Analytical optimisation of industrial systems and applications to refineries, petrochemicals

A new method for optimising process networks is presented in this paper. The method uses economic analysis of existing systems based on the new value analysis method (Ph.D. Dissertation, UMIST, Manchester, UK, 2002) as the basis to derive the optimum network design. The analytical optimisation method comprises of three steps. Market integration is the first step that fully exploits the available market opportunities for selling and purchasing streams based on individual marginal contributions from productions and processing of streams. Market integration is an easy and straightforward way of achieving quick benefits. The second step deals with optimisation of network flowsheet/connections. The economic margins of various paths of network are used to determine the weaker paths and the stronger paths where the loads of weaker paths can be shifted. This load shifting among paths leads up to the overall benefits of a system. Finally, the non-profitable or less profitable process units are optimised to improve their individual marginal contributions. Analytical optimisation turns the traditional back box approach into a clear and transparent procedure and is simple to understand and easy to use. The application of analytical optimisation is demonstrated with industrial cases from refining. In the end, a generalised methodology has been illustrated on how to design the optimum flowsheet of a petrochemical complex in a changing market price scenario.     

Optimal synthesis of multi-plant heat exchanger networks considering both direct and indirect methods

Heat exchanger networks (HENs) in separated plants can be simultaneously synthesized through process streams directly or intermediate fluids indirectly. However, the direct and indirect methods are studied separately in most existing researches. As the result, conventional designs are probably suboptimal, because optimal solutions may call for hybrid approach wherein direct and indirect integration methods are used synchronously. To circumvent this drawback aforementioned, we propose in this research a novel methodology to synthesize multi-plant HENs considering both direct and indirect approaches. The methodology employs a novel superstructure covering most potential topologies for both interplant and intra-plant heat integration. We also take into account multiple kinds of intermediate fluids for indirect integration and this has not been fully addressed in previous research. A mixed-integer nonlinear programming (MINLP) is formulated to optimize multi-plant HENs involving indirect and direct methods. One example from existing literature and one industrial problem are solved to demonstrate the methodology's capability.     

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

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 level of detail of a piping and instrumentation diagram. In this paper, a subgoaling procedure is developed using the State Task Network representation which decomposes the procedure synthesis goals into simpler subgoals by means of an efficient Mixed Integer Linear Programming (MILP) technique. Detailed control sequences are then generated for each subgoal using a set of rules and algorithms specific for each type of subgoal. The procedure sequences thus generated are validated by simulation on a plant model with checking of physical and operational constraints at each new plant state. We have found that this hierarchical approach to the procedure synthesis problem greatly reduces the problem complexity

The usefulness of the general approach and of the subgoaling procedure in particular are demonstrated through a multiproduct batch plant example.     

A modeling strategy for integrated batch process development based on mixed-logic dynamic optimization

This paper introduces an optimization-based approach for the simultaneous solution of batch process synthesis and plant allocation, with decisions like the selection of chemicals, process stages, task-unit assignments, operating modes, and optimal control profiles, among others. The modeling strategy is based on the representation of structural alternatives in a state-equipment network (SEN) and its formulation as a mixed-logic dynamic optimization (MLDO) problem. Particularly, the disjunctive multistage modeling strategy by Oldenburg and Marquardt (2008) is extended to combine and organize single-stage and multistage models for representing the sequence of continuous and batch units in each structural alternative and for synchronizing dynamic profiles in input and output operations with material transference. Two numerical examples illustrate the application of the proposed methodology, showing the enhancement of the adaptability potential of batch plants and the improvement of global process performance thanks to the quantification of interactions between process synthesis and plant allocation decisions.     

