(466b) Safeparking of Multi-Unit Process Systems

The operation of chemical processes often encounters faults in process equipments such as actuators and sensors. Equipment failure can have a serious impact on the product quality, lead to undesirable pollutant emission and impact the overall plant productivity and economy negatively. To overcome these problems without interrupting nominal process operation, significant research efforts have focussed on devising automated methods for online diagnosis and isolation of faults and in developing online strategies for performance recovery from the degraded conditions. The extensive research in fault-tolerant control (FTC) is based on the assumption of availability of sufficient control effort or redundant control configurations to preserve operation at the nominal equilibrium point in the presence of faults (see, e.g., [1,2,3]). In contrast, the problem of faults that do not allow continuation of operation at the nominal operating point has not received sufficient attention. In the absence of a framework for handling such faults, ad-hoc approaches could result in temporarily shutting down the process which can have substantially negative economic ramifications.

In [4], a safe-parking framework was developed to address the problem of determining how to run the process during fault-rectification to prevent onset of hazardous situations and enable smooth transition to nominal operation upon fault recovery. In [5], the safe-parking framework is extended to handle uncertainty and unavailability of measurements. The results in [4,5], however, consider safe-parking in the context of single unit. The connected nature of chemical processes via material, energy or communication lines can have significant impact on the ability of safe-parking the process. In particular, a change in operating condition of one unit naturally acts as disturbance to the downstream units and hence large changes in operating conditions of one unit, while possibly enabling safe-parking of the unit in question, can jeopardize the operation of the downstream units, and therefore of the whole plant. This necessitates that the safe-park point for a unit in multi-unit process be chosen with adequate consideration of its effect on downstream processes.

Motivated by above consideration, this work addresses the problem of handling faults in the context of multi-unit networked processes. We consider a multi-unit nonlinear process system subject to input constraints and faults in one unit that preclude the possibility of operating the unit at its nominal equilibrium point. We first consider the case where there exists a safe-park point for the faulty unit such that its effect can be completely absorbed (via changing the nominal values of the manipulated inputs) in the downstream unit. Steady-state as well as dynamic considerations (including the presence of input constraints) are used in determining the necessary conditions for safe-parking the multi-unit system. We next consider the problem where no viable safe-park point for the faulty unit exists such that its effect can be completely absorbed in the subsequent unit. A methodology is developed that allows simultaneous safe-parking of the consecutive units. Finally, we incorporate performance considerations in the safe-parking framework for multi units systems and illustrate the implementation of the safe-parking framework using a multi-unit chemical reactor system.

References

[1] Mhaskar, P., A. Gani, N. H. El-Farra, C. McFall, P. D. Christofides and J. F. Davis, "Integrated Fault Detection and Fault-Tolerant Control of Nonlinear Process Systems,'' AIChE J., 52, 2129-2148, 2006.

[2] P. Mhaskar, ``Robust Model Predictive Control Design for Fault-Tolerant Control of Process Systems,'' Ind. Eng. & Che. Res., 45, 8565-8574, 2006.

[3] Mhaskar, P., C. McFall, A. Gani, P. D. Christofides and J. F. Davis, "Isolation and Handling of Actuator Faults in Nonlinear Systems,'' Automatica, 44, 53-62, 2008.

[4] Gandhi, R. and P. Mhaskar. Safe-parking of nonlinear process systems. Comp. & Chem. Eng., accepted for publication.

[5] Mahmood, M., Gandhi, R. and P. Mhaskar. Safe-parking of nonlinear process systems: Handling uncertainty and unavailability of measurements, Chem. Eng. Sci., submitted.