Introduction
In 1992, Eka Engineering and Eka Chemicals introduced the HP-A® system to the market. This proprietary technology has several advantages over earlier atmospheric chlorine dioxide processes.
These advantages include:
- Existing facilities can achieve significant increases in capacity with minimal capital investment.
- The amount of spent acid, containing by-product sodium bisulfate, is reduced.
- The HP-A® system uses an inorganic reducing agent, which means no additional load on the mill secondary effluent treatment system.
- The HP-A® system does not produce any significant amount of chlorine as a by-product. Therefore, organo-chlorine formation in the bleaching process is minimal.
HP-A® Chemistry
In the HP-A® chlorine dioxide process, sodium chlorate (NaCIO3) reacts with a proprietary hydrogen peroxide (H2O2) based reducing agent in sulfuric acid solution (H2SO4) to produce chlorine dioxide (ClO2). The reaction by-products are oxygen (O2) and spent acid containing sodium bisulfate (NaHSO4) as shown in Equation 1.
2NaClO3 + H2O2 + 2H2SO4 2ClO2 + 2NaHSO4 + 2H2O + O2
The older atmospheric ClO2 processes, salted Mathieson, Solvay, and R2, use sodium chloride which reacts to produce chlorine as by-product. The HP-A® process does not require sodium chloride, therefore producing essentially chlorine-free chlorine dioxide.
HP-A® System Description
Please refer to the HP-A® schematic while reviewing the process description.
Primary generator system:
The Primary Generator Loop for the HP-A® system consists of the Primary Generator (usually titanium), the Primary Generator Cooler (usually integral with the Generator), a gas relief lid, and the associated piping and instrumentation.
Individual feed streams of sodium chlorate, hydrogen peroxide reducing agent, and sulfuric acid are injected into the Generator. ClO2 is produced and process air is sparged through the vessel to mix the chemicals together and to keep the ClO2 concentration in the gas phase within the safe operating range. Generator cooling, if required, is achieved by coils or a water jacket. The liquor overflows by gravity into the Secondary Generator.
Secondary Generator System:
The Secondary Generator System for the HP-A® unit operates in the same manner as the Primary Generator, except that the chlorate feed stream is replaced by the feed of the overflow solution from the Primary Generator. The Secondary Generator overflows to the Stripper.
Stripper:
The purpose of the Stripper is to remove any remaining ClO2 from the spent acid solution before it is discharged from the process. Liquor overflows from the Secondary Generator and is fed into the bottom of the Stripper. This is done by sparging air through the liquor under controlled temperature. ClO2 gas generated in the Stripper is collected in the gas header before entering the ClO2 Absorber.
ClO2 Absorption System:
Chlorine dioxide, oxygen, air, and water vapor collected in the gas header enter the bottom of the ClO2 Absorber and flow up the column. Chilled water is added to the top of the ClO2 Absorber and flows down the column. As the water and gas pass each other, the ClO2 in the gas is absorbed into the water to form the ClO2 product solution. This solution flows by gravity to the ClO2 Storage Tanks. Oxygen, air, and traces of chlorine dioxide exit the top of the ClO2 Absorber and are usually forwarded by the Vent Fan to the mill scrubber.
Emergency Stop Interlock System:
The Emergency Stop Interlock System is a fully automated shutdown sequence which ensures that the system will automatically shut down in a safe manner should a process upset occur. Process instrumentation monitoring pressures, temperatures, and flows provide the input to the Interlock System.
Information
For further information and technical details, contact a Eka Engineering Sales Representative.