Multivariable Control Structure Design Based on Mixed-Integer Quadratic Programming

In this work a new approach to address multivariable control structure (MCS) design for medium/large-scale processes is proposed. The classical MCS design methodologies rely on superstructure representations which define sequential and/or bilevel mixed-integer nonlinear programming (MINLP) problems. The main drawbacks of this kind of approach are the complexity of the required solution methods (stochastic/deterministic global search), the computational time, and the optimality of the solution when simplifications are made. Instead, this work shows that, by using the sum of squared deviations (SSD) as well as the net load evaluation (NLE) concepts, the control structure design problem can be formulated as a mixed-integer quadratic programming (MIQP) model with linear constraints, featuring both optimality and improved computational performance due to state-of-the-art solvers. The formulation is implemented in the GAMS environment using CPLEX as the selected solver and two typical case studies are presented to show the benefits of the proposed approach

A Sequential Integration between Optimal Flexible Heat Exchanger Network Synthesis and Control Structure Design

In this work, the optimal synthesis and control structure design (CSD) problems for flexible heat exchanger networks (HENs) are integrated into a new sequential methodology. The proposed approach relies, on one hand, on a convexification and outer-approximation strategy to solve the synthesis stage and, on the other hand, on the sum of squared deviations (SSD) method for the optimal CSD. These methods guarantee the optimality of the synthesis process, as well as the proper operation of the HEN in several operating points. The first stage of the proposed approach, which focuses on the flexible HEN synthesis problem, considers both temperature and flow rate modifications in the inlet streams. A multiperiod synthesis formulation is proposed where critical points are iteratively incorporated to fulfill the flexibility requirements. Because the problem size and the nonconvexities increase when additional critical points are considered, both the convexification of nonlinear terms and an outer approximation strategy are used to guarantee the optimality of the solutions at this stage. The second stage handles the decisions associated with the design of the control structure. This stage is critical because the network is required to work in a wide range of operating points. If the classical CSD method based on the well-known relative gain array (RGA) is applied, and only the nominal operating point is considered, such requirements are not fullfiled. In fact, this work demonstrates that such classical CSD approaches are not sufficient to operate the HEN in the range of variation considered by the multiperiod synthesis phase. As an alternative method, the application of the SSD approach to multiple operating points is proposed. Thus, several optimal control structures are developed to ensure the operability of the HEN. Three academic case studies are presented to illustrate the application of the proposed methodology

Simultaneous Production and Distribution of Industrial Gas Supply-Chains

In this paper, we propose a multi-period mixed-integer linear programming model for optimal enterprise-level planning of industrial gas operations. The objective is to minimize the total cost of production and distribution of liquid products by coordinating production decisions at multiple plants and distribution decisions at multiple depots. Production decisions include production modes and rates that determine power consumption. Distribution decisions involve source, destination, quantity, route, and time of each truck delivery. The selection of routes is a critical factor of the distribution cost. The main goal of this contribution is to assess the benefits of optimal coordination of production and distribution. The proposed methodology has been tested on small, medium, and large size examples. The results show that significant benefits can be obtained with higher coordination among plants/depots in order to fulfill a common set of shared customer demands. The application to real industrial size test cases is also discussed</p