Sodium chlorate is produced by passing an electric current through a solution that contains sodium chloride (salt). The salt for this reaction is a recycled by-product from the chlorine dioxide generation reaction. Hydrogen gas is co-produced with the sodium chlorate and is used as a feedstock for hydrochloric acid production.
Hydrochloric acid is produced by burning chlorine gas and hydrogen gas. The hydrogen gas comes from the sodium chlorate electrolysis area. Make-up chlorine gas comes from the Plant Battery Limits. Recycled chlorine gas, a by-product of the chlorine dioxide generation reaction, is combined with this chlorine make-up stream prior to being burned with the hydrogen gas.
Chlorine dioxide gas is produced, along with chlorine gas and sodium chloride (salt), by combining strong chlorate liquor and hydrochloric acid in the chlorine dioxide generator. The chlorine dioxide gas is absorbed in chilled water to produce the chlorine dioxide solution for use in the pulp mills’ bleach plant. The liquor leaving the generator contains unreacted sodium chlorate and the by-product salt. This solution, called weak chlorate liquor, is recycled back to the sodium chlorate electrolysis area for reconcentration. The chlorine by-product, as mentioned above, is sent to the hydrochloric acid synthesis area to be used as a feedstock for HCl production.
As a result of the integration of these three production areas, the only two major inputs to the process are the make-up chlorine gas (from a nearby chloralkali plant or other sources) and electrical energy.
Sodium Chlorate Production Area
Sodium chlorate liquor is produced in the sodium chlorate production area.
In the electrolyzers, which consist of a number of cells connected together, sodium chloride and water are electrochemically converted to chlorine, sodium hydroxide and hydrogen gas. The liquor/gas mixture rises to the degasifiers where the hydrogen gas is separated from the liquor. The liquor then passes to the chlorate reactor where the reaction to form sodium chlorate is completed. The electrolyte cooler removes the heat generated during electrolysis.
When direct current is applied by the rectifier to the electrolyzers, the hydrogen gas produced in each cell reduces the average specific gravity of the liquor in the cell. The resulting specific gravity difference between the liquor in the electrolyzers and that in the chlorate reactor creates a high rate of liquor circulation. Figure 2 illustrates this concept.
Figure 2 - Intricate design, simple operation. Chlorate liquor circulation is achieved by natural gas lift without the use of a pump.
Weak chlorate liquor returning from the chlorine dioxide generation area displaces strong chlorate liquor from the chlorate reactor, causing an overflow into the strong chlorate feed tank. This tank provides strong chlorate surge volume for feed to the chlorine dioxide generator.
The chlorate liquor holding tank provides storage for chlorate liquor drained from the electrolyzers or the chlorine dioxide generator during plant maintenance.
Chlorate liquor is cooled and filtered before introduction to the chlorine dioxide generator by pumping it through the chlorate cooler and chlorate filter, using the strong chlorate feed pump.
Hydrogen, containing small quantities of chlorine and oxygen, is co-produced with sodium chlorate. Most of the hydrogen is used for HCl synthesis while the remainder is passed through the hydrogen scrubber for chlorine removal before venting to atmosphere.
The chlorine is absorbed in the hydrogen scrubber using a circulating stream of sodium hydroxide solution. The hydrogen scrubber pump provides circulation while the hydrogen scrubber cooler removes the heat produced in the hydrogen scrubber. The chlorine scrubbing reaction is as follows:
Cl2 + 2 NaOH ® NaCl + NaOCl + H2O
Prior to start-up and during shutdown, the gas space in the sodium chlorate system is purged with nitrogen. This is to prevent an explosive mixture of hydrogen and air from forming.
During shut‑down periods, the steel cathodes in the electrolyzer must be protected to prevent attack by the corrosive hot chlorate liquor. The cathodic protection unit provides a small current that maintains the cathodes at a negative voltage relative to the cell liquor to prevent corrosion. During cathodic protection, a hydrogen / oxygen mixture is produced in the cells. This gas mixture must be continuously diluted with nitrogen to avoid accumulating an explosive mixture.
Features and Benefits of the Aker Solutions Sodium Chlorate Production Area
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High efficiency sodium chlorate production. The design of the sodium chlorate plant area has been optimized to maximize current efficiency and minimize chlorate cell voltage, the two main determinants of the power consumption. The use of carrier plates instead of inter-cell flexible bus bars, a small electrode gap, good process control, selected anode coatings and advanced chlorate cell design all contribute to high performance. The result is minimum power consumption for the production of sodium chlorate.
