Antonis C. Kokossis

Wastewater minimisation of industrial systems using an integrated approach

The design of an industrial water system, which makes the most efficient use of the water resources available, is a complex problem that involves different trade-offs. If we assume that no fundamental process changes can be performed (i.e. wet cooling towers cannot be replaced by air-coolers, etc.), then we can improve the efficiency of the water system through practices as water re-use, regeneration of water prior to re-use, or regeneration and recycling. The inherent combinatorial nature of the problem calls for the development of a systematic methodology that can deal with the high dimensionality of the design problem. In this paper we propose an integrated methodology for the design of industrial water systems. This approach brings the engineering insights provided by the water-pinch analysis together with powerful mathematical programming tools. The method is based on a decomposition scheme for the optimisation of a superstructure model that includes all the possible features of a design. The proposed decomposition strategy is based on a recursive procedure. With this new approach, a network featuring minimum total annualised cost can be found where the complexity of the network structure is under the control of the designer and many practical constraints can be incorporated.

A multi-contaminant transhipment model for mass exchange networks and wastewater minimisation problems

Abstract The methodology presented in the paper combines insights from Water Pinch with mathematical programming. The approach applies to mass exchanger network and wastewater minimisation problems. Its purpose is the development of targeting models at a conceptual stage where the process network is not yet developed. For wastewater minimisation, the concept of the limiting water profile is employed. The development of utility targets for multiple contaminants is developed as a mixed integer linear transhipment formulation that enables easy screening and scoping ahead of the network development. The paper describes the foundation of the model, its variants for the different problems considered, and literature examples that explain its functions. The paper reports particularly encouraging results and improvements against reported solutions in the literature.

Conceptual optimisation of utility networks for operational variations—I. targets and level optimisation

The design of utility networks involves two major tasks: the selection of the steam levels, with respect to their pressure or saturation temperature, and the determination of the configuration of the operating units between the steam levels. Both tasks require models that enable the conversion of the cogeneration potential into shaftwork targets, on the basis of which the profitability of the proposed designs is decided. This paper introduces a new targeting model and procedures to analyse existing design options ahead of design. The turbine hardware model (THM) accounts for the variation of efficiency with the turbine size, load and operating conditions in a simple, yet accurate way. The model is derived by exploiting basic steam turbine principles and engineering knowledge. It is applicable to any type of units and can provide accurate estimates of its performance over the entire operation range. As such, it can be used for setting realistic targets for shaftwork at the early stages of design, as well as for selecting the steam levels that maximise the potential for power cogeneration. The application of the THM is illustrated through a number of examples involving simple and multiple operations, and a comparison is made with a constant efficiency model.

On the use of systems technologies and a systematic approach for the synthesis and the design of future biorefineries

Systems technologies emerge with a powerful potential to support the deployment and design of future biorefineries. The chemical industry experiences a steady growth in the use of renewables induced by the gradual depletion of oil, uncertainties in energy supplies and a commanding requirement to reduce GHG emissions and save the planet. Renewables introduce an impressive range of options with biorefining at the center of attention as an emerging industrial concept, uniquely attached to chemical engineering and aiming to transform plant-derived biomass into a variety of products including transport fuels, platform chemicals, polymers, and specialty chemicals. In competing with conventional processes, biorefineries should match maximum efficiencies with better design and process integration. The paper highlights the pivotal role of systems technology to foster innovation, preview options, and support high-throughput computational experimentation, arguing that systems tools are largely under-deployed. Systems-enabled platforms could instead function as powerful environments to generate ideas for integrated designs and offer tremendous services to the complex and large problems produced by the numerous portfolios of feedstocks, unknown portfolios of products, multiple chemistries, and multiple processing paths. Complexities certainly exceed capabilities of previous methodologies but established achievements and experience with similar problems are excellent starting points for future contributions. Besides a general discussion, the paper outlines opportunities for innovation in design, concept-level synthesis, process integration, and the development of supply chains.

