Rafael Maya-Yescas

Temperature control in catalytic cracking reactors via a robust PID controller

The stabilization of fluid catalytic cracking reactors is tackled in this paper. A robust PID control law is developed in order to control the outlet reactor temperature. The suggested control is based on a reduced order model of the reactor given by a system of ordinary differential equations. The controller is synthesized using an input/output linearizing control law coupled to a proportional-derivative reduced order observer to infer on-line the unknown heat of reaction. The proposed control algorithm leads to a classical PID structure. New tuning rules are given, based on the system structure, estimations and closed-loop time constants. This control strategy turns out to be robust against model uncertainties, noisy temperature measurements and set point changes. The performance of the reaction temperature in a tubular riser reactor is numerically compared when the proposed control scheme and standard PID controllers are applied. # 2002 Elsevier Science Ltd. All rights reserved.

Controllability Assessment Approach for Chemical Reactors: Nonlinear Control Affine Systems

Abstract Although there have been several attempts to develop systematic evaluation of control schemes, the choosing and pairing of control variables in chemical reactors is not an easy task. Intrinsic nonlinearities of this kind of systems provoke dynamic responses that are difficult to predict. In this paper, a simple proposition for the evaluation of pairs of control and manipulated variables is developed for nonlinear control affine systems. It compliments the RGA analysis for nonlinear systems because is based on the relationship between zero dynamics and control stability. The resulting strategy is simple, easy to understand and easy to apply for the analysis of control schemes. Also, it is independent of the type of controller used. As an example, it is probed in the evaluation of four control options for industrial FCC regenerators; two of them applied in industry. The results obtained when evaluating the control strategies proposed in four different situations are coherent with industrial practice and operating experience.

DESIGN STUDY OF THE CONTROL OF A REACTIVE THERMALLY COUPLED DISTILLATION SEQUENCE FOR THE ESTERIFICATION OF FATTY ORGANIC ACIDS

This article reports the esterification of lauric acid and methanol studied using a thermally coupled distillation sequence with a side rectifier and the Petlyuk distillation column. The product of the esterification can be used as biodiesel. It was found that the thermally coupled distillation sequence with a side rectifier can produce ester with a high purity (around 0.999) and also pure water, and the excess of methanol is recovered in the side rectifier. The results indicate that the energy requirement of the complex distillation sequence with a side rectifier can be reduced significantly by varying operational conditions. These reductions in energy requirements can be interpreted as reductions in carbon dioxide emissions. Moreover, dynamic tests for control of the composition of the ester and control of two temperatures for the thermally coupled distillation sequence with a side rectifier indicate that it is possible to eliminate disturbances in the feed composition, while the composition of the biodiesel remains at the desired value.

Robust PI2 controller for continuous bioreactors

Abstract An estimation algorithm related to the kinetics terms of the main uncertainties present in a continuous stirred tank bioreactor, is developed. This algorithm is based on a proportional–integral reduced order uncertainty observer. With the estimate generated of the uncertain term, an input–output linearizing feedback control is designed which provides robust regulation model uncertainties, noisy measurements and sustained disturbances. This control strategy could be represented as PI 2 controller where new tuning rules of the controller gains are given in terms of the estimation and closed-loop time constants. The performance of the estimation algorithm and the corresponding closed-loop behaviour of the system are compared to a standard PI controller and they are illustrated by means of numerical simulations.

Reactivity of Fluid Catalytic Cracking Feedstocks as a Function of Reactive Hydrogen Content

Abstract Detailed feedstock composition analysis is a reasonable support for predicting product yields and quality parameters from the chemistry involved in FCC processing. Nonetheless, given the huge number of individual hydrocarbons present in common FCC feedstocks, it is necessary to lump them in Hydrocarbon Types (HT). In order to understand the interactions during FCC processing between these different hydrocarbon types and its relationship to yield and quality of end products, it is necessary to consider relative content and reactivity of these carbon families in the raw material. Hydrogen content of each hydrocarbon is a good measure of the cracking performance of this hydrocarbon. As hydrogen content increases in a molecule, the yields to LPG and gasoline increase. In this article, based on HT identification and refinery-measured feedstock properties, an estimation of reactivity for any feedstock is obtained. Two types of hydrogen, which are commonly part of the same molecule were defined as refractory (nonreactive) and reactive. Based on these types of hydrogen, a Reactivity Factor (RF) was introduced and defined as a relative index that measures cracking potential of FCC feedstocks. The described approach to classify FCC feedstocks, will be useful in future developing of correlations predicting yields and quality of end products.

