SVP-LITE® Overall Flow Diagram »
SVP-HP® Overall Flow Diagram »
The SVP® Processes
The SVP® Chlorine Dioxide Process family consists of a number of similar processes all performed in basically the same set-up of equipment. It is relatively easy to switch from one to the other. The different processes are distinguished from each others by use of different reducing agents for converting chlorate to chlorine dioxide and different acids. The most common process today is the SVP-LITE® process based on use of methanol.
The SVP-LITE® process is an ideal choice for pulp mills:
-That want to eliminate also the chlorine in the chlorine dioxide water in order to produce real ECF pulp
-That have low saltcake make-up requirements.
-That want to economically expand the capacity of existing SVP/R3 plants.
-That can not utilize chlorine water or sodium hypochlorite.
Chemistry
The production of chlorine dioxide in the SVP-LITE® process is based upon the reaction between sodium chlorate, sulfuric acid, and methanol, per the following overall reaction:
9NaCIO3 + 2CH3OH + 6H2SO4 9ClO2 + 3Na3H(SO4)2 + ½CO2 + 3/2HCOOH+7H2O
To maintain the highest yields and generator efficiencies possible, reactant concentrations are carefully controlled. The SVP-LITE® process, like the whole SVP® family of processes can be converted to other modes of operation to meet the mill's by-product requirements.
Chlorine Dioxide Generation
The heart of the SVP-LITE® process is an all titanium chlorine dioxide generator, a large vessel in which the sodium chlorate is reduced to form chlorine dioxide. The generator is sized to optimize reaction efficiency, promote saltcake crystal growth, provide efficient liquid-gas separation and to provide sufficient capacity for optimum control of chemical compositions.
A titanium pump circulates the generator solution through a titanium shell and tube heat exchanger, where water is evaporated, through indirect contact with low pressure steam. The vapor in turn leaves the generator combined with chlorine dioxide gas.
The design of all the components in this loop is critical to eliminate boilouts and is unique to our process. The water evaporation rate is matched to the rate at which water enters the generator with the chemical feeds and from other sources. Chemical feeds are injected into the recirculation cycle at the optimum points, to maximize efficiency and to insure adequate mixing. The entire generating system is operating under vacuum. It is designed to assure reliable, fail-safe operation with minimal operator attention. A dump tank is supplied, to allow the generator system to be emptied for maintenance without loss of chemicals.
Chlorine Dioxide Recovery
The gas leaving the generator is a mixture of chlorine dioxide and water vapor. The heat in this gas is removed in the generator condenser, where water vapor is condensed, thereby enriching the chlorine dioxide concentration of the gas. The chlorine dioxide is absorbed in a packed tower, the chlorine dioxide absorber, where water is forced into intimate contact with the gas. The resulting product is a strong solution of chlorine dioxide.This solution is then pumped to storage tanks for further use in the bleaching process.
Contrary to other SVP® processes, the gas from the chlorine dioxide generator does not contain any diluting gases like air or chlorine. This means that the absorption efficiency will be high. Chlorine dioxide concentrations up to and even exceeding 10 g/l can easily be reached when using chilled absorption water.
Tail Gas Treatment
The tail gas from the absorption tower consists only of very small amounts of different gas components. Normally, the vacuum in the chlorine dioxide generation system is created by means of a steam ejector, but a titanium liquid ring vacuum pump could also be used. After passing the ejector the tail gas is washed at atmospheric pressure in a scrubber. This vent scrubber is also used for removal of chlorine dioxide in vent gases from the saltcake filter and from the chlorine dioxide water storage tanks.
Chilled water is also used in the vent scrubber for absorption. The effluent water from the scrubber is transferred to the main absorption tower and thus all the chlorine dioxide in the vent gases is recovered and added to the produced chlorine dioxide water.
