(740e) Safe-Parking of Networked Process Systems with Recycle Streams

Faults are ubiquitous in chemical process industries and can occur either in processing equipments or in auxiliary equipments such as actuators or sensors. Faults can have a serious impact on the plant economy, product quality and safety, productivity and pollutant emission. To overcome these problems without interrupting the process operation, significant research efforts has been made to devise automated methods for on-line assessment of the process performance, diagnose the cause for the performance degradation, and implement an on-line strategy for performance recovery from the degraded conditions. The extensive research in Fault Tolerant Control System (FTCS) 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. The problem of handling faults that do not allow continuation of operation at the nominal operating point is relatively neglected, even though there are number of fault scenarios in chemical industries that can render such situation. In [1], a safe-parking framework was proposed to address the problem of determining how to run the plant during fault-rectification to enable smooth transition to nominal operation upon fault recovery. In [3], the safe-parking framework is extended to handle uncertainty and unavailability of measurements. In [2], a safe-parking framework is extended to handle faults in the plant-wide setting, where multiple process units are connected via material and energy streams, to account for interaction between units while choosing safe-park point for the faulty unit. In [2], the safe-park point for the faulty unit is chosen such that when the faulty unit is safe-parked it has no or minimum effect on downstream units. Also, a methodology to carry out simultaneous safe-parking was proposed when the disturbance caused by safe-parking of faulty unit cannot be compensated in the downstream unit. In simultaneous safe-parking, safe-park points for each unit that needs to be safe-parked are chosen sequentially. The work in [2] considers multi-unit process where units are connected in series without recycle streams and hence, sequential choice of the safe-park points is possible. Most of chemical industries uses recycle streams to process unprocessed feedstock or to improve heat economy of the plant. The recycled (material/energy) stream makes it impossible to use sequential approach to choose safe-park points when multiple units need to be safe-parked and, hence, all the safe-park points should be chosen simultaneously using overall process plant model or the effect of recycle stream should be accounted while choosing safe-park points.

Motivated by above consideration, this research considers the safe-parking in the context of multi-unit networked process with material/energy recycle streams to account for the effect of recycle stream while choosing the safe-park points. We consider multi-unit nonlinear process system with recycle streams and the units connected in parallel & series. We consider an actuator fault in one of the unit such that it precludes the possibility of operating that unit at its nominal operating point. We first characterize the state-space region D, as proposed in [2], for the faulty unit such that, if the faulty unit is safe-parked at any safe-park point which is inside D, then the disturbance caused by it can be absorbed in downstream unit. This region D in conjunction with the stability region is used to choose safe-park point. This ensures the continuation of the nominal operation in the downstream unit and thus the nominal operation of entire plant except the faulty unit.

Next, we consider a scenario where the disturbance caused by the safe-parking of the faulty unit cannot be compensated/absorbed in the downstream unit. This situation requires safe-parking of more than one unit and in worst case can require safe-parking of the set of units with recycle stream. When safe-parking set of units with recycle stream, sequential approach presented in [2] cannot be used and the feedback effect of recycle stream should be accounted while choosing safe-park points. In this work, we use the overall steady state model of the plant (or a set of process units) to simultaneously enumerate candidate safe-park points for the units that need to be safe-parked. The dynamic models for individual units are used to verify whether a candidate safe-park point satisfies safe-parking requirements or not. We use stability region characterization proposed in [3] to estimate stability region for each unit and robust model predictive controller of [3] is used to execute safe-parking. Safe-park points for the units are chosen such that the end product quality is close to desired product quality and the safety of overall plant is preserved.

We demonstrate the efficacy of proposed methodology by simulation study on a process comprising of three CSTRs and a separator. The process has three CSTRs connected in parallel & series followed by a separator and the unprocessed feed from separator is recycled to the first reactor for reprocessing. We first assume a fault in first CSTR which doesn't allow the nominal operation in first unit and, therefore, it needs to be safe-parked. We show that by proper choice of safe-park point for CSTR-1, nominal operating point in the downstream unit can be maintained, resulting in same product properties from the separator during fault rectification. We also show that the nominal operation for entire process can be resumed after the fault is rectified. Then, we consider a fault in CSTR-2 such that there is no safe-park point available for CSTR-2 so that the nominal operation in other units can be maintained. For this case, we demonstrate the simultaneous safe-parking of entire plant considering the effect of recycle. We show that proposed approach guarantees the safety of overall system and, also, resumption of nominal operation after fault recovery.

References

[1] Gandhi, R. and Mhaskar, P. (2008). Safe-parking of nonlinear process systems. Comp. & Chem. Eng., 32:2113-2122.

[2] Gandhi, R. and Mhaskar, P. (2009). A safe-parking framework for plant-wide fault-tolerant control. Chem. Eng. Sci., 64:3060-3071.

[3] Mahmood, M., Gandhi, R., and Mhaskar, P. (2008). Safe-parking of nonlinear process systems: Handling uncertainty and unavailability of measurements. Chem. Eng. Sci., 63:5434-5446.