Computer Model of a Typical Bleaching Sequence Predicts Effect of Partial Closure on Chlorine and Steam Consumption

A descriptive computer simulation model based on University of Idaho's PCGEMS (Edwards et al,1987) package for the IBM PC was developed for a typical bleaching sequence (DC) (EO) DED in a New Zealand pulp and paper mill. The model was used for the following purposes: 1. To predict the dissolved solids build-up as a result of partial closure. 2 . To provide dissolved species concentration data for bench scale studies on decolourisation of alkali effluent. 3. To determine the optimum recycle ratio of acid effluent. 4. To generate data for a proposed bleach plant revamp. For this plant, recycle of acid effluent (DC-stage filtrate) was found to increase the operating cost based on steam, fresh water and chemical consumed. As the acid effluent recycle is increased steam and water costs diminish but not enough to offset the increase in chlorine cost due to consumption by the recycled E-stage and C-stage organics. The model is capable of tracking up to twelve chemical subspecies including C1- ion. Some corrosion assessment could be made for upstream plant components under conditions of higher chemical concentration at up to 44% recycle of acid effluent. This is the upper limit of fresh water substitution for this plant. The model was extended to assess mass and energy flows for a proposed future revamped bleach plant configuration designed to increase productivity by doubling DC stage capacity at the front end and splitting the subsequent bleaching sequence into two parallel lines. Since this study is still in progress results from the extended model have not been included.