Simultaneous water and energy optimization for a pulp and paper mill

In this paper, simultaneous water and energy reduction in a brown stock washing system (BSWS) for an existing pulp and paper mill is analyzed. In the BSWS, seven counter-current washers are used for pulp cleaning, where the dissolved solid (DS) contents in the pulp liquor is reduced prior sending to the bleach plant. Meanwhile, hot steam is consumed in black liquor heating and treatment, as well as delignification process in the oxygen (0₂) reactor. The mass and energy balances of the water network for the BSWS is first simulated with Cadsim, and adopted as the base case of the study. Next, a non-linear programming (NLP) optimization model is proposed to minimize the water and energy consumptions through wash water reuse/recycle. The NLP model is solved using LINGO optimization software. The proposed optimization approach reduces both energy and water consumption requirements in the BSWS significantly. A new water network configuration which achieved the abovementioned utility savings is proposed in this paper.