Multisite facility network integration design and coordination: An application to the refining industry

This paper addresses the design and analysis of multisite integration and coordination strategies within a network of petroleum refineries using different crude combination alternatives. In addition, production capacity expansion requirements are also accounted for. The main feature of the paper is the development of a methodology for simultaneous analysis of process network integration alternatives in a multisite refining system through a mixed-integer linear program (MILP) with the overall objective of minimizing total annualized cost. The State Equipment Network (SEN) representation was used for modeling the network as it provides a consistent modeling strategy and proper handling of units that operate under different operating modes, which is common in the refining industry. The integrated network design specifically addresses intermediate material transfer between processing units at each site. The performance of the proposed model was tested on several industrial-scale examples to illustrate the economic potential and trade-offs involved in the optimization of the network. The use of mathematical programming models on an enterprise-wide scale to address strategic decisions considering various process integration alternatives yielded substantial benefits. These benefits not only materialize in terms of economic considerations, but also in terms of process flexibility and improvements in the understanding of the process interactions and systems limitations. Although the methodology was applied on a network of refineries, it can be readily extended to cover any network of continuous chemical processes.     

Optimal synthesis of integrated refrigeration systems—I: Mixed-integer programming model

This paper deals with the problem of synthesizing minimum-cost refrigeration systems that are integrated with heat-recovery networks. An efficient network representation is proposed for embedding multistage cycle configurations that operate over a discrete set of potential temperature levels, and which account for the heat integration of a set of hot and cold process streams. Based on this network representation, the prediction of a lower bound for the utility costs is formulated as a linear programming problem. The synthesis of integrated refrigeration systems that minimize both capital and utility costs is formulated as a mixed-integer linear programming problem. The application of this method is illustrated with a test problem.     

Systematic optimisation technology for high-level screening and scoping of complex distillation systems

Established procedures for complex distillation synthesis employ detailed models that are unable to function for high-level screening. In this article, a new approach is presented in the form of Conceptual Programming for the preliminary screening of complex distillation systems. Conceptual Programming employs a generic supertask representation that replaces the need of superstructures. Tasks stand for simple distillation columns and hybrids represent complex distillation arrangements. A supertask accounts for all possible configurations without introducing representation and modelling difficulties. The basic tasks are then modelled using appropriate shortcut or semi-rigorous methods. A mixed integer linear programming (MILP) problem is formulated to obtain the optimum structure and performance targets. The strengths of the approach are highlighted with a light alcohol separation problem.     

The FluxMax approach for simultaneous process synthesis and heat integration: Production of hydrogen cyanide

Resource and energy efficiency are essential in process synthesis of chemical plants as they combine economic with ecological benefits. The two main targets of the process synthesis problem—mass and energy flux optimization—are typically split into two steps: single unit optimization and subsequent energy integration preventing the identification of the globally optimal solution. This article presents a single-step procedure for resource-efficient process synthesis through simultaneous heat and mass flux optimization called FluxMax approach (FMA), which is demonstrated for the production of hydrogen cyanide (HCN). The impact of simultaneous heat integration on the optimal process structure is demonstrated and two resource-optimal processes for HCN production are identified consisting of a combination of different reactor and recycling strategies reducing total variable cost by 68%. For convex objective functions, the globally most resource-efficient process is identified highlighting the potential of the FMA for site planning and retrofitting of existing plants.     

多流股换热器网络综合方法在原油预热系统中的应用

以原油常减压蒸馏装置原油预热网络为研究对象,进行了多流股换热器网络的实例综合. 基于超结构物理模型建立了改进的多流股换热器网络综合数学模型,提出将该网络综合问题由混合整数非线性规划问题转化为简单的非线性规划问题的求解策略,并利用改进的遗传/模拟退火新算法进行了原油预热网络的综合. 与Hextran软件的综合结果以及现场换热网络的对比表明,本模型和求解策略可以应用于工业规模的多流股换热器网络综合,有可能取得较好的经济效益.     

