The use of secondary condensates from evaporation plant in pulp bleaching

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Kemian tekniikan korkeakoulu | Master's thesis
Biomass Refining
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Master's Programme in Chemical, Biochemical and Materials Engineering
High water consumption is a key concern in pulping industry. To resolve the issue, secondary condensates from evaporation plants have been reutilized in pulp washing between the bleaching stages. In order to explore further their exploitation, secondary condensates were applied in bleaching, rather than after bleaching, in this thesis work. Pure water was partially substituted with four types of evaporation condensates as diluents in chlorine dioxide (D) and ozone (Z) bleaching stages. Mass and COD balance of the bleaching stages and the effects of secondary condensates on pulp quality and chemical composition were studied. Headspace gas chromatography - mass spectrometry was applied for quantifying volatile organic compounds (VOCs) in the condensates, bleaching filtrates and the vapor phase of the bleaching reactor. Meanwhile, COD of pulp filtrates and condensates and brightness, viscosity and kappa number of the pulps were examined to study the possible impacts of the condensate use on environmental load and pulp properties. No noticeable effect of the condensate used on pulp brightness, viscosity and kappa number in D0 and Z stages were observed. The main VOCs in the condensates were methanol, ethanol, acetone and some sulphur components, such as dimethyl disulfide. VOCs detected in pulp filtrates were mainly methanol. Also the condensate-free pulp filtrates contained significant amounts of methanol, thus secondary condensates are not the only source of methanol. It could also be formed through demethoxylation of lignin in pulp bleaching. Instead of the secondary condensates, the unbleached kraft pulp was the main source of sulphur in pulp bleaching. Secondary condensates and the methanol in them contributed quite little on COD of pulp filtrates. In conclusion, it seemed to be feasible to substitute fresh water with secondary condensates in pulp bleaching.
Vuorinen, Tapani
Thesis advisor
Vehmaa, Janne
Oljakka, Tuuli
secondary condensate, chlorine dioxide, ozone, mass balance, ISO Brightness, viscosity
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