LINEAR PI TEMPERATURE-CONCENTRATION CASCADE CONTROL FOR TUBULAR REACTORS

In this article we explore the application of linear PI cascade control schemes to improve the performance of industrial PI/PID controllers for controlling outlet reactor concentration. By departing from simple I/O first-order dynamical models obtained from step responses, it is shown that the incorporation of a secondary loop for regulating the reactor temperature at a given interior position significantly improves the control performance in the face of feed composition and temperature disturbances. The effects of the temperature sensor location and the usage of multiple temperature measurements are also evaluated.     

THE MASS BALANCE FOR GAS PHASE REACTIONS IN TUBULAR REACTORS

In this paper we discuss the use of an approximated form of the mass balance equation for a tubular reactor in which a gas phase reaction is carried out. By means of three examples it is shown that if density variations are not taken into account in a correct way, surprisingly high errors can occur. More specifically, if the mass balance is written down in a generally applied approximate form, i.e. using udc/dz instead of d(uc)/dz—u, c and z respectively being the gas velocity, concentration and length coordinate—even for density variations as low as 5%, errors of a magnitude higher than 5% can be introduced. This is particularly true for multiple reaction systems. The inexpediency of approximating d(uc) by udc is stressed: the approximation is in no way a simplification. Therefore, this commonly seen approximation should never be applied.     

MODELLING OF AXIALLY DISPERSED NON-ISOTHERMAL TUBULAR REACTORS FOR HYDROPROCESSING OF SOLID-LIQUID MIXTURES

A mathematical model for hydroprocessing of coal-oil slurries in non-isothermal, axially dispersed tubular reactors is developed and numerically solved, considering a five component-six reaction model representing the chemistry of the process, and, dispersive, corrective and interfacial mass and energy transfers. Several simulation results are presented.     

OPTIMAL TUNING OF PID CONTROLLERS FOR SINGLE AND CASCADE CONTROL LOOPS

Design of one parameter tuning of three-mode PID controller was developed in this present study. The integral time and the derivative time of the controller were expressed in terms of the time constant and dead time of the process. Only the proportional gain was observed to be dependent on the implemented tunable parameter in which the stable region could be predetermined by the Routh test. Extension of the concept towards designing cascade PID controllers was straightforward such that only two parameters for the inner and outer PID controllers required to be tuned, respectively. The optimal tuning correlative formulas of the proportional gain for single and cascade control systems were obtained by the least square regression method.     

SIMPLIFIED MODEL PREDICTIVE CONTROL FOR SINGLE AND CASCADE LOOPS

An analysis of the simplified model predictive control (SMPC) has been carried out so that the digital algorithm involved could be conveniently applied to single loop control of complex higher order processes with noise and dead time. Simulation results are presented to show the excellent capability of the control technique particularly in the presence of noise. Certain aspects related to SMPC tuning, such as development of a simple closed loop tuning procedure, have been discussed. Finally, the control law is extended to cascade control structure involving a primary and a secondary loop.     

A SIMPLE METHOD FOR TUNING CASCADE CONTROL SYSTEMS

A simple method for tuning controllers in a cascade system is presented, In this method, all the relations that facilitate the tuning procedures are well prepared in terms of figures or simple equations. Using these figures and equations, the controller tuning for different configurations of cascade systems becomes easy and straightforward when process models are available. On the other hand, when process models are not available, a simple method that uses one single run of step input experiment to develop such models is proposed. Based on these developments in the controller tuning and process models, an autotuning system that uses relay feedback is presented. Unlike the existing autotuning systems, this proposed system conducts identification and controller tuning in a decoupled manner. As a result, no excessive trial- and-error efforts for modeling and tuning are required. Simulation results show the potential usage of such a method, It is interested to see that the resulting systems have almost compatible responses to those systems which have been designed optimally in one way or another as reported in the literature. It is not, however, the purpose of this article to emphasize on obtaining superior performance to all other existing methods, but to emphasize on its effectiveness and simplicity for application.     

