X.X. Zhu

An automated approach for heat exchanger network retrofit featuring minimal topology modifications

This paper describes a new automated procedure for retrofit heat exchanger network (HEN) design which aims to minimise modifications to the existing HEN structure. The new procedure employs a two-stage approach for retrofit HEN design, consisting of a modification selection stage and an optimisation stage. During the modification selection stage a minimum number of promising HEN topology modifications is obtained to enable a desired heat recovery target to be achieved. The resulting HEN is then optimised during the optimisation stage, using non-linear optimisation techniques, to minimise the cost of additional surface area employed.

A novel modelling and decomposition strategy for overall refinery optimisation

Abstract In this paper, a novel decomposition strategy is presented to tackle large-scale overall refinery optimisation problems. This decomposition approach is derived from analysis of the mathematical structure of a general overall plant model, which contains common elements and independent elements. This understanding forms the basis for decomposing the overall plant model into two levels, namely a site level (master model), and a process level (submodels). The master model determines common issues among processes, such as allocation of raw materials and utilities, etc. With these common issues determined, submodels then optimise individual processes. The results from submodel optimisation are fed back to the master model for further optimisation. This procedure terminates when convergence criteria are met. In this way, individual process optimisations are effectively controlled and co-ordinated by the centre master model. This decomposition provides the flexibility of replacing existing process models by using better models supplied by vendors without affecting the optimisation infrastructure.

A novel approach for retrofit and operations management of existing total sites

Abstract A new methodology is developed for analysis of total sites which expedites the energy retrofit and operations management. The directions for improvement of site efficiency, and their limits are first identified by conducting a utility system analysis termed as the top-level analysis. The results are represented graphically on the novel “power generation efficiency curves.” These curves enhance the understanding of performance of an existing utility system, and thus enable the identification of the most promising modifications is the retrofit of process heat exchanger networks (HENs). A significant advantage of the approach is that it minimises the amount of data required, a big obstacle in total site retrofit, without compromising on the optimality of the results. The power generation efficiency curves also prove useful for optimisation of utility system operations to reduce the energy costs on a site. An industrial case study is solved for demonstration.

Thermodynamic analysis and mathematical optimisation of power plants

Abstract A new methodology is presented for a new design of power plants, which combines the benefits of thermodynamics, economics and mathematical optimisation. The design process is divided into two mains stage, namely analysis stage and design stage. The analysis stage comprises thermodynamic analysis, sensitivity analysis and economic analysis, and the design stage includes mathematical optimisation. Thermodynamic analysis is based on the Combined Pinch and Exergy Representation (CPER) (Zheng 1996) and σ - E p diagram. CPER gives an overview of the exergy losses in the heat exchange system and provides information on exergy input for shaftwork production. σ - E p diagram on the other hand provides more detailed information on the exergetic efficiencies of subsystems, including not only heat exchangers but also turbines, pumps, etc. This exergetic efficiency is based on the concept of avoidable exergy loss (Feng and Zhu, 1997), thus providing realistic targets on exergy loss reduction in the processes. These two tools together enable the designer to quickly locate the inefficiencies in the process and evaluate the effects of process changes on shaftwork. Once the promising modifications from the thermodynamic analysis are found, sensitivity and economic analyses are performed to evaluate the effect of individual changes on the performance of the plant and its economics. The changes with good economic potential together with the base case form a superstructure leading to a MINLP formulation. Because only the economically viable changes are included, the superstructures will be potentially much simpler and hence the number of integer variables is reduced. Another benefit of the analysis stage is that the results give upper and lower bounds and initial points for the optimisation. With a reduced superstructure, good initial starting point and feasible bounds industrial problems can be tackled effectively.

Pressure drop considerations for heat exchanger network grassroots design

Pressure drop is an important issue in design of a heat exchanger network (HEN), which has yet to be addressed properly. To overcome pressure losses incurred when streams flow through heat exchangers, pumps/compressors must be installed. The total cost for a system of pumps and compressors consists of the purchase cost of equipment and the electricity cost to run these equipment. This cost could occupy a significant part of the overall cost for a HEN design. Therefore, the pressure drop aspect should be considered together with the costs for heat exchanger area and utility consumption. A new approach is proposed to consider the pressure drop aspect in the overall context of a HEN design. Firstly, the optimal ΔTmin is determined through three-way trade-offs between area, utility and pressure drop at the targeting stage. As a result of targeting, targets for area, utility and pressure drops can be established ahead of the network design. Then a network structure is initialised at the determined ΔTmin and optimised to achieve a final design. In this procedure, the pressure drop is considered at both the targeting stage and the design stage in a systematic manner.

