SVP-LITE process

 

The SVP-LITE® process is an ideal choice for pulp mills:

• wanting to eliminate also the chlorine in the chlorine dioxide water in order to produce real ECF pulp
• having low salt cake make-up requirements. 
• wanting to economically expand the capacity of existing SVP/R3 plants. 
• which cannot 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 salt cake 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 boil outs 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 salt cake 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 sesquisulfate 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 salt cake. The wash water and mother liquor are withdrawn from the filter and returned to the generator, leaving behind a dry, high quality salt cake. A steam operated titanium ejector provides the motive force to remove the wash water and reaction chemicals from the crystals. The sodium sesquisulfate 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, it could also be used for acidification purposes in the bleaching plant or elsewhere. If a neutral salt cake is required without 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 boil outs
• No "white-outs"
• By-product chlorine is reduced to negligible level
• Very low airborne emissions
• No by-product hypochlorite from tail gas system
• Significant low-cost capacity increases in existing SVP®/R3 plants
• A 40% reduction in salt cake compared to a SVP®/ R3 process

This process is a trademark of Eka Chemicals, registered in one or more countries.

Related Links
Chlorine dioxide process
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