Optimization of multiple-effect evaporation in the pulp and paper industry using response surface methodology

The steady-state evaporation process was modeled by means of a phenomenological model based on mass and energy balances. The Newton method for nonlinear systems was adopted and then statistical methods were used for the preparation of an empirical model. The mathematical model was validated through a comparison with data reported in the literature. Surface responses were used for the optimization of the process by considering the output composition of the black liquor and the coefficient performance of the evaporator system (COP) as the response variables. The variables evaluated in the planning were the live steam ratio and the temperature of the live steam in evaporators 1 and 2. The temperature of the live steam entering the second evaporator was shown to be the most influential variable for the output composition, followed by the live steam flow ratio. For COP, the live steam flow ratio was the most influential variable, followed by the temperature of the live steam entering the first and second evaporators. The maximum value of the COP was achieved for a live steam flow ratio of 0.47 with the temperature of the live steam in evaporator 1 at 128.24 °C and the temperature of the live steam in evaporator 2 at 137.52 °C with a desirable function of 5.29.