SBR工艺过程控制策略研究进展

将SBR过程控制策略分为定时控制、实时控制和智能控制3类,分别进行了简要的介绍,并介绍了采用新工艺机理的SBR控制技术,重点对SBR实时控制策略原理和应用进行了分析、归纳和总结.并对SBR工艺过程控制策略的未来研究方向进行了展望.     

ADAPTIVE LINEAR QUADRATIC CONTROL OF BATCH AND SEMI-BATCH CEHMICAL REACTORS WITHIN THE DELTA OPERATOR FORMULATION

Batch and semi-batch chemical reactors are extensively used in polymer, fine chemical and pharmaceutical industries. The temperature control is of fundamental importance from product quality and reproductibility points of view. It is then necessary to improve the automation of these plants which are typically controlled using standard PID controllers. An autotuning PID controller is proposed and evaluated by application to a batch and semi-batch pilot plant reactor to investigate advanced control techniques. The involved control design is derived in the spirit of the partial state reference model adaptive control (PSRMAC) approach within the delta operator formulation. PSRMAC is mainly motivated by its suitable tracking capability when only a crude knowledge about the plant to be controlled is available. The delta operator formualtion is particularly motivated by the convergence of the performances to their continuous time counterpart as well as its numerical robustness when the sampling is required to be fast.     

PERIODIC OPERATION OF ISOTHERMAL PLUG-FLOW REACTORS FOR AUTOCATALYTIC REACTIONS

The performance of isothermal plug-flow tubular reactors under periodic inlet concentration is theoretically analyzed for improvement in yield for liquid phase, homogeneous, autocatalytic reactions of both quadratic and cubic forms. The system of two hyperbolic partial differential equations describing the reactor is solved by the topological method suggested by Bailey (1977). The negligible yield of product obtained at steady-state operation is enhanced to 100 percent yield under periodic operation. The performance of the periodically forced reactor with autocatalytic reactions is compared with that of ordinary simple reactions. The improvement in the yield is much more (at least five times) than that of ordinary simple reactions. The effect of decay of autocatalyst to stable product and the effect of reversibility of reactions on the performance of the periodically forced reactor are evaluated. For irreversible reaction with decay, the yield shows a resonance with inlet pulse width. This is due to complete conversion of the reactant and further only the decay of the autocatalyst.     

ON THE RESPONSE FUNCTION AND RESIDENCE TIME DISTRIBUTION OF THE AXIAL DISPERSION MODEL FOR TUBULAR REACTORS

In view of confusion on the residence time distribution and transfer fucntions for dispersion models, the solution of the model subject to possible combinations of closed and open type boundaries at the ends of a chemical reactor is investigated systematically. In particular, some new results are obtained for the closed-open and open-closed types of reactors. It is found that the interchange of boundary conditions for these reactors is not immaterial. In several cases, caution is necessary to distinguish whether the transfer function is applied to the concentration or the flux. When the dispersive flux is notable, measurement techniques must be carefully selected to match the kind of transfer functions derived.     

STEADY AND NON-STEADY STATE MODELING OF TUBULAR FIXED BED REACTORS

In this work a detailed non-steady state two dimensional heterogeneous model for a non-isothermal, non-adiabatic packed bed tubular reactor is discussed. The model has the following features: i) axial and radial heat and mass dispersion, ii) heat transfer balance for the cooling fluid, iii) comprehensive treatment of the convective term, iv) spatially-dependent physical parameters, v) radial porosity and velocity profiles. Results are given, for both steady and non-steady state situations, in the context of an industrial unit for phthalic anhydride production. Co-current flow of reactive and cooling mediums is assumed.     

ROBUST ANALYSIS AND PID TUNING OF CASCADE CONTROL SYSTEMS

The robustness and performance of the cascade control system are analyzed in this article. A robustness measure is defined, and it is shown that it can reflect the interaction between the inner loop and the outer loop and has a clear indication of the robustness of each loop. The measure can serve as a graphic aid for tuning the cascade controller. Moreover, it is observed that the cascade controller can be tuned individually for each loop instead of sequentially.     

