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The mixing sensitivity of polysulphide generation Dobson, Heather

Abstract

Increasing the yield of kraft pulping processes is desirable for both environmental and economic reasons. The addition of polysulphide to the cooking liquor is a solution, which has been shown to increase pulp yield by 1 % or more. One economical approach used to generate polysulphide is the PAPRILOX™ process. In this process, sulphide in white liquor is oxidized to polysulphide in a sparged stirred tank reactor using lime mud and MNO2 catalysts. This thesis investigates the mixing-sensitivity of polysulphide generation in the PAPRILOX™ process, with the goal of determining the optimum generating conditions needed to maximize polysulphide yield and selectivity. Parameters affecting mixing and mass transfer, including impeller speed, gas flowrate, oxygen partial pressure, catalyst loading and catalyst type, were studied in two laboratory batch-operated reactors; one sparged (scale: 1:100 000 by volume, liquid volume 1.4 L, gas hold-up 2.4-28%, gas superficial velocity: 0.066-0.26 cm/s), at atmospheric pressure, and having low-intensity mixing and the other not sparged (liquid volume 1.25 L, gas fraction 20%), pressurized (1.8-20 psig), and having high-intensity mixing. In order to mimic the liquor conditions used in the PAPRILOX™ process the reactions were performed at 90 °C with white liquor compositions similar to those typically found in kraft pulp mills. Polysulphide yield and selectivity increased during oxidation, reaching a maximum level in a time that depended on reaction parameters. In the sparged reactor, regardless of gas flowrate, oxygen partial pressure, catalyst type or catalyst loading, increasing the mixing intensity by increasing the impeller speed gave higher polysulphide yields, selectivities and initial rates of formation. Increasing the gas flowrate only impacted the initial polysulphide formation rate, whereas increasing the catalyst loading improved the yield, selectivity and initial rate of polysulphide formation. Reactions performed at higher oxygen partial pressures (sparging with O2 instead of air) were faster, however the maximum polysulphide yield and selectivity reached were lower, 47 and 75% respectively at 2000 rpm, 2.0 g/L Fisher MnO2 catalyst and 2.0 SLPM air versus 30 and 43% under the same conditions with 2.0 SLPM O2. With the high-intensity reactor higher impeller speeds gave higher polysulphide yields regardless of the oxygen partial pressure. The initial rate of polysulphide formation increased with impeller speed up to 2000 rpm, after which the rate levelled off. Polysulphide selectivity was affected to the largest extent, in the high-intensity reactor, by O2 partial pressure and was only minimally affected by mixing. In the sparged reactor, with use of the Fisher MnO2 catalyst, the maximum polysulphide yield and selectivity, 47% and 75% respectively, were obtained during 50 min of oxidation at an impeller speed of 2000 rpm and air flowrate of 2.0 SLPM. Results with oxygen sparging were only comparable when the concentration of the Fisher MnO2 catalyst was 6.5 g/L or when the proprietary PAPRILOX™ catalyst was used. At a Fisher MnO2 concentration of 6.5 g/L polysulphide formed in 39% yield and 69% selectivity in 9 min of oxidation at an impeller speed of 2000 rpm and oxygen flowrate of 2.0 SLPM. Under the same conditions, using the proprietary catalyst in a concentration of 2.0 g/L gave polysulphide in 46% yield and 85% selectivity after 8 min of oxidation. With the high-intensity reactor reactions were much faster and polysulphide yield and selectivities of 43-45% and 66-76%, respectively, were obtained in 1.7-2.7 min of oxidation at oxygen partial pressures close to atmospheric. However, the energy required to reach these yields and selectivities in the high-intensity reactor was twice that of the sparged reactor. In all of the cases studied lime mud settling was not adversely affected by the higher mixing intensities.

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