquisulphate filter drum rotates through the vat full of generator solution while
vacuum is pulled inside the drum. This vacuum pulls solids onto the face of the drum and the drum rotation
carries the cake out of the vat. Wash water is sprayed onto the cake to remove the chemicals (sulphuric acidand sodium chlorate) as air is drawn through the cake, drying the solids. The combined mixture of air andliquid pulled through the cake drains into the center of the drum. This combined stream flows into thesesquisulphate filter separator where the gas (air and traces of ClO2) and liquid (filtrate: sodium chlorate andsulphuric acid solution) are separated: the gas is drawn out of the sesquisulphate filter separator by thevacuum system and the filtrate drains back to the generator circulation pipe.
After the cake has been washed and dried, it rotates past the blow-off shoe. The blow-off shoe, located inside
the filter drum, uses air pressure to blow the cake off the face of the drum and into the sesquisulphate filter
chute. The acidic salt cake falls down the chute into the dissolving tank where it is dissolved and pumped tothe pulp mill.

2.3 SVP-SCW process
The SVP-SCW process benefits are: (a) ease of operation, (b) minimal amount of by-product chlorine isformed, (c) the salt cake product is less than with atmospheric process and (d) sulphuric acid is recovered
and recycled to the generator.
The difference between the SVP-LITE and the SVP-SCW processes is that the acidic sesquisulphate solution
is separated in the metathesis tank to a weak acid solution and a neutral sodium sulphate by the following
reaction:

2Na3H(SO4)2 → H2SO4 + 3Na2SO4
The overall reaction of the SVP-SCW process is:
12 NaClO3 + 6 H2SO4 + 3 CH3OH → 12 ClO2 + 3 HCOOH + 6 Na2SO4 + 9 H2O
Typical chemical feed consumption and products for the SVP-SCW process are presented in the tables below:

Feed chemical

Expected consumption

Sodium chlorate

1,65 ton NaClO3 (100%) / ton ClO2

Sulphuric acid

0,80 ton H2SO4 (100%) / ton ClO2

Methanol

0,18 ton CH3OH (100%) / ton ClO2

Product / Main by-productExpected production

Chlorine dioxide solution

10 g ClO2/l

Sodium sulphate

1,10 ton (100%) / ton ClO2

The figure below shows a simplified SVP-SCW process flow diagram:
Vacuum pump /EjectorVentilation

Vacuum pump /Ventilation
Ejector

Chilledwater

Vacuum pump /Ejector
CWR

Salt cake filter
SesquisulphatefilterCW
SVP
generator

Gas
condenser

H2SO4
LP
steam

Reboiler

Absorption
tower

Hot water
Metathesis
tank

Cond.

NaClO3
CH3OH
Circulationpump

Chilledwater(weak ClO2)

Hot water

DissolvingtankSulphate
solution

ClO2 water
to storage

The metathesis tank and a second filter (salt cake filter) are added in the SVP-SCW process. The acidic
sesquisulphate solution is separated in the metathesis tank to form a slurry containing weak acid solution andneutral sodium sulphate. A pump feeds the slurry (from the metathesis tank) to the second filter where therecycled acid is separated from the neutral sodium sulphate. In the salt cake filter the solids (now neutral
sodium sulphate) are separated. The filtrate is the valuable weak acid that will be recycled into the generator
system. The salt cake filter has the same construction and operation principle as the sesquisulphate filter.
The neutral sodium sulphate crystals fall through the chute into the dissolving tank and is dissolved in hot

water and pumped to the pulp mill.

3. ClO2 decompositions
ClO2 decomposition occurs occasionally due to the natural instability of ClO2 gas. During decomposition,
the chlorine dioxide decomposes to form chlorine, oxygen and heat. The decomposition reaction is
spontaneous and results in the release of heat and pressure. An increase in generator gas temperature andpressure indicates a ClO2 decomposition.
Contamination caused by the raw chemicals, utilities or wrong materials in contact with generator solution
or ClO2 gas are a very common cause of decompositions. Bad analysis results, instrumentation problems andfoam are also among the more common causes of decompositions. The pictures below show examples offoam and bad quality sulphuric acid and sodium chlorate solution:

Picture 1

Picture 2

Picture 3

Picture 4

Picture 1: bad quality sodium chlorate solution. Contaminants (iron/organic material) are trapped in the filter
element.
Picture 2: bad quality sulphuric acid. Contaminants (iron/organic material) are trapped in the filter elements.
Picture 3: bad quality sulphuric acid samples that were taken before unloading the tanker truck.
Picture 4: foam in the line between the primary and secondary reactors (HP-A process).

4. Other application for ClO2
The Application RD&I division within Nouryon Pulp and Performance Chemicals is working on a projectfocused with assisting heavy polluting industries (waste combustion plants) for cleaner air emissions withthe objective to reduce NOx in flue gases by the use of ClO2 gas.
The technology has been scaled-up and the full-scale phase is been analyzed.
A simplified process flow diagram and a picture of the 40 ft transportable container of the slip streams trials
are shown below:

Vent gas
to stack
Quench tower
ScrubberReactorFlue gasfrom tie-in
ClO2 gas
ClO2 generation

The transportable container includes the ClO2 generator and mixing zone, the quench tower, the scrubber,
analyzing system for flue gas and instrumentation needed for operation.