ROBUST ANALYSIS AND PID TUNING OF CASCADE CONTROL SYSTEMS

The robustness and performance of the cascade control system are analyzed in this article. A robustness measure is defined, and it is shown that it can reflect the interaction between the inner loop and the outer loop and has a clear indication of the robustness of each loop. The measure can serve as a graphic aid for tuning the cascade controller. Moreover, it is observed that the cascade controller can be tuned individually for each loop instead of sequentially.     

ON OPTIMIZATION IN TUBULAR REACTOR SYSTEMS

The Laguerre polynomial approximation technique is employed in obtaining optimal control variable profiles in distributed parameter systems. The classic problem posed by Denn et al. (1966) is presented and optimal cooling flux profiles obtained by several methods are compared.     

ROBUSTNESS ANALYSIS OF A FIXED-BED TUBULAR REACTOR: IMPACT OF MODELING DECISIONS

A robustness analysis technique is applied to the control of a tubular fixed-bed reactor. Critical modeling decisions such as lumping and order reduction are evaluated in terms on their impact on the model/plant mismatch and its consequences in closed-loop stability and performance. It is shown that a previously proposed control scheme lacks robustness and thus is very difficult to control in the presence of modeling errors.     

A GENERAL RUNAWAY CRITERION FOR FIXED-BED REACTORS

The safe design and operation of fixed bed reactors require the consideration of potential runaways. The steep temperature rise may affect the selectivity, activity and stability of catalysts and in certain cases the reactor safety.

Mathematical models simulate the runaway conditions. Such simulations require extensive calculations to define the range of safe operation. A new runaway criterion was developed to replace the need to solve the differential reactor balances. The criterion is simple, defined by one algebraic equation. It can be applied to any kinetic expression. Extensive testing shows an excellent agreement between the runaway criterion and the two-dimensional model predictions. It also predicts the runaway conditions in agreement with experimental data.     

固定床反应器的控制

主要综述了在实验反应器或仿真反应器上实现的,并具有工业应用前景的有关控制文献,介绍了固定床反应器控制的研究概况.最后,对当前的固定床反应器控制研究提出了一些见解和展望.     

ROBUST CONTROL OF BATCH REACTORS

The design of robust nonlinear feedback controller is analysed for a trajectory tracking in a single-input single-output nonlinear state variable system x = f(x) + g(x)u, y=cx which arises in nonlinear chemical processes particularly in batch reactor control problems. Simulation results for the batch reactor temperature tracking problem show the effectiveness of the control scheme and its robustness to modelling errors. The method is also applicable to multi-input multi-output system where the number of inputs is equal to that of outputs. The controller design is also analyzed for situations wrier: the kinetics, the activation energy and Ihe heat of reaction are unknown and also only limited measurement of state-variables are available. The method of Youcef-Toumi and Ito (1987) is applied to such problems and the effectiveness of control system is shown by simulation.     

HOMOGENEOUS GAS-SOLID SUSPENSIONS FOR EFFICIENT DOWNFLOW REACTORS

In order to increase heat and mass transfer and to accelerate high temperature gas-solids reactions of fine powders, downflow tube-reactors have attracted growing attention. Short reaction times of fractions of a second require short term constant feeding of gas and solids followed by the continuous and rapid formation of homogeneous suspensions with low fluctuations in time to meet required mole ratios. The objective of the present study is the preparation of short-term constant, homogeneous gas-solid suspensions under gravitational flow conditions in downflow reactors. The dynamics of dispersing a very compact dense solids jet lo achieve a homogeneous gas-solid suspension was investigated in a semi-industrial test unit with a maximum solids throughput of 5 t/h and different tube geometries.     

A MODEL FOR PULSING FLOW IN COCURRENT DOWN-FLOW TRICKLE-BED REACTORS

Based on macroscopic mass and momentum balances, a model is developed to characterize the hydrodynamic behavior of pulsing flow in cocurrent down-flow trickle-bed reactors. Predictions of the model regarding liquid saturation, length and velocity of both the liquid rich slug and gas rich pulse are reported. Also presented are experimental data of an air-water system for the effects of varying gas and liquid flow rates on pulse frequency, total pulse length and apparent slug velocity. To confirm the theory, predictions are compared with measured overall pressure gradient. Agreement is reasonably good.