A study of chemical equilibria in the kraft smelt dissolving system Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3b591d295

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  • Hydrogen sulfide emissions from the smelt dissolving tank area of some kraft pulp mills exceed environmental limitations. There are two main factors to consider in determining the cause of these emissions: chemical equilibria and rate processes. This investigation considers the chemical equilibria in the smelt shatter jet area (system A) and the dissolving tank (system B) of a kraft pulp mill. In each system, the chemical species and operating conditions are described. The thermodynamic constants, which include activity coefficients and equilibrium constants, are determined. A detailed analysis is made of the errors in these thermodynamic constants, particularly the activity coefficients in the dissolving tank. An iterative calculation procedure to calculate the equilibrium composition is given for each system. In both cases, the system of equations includes conservation of mass in each phase and equilibrium expressions describing the interactions between species. The calculated equilibrium values are compared to measured values to determine if either system is at equilibrium. The calculated equilibrium partial pressures of hydrogen sulfide for system A greatly exceed the measured values obtained from a kraft pulp mill. Thus, the evolution of hydrogen sulfide in system A is not limited by equilibrium, but by rate processes (i.e., kinetics or mass transfer). The major variables affecting hydrogen sulfide release are steam, carbon dioxide, and oxygen partial pressures. In system B, the equilibrium partial pressure of hydrogen sulfide is stable over the typical range of operating conditions. The uppermost limit of calculated equilibrium partial pressures of hydrogen sulfide is slightly below the measured values. As a result of these equilibrium calculations, it is concluded that the major source of hydrogen sulfide emissions is system A. Future work is recommended involving experimentation to determine the rate processes controlling the release of hydrogen sulfide. Process controls and modifications for a kraft pulp mill that may help decrease emissions are also presented.
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