S. Macchietto

Detailed design of multipurpose batch plants

Abstract A formulation is presented for the optimal selection of both the equipment units and the network of connections for multipurpose batch plants so as to satisfy given production requirements for a variety of products. The model includes very general constraints and objective functions and permits to optimize simultaneously the structural aspects of the plant and the associated production schedule, accounting for both capital costs of equipment units and pipework, operating costs and revenues. A special novelty of the formulation is that it provides the possibility of generating directly the optimal connectivity between the process units, optimal sizing and costs of the transfer lines, and the optimal storage policies (and associated vessel sizes, if any) for stable intermediates without the need for a priori assumptions (e.g. unlimited storage, etc.). The resulting MILP problem is solved using a branch and bound method. Several examples are detailed, demonstrating the generality of the approach and the importance of including structural and operational aspects at the design stage. The flexibility of the formulation permits utilizing various design strategies, from a simultaneous optimization of all problem decisions (equipment, connections and schedule) to a sequential optimization of main equipment, followed by the design of connections etc. These strategies are also illustrated by an example.

Optimal control and on-line operation of reactive batch distillation

While both the dynamic modelling, and to a lesser extent, the optimisation of reactive batch distillation have been studied in the past, issues related to control and on-line operation of such a process have not been properly addressed. In this study, a priori optimal profiles of the operating variables are established for an industrial reactive distillation process. The optimal profiles are found either by maximising the profitability or by minimising the operating time subject to constraints on the reactor temperature and the loss of volatile reactant in the distillate. The control properties at optimal and non-optimal conditions are assessed. A one-point column control scheme with a PI-controller is designed to implement the optimal profiles. The controller performance is tested using a simulation model including disturbances in the reaction model and in the reboiler heat duty. The fully automatic operation of the process according to the optimal policies is demonstrated in conjunction with both a continuous controller and a real time control system. Good performance is achieved in both cases.

Simultaneous optimization of design and operation of multicomponent batch distillation column—single and multiple separation duties

A method is presented for the simultaneous optimization of a batch distillation column design and its operation, for single and multiple separation duties, each involving different multicomponent mixtures and complex operations with intermediate cuts. For operation structures selected a priori, the formulation presented permits the use of general distillation design and cost models. The objective function and constraints include capital and operating cost. In particular, the number of internal plates is optimized along with the most significant operating variables (recoveries in various cuts and reflux ratio profiles and times). The multiple duty formulation presented accounts for the different importance of each duty and setup time between batches. Application of the method to single duty multicomponent separation from the literature shows that significant profit improvements can be achieved within acceptable computing times. For multiple separation duties (two binary mixtures), the method clearly shows the importance of including allocation time to each duty and setup time for each batch in the objective function.

Some issues in the design and operation of a batch distillation column with a middle vessel

Some issues in the design and operation of a batch distillation column with a middle vessel are addressed. Simulation results indicate that operation at infinite reflux and reboil ratios may be more profitable than conventionally considered finite reflux and reboil ratios policies. From an experimental point of view, the possibility of running a pre-existing continuous pilot plant column in a complex batch mode is investigated. Practical suggestions for column design and process control are given.

Design of procedural controllers for chemical processes

Abstract A four step method is presented for the formal specification and synthesis of procedural controllers. These controllers are used in the process industries for safety and alarm operations, interlocking, batch sequencing, etc. The procedural controller is based on a process model that takes the form of a labelled finite state machine, termed a - machine . Desired process behaviour specifications are constructed using Predicate (PL) and Linear Temporal Logic (LTL) formalisms. The resulting controller structure can then be translated into a target sequential control language for implementation. The method is demonstrated with a small example.

Model based control of a multipurpose batch reactor - an experimental study

Abstract An experimental study of two nonlinear model based control methods for the temperature control of a multipurpose batch reactor is presented. A combination of a cascade control structure and suitably chosen controllers is shown to provide robust control in the presence of severe measurement error and model mismatch. Various nonlinear or linear control techniques can be accommodated in the cascade structure. Furthermore, the theoretical equivalence of GMC and GLC controllers is confirmed experimentally.

An application of automated operating procedure synthesis in the nuclear industry

Abstract The synthesis of operating procedures for the transient operation of chemical processes is a formidable and expensive task during the design of a plant. It is time-consuming and error-prone both for the process engineering department to specify the procedures and for the automation department to generate sequence control code from the specifications. A further source of error arises at the interface between the two departments since the boundary is typically ill-defined and engineers of different backgrounds often make assumptions without adequate understanding of the other discipline. In this paper, we describe the application of the CAPS (computer-aided procedure synthesis) system (Crooks et al ., 1992a,b,c) to a case study in the nuclear industry — British Nuclear Fuels (BNFL) Thermal Oxide Reprocessing Plant (THORP), which will be the largest facility in the world for reprocessing spent reactor fuel pins. The case study considers the “head end” plant of THORP which involves reagent preparation and fuel pin dissolution and is a single product batch and semi-continuous process. The CAPS system successfully generated realistic supervisory procedures and control sequence specifications which were in close agreement with the actual control sequences developed by BNFL engineers. Furthermore, the modelling effort required as input to CAPS was reasonable and all of the information was readily available early in the design life cycle.

PARTITIONING-SPACE APPROACH FOR NONLINEAR PROCESS CONTROL

Abstract A partitioning-space approach is presented for the design of nonlinear controllers. Easy to check conditions are presented for partitioning a general nonlinear SISO system into two subsystems, resulting in a cascade control structure which can accommodate standard or nonlinear control techniques in each subsystem. A particularly useful application is to a class of nonlinear systems for which model uncertainties can be restricted to just one subsystem. The resulting cascade structure is robust with respect to measurement errors, sever model mismatch and disturbance rejection. Simulation and experimental results on a batch reactor are presented.

A branch and bound procedure for the design of multipurpose batch plants with uncertain demands

A systematic procedure for the design of multipurpose batch plants subject to uncertain demands is proposed. The approach relies on the sequential refinement of upper and lower bounds on a stochastic flexibility index (SF). An optimization procedure is presented for the case of uncertain demands and discrete sizes of processing units. The procedure consists of the solution of a sequence of MILP problems. It guarantees the selection of the plant design with the best flexibility index value (within a given tolerance). The method is demonstrated on a case study.

SYNTHESIS OF PROCEDURAL CONTROLLERS FOR BATCH CHEMICAL PROCESSES

Abstract Procedural controllers are employed for the automation of event-driven operations in process systems. In this paper a formal definition for these controllers is proposed, using concepts developed from Supervisory Control Theory (SCT). Some of the limitations of the original SCT for the characterisation of the proposed controllers are identified and suitable theoretical modifications are proposed. An example is used to illustrate the ideas presented. Finally, a controller synthesis method is outlined.