Control of Chemical Reactors

In any reaction system operating at steady state, there will always be upsets to the system—some large, some small—that may cause it to operate ineflfi-ciently, shut down, or perhaps explode. Examples of upsets that m occur are variations of the feed temperature, composition, and/or flow rate, the cooling jacket temperature, the reactor temperature, or some other variable. To correct for these upsets, a control system is usually added to make adjustments to the reaction system that will minimize or eliminate the effects of the upset. The material that follows gives at most a thumbnail sketch of how controllers help cause the reactor system to respond to unwanted upsets. 

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There are several control problems in chemical reactors. One of the most commonly studied is the temperature stabilization in exothermic monomolec-ular irreversible reaction A B in a cooled continuous-stirred tank reactor, CSTR. Main theoretical questions in control of chemical reactors address the design of control functions such that, for instance (i) feedback compensates the nonlinear nature of the chemical process to induce linear stable behavior (ii) stabilization is attained in spite of constrains in input control (e.g., bounded control or anti-reset windup) (iii) temperature is regulated in spite of uncertain kinetic model (parametric or kinetics type) or (iv) stabilization is achieved in presence of recycle streams. In addition, reactor stabilization should be achieved for set of physically realizable initial conditions, (i.e., global... 

De Wolf, S. Jager, J. Kramer, H.J.M. Eek, R. Bosgra, O.H. Proc. IFAC Symp. on Dynamics and Control of Chemical Reactors. Distillation Columns and Batch Processes. Maastricht, The Netherlands, I989. 

Raw-lings, J.B. Dwwm/i and Control of Chemical Reactors. Distillation Columns and Batch Ptrtcesses (Dycord 95) A Postprint Volume Emm the 4th I FAC Symposium. Htlsingor, Denmark. 1995. Elsevier Science, New York. NY. 1995, Stichlmair. J. and J.R. Fair Distillation PrinciplesandPractice. John Wiley Sons. Inc.. New York. NY, 1998. 

Questing control of chemical reactors. Paper presented at the Annual Meeting of the American Institute of Chemical Engineers, Mexico City, October 1966. 

The key element in temperature control of chemical reactors is to provide sufficient heat transfer surface area or some other heat removal mechanism so that dynamic disturbances can be safely handled without reactor runaways. 

Padmanabhan, L. and Lapidus, L., "Control of Chemical Reactors" in Chemical Reactor Theory A Review, Ed. by L. Lapidus and N. R. Amundson, Prentice-Hall, Englewood Cliffs, N. J., 1977. 

The suppression or the limitation of intermediate storage tanks in order to improve the overall dynamics and/or increase the safety. The result is that the control of chemical reactors and separators is submitted to more frequent and larger disturbances. 

Here we have dealt with the control of chemical reactors. We covered some of the fundamentals about kinetics, reactor types, reactor models, and open-loop behavior. In particular we have shown that reactors with recycle or backmixing can exhibit multiple steady states, some of which are unstable. Nonlinearities in reactor systems also frequently give rise to open-loop parametric sensitivity. 

Sorensen, J. P., Simulation, Regression and Control of Chemical Reactors by Collocation Techniques, Doctoral Thesis, Danmarks tekniske Hpjskole, Lyngby (1982). 

We will then discuss reactor start-up, falling off the upper-steady state, the control of chemical reactors, and multiple reactions with heat effects. 

In chemical process control the variable that is most commonly inferred from secondary measurements is composition. This is due to the lack of reliable, rapid, and economical measuring devices for a wide spectrum of chemical systems. Thus inferential control may be used for the control of chemical reactors, distillation columns, and other mass transfer operations such as driers and absorbers. Temperature is the most common secondary measurement used to infer the unmeasured composition. 

J. B. Rawlings, N. F. Jerome, J. W. Hamer, and T. M. Bruemmer. End- point control In semi-batch chemical reactors. In Proceedings of the IFAC Symposium on Dynamics and Control of Chemical Reactors,. Distillation Columns, and Batch Processes, pages 323-328,1989. 

Ellingsen, W. R., "Diagnosing and Preventing Tray Damage in Distillation Columns, DYCORD 86, IF AC Proceedings of International Symposium on Dynamics and Control of Chemical Reactors and Distillation Columns, Bournemouth, U.K., Dec. 8-10, 1986. 

The participants had the opportunity to contribute to the NATO-ASI during the Poster Session, Posters covering a wide range of subjects were presented non-isothermal trickle beds, catalyst deactivation, mixing in fluidized beds, control of chemical reactors, maldistribution in chemical reactors, cyclic operation in trickle beds, polymerization reactors, etc. The most relevant contributions of the Poster Session were selected to be included in this NATO-ASI Proceedings,... 

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