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Natural chlorate liquor circulation. The natural circulation system, originally developed by Aker Kvaerner Chemetics, inherently achieves the large flow of chlorate liquor necessary for efficient electrolyzer performance. This design improves plant reliability and reduces maintenance by eliminating the large titanium pump that would otherwise be required.
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Superior materials of construction. The corrosive nature of high temperature chlorate liquor presents a challenge in material selection for the chlorate plant area. Aker Kvaerner Chemetics has met this challenge by designing equipment that is expected to operate maintenance-free for at least five years. With the exception of the electrolyzer cathodes, all wetted parts in the chlorate cellroom are constructed of either titanium, FEP, PTFE or fluoro-elastomer material.
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Single chlorate reactor. There is only one chlorate reactor system in the Plant. The number of operator checks required for pH, product liquor composition, and hydrogen gas analysis are minimized.
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Pre-Assembled Electrolyzer. The electrolyzers are pre-assembled and hydrostatically tested prior to shipment. The time and effort required for installation and servicing is minimized, with only piping and bus bar connections to be completed on site.
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Electrolyzers are compact and easily transported. The use of carrier plates between cells eliminates the need for inter-cell flexible bus bars and associated hardware. This not only reduces power consumption, but also allows the electrolyzer to be supplied as a single, compact equipment item. Wheels can be installed on the base of the electrolyzer to allow it to be moved within the plant.
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Easy electrolyzer maintenance. The sodium chlorate electrolyzer is designed to permit fast and easy access to its internals. If necessary, individual cells can be readily examined without dismantling a complete electrolyzer. An overhead crane is not required to install or remove an electrolyzer.
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STAHRMETâ steel cathodes. Steel is used for chlorate cell cathodes because of its electrolytic properties. Atomic hydrogen permeates through ordinary carbon steel cathodes and accumulates as molecular hydrogen at imperfections within the steel sheet. If an accumulation occurs, the swelling and blistering of the steel can result in eventual cathode contact with the adjacent anodes. The resulting short‑circuit damages the anodes while the blisters make cell disassembly difficult. STAHRMETâ is specially produced to Aker Kvaerner Chemetics' specifications and is free of imperfections that could cause blistering.
Hydrochloric Acid Synthesis Area
Aker Solutions' hydrochloric acid plant experience extends beyond basic procurement of HCl synthesis equipment, to overall system design and its full integration with the chlorine dioxide and sodium chlorate plants.
Hydrochloric acid is produced by the combustion of hydrogen gas and chlorine gas, followed by absorption of the hydrogen chloride vapours in demineralized water. Hydrogen from the chlorate production area, recycled chlorine from the chlorine dioxide generator and strong chlorine are fed to the HCl synthesis unit.
Aker Solutions specifies and purchases the proprietary HCl synthesis unit and tail gas scrubber, designs the peripheral equipment, develops the process control philosophy, and integrates this system into the overall integrated chlorine dioxide system.
The HCl Synthesis Unit has the dual purpose of burning the hydrogen and chlorine gases and absorbing most of the resulting hydrogen chloride gas in the weak acid stream from the tail gas scrubber. Product acid flows by gravity from the HCl synthesis unit to the HCl pump tank from where it is pumped to storage by the HCl transfer pump.
The HCl synthesis unit consists of a series of graphite blocks enclosed by a carbon steel jacket. Hydrogen and chlorine gases are introduced into the HCl synthesis unit through separate inlet ports and are combined in a burner assembly. Cooling water is supplied to the HCl synthesis unit to remove the of heat that is generated by the combustion of hydrogen with chlorine and by the absorption of HCl.
A chlorine flame arrestor is installed in the chlorine feed line to the HCl synthesis unit to prevent propagation of the flame back to the chlorine supply.
Hydrogen gas from the sodium chlorate plant is hot and saturated with water vapour. Droplets of condensate must be removed from the hydrogen stream as they can cause damage to the HCl synthesis unit. This is accomplished by cooling the hydrogen in the hydrogen cooler and then passing the gas through the hydrogen demister.
A hydrogen flame arrestor is installed in the hydrogen feed line to the HCl synthesis unit to prevent propagation of the flame back to the hydrogen supply.
The tail gas scrubber absorbs the residual hydrogen chloride gas from the HCl synthesis unit in demineralized water. The resulting weak acid flows to the HCl synthesis unit where it absorbs more hydrogen chloride gas. The vent gas from the tail gas scrubber consists of excess hydrogen and inerts (such as nitrogen) which are present in the chlorine feed streams.
The tail gas start-up fan is used to provide the required air flow to the HCl synthesis unit during start-up. The HCl synthesis unit is started by burning hydrogen in air until a flame is established. The air is then replaced by chlorine.