Sodium Sulfate Recovery
The sodium sesqui-sulfate crystals, formed in the generator, are pumped to a titanium bottom feed, rotary drum filter for removal as a near dry solid. The filter is equipped with a hot water wash system, to assist in recovering reaction chemicals trapped in the saltcake. The wash water and mother liquor are withdrawn from the filter and returned to the generator, leaving behind a dry, high quality saltcake. A steam operated titanium ejector provides the motive force to remove the wash water and reaction chemicals from the crystals. The sodium sesqui-sulfate is discharged into the dissolving system, where it is dissolved in hot water.
The concentrated acid sulfate solution could be transferred to the pulp mill recovery system as sodium and sulfur make up either after neutralization or directly. If not all the sulfate is needed for make up, it could also be used for acidification purposes in the bleaching plant or elsewhere. If a neutral saltcake is required whitout use of caustic soda the SCW add-on process could be applied (se below).
Summary of the Advantages of the SVP-LITE® Process
Very high yield
High strength chlorine dioxide water produced
Easy to operate
Extremely fast reaction rate, resulting in minimum start-up time
No boilouts
No "white-outs"
By-product chlorine is reduced to negligible level
Very low airborn emissions
No by-product hypochlorite from tail gas system
Significant low-cost capacity increases in existing SVP®/R3 plants
A 40% reduction in saltcake compared to a SVP®/ R3 process
Other Modifications of the SVP® Process
As indicated in the presentation of the SVP-LITE® process, it is possible with only minor equipment changes, to convert any SVP® process to another. They are all vacuum Single Vessel Processes (SVP®), producing more or less a crystalline saltcake as by-product. Below we have summarized the main features of each of these modes of operation.
SALTCAKE WASH (SCW) System, SVP®-SCW
Production of chlorine dioxide in an SVP® system can be accomplished by reducing sodium chlorate with methanol in the presence of sulfuric acid. Methanol technologies require generator acidities of about 6 N or above to achieve high reaction efficiencies. High acidities result in the production of an acidic saltcake by-product. This saltcake is sodium sesquisulfate which is an acidic version of sodium sulfate. Sodium sesquisulfate has a sulfuric acid content of about 18 wt%.
Recovery of the sulfuric acid content in sodium sesquisulfate is possible while operating Eka Engineering's SVP®-SCW system. SVP®-SCW stands for Single Vessel Process SaltCake Wash. The SVP®-SCW process is basically an SVP-LITE® process with an additional saltcake wash stage which involves a secondary saltcake filter system.
In the SVP®-SCW process, sodium sesquisulfate is filtered from the generator solution by the first saltcake filter and transferred to a metathesis tank. Here the sodium sesquisulfate is separated into its sodium sulfate and sulfuric acid components. The metathesis tank slurry is pumped to a second saltcake filter where the sodium sulfate crystals are removed and sent to the mill's recovery system. The filtrate from the second filter, which contains the sulfuric acid to be recovered, is transferred to a filtrate tank. The filtrate tank solution is pumped back to the generator and/or to any other acid consumer in the pulp mill.
SVP®-SCW Chemistry:
In the SVP®-SCW ClO2 process, sodium chlorate (NaClO3) reacts with methanol in sulfuric acid solution (H2SO4) to produce chlorine dioxide (ClO2). The reaction by-products are formic acid (HCOOH) and acidic saltcake, sodium sesquisulfate [Na3H(SO4)2].
Equation 1:
12 NaClO3 + 8 H2SO4 + 3 CH3OH 12 ClO2 + 3 HCOOH + 9 H2O + 4 Na3H(SO4)2
Sodium sesquisulfate is dissolved in hot water in the metathesis tank where it dissociates into sodium sulfate crystals and a weak sulfuric acid solution. The weak sulfuric acid solution is recycled to the generator.
Equation 2:
H2O 4Na3H(SO4)2 2 H2SO4 + 6Na2SO4
SVP-HP®
The newest patented SVP® process is based on use of hydrogen peroxide for reduction of chlorate to chlorine dioxide. The first SVP-HP® plants were started up in 1993.
NaCIO3 + ½H2O2 + ½H2SO4 ClO2 + ½Na2 SO4 + ½O2 + H2O
The main advantages for the SVP-HP® process compared with methanol based processes are:
30% less saltcake.
