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Effect of mixing on peroxymonosulfate generation

University of British Columbia

Efficient generation of pulping and bleaching chemicals is essential to economic production of pulp. In some cases the success of a proposed process will depend on whether the key chemical can be generated economically at an industrial scale. Peroxymonosulfate (PMS, Na₂S0₅) has shown to be an effective and selective TCF bleaching agent for both delignification and brightness development. The commercial production of PMS is achieved by reacting concentrated H₂SO₄ with 70% H2O2. Conversion is limited to 70% by equilibrium, and production costs are high for commercial bleaching applications. With the increase of oxygen delignification throughout the world, a cheaper method to generate alkaline PMS is desired. This is because an acidic PMS stage, if placed between two alkaline oxygen treatments, would require additional alkali to reach the required bleaching pH. The additional cost of the added alkali would significantly increase the total cost associated with use of acidic PMS. PMS can also be generated by the oxidation of sodium sulfite with oxygen or air in alkali media. This shows promise as an economic alternative for basic PMS generation. Past generation of PMS in the laboratory was limited to 20% yield and 3.8 g/L Na₂S0₅. However, the reaction is mixing-sensitive and optimization of mixing and reaction conditions allow both the yield and chemical concentration to be increased. The effects of key parameters (feed-time, energy dissipation rate, sodium sulfite concentration, reactant volume ratio, oxygen pressure and temperature) were verified in a number of different batch reactors (medium-intensity, P[sub HT], high-intensity, rotor-stator). The addition time of sodium sulfite (feed time) was found to affect both the PMS yield and concentration. For very short feed times (<60 sec), there is a point where the yield of PMS is a maximum. Experiments indicate that for each set of reaction conditions an optimal feed time must be found to achieve the maximum PMS yield. An increase in energy dissipation (0.33 W/kg - 1000 W/kg) rate was always found to increase PMS generation efficiency. A weaker sodium sulfite solution was useful to generate PMS at higher yield and lower concentration. By manipulating mixing parameters (0.5M Na₂S0₃ 4°C, 950 W/kg, V[sub oxygenated water]/V[sub sodium sulfite soiution]=25), PMS yield was increased to 54.4% at 1.6 g/L. The concentration (2.0M Na₂S0₃ , 4°C, 770 W/kg, V[sub oxygenated water]/V [sodium sulfite solution]= 25) was increased to 9.8 g/L, but the PMS yield dropped to 33%. Mathematical modelling shows that by adjusting mixing, both the yield and concentration of PMS can be increased. Generation limits are expected to be about 80% yield at 10.0 g/L PMS at 5000 W/kg. Oxygen solubility was found to be a limiting factor as observed experimentally in the PHT reactor by performing experiments at increased pressure. The same conclusion was made from mass transfer calculations. Both indicate that oxygen depletion can be a major barrier in running experiments at higher (1.5 - 2.0M) sodium sulfite concentration. Sulfite solutions could not be prepared at concentrations higher than 2M because of the solubility limitation of sodium sulfite. PMS yield was decreased by almost 20% when reactions were carried out at room temperature (20 - 25°C) as opposed to 4°C. An engulfment model (E-model) predicts that a high yield and concentration of PMS is possible. However, our experimental results still lag behind the model prediction. This is possibly due to limitation in achieving maximum average energy dissipation (950 W/kg) in a medium-intensity reactor. A target of 50% yield at 10.0 g/L of PMS might be possible in a reactor that takes into account of all the mixing parameters together.

Thesis/Dissertation

application/pdf

2009-06-27

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http://hdl.handle.net/2429/9777

Chemical and Biological Engineering

University of British Columbia

UBCV

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