Sandro Macchietto

Computer aided molecular design: a novel method for optimal solvent selection

Abstract The task of selecting a solvent or solvent mixture with desirable combination of physical properties to meet the needs of specific applications, has largely been tackled using a combination of heuristics and costly experimental studies. This material selection problem is here formulated as the combinatorial molecular design problem of choosing a set of structural groups making up a target molecule with the desired properties as predicted by available group contribution techniques. A novel mixed-integer nonlinear programming (MINLP) technique is used to solve the problem yielding compounds with optimum value of an appropriate performance index, subject to material balances, process and design limitations and feasibility of molecular structures. The strategy is applied with excellent results to solvent design examples for liquid-liquid extraction and multicomponent gas absorption using varying combinations of objective functions and constraints to reflect directly a multiplicity of operational objectives.

Dynamic modelling and simulation of plate heat exchangers under milk fouling

Abstract This work presents the mathematical modelling and simulation of complex plate heat exchanger arrangements under milk fouling, using detailed dynamic models. A complex fouling model based on a reaction/mass transfer scheme is coupled with a general thermal dynamic model of plate heat exchangers. All the important factors affecting milk heat treatment are formally quantified. The final model comprises a set of partial differential, integral and algebraic equations. Parameter estimation analysis is performed based on the solution of a dynamic optimization problem. The simulation results are in a good agreement with available experimental work. Three different configurations with complex flow arrangements are considered to illustrate aspects of fouling behaviour. The simulation results provide significant insight into the key factors affecting milk fouling.

A general mathematical programming approach for process plant layout

The generation of a good layout is an important stage in the design of a new plant or the retrofit of an existing facility. Layout decisions affect piping, electrics, instrumentation and therefore have a great impact on the total plant cost. Moreover, layout has a large impact on the safety, operability and maintainability of any chemical plant. This paper presents a general mathematical programming approach for addressing the problem of allocating items of equipment in a given two or three dimensional space. The problem is formulated as a mixed integer linear programming model where equipment of various sizes and geometries are taken into account. The objective function to be minimized accounts for the total transport, connection, land and floor construction cost. This optimization procedure results in the coordinates of each unit (location), the total piping length, and the land occupied. Three case studies are presented to illustrate the applicability of the proposed approach.

A general framework for the integration of computational fluid dynamics and process simulation

Abstract Computational fluid dynamics (CFD) and process simulation are widely used in the process industry. The two technologies are largely complementary, each being able to capture and analyse some of the important process characteristics. Their combined application can, therefore, lead to significant industrial benefits. This is especially true for systems, such as chemical reactors, in which steady-state performance, dynamics and control strategy depend on mixing and fluid flow behaviour. This paper presents a new approach for the integration of the capabilities of CFD technology and process simulation via a general interface that allows the automatic exchange of critical variables between the two packages, leading to a simultaneous solution of the overall problem. The approach applies to both steady-state and dynamic problems. The feasibility of the approach and its first practical implementation are demonstrated by integrating a widely used CFD package (Fluent 4.5, by Fluent Inc.) within a general-purpose advanced process simulator (gPROMS 1.7, by Process Systems Enterprise Ltd. (1999)). One case study involving a batch reactor is used to illustrate the ability of the combined tool to provide information on the detailed interactions between fluid mechanics, heat transfer, reaction and control strategy, and to provide insights on important design and operational decisions.

Optimal operation of multicomponent batch distillation : multiperiod formulation and solution

Abstract A method is proposed to determine optimal multiperiod operation policies for binary and general multicomponent batch distillation of a given feed mixture, with several main products and intermediate off-specification cuts. A two-level optimal control formulation is presented so as to maximize a general profit function for the multiperiod operation, subject to general constraints. The solution of this problem determines the optimal amount of each main and off cut, the optimal duration of each distillation step and the optimal reflux ratio profiles during each production period. The outer level optimization maximizes the profit function by manipulating carefully selected decision variables. These are chosen in such a manner that the need of specifying the mole fractions of all the components in the products, as required by previous methods is avoided. For values of the decision variables fixed by the outer loop, the multiperiod operation is decomposed into a sequence of independent optimal control problems, one for each production step. In the inner loop, a minimum time problem is then solved for each step to generate the optimal reflux ratio values, reflux switching times and duration of the step. The procedure permits the use of very general distillation models described by differential and algebraic equations, including rigorous thermodynamics if desired. The model equations are integrated by using an efficient Gears type method, the inner loop optimal control problems are solved using a variational method, and all optimisations are solved using a robust and efficient successive quadratic programming code (Chen, Ph.D. Thesis, Imperial College, 1988). Several example problems (involving binary and multicomponent mixtures) are used to demonstrate the idea and to show the effect of the cost functions used (in particular the value of the main products) on the optimal solutions.

