Fundamental modeling and control of falling film evaporators.

摘要

Evaporators are a common unit operation that can be found in many industries. The evaporator plant, in the pulp and paper industry provides a major role of regenerating the process chemicals from the fiber line waste liquor. The effectiveness of the recovery, determines the overall mill economy. Consequently, the recovery cycle must be fully operational because it is unacceptable to discard the waste liquor due to its highly negative effect on the surrounding ecosystem.; The product of the evaporator plant, the concentrated black liquor serves as a fuel to the recovery boiler which is a combination of a chemical reactor and a power boiler. The dry solids concentration of the black liquor affects the recovery boiler performance from an economical point of view and for safety reasons.; Evaporation of the waste liquor is usually accomplished in a multiple effect evaporator plant. The most modern and efficient design is the falling film plate evaporator. This design is characterized with very high heat transfer rates at small temperature differences and high resistance to scaling due to low residence times.; This research has two main objectives: to develop a rigorous fundamental distributed parameter model of the falling film evaporator and to synthesize an effective control structure for the evaporator and the evaporator plant. A bench-scale experiment has shown that one-dimensional distributed model of the evaporator plate satisfactory to describe the important transfer processes on the plate accurately.; Investigations into simple and advanced control approaches have revealed that the closed-loop performance of a proportional-integral-derivative (PID) controller design in feedback with a single evaporator can provide satisfactory compensation. However, in the case of the entire evaporator plant, the advanced control approach of model-predictive control (MPC) provides better control because the MPC centralized controller can address multiple interactions, input and output constraints, and unmeasured disturbances.; This work presents the development of the distributed parameter model and the synthesis of the control structure; and demonstrates the performance of the closed-loop system to measured and unmeasured disturbances and parameter uncertainty.