| China is the third largest sugar producer in the world,but it's sugar industry is not strong and efficient.The industry is dominated by the cane sugar process adopting mainly the carbonation and sulfitation processes,with the sulfitation accounting for over 90%.While the carbonation process has the drawbacks of high production costs,and filter mud pollution,etc,with the sulfitation process,the white sugar product has problems with high color value,high levels of sulfur dioxide,formation of floc in acidic conditions and color formation during storage,weakening China's sugar international competitiveness.Therefore,continuous questing for improvement in production process and thus in quality of white sugar and reducing production costs are key issues to the rapid development of China's sugar industry.In this thesis,combining the advantages of the carbonation and sulfitation clarifying pprocesses,the CO2 of the boiler flue gas is used to improve the traditional sulfitation process,increasing the removing rate of impurities in the clarifying process,improving the quality of white sugar while maintaining the Filter mud utilization characteristics of the sulfitation process,and reducing the emission of CO2 at the same time.The research further develops and improves the clarification theory of sugar process,providing references for the green development of sugar industry.The research is of positive significance in increasing the international competitiveness of the white sugar product and promoting the cleaner production of China's sugar industry.Contents and results of the research include:1.The mechanism of the flue gas CO2 carbonation improved sulfitation clarifying process is investigated.The adsorption of colloids by CaCO3,Ca3(PO4)2 and CaSO3 and their co-precipitation with colloids,as well as their properties of adsorption of methyl orange pigment are investigated.The mechanism of eliminating colloids,colors and other non-sugars in the juice during the flue gas CO2 carbonation improved sulfitation clarifying process is investigated.It was shown that adsorption isotherms for adsorptions of pectin and the methyl orange by Calcium phosphate,calcium carbonate,calcium sulfite were in accordance with the Langmuir model,with the pseudo-first-order reaction kinetics.The calcium phosphate had the largest adsorption capacity of pectin with qm=902.37 mg/g.The calcium carbonate had the largest adsorption capacity of the methyl orange with qm=43.23 mg/g.The adsorption rates of pectin by Calcium phosphate,calcium carbonate,calcium sulfite were in the order of calcium sulfite>calcium phosphate>calcium carbonate,the obtained adsorption isotherms respectively beingln(685.12-qt)=4.176-0.0633tln(548.67-qt)=6.307-0.0539tln(343.06-qt)=5.838-0.0716tThe adsorption rates of methyl orange were in the order of calcium carbonate>calcium phosphate>calcium sulfite,the obtained adsorption isotherms respectively beingln(11.99-qt)=2.484-0.0408tln(44.65-qt)=3.799-0.0535tln(14.20-qt)=2.653-0.0314t2.The adsorptions of pectin and methyl orange by calcium phosphate,calcium carbonate,calcium sulfite mainly appeared as the endothermic physical adsorption process.During formation,calcium carbonate and calcium sulfite not only absorb colloids in the juice,but also undermine their stability and precipitate them,forming co-precipitations,thus to improve the removal efficiency of colloids.3.Through the comparison of the clarifying effects and the influences on following processes between clarifications by the carbonation to sulfitation process and the sulfitation to carbonation process,the carbonation to sulfitation process was confirmed as the best process for the flue gas CO2 carbonation improved sulfitation clarifying process.4.After chemically modified with urea,the phosphate intensified both the adsorption of colloids and colors in the juice,and the co-precipitations by Ca3(PO4)2.The purification efficiency was increased by 0.5%,the purity difference between clarified juice and the mix juice increased by 0.40AP,the color value of the clarified juice reduced by 15.4%.When added both in prelimining and first clarified juice,the forming Ca3(P04)2 shown better clarifying effect.The purification efficiency was increased 0.2 percentage point,the purity difference between the clarified juice and the mixed juice increased by 0.24AP,and the color of clarified juice reduced by 10%.5.The flue gas CO2 carbonation improved sulfitation clarifying process was investigated through single factor experiments and orthogonal experiments.The influences of the temperature,pH value,and duration of carbonation,the CO2 concentration of flue gas,the first and second phosphate dosages,the pH value of sulfitation and neutralization and the sulfitation intensity on the clarification were investigated.The conditions were optimized and are concluded as:the temperature,pH value,and duration of carbonation were 50?,11.0 and 8 min respectively,the CO2 concentration was 22%,pH value of sulfitation and neutralization was 7.0,the first and second phosphate dosages were 50mg/kg and 150 mg/kg respectively,and the sulfitation intensity was 1.2g/L.6.Industrial application of the flue gas CO2 carbonation improved sulfitation clarifying process was carried out.An intensified sulfitation reactor was developed and optimized by the FLU software.A flash tank with surfacial and interial emission of gases were designed.The application shown that the clarification and thus the product quality was improved,and the sugar production rate was increased.The results were:the purity difference between clarified juice and the mixed juice was increased from 0.66 AP to 2.02 AP,the decoloration rate increased by 18.06%,the sugar production rate increase 0.1%,the color value of the white granulate sugar reduced by 16%,the grade one white sugar production rate increased by 22.78%,Grade one A/B sugar production rate increased 46.95%,with good economic and environmental benefits.7.In order to improve the stability and adaptability of industrial applications,the improved flue gas CO2 absorption process was investigated.The process includes that lime milk is used to complete the absorption of the flue gas CO2,which is then activated,the activated liquid is used in the juice charification.Orthogonal tests in laboratory and trials in factory were carried out to study the optimum conditions for clarification and the application effect.The optimum process conditions were:the addition of activated liquid being 0.4%(CaO%C),the second defecation pH 8.5,and the sulfitation intensity 1.4g/L.The application results were:Grade one A/B sugar production rate being 100%,including grade one A 92.94%.The result shows that the improved flue gas CO2 absorption technology can be a good solution to the problem of low CO2 absorption rate,showing better results in production than the direct flue gas CO2 carbonation mode.8.The investigation on the comprehensive utilization characteristics of the filter mud of the new process shown that organic matter content of the mud was 48%,pH value was 7.7,the total nutrient(N+P+K)was 3.8%,and CaO content was 10.6%,which are in line with the technical requirements of the domestic organic fertilizer. |
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