Fouling in Crude Oil Preheat Trains: A Systematic Solution to an Old Problem

A major cause of refinery energy inefficiency is fouling in preheat trains. This has been a most challenging problem for decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts on design/operations. Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally preventing it. To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College London and, through IHS ESDU, industry. The research, coordinated in eight subprojects blending theory, experiments, and modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger network, in an integrated way. To make the outcomes of the project relevant and transferable to industry, the research team is working closely with experts from many world leading oil companies. The systematic approach of the CROF project is presented. Individual subprojects are outlined, together with how they work together. Initial results are presented, indicating that a quantum progress can be achieved from such a fundamental, integrated approach. Some preliminary indications with respect to impact on industrial practice are discussed.

Modelling and simulation of complex plate heat exchanger arrangements under milk fouling

This paper presents the detailed mathematical modelling and simulation of complex plate heat exchangers under milk fouling, using computational fluid dynamics models. A complex fouling model based on a reaction/mass transfer scheme is described where all the important factors during milk heat treatment are quantified in a formal way. This model is coupled with detailed dynamic models of plate heat exchangers (PHEs). The final model comprises a set of partial differential, integral and algebraic equations. Parameter estimation analysis is performed based on the solution of a dynamic optimization problem. The simulation results have been compared with available experimental work and a satisfactory agreement has been found. Three different industrial configurations with complex flow arrangement are considered to illustrate aspects of fouling behaviour.

An investigation on integration of aggregate production planning, master production scheduling and short-term production scheudling of batch process operations through a common data model

Abstract A prototype system has been developed by integrating two higher-level hierarchical production planning application programs (aggregate production plan (APP), master production schedule (MPS)) using a common data model integration approach into an existing planning system for short-term scheduling and supervisory batch management which was originally proposed and developed by Richard, J. G., Macchietto, S. & Shah, N. (1999). Integrated decision support in flexible multipurpose plants. Computers & Chemical Engineering, 23, S539–S542. The hierarchical production planning system has been modelled and validated around an industrial scenario concerning multi-site, multipurpose batch process operations making household chemicals from start to transportation to regional warehouses. This preliminary work suggests that the idea of integrating software applications through a common data model, which was originally described by Stanley, G. M. (1994). The emerging trend towards knowledge-based frameworks for computer-integrated manufacturing. Advances in Instrumentation & Control, 49, 1121–1133 and further developed by Rickard, J. G., Macchietto, S. & Shah, N. (1999). Integrated decision support in flexible multipurpose plants. Computers & Chemical Engineering, 23, S539–S542 is feasible. However further research work, improvements and validations are required using varieties of industrial batch process operations and distribution problems to prove its viability.

A COMBINED MILP AND LOGIC-BASED APPROACH TO THE SYNTHESIS OF OPERATING PROCEDURES FOR BATCH PLANTS

Abstract Synthesizing a set of operating procedures for the safe and efficient transient operation of chemical plants is a difficult problem owing to the enormous number of possible combinations of actions in a typical plant. In most current industrial plant design practice, there are no formal methods for systematically transforming process specifications into operating procedures for the plant operators and into sequence control instructions for the control computers. There is much scope for a formalized computer-based procedure synthesis methodology to assist the design engineer/plant operator with both the formulation and assessment of procedures off-line and, eventually, with the on-line problem of procedure synthesis in response to unexpected situations A recently developed approach for operating procedure synthesis for multipurpose batch plants is considered. The modelling formalism used includes the separate definition of process operations, as State Task Networks, and of physical plant, at the le...

SAHPPA: a novel power pinch analysis approach for the design of off-grid hybrid energy systems

This work proposes a novel approach called stand-alone hybrid system power pinch analysis (SAHPPA), which is particularly applicable for the design of off-grid distributed energy generation systems. The enhanced graphical tool employs new ways of utilising the recently introduced demand composite curve and supply composite curve while honouring and adapting fundamental energy systems engineering concepts. The SAHPPA method is capable of optimising the capacity of both the power generators and energy storage for biomass (i.e. non-intermittent) and solar photovoltaic (i.e. intermittent) energy technologies, which is a contribution to the emerging area of power pinch analysis. In addition, the procedure considers all possible efficiency losses in the overall system encompassing the charging–discharging and current inversion processes.