Reactive Hydrogen Content: A Tool to Predict FCC Yields

Fluid Catalytic Cracking is a process conceived to produce gasoline as the main product. Feed stocks to the process are vacuum gas oils (VGO) showing boiling point ranges typically between 343°C and 538°C. Since these boiling range cut points are usually fixed, the carbon number range of encountered hydrocarbons is approximately constant and this is also true for the relative amounts of paraffins, naphthenes and aromatics, regardless of changes from feed to feed. Under FCC reaction conditions, each of the above hydrocarbon groups exhibit different crackability. In order to explain the existing relationship between feed composition and yields and quality of end products, it is necessary to establish the effect of composition and operational conditions on cracking behavior of feedstocks, expressed in terms of a relation between some specific properties, or as in our case, by its Reactive Hydrogen Content (RHC). Therefore, yields should then be dependent on three factors, RHC value, as a measure of feed quality, catalysts and applied operational conditions. In this work, a RHC for a series of feeds of known composition is determined and correlated to conversion and yields obtained under different operational conditions, using the same catalyst. The resulting correlations are then applied to unknown feeds with the RHC being calculated from physical properties, and yields.

Feedback Regulation of Temperature in FCC Regenerator Reactors

Abstract Design of regulative control laws for stabilization of regenerator temperature in adiabatic Fluid Catalytic Cracking Units is performed. This is achieved despite the fact that kinetic terms and heat generation are, commonly, unknown. This article presents a control law that makes use of on-line discrete measurements of temperature to estimate the heat of reaction, obtained from a neural-estimation algorithm. The controller design is similar to discrete PID controllers. Some new tuning rules are given in terms of closed loop and measured characteristic times. In order to obtain the simplest neural network and the minimum number of parameters to be adjusted, only one perceptron is used. Moreover, the resulting control law has physical meaning and can be interpreted as a standard PID controller with time varying gain. Performance of the neural-estimation technique is analyzed under closed-loop behavior using numerical simulations considering disturbances to the process and set point changes. The performance obtained when using this new control is compared with the one obtained by using a PID controller tuned following the IMC rules.

Impact of Production Objectives on Adiabatic FCC Regenerator Control

Abstract In adiabatic FCC regenerators, the classical approach of design and control of continuous stirred tank reactors identify stable steady states. This kind of reactors is a particular case of lumped parameter nonlinear systems. The easiness of regulation of the steady operating states depends on the relationship between system dynamics and control scheme. There have been several attempts for the evaluation of this relationship by systematic approaches, however the nonlinearities of these systems have troubled the evaluation goals. In partially controlled systems, such as chemical reactors, dynamic interactions among process variables might exhibit unexpected responses to control actions when the system is operating in some particular regions. These responses depend on the selection of control and manipulable variables. In this work, a methodology for the evaluation of control strategies is used to identify possible troubles when production objectives and process dynamics are noncompatible. The methodology is easy to understand and to apply to nonlinear lumped parameter systems that present control affine structure. Because the proposed methodology is independent of the kind of control used and the full process model is evaluated, it is possible to study the system behavior at any operating condition. An adiabatic FCC regenerator is used as example of the methodology application; three common production objectives are evaluated. Recommendations for the selection of operation objectives are given.

Comparison of two dynamic models for FCC units

Modelling of FCC units has been an interesting activity because of the complexity of the system and the economic incentives associated. Models have been classified depending on either the used kinetic scheme or the proposed reactor configuration. The main problems to assess are the elimination of coke when catalyst is regenerated and the delicate interaction between reactor and regenerator due to the global energy balance. In this work two models, each one using a different kinetic scheme, a different formulation for the catalyst activity and a different conception of the regenerator are compared when simulating the same FCC unit. Steady-state predictions and dynamic phenomena such as stability and multiplicity are simulated. In order to emphasise the ability for predicting the dynamic phenomena, the obtained results are compared in terms of accuracy and computing resolution time. It is pointed out that even though the complexity of a model could be a limiting factor for control purposes, in the case of dynamic studies this factor is not a constraint and it is possible to use more complex models. It is also emphasised that the most useful model will be the one which fulfils the requirements of the researcher, the situation to be modelled and the results that could be necessary.

Temperature Regulation via PI High-Order Sliding-Mode Controller Design: Application to a Class of Chemical Reactor

The main issue of this paper is the synthesis of a robust control law for regulation purposes, which is applied to a class of chemical reactor which exhibits highly nonlinear and oscillatory behavior. The considered methodology employs the typical structure of Proportional-Integral controllers, where the corresponding integral term is now proposed as an integral high order sliding-mode compensator, which deals with the intrinsic nonlinearities of the system to be regulated. A theoretical frame is provided to demonstrate that the proposed controller produces semi-global practical stability; performance of the proposed methodology is assessed via comparison with other controllers.