Tablas Heuristicas

I NTERNATIONAL J OURNAL OF C HEMICAL
R EACTOR E NGINEERING
Volume 8

2010

Note S3

Cascade Control Scheme for Tubular
Reactors with Multiple Temperature
Measurements
Eliseo Hernandez-Martinez∗

Hector Puebla†

Jose Alvarez-Ramirez‡

∗

Universidad Autonoma Metropolitana Iztapalapa, elijazfan@yahoo.com
Universidad Autonoma Metropolitana Azcapotzalco, hpuebla@correo.azc.uam.mx‡
Universidad Autonoma Metropolitana Iztapalapa, jjar@xanum.uam.mx
ISSN 1542-6580
Copyright c 2010 The Berkeley Electronic Press. All rights reserved.
†

Cascade Control Scheme for Tubular Reactors with
Multiple Temperature Measurements
Eliseo Hernandez-Martinez, Hector Puebla, and Jose Alvarez-Ramirez

Abstract
In this work, we introduce a new structure for cascade control schemebased
on a weighted average temperature of three-temperature measurement distributed
along the axial position of the tubular reactor. The control configuration exploits
the information provided by two additional temperature sensors located close to
the feed and the output positions of the tubular reactor. The configuration with
the averaged temperature improves the behavior of the classicalcascade control
scheme by enhancing the disturbance rejection. Numerical simulations of two
axial dispersion models of tubular reactors are used to illustrate and compare the
control performance.
KEYWORDS: tubular reactors, cascade control, hot spot

Hernandez-Martinez et al.: Control for Tubular Reactors with Multiple Measurements

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1

Introduction

Tubular reactors are difficult tooperate due the intricate coupling between
transport processes, nonlinear chemical kinetics, and their distributed nature.
Nevertheless, due to numerous industrial applications for chemical tubular
reactors, the problem of monitoring and controlling them is of great safety
and economical importance. The essential aim of the control in tubular reactors is the regulation of the outlet concentration.The composition control
problem in tubular reactors has been extensively studied with linear (Pellegrine et al., 1993; Alvarez-Ramirez et al., 2000) and nonlinear control schemes
(Christofides, 2001; Hoo and Zheng, 2002; Wu and Huang, 2003; Li and
Christofides, 2007). Given that composition measurements generally are delayed because of constraints in the response of measurement devices andinternal transport, a feedback loop, based only on composition measurement
can suffer from poor performance and even instabilities. To alleviate this
situation, composition-temperature cascade control scheme have been proposed. Hua and Jutan (2000) designed a nonlinear cascade control with inferential schemes based on model reduction. The proposed cascade control is
composed of a nested configurationof an internal temperature control loop
aimed at achieving stabilization tasks and an external composition control
loop oriented to exit composition objectives. The result is an efficient control scheme that is robust against feed disturbances while introducing good
regulation properties. Urrea et al. (2008) proposed a cascade control scheme
temperature-composition, where the temperature controlloop employed an
average of two-temperature measurements located before and after of the expected hot spot position in tubular reactors. This cascade control scheme
improves the performance control in the face of feed composition and temperature disturbances. More sophisticated control schemes have been reported in
the literature (Christofides, 2001; Hoo y Zheng, 2002; Wu y Huang, 2003; Li
andChristofides, 2007). However, in general, they have limited application for
industrial application because their complexity and dependency on an exact
mathematical model of the tubular reactor. On the other hand, the PI/PID
compensators are accepted well in the industry due to his facility of design
and implementation (Alvarez-Ramirez et al., 2000).
Diverse studies demonstrate that in...