Waste minimization in a bleach plant

In this study a steady-state model was developed for a six stage bleaching system in a pulp and paper industry. A FORTRAN source code was written to simulate the whole bleach plant. Different case studies were performed with the aim of decreasing the total amount of dissolved solids coming out from the six washers. Each stage of the sequence is composed of chemical additions and mixing, reaction in a retention tower, and washing. Unit operation models were formulated using mass balances on liquor, fibres, chromophores, chemicals and dissolved solids. In order to compare the simulation results with the mill data, Dalaman SEKA Pulp and Paper Mill bleaching unit was chosen. A good agreement between the simulation results and the mill data was achieved. In the case studies, the optimization of the process variables were performed in which the effects of retention tower operating temperature, residence time, consistency, bleaching liquor flow rate and the wash water flow rates in the washers were studied. Also, the effect of 100% chlorine dioxide substitution in the chlorination stage was analyzed. It was concluded that 100% chlorine dioxide substitution was the best way to decrease the amount of dissolved solids