Fundamental modeling and control of falling film evaporators

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Title: Fundamental modeling and control of falling film evaporators
Author: Stefanov, Zdravko Ivanov
Abstract: 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 operation 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 . The recovery boiler is a combination of a chemical reactor and a power boiler . The dry solids concentration of the black liquor affects the recovery boiler performance not only from an economical point of view but also for safety reasons . It is known , that if the dry solids concentration of the black liquor falls below a lower limit , there is the possibility of an explosion in the recovery boiler . Evaporation of the waste liquor is usually accomplished in a multiple effect evaporator plant . While there are more than one type of evaporator design , 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 . The first is to develop a rigorous distributed parameter model of the falling film evaporator using the fundamental principles of mass , energy , and momentum conservation . The second is 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 is satisfactory to describe the important transfer processes on the plate accurately . Additionally , it was confirmed by experimentation that two different hydrodynamic regimes (turbulent and wavy -laminar ) can exist in the multiple effect black liquor evaporator plants . 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 disturbance and parameter uncertainty .
URI: http : / /hdl .handle .net /2346 /9054
Date: 2004-05


Stefanov, Zdravko Ivanov Fundamental modeling and control of falling film evaporators. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /9054 .

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