Automated design method for heat exchanger network using block decomposition and heuristic rules

Abstract The research in HEN synthesis has achieved significant progress over the last two decades, particularly through the discovery of the pinch concept and the research in targeting energy and capital. Normally, the HEN synthesis is carried out with two stages, targeting and design. In the past, many researchers mainly focused on developing manual design methods. Having realised the significance of saving engineering time and the possibility of using mathematical optimisation techniques to enhance the search for good designs, much research has been carried out to develop automated HEN design methods and significant progress has been achieved. However, there is still no design methods available which can fulfil the task of automated synthesis for practical applications. In the present paper, a simple method for automated synthesis of HEN is presented, which is based on the block concept proposed by Zhu et al. (1995a). The basic idea is to simplify a design problem by decomposing it into a number of blocks. In each block, piece-wise stream composites present similar profiles and hot streams are in energy balance with cold streams. After the block decomposition, design is carried out using area targeting principles and a number of newly developed heuristic rules. An MILP model and a simple MINLP model are used for selecting a best set of matches and determining the optimal split ratios, respectively. The initial designs produced by such a method can approach the energy and area targets closely coupling with a small number of units. Such good initial solutions are then provided for subsequent cost optimisation. The main advantage of this design strategy is that designs with good quality can be achieved efficiently and effectively. In addition, the block concept can be easily extended to handle problems with different film coefficients by combining the current method with the diverse pinch approach proposed by Rev and Fonyo (1991) [(Chem. Eng. Sci. 46, 1623–1634 (1991)] and thus the above strategy for HEN synthesis can be applied for such problems.

Novel modelling and decomposition strategy for total site optimisation

Overall plant optimisation is one of the most difficult problems in the field of applied optimisation due to the size and mathematical complexity of the problem. The state-of-the-art approaches mainly rely on linear programming or mixed integer linear programming. Developing and solving rigorous site-wide models is still at research stage. In this work, a novel decomposition strategy is presented to tackle large-scale total site optimisation problems. This decomposition approach is derived from analysis of the mathematical structure of a general total site model, which features an angular structure with common elements and independent elements. This understanding forms the basis for decomposing the overall plant model into two levels, namely a site level (master model) and a process level (submodels). The master model determines common issues among processes, such as allocation of raw materials and utilities, etc. With these common issues determined, submodels then optimise individual processes. The results from submodel optimisation are fed back to the master model for further optimisation. This procedure terminates when convergence criteria are met. In this way, individual process optimisations are effectively controlled and co-ordinated by the centre master model. Case studies are carried out to demonstrate the effectiveness of the approach.

Automated synthesis of HENs using block decomposition and heuristic rules

Abstract Heat Exchanger Network (HEN) synthesis is normally carried out in two stages, targeting and design. In the past, most researchers have focused on developing design methods for hand calculations (e.g. the Pinch Design Method). In the present paper, an algorithm for automated synthesis of HEN is presented, which is based on the block concept proposed by Zhu et al (1994a). The basic idea is to simplify a design problem by decomposing it into a number of blocks. In each block, piece-wise stream composites present similar profiles and hot streams are in energy balance with cold streams. After the block decomposition, design is carried out using area targeting principles and a number of newly developed heuristic rules and a best set of matches is selected simultaneously. The initial designs produced by such a method can closely approach the energy and area targets with a small number of units. Good initial solutions are then provided for subsequent cost optimisation. The main advantage of this design strategy is that designs with good quality can be achieved efficiently.

Economic achievement of environmental regulation in chemical process industries

This paper presents an integrated approach for economic achievement of environmental regulation from existing total sites. The approach follows a hierarchical method that combines physical insights with mathematical methods. An understanding of the problem is used to generate options in the first step. The second step uses economics and site feasibility to screen options. The options left after screening are formulated in a superstructure. The optimisation of the superstructure gives an optimum mix of options that are then translated into the retrofit design. An important feature of the method is that understanding of the problem is used to generate options and proper bounds on different variables. This reduces the size, complexity and the solution space of the superstructure. Thus even large industrial problems can be solved effectively.