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Plantwide Optimization of a Pulp Mill Process

An economic optimization study of a pulp and paper mill is presented. Costs and revenues associated with all unit operations are identified to formulate a suitable objective function. The existing control structure of the pulp mill is analyzed and used to develop an optimization-relevant model through a series of step tests. This model is then used with the objective function and the information about other constraints for process variables to devise a linear programming (LP) problem to find the most profitable operating conditions for the pulp mill process. The LP solution predicted an improvement in profitability of 47% compared to the nominal operating conditions of the pulp mill. In the next stage, an optimizer is interfaced with the existing control structure to transition the plant to the newly calculated, more profitable operating point. Simulation results showed a profitability improvement of 24% at the new operating conditions. The discrepancy is attributed to the structural limitation of the optimization-relevant model and the complex nonlinear behavior of the pulp mill process.