Neutral saltcake, no caustic required.
No additional equipment required to produce neutral saltcake.
25% decrease in H2SO4 feed requirements.
No organic by-products or reducing agents.
By-product oxygen can be utilized in the bleach plant.
In addition to these new improvements, SVP-HP® chemistry maintains all the advantages of the SVP-LITE® process, including:
No by-product chlorine.
No downtime due to boil-outs.
No "white-outs".
High reaction efficiency and yield.
Rapid start-up and shutdown.
SVP® STANDARD
Sodium chlorate is reacted with sodium chloride in a sulfuric acid solution. The main reaction is:
NaCIO3 + NaCl + H2SO4 ClO2 + ½Cl2 + Na2SO4 + H2O
The unwanted side reaction which has to be suppressed by different means only produces chlorine:
NaCIO3 + 5NaCl + 3H2SO4 3Cl2 + 3Na2SO4 + 3H2O
About 1/3 of the chlorine is absorbed in the chlorine dioxide water and the rest can be handled in different ways. After compression of the chlorine in a steam ejector, it can be absorbed in chilled water. This chlorine water can then be used in the chlorination stage in the bleaching plant. Alternatively, the chlorine could be absorbed in caustic soda, if hypochlorite has any use in the bleaching process. The saltcake by-product consists of neutral sodium sulfate.
SVP® PARTIAL HCI
In this process sodium chloride is replaced as the chloride source in the SVP® Standard Process with hydrochloric acid. This will give a substantial reduction in by-product sodium sulfate. The chlorine can be handled in the same way as described under the SVP® STANDARD process. The SVP® PARTIAL HCI process is of interest when reasonably cheap hydrochloric acid is available and when less sodium sulfate is required for the pulp mill's make up needs.
SVP® TOTAL HCI
Here all the sodium chloride and sulfuric acid is replaced with hydrochloric acid. Sodium chloride is produced as a by-product. As there is no need for sodium chloride in the pulp mill, it can only be used as raw material for making, for instance, sodium chlorate or chlorine or be sewered. This process is thus of interest basically only in combination with on-site chlorate production. The excess chlorine produced will, in the integrated chlorate/chlorine dioxide concept, be converted to hydrochloric acid by burning it with hydrogen from the chlorate electrolysis.
Information
For further information and technical details, contact a Eka Engineering Sales Representative. Material Balance Sheet of different CIO2 processes, all available through Eka Engineering. Information
For further information and technical details, contact a Eka Engineering Sales Representative.
Material Balance Sheet of different processes, all available through Eka Engineering
The below given figures may vary depending on site specific conditions
Consumption/production in ton/ton ClO2
PROCESS SVP SVP SVP SVP SVP HP-A Mathieson Integrated
LITE SCW HP Standard Partial HCl SO2 SVP
Chemicals in Total HCl
NaClO3 1,65 1,64 1,66 1,68 1,68 1,65 1,78 -
H2SO4 1,0 0,8 0,78 1,57 0,75 2,05 1,4 -
NaCl - - - 0,97 - - - -
HCl - - - - 0,58 - - -
Red. agent/
H2O2 - - 0,31 - - 0,29 - -
SO2 - - - - - - 0,75 -
Cl2 - - - - - - - 0,73
MeOH 0,18 0,18 - - - - - -
Utilities
Electrical
power kWh 100 120 100 120 120 80 80 8900
Steam 4,2 5,5 5,5 8,7 7,5 - - 8
Chemicals out
Na2SO4 1,1 1,1 1,1 2,3 1,1 1,1 1,2 -
H2SO4 0,25 - - - - 1,3 1,5 -
Cl2 in
ClO2-water - - - 0,2 0,2 - - 0,2
Cl2 - - - 0,4 0,4 - - -
O2 - - 0,27 - - 0,26 - -
Total Na 0,35 0,35 0,36 0,74 0,36 0,36 0,38 -
Total S 0,33 0,26 0,25 0,52 0,25 0,66 0,75 -