Integrated Modelling of Process Operation Systems Using the Agent‐Oriented Approach

For the past years, several software and computer tools have been developed to aid the chemical process operations including real‐time simulation, on‐line optimization, fault diagnosis, process monitoring, and many other functions. These tools were designed separately and did not collaborate efficiently, making it difficult to integrate different engineering tasks for the optimal process operation. In this paper, an agent‐oriented modelling approach is presented to address this problem. Elements in the process operation systems are divided into two classes. One class consists of equipment, units and processes, while the other class consists of production operation tasks. The two classes of elements are modelled as objects and agents, respectively. Then, three strategies are presented to implement the integration of the whole process operation system, which are integration of object models, integration of agent models and supervision of operator. Also presented is a case study of integration of process operation decision optimization and abnormal situation management using the proposed agent oriented approach for TE challenge problem.     

A rigorous targeting algorithm for resource allocation networks

Techniques of process integration can be applied to conserve resources such as energy, freshwater, cooling water, hydrogen, solvent, etc. Process integration methodologies are broadly classified into two categories: methodologies based on the mathematical optimization techniques and methodologies based on the conceptual approaches of pinch analysis. In this paper, a mathematically rigorous methodology is proposed to minimize the requirement of a natural resource in a chemical process industry. The proposed methodology combines the simplicity of the pinch analysis with the mathematical rigor of mathematical optimization techniques. Conservation of resource in a chemical process industry is posed as a network flow optimization problem and a simple algebraic methodology is proposed to solve the optimization problem. The proposed algebraic methodology is mathematically proved in this paper. The proposed algorithm is numerically faster than the general mathematical optimization methods used for solving optimal resource allocation problems.     

Energy optimization of bioethanol production via gasification of switchgrass

In this article, we address the conceptual design of the bioethanol process from switchgrass via gasification. A superstructure is postulated for optimizing energy use that embeds direct or indirect gasification, followed by steam reforming or partial oxidation. Next, the gas composition is adjusted with membrane‐PSA or water gas shift. Membrane separation, absorption with ethanol‐amines and PSA are considered for the removal of sour gases. Finally, two synthetic paths are considered, high alcohols catalytic process with two possible distillation sequences, and syngas fermentation with distillation, corn grits, molecular sieves and pervaporation as alternative dehydration processes. The optimization of the superstructure is formulated as an mixed‐integer nonlinear programming problem using short‐cut models, and solved through a special decomposition scheme that is followed by heat integration. The optimal process consists of direct gasification followed by steam reforming, removal of the excess of hydrogen and catalytic synthesis, yielding a potential operating cost of $0.41/gal. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

绿色过程系统合成与设计的研究与展望

过程工业的高速发展导致环境污染不断加剧，对传统工业的改造和实现清洁生产离不开绿色过程系统合成的理论和方法. 本工作对绿色过程合成的相关研究进展进行了综述. 首先对现有的环境影响评价体系进行了分类、归纳和总结，评述了其各自的特点和作用，以及具体应用；其次对绿色过程合成的模型化和算法的研究进展进行了详细论述，介绍了其在废物(水)最小化、分离系统集成等方面的应用；论述了绿色过程合成在化工过程绿色设计中的应用和发展趋势. 将绿色化学原理和系统集成的普适性理论相结合，提出了绿色度的理论和方法，阐明了其研究内容和拟解决的关键科学问题，提出量化物质、能量、过程和系统的绿色度的原则方法，通过多目标优化实现不同层次系统的生态、经济和社会效益的全局最优.     

Synthesis,optimal planning and scheduling for process plant utility systems

A systematic methodology is presented for the synthesis, optimal planning and scheduling of plant utility systems. The methodology first addresses long-term synthesis and planning problems and continues with short-term maintenance scheduling applications. The synthesis approach determines new units, their layout, their number and capacities, as well as the operating strategies appropriate in view of process variations. The scheduling problem methodology determines maintenance, along with shut-down and start-up schedules for uncertain operational scenarios. The optimisation technology is applied in the form of natural decomposition schemes.     