The product hydrochloric acid is stored in the HCl storage tank and fed to the chlorine dioxide generator by the HCl supply pump.
The hydrochloric acid synthesis unit is started up and operated from a locally-mounted field control panel. Any serious process upset or utility failure will automatically shutdown the hydrochloric acid plant and purge it with nitrogen. Critical process parameters including alarms and interlocks are displayed at the main control system.
Chlorine Dioxide Generation Area
Chlorine dioxide is produced by reacting sodium chlorate liquor and hydrochloric acid in the Aker Solutions chlorine dioxide generator.
Chlorine dioxide, chlorine, sodium chloride, and water are formed in the chlorine dioxide generator from the reaction of sodium chlorate and hydrochloric acid:
NaClO3 + 2HCl ® ClO2 + ½ Cl2 + NaCl + H2O
Strong chlorate liquor from the electrolyzer area is introduced into the generator prior to the suction of the circulation pump. The circulating liquor is then heated in a shell and tube heat exchanger so that the water will evaporate upon return to the generator body. Hydrochloric acid is added at the injection module where the conditions are optimum for the efficient generation of chlorine dioxide.
The reacting mixture, upon returning to the generator body, flashes. Chlorine dioxide, chlorine and water vapour are released. The water vapour dilutes the chlorine dioxide to below its decomposition limit.
Weak chlorate liquor is removed from the generator by the chlorate return pump, and is heated in the weak chlorate heater before returning to the electrolyzer area. The heating converts the excess HCl in the returning liquor to chlorine so that the acidity balance within the circuit is maintained. The small amount of chlorine produced is sent to the weak chlorine vacuum pump for recycle.
The generator off gas containing chlorine dioxide, chlorine and water vapour leaves the generator through an exit duct and is fed to an absorber-stripper, after passing through a condenser. The top portion of this two stage column serves as a standard absorption unit with chilled water cascading downward. Both chlorine and chlorine dioxide are absorbed within this section of the tower. The bottom section however, acts as a preferential chlorine stripper. Chlorine that already has been absorbed is subsequently stripped out by the small amount of air introduced at the bottom of the column. Some chlorine dioxide is also stripped out but is reabsorbed higher up in the column. The final liquid product containing very low level of chlorine is pumped from the absorber-stripper to storage.
The chlorine dioxide supply pump provides chlorine dioxide solution to the mill from the chlorine dioxide storage tanks.
The hypo system absorbs the weak chlorine from the absorber in emergency situations or when the HCl synthesis unit is shut down. Weak chlorine from the chlorine dioxide absorber is transferred to the hypo system by the weak chlorine vacuum pump.
The chlorine in the weak chlorine stream reacts with sodium hydroxide in the hypo tower according to the following reaction:
Cl2 + 2 NaOH ® NaCl + NaOCl + H2O
Sodium hydroxide solution is circulated around the hypo tower and hypo tower pump tank by the hypo tower pump. The hypo tower cooler removes the heat generated by the above reaction. The hypo fan maintains the hypo system under vacuum to draw in the weak chlorine.
Features and Benefits of the Aker Solutions Chlorine Dioxide Generation Area.
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High purity product. The Aker Kvaerner Chemetics chlorine dioxide generator system produces a high purity chlorine dioxide solution, with a chlorine level the same or lower than those of non-integrated processes. Furthermore, the chlorine dioxide solution does not contain methanol or methanol by-products found in most non-integrated processes, which impose a BOD load to the mill’s effluent treatment.
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Simple & safe generator design. The reactions in the generator can be easily quenched with water using a simple shower system. Unlike other generator configurations, dilute caustic soda is not necessary for generator quenching. The use of caustic soda is undesirable because it causes a sodium ion imbalance in the closed-loop integrated system. This will result in an increase of liquor in the system and a need for the plant to dispose of solution containing sodium chlorate and sodium dichromate.
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Unimpeded generator gas space. During a process upset, the gases over the liquor can be quickly swept from the generator. In the event of a chlorine dioxide decomposition, the rapidly expanding gases can damage the generator if they are restricted by generator internals. An unimpeded gas space over the liquor allows the gases to safely escape from the generator during a decomposition.
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High efficiency ClO2 absorber-stripper. Aker Kvaerner Chemetics uses a "Linder" Column for chlorine dioxide absorption. Unlike conventional packed towers, this highly efficient (99.5%) proprietary absorber allows for a turn down to 30% of design capacity without loss of absorption
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No generator boil-outs are required. Plant shutdowns for generator scale removal are not required as the integrated system is a completely liquid-based system.
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There are no ‘whiteouts’. The loss of chlorine dioxide production via "whiteouts" does not occur as the generator operates with sodium chloride present at all times.
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