Synthesis of isothermal reactor—separator—recycle systems

Abstract A systematic synthesis approach is presented for isothermal reactor—separator—recycle systems. The approach proposes a general superstructure of different reactors and separation tasks and features all the potential interconnections among the proposed units. The synthesis problem based upon the proposed superstructure results in a mixed integer nonlinear programming (MINLP) formulation in which the objective function involves both integer and continuous variables and is subject to a nonlinear set of constraints. A variety of objectives was selected for the synthesis problem such as the minimization of the total annual cost of the plant and the maximization of its profit, as well as objectives traditionally used for optimizing the performance of a reactor network such as the product yield and selectivity. Discussion of the results and comparison among the different solutions obtained provided the ground for conclusions related to the potential trade-offs and the performance of the isothermal chemical systems under consideration.

On the development of novel chemicals using a systematic optimisation approach. Part II. Solvent design

Abstract A new approach is presented for the optimal selection of solvents. The approach is using group contribution methods and a vector representation to formulate an optimisation problem with respect to desired properties for the solvent. The formulation can employ conceptual flowcharts or detailed flowsheets. The synthesis objectives are expressed either as a function of the solvent properties or alternatively as functions of design parameters and variables of the flowsheet. The optimisation is facilitated with a stochastic tool in the form of simulated annealing. The examples address literature problems and industrial case studies involving the design of separation enhancing materials. Applications are presented for liquid–liquid extraction processes, extractive distillation problems, retrofit designs and gas absorption processes.

On the development of novel chemicals using a systematic synthesis approach. Part I. Optimisation framework

This paper presents a computer-aided technology for the synthesis of molecules with optimal properties. The approach combines stochastic optimisation and group-contribution methods to search and select chemicals of desirable performance. The stochastic search is in the form of a simulated annealing algorithm. For the application of the annealing algorithm the molecular system is mapped in terms of problem states. A set of moves is applied to generate alternatives and monitor the synthesis search. Molecules are represented by molecular and composition vectors of UNIFAC groups, but most appropriate group-contribution methods are also considered. Size and composition of the molecular vector are optimised on the basis of the selected synthesis criteria without limitations on the type of calculations required. The application of the approach is illustrated with three synthesis examples.

A transhipment model for the optimisation of steam levels of total site utility system for multiperiod operation

A new approach for the optimisation of steam levels of total site utility systems satisfying varying utility demands is presented, accounting for interactions between total site utility systems and chemical processes. The optimisation problem involves the selection of the steam levels with respect to their temperatures. In this paper, by exploiting total site analysis techniques, a new transhipment network is developed to represent the heat flows of a total site. Base on the transhipment network representation of the total site, a general multi-period mixed-integer linear programming (MILP) model is presented for identifying the optimal steam levels of the total site utility system. By using engineering and thermodynamic knowledge, a boiler hardware model (BHM) is developed to describe the performance of boilers, and the turbine hardware model (THM) is applied for the shaft-work targeting of steam turbines. Both models are capable of predicting the real efficiency trends of the units. The application of the proposed optimisation approach is illustrated through two case studies including single operation scenario and multiple operation scenarios.

Molecular design synthesis using stochastic optimisation as a tool for scoping and screening

This paper presents optimisation technology for the computer-aided design of molecules. A new approach is presented that combines stochastic optimisation and group-contribution methods to select chemicals with optimised properties. Each molecule is represented as a set of functional groups. The search follows an iterative procedure, where new molecules are generated, evaluated and subjected to acceptance. The evaluation stage calls upon calculation of molecular properties using available group-contribution expressions and databases. The proposed methodology is illustrated with literature examples involving the design of refrigerants and liquid-liquid extraction solvents. The efficiency of the search and the thermodynamic models employed are validated through process simulation studies. The work reports novel molecular structures and significant improvements over conventional techniques.

On the robust application of stochastic optimisation technology for the synthesis of reaction/separation systems

Stochastic optimisation technology is presented for the synthesis of integrated reaction and separation process superstructures. The optimisation techniques enable the systematic exploration of process design options hidden in the network formulations. The implementation of two stochastic algorithms, simulated annealing and Tabu search, is presented in the context of the synthesis problem. The paper presents a design transition framework at the interface between the optimisation technology and the process synthesis representation as well as customisation aspects that enable an efficient search of the vast number of processing candidates generally comprising the solution space of the optimisation problem. Both stochastic algorithms are easily implemented as they utilise the same state transition and simulation framework. Asymptotic convergence to the globally optimal domain is observed for simulated annealing as well as Tabu search. In view of the numerical evidence obtained, Tabu search appears to arrive at high-quality solutions on more efficient search paths as compared with simulated annealing.