A systematic visual methodology to design ionic liquids and ionic liquid mixtures: Green solvent alternative for carbon capture

Ionic liquids (ILs) have gained great interest recently to substitute volatile organic compounds (VOCs), since their properties can be tuned to match certain targets and applications. Further to this, another possibility to optimise ILs for their specific application is through IL mixtures. In this work, an insightful and yet simple systematic approach to design pure ILs and their mixtures is presented. This newly presented approach allows the visualisation of IL mixture design problem, and hence provides insights and allows users to solve the problem visually. The visualisation of problem and solutions is achieved by applying property integration framework in this proposed methodology. In property integration framework, IL products design problem is mapped from property domain into cluster domain through property clustering technique. Therefore, the proposed methodology provides a property based platform to visualise the overall performance of the designed IL products with graphical tools. A feasible IL product is always designed to fit a purpose based on consideration of multiple target properties, but these properties can be contradicting one another. The presented approach allows multiple target properties consideration during the design process, by portraying these properties and target of each clearly on a single graphical tool. To date, the study of properties of pure ILs and IL mixtures is still in the infant phase, and these data are still scarce. Hence, some of the prediction models do not cover all available ILs. To overcome this problem, the proposed approach is developed to adapt property data of pure ILs directly, together with existing property prediction models to predict the properties of the designed IL mixtures. The presented approach is able to generate a list of potential solutions to users, and the final decision can be made by users accordingly, through further screening and experimental validations. An illustrative case study, which focuses on the design of carbon capture solvents, is solved to demonstrate the proposed approach.     

A hybrid optimisation model for the synthesis of sustainable gasification-based integrated biorefinery

Depletion of fossil fuels and increasing public awareness of environmental issues has stimulated the search for alternative energy sources. Biofuels are recognised as one of the most promising alternatives to fossil fuels, as they can be produced from various types of feedstock. The efficiency and sustainability of biomass-based production can be maximised by producing biofuels along with other valuable coproducts in a “biorefinery”. This concept was proposed to make the production of biofuels and biochemicals more economically viable by taking advantage of opportunities for process integration and waste recovery. In this work, a novel hybrid optimisation model that combines superstructure-based optimisation approach and insight-based automated targeting for the synthesis of a sustainable integrated biorefinery is presented. In addition, fuzzy optimisation is also adapted to synthesize such integrated facility with the simultaneous consideration of both economic and environmental performance. Note that the proposed approach is a generic synthesis strategy that can be applied even without detailed modelling of individual processes.     

柔性换热网络综合与清洗计划同步优化(英文)

A novel methodology is presented for simultaneously optimizing synthesis and cleaning schedule of flexible heat exchanger network (HEN) by genetic/simulated annealing algorithms (GA/SA). Through taking into account the effect of fouling process on optimal network topology, a preliminary network structure possessing twofold oversynthesis is obtained by means of pseudo-temperature enthalpy (T-H) diagram approach prior to simultaneous optimization. Thus, the computational complexity of this problem classified as NP (Non-deterministic Polynomial)-complete can be significantly reduced. The promising matches resulting from preliminary synthesis stage are further optimized in parallel with their heat exchange areas and cleaning schedule. In addition, a novel continuous time representation is introduced to subdivide the given time horizon into several variable-size intervals according to operating periods of heat exchangers, and then flexible HEN synthesis can be implemented in dynamic manner. A numerical example is provided to demonstrate that the presented strategy is feasible to decrease the total annual cost (TAC) and further improve network flexibility, but even more important, it may be applied to solve large-scale flexible HEN synthesis problems.     

Computer aided flowsheet drawing—II: Stream layout and drawing

A methodology for machine layout of process streams involving multiple directional changes given the equipment layout and stream connection information is formulated. Also considered are the drawing of these streams and of various process equipment symbols as well as the layout and drawing of peripheral flowsheet symbols and messages. The key concepts involved are the use of a coded grid representation of the drawing space, a structural classification of stream configurations, efficient constructions for generating stream paths, and the formulation of a ranking of stream paths. A program package implementing the methodology which includes a simple yet flexible data structure for generating and storing equipment symbols is discussed and example flowsheet are given.