The computer simulation of NZFP no. 2 bleach plant

A steady state descriptive GEMS computer model, established by Edwards et al, 1988, for the simulation of NZFP No.2 bleach plant was further tuned to match the existing plant conditions. The initial GEMS model incorporated assumptions estimated by Edwards, for chlorine chemical adjustment which resulted from the accumulation of recycled organics. Laboratory studies, applied to organics generated from bleaching of Pinus Radiata pulp were carried out to verify these assumptions. Bench scale experiments were conducted in D/C-stage and E1-stage filtrates for both laboratory generated filtrates and mill samples, whereas for the D1-stage filtrate, only the generated filtrate was investigated. Chlorine consumption of the generated D/C-stage and E1-stage filtrates was measured as 0.33kg and 0.20kg active chlorine per kg of organics, at the temperatures of 50°C and 70°C, respectively. The quantity of organics in each filtrate was determined as Total Volatile Solids. For the D/C-stage filtrate, the organics-chlorine reaction was shown to be temperature dependent with a maximum consumption occurring at approximately 50°C, whereas for the E1-stage filtrate, the organics chlorine reaction as a function of temperature was not investigated. For the mill filtrate samples, chlorine consumption measured as kg active chlorine per kg of organics was 0.38 and 0.14 for D/C-stage and E1-stage organics, respectively. The chlorine consumption of D1-stage organics was negligible compared to the D/C-stage and the E1-stage organics, due to only a minor concentration of volatile solids present in the effluent, i.e., approximately 20-30mg per litre of effluent. The generated D/C-stage and E1-stage filtrates were also found to consist of 17.8kg and 17.0kg of organics per tonne of O.D. pulp, respectively. The measured chlorine consumption values were incorporated into the fine tuned GEMS model (total closure case) to assess the effect of various amounts of fresh water substitution, by recycling D/C-stage and E1-stage effluents, on the economics of bleach plant operation with respect to chemical, water, and steam usage. The studies indicated a lowering of the total cost of these utilities, with increasing amounts of recycled effluent. Using the existing bleach plant model (open case) as a base case, the annual savings in cost of utilities for total mill closure was estimated to be NZ$0.2 million. Experience gained from this study has been applied to the formation of a GEMS model for a proposed bleach plant expansion with double capacity, using two three stage short sequences in parallel, and using recycled effluent. Initial runs with this model predict potential savings of utilities of approximately NZ$1.04 million p.a. compared to the existing open case configuration