Validation of plant dynamic model by online and laboratory measurements – a tool to predict online COD loads out of production of mechanical printing papers

Tommi Lappalainen Validation of plant dynamic model by online and laboratory measurements – a tool to predict online COD loads out of production of mechanical printing papers Lappeenranta 2008 112 p. Acta Universitatis Lappeenrantaensis 319 Diss. Lappeenranta University of Technology ISBN 978-952-214-632-8, ISBN 978-952-214-633-5 (PDF ), ISSN 1456-4491 COD discharges out of processes have increased in l ine with elevating brightness demands for mechanical pulp and papers. The share o f lignin-like substances in COD discharges is on average 75%. In this thesis, a pla nt dynamic model was created and validated as a means to predict COD loading and dis charges out of a mill. The assays were carried out in one paper mill integrate produc ing mechanical printing papers. The objective in the modeling of plant dynamics was to predict day averages of COD load and discharges out of mills. This means that o nline data, like 1) the level of large storage towers of pulp and white water 2) pulp dosa ge , 3) production rates and 4) internal white water flows and discharges were used to create transients into the balances of solids and white water, referred to as “plant dynamics”. A conversion coefficient was verified between TOC a nd COD. The conversion coefficient was used for predicting the flows from TOC to COD to the waste water treatment plant. The COD load was modeled with simi lar uncertainty as in reference TOC sampling. The water balance of waste water trea tm nt was validated by the reference concentration of COD. The difference of C OD predictions against references was within the same deviation of TOC-pre dictions. The modeled yield losses and retention values of TOC in pulping and b leaching processes and the modeled fixing of colloidal TOC to solids between t he pulping plant and the aeration basin in the waste water treatment plant were simil ar to references presented in literature. The valid water balances of the waste water treatme nt plant and the reduction model of lignin-like substances produced a valid predicti on of COD discharges out of the mill. A 30% increase in the release of lignin-like substances in the form of production problems was observed in pulping and bleaching proc esses. The same increase was observed in COD discharges out of waste water treat m nt. In the prediction of annual COD discharge, it was noticed that the reduction of lignin has a wide deviation from year to year and from one mill to another. This mad e it difficult to compare the parameters of COD discharges validated in plant dyn amic simulation with another mill producing mechanical printing papers. However, a trend of moving from unbleached towards high-brightness TMP in COD disch arges was valid.

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