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Modelling of kraft pulp mill total reduced sulphur emissions Jensen, Allan Stewart

Abstract

Atmospheric release of odorous total reduced sulphur (TRS) emissions from kraft pulp mills has been an ongoing concern worldwide. Organic TRS compounds are formed in the digester by undesired side reactions during the kraft pulping of wood. These, along with H₂S, being highly volatile, are released from mill processes, such as brown stock washing. TRS gases are extremely noxious, described as rotting eggs or rotting cabbage, and have a threshold of odour detectability in the range of a few parts per billion. The general objective of this work was to develop and test a method to predict emissions of these TRS compounds from kraft pulp mills using a vapour-liquid equilibrium (VLE) model. This technique could then be utilized to optimize the process to minimize emissions, or to optimize the design and operation of TRS control systems. A mill TRS sampling and testing program was conducted around the brown stock washing area of the Howe Sound pulp mill located in Port Mellon, British Columbia. Over 90% of the sulphur emissions from the TRS compounds were in the form of dimethyl sulphide. This compound was subsequently chosen as a surrogate compound for modelling of TRS. Analytical testing was done on black liquor and various solutions containing the substances found in the highest concentrations in black liquor, including mixed sodium salts solutions to 6 wt% and lignin mixtures to 6 wt%. The VLE of dimethyl sulphide, in the form of an activity coefficient, for these solutions was determined using a headspace gas chromatographic method. The adjustable parameters that provided the best fit for the electrolyte non-random two-liquid (eNRTL) equation for the experimental activity coefficients were determined using statistical regression techniques. Simulation software, incorporating a VLE model using the eNRTL correlation, was used to construct a base-case heat and mass balance representing current operation of the Howe Sound brown stock washing process. This balance was used to evaluate potential process modifications and their effect on TRS emissions.

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