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Licensed Unlicensed Requires Authentication Published by De Gruyter December 13, 2018

Application of Adaptive Feedforward-Feedback Control on Multiple Effect Evaporator Process

Felipe Matheus Mota Sousa and Rodolpho Rodrigues Fonseca

Abstract

Evaporation is one of the most standard procedures in many industrial processes. Although its importance, this operation is quite expensive and, to minimize costs, it is applied multiple effect evaporators. In terms of process control, this configuration is more susceptible to disturbances and also more complicated to control due to process non-linearity. In this paper, a two effects evaporator in a feedforward pattern of feeding was controlled by different adaptive strategies of regulatory control. It was developed feedback and feedforward-feedback control loops in which their PID and lead-lag control parameters were modified according to a gain scheduling strategy based on process variable value. It was shown that adaptive strategy used in feedback control led to a better performance against non-adaptive control, reducing ISE, IAE and ITAE criteria by 12.60%, 7.86% and 13.98% respectively, but also the settling time. However, the final control element was affected leading to 59.77% increase of control effort (ISU). On the other hand, feedforward-feedback allowed even more disturbance rejection than adaptive feedback. As example, IAE values were reduced by 29.96% and 48.17% for feedforward-feedback using non-adaptive and adaptive lead-lag control, respectively. Although their better performance, both feedforward-feedback loops increased control effort, reaching even 70 times of feedback ISU value.

Appendix

A Correlations for thermal and physical properties of process stream and steam

Density of solution [kg/m3]

(20)ρ=1000+Bxi200+Bxi5410,036Ti20160Ti

Boiling point rise [° C]

(21)BPRi=0,03+0,018PiTv,i+84Bxi100Bxi

Specific heat capacity [J/(kg° C)]

(22)Cp,i=4186,829,7Bxi+4,16BxiPi+0,075BxiTi

Latent heat of vaporization [J/kg]

(23)ΔHvi=226328058210log(Pi)

Enthalpy of saturated steam [J/kg]

(24)hvi=1766,0799,65+27,55logPi+1,8log(Pi2)+2501800

B Multiplicative factors for mathematical model

C1=1+BPRiTviC2=BPRiBxiC3=AiρiC4=AiLiρiTi1+BPRiTviC5=AiLiρiBxi+BPRiBxiρiTiC6=AiρiBxiC7=AiLiBxiρiTi1+BPRiTviC8=AiLiBxiρiBxi+BPRiBxiρiTi+miC9=AiρihiC10=AiLihiρiTi1+BPRiTvi+1+BPRiTvihiCp, iCp, iTi+hiTimiC11=AiLihiρiBxi+BPRiBxiρiTi+hiCp, iCp, iBxi+BPRiBxiCp, iTi+BPRiBxihiTimi

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Received: 2018-07-16
Revised: 2018-11-06
Accepted: 2018-11-09
Published Online: 2018-12-13

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