Heat transfer intensification for retrofitting heat exchanger networks with considering exchanger detailed performances

The challenging of this work is to present a thorough study of implementing heat transfer intensification in heat exchanger network (HEN) retrofitting, including all details of exchanger geometry, stream bypassing and splitting, temperature‐variation of properties, LMTD and its correction, and pressure drops. This leads to very complex mixed integer nonlinear programming (MINLP) problems rarely reported before. By adopting the MILP‐based iterative approach proposed in the earlier work (Pan et al., Chem Eng Sci. 2013;104:498–524.), temperature‐variation of properties, LMTD and its correction are initialized to parameters at first, and the rest nonlinear terms are then linearized and expressed as first‐order Taylor series expansions. Finally, two iteration loops are executed to find optimal solutions. A small‐scale motivating problem and an industrial scale problem are presented to demonstrate the validity and efficiency of the proposed methods. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2052–2077, 2018     

Large‐scale heat exchanger networks synthesis using simulated annealing and the novel rocket fireworks optimization

Heat Exchanger Network (HEN) synthesis is an important field of study in process engineering. However, obtaining optimal HEN design is a complex task. When mathematically formulated, it may require sophisticated methods to achieve good solutions. The complexity increases even more for large‐scale HEN. In this work, a hybrid meta‐heuristic method is presented. A rather simple Simulated Annealing approach is used for the combinatorial level, while a strategy named rocket fireworks optimization is developed and applied to the continuous domain. An advantage over other approaches is that the algorithm was written in C++, which is free and faster when compared to many other languages. The developed method was able to provide the lowest costs solutions reported so far to six cases well studied in the literature. An important feature of the approach here proposed is that, differently from other approaches, it does not split HEN into smaller problems during the optimization. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1582–1601, 2017     

柔性换热器网络综合与清洗时序安排同步优化

提出在柔性换热器网络设计时考虑设备投入运行后实施在线清洗时序安排的同步优化方法,实现换热器网络动态综合。为降低该问题计算的复杂性,提出采用两阶段法。第一阶段利用虚拟温-焓图法初步综合获得考虑设备结垢过程和流股入口温度、热容流率发生变化时的多周期柔性网络,在同步优化之前识别出较优匹配候选。第二阶段,换热网络结构、设备面积和清洗时序安排利用遗传/模拟退火算法进行同步优化。通过积分方法来计算随时间变化的公用工程消耗,所建立的数学模型因此比以往文献更加严格。计算实例表明,所提方法有效降低年度总费用,同时能够解决大规模柔性换热器网络综合问题。     

有分流换热网络的弹性设计-基于温度波动情形

提出了基于温度波动的有分流换热网络的超结构模型和同步优化的弹性设计方法。首先对于起始温度在一定范围内波动的物流进行预处理。即分段处理。然后建立换热网络同步优化的数学模型,最后对分段物流进行合并,并通过能量松弛方法得取满意一定弹性要求的换热网络结构。     

Synthesis of flexible multi-stream heat exchanger networks based on stream pseudo-temperature with genetic/simulated annealing algorithms

The synthesis of flexible heat exchanger network (HEN) is generally regarded as an over-design of process units over a specified range of deviations in process parameters from their nominal values. The HEN obtained is more costly because of the over-design of HEN. The global solution to flexible design problems cannot be guaranteed because of the resulting non-differentiable, non-convex, max–min–max constraint of mixed integer nonlinear programming (MINLP) models. In this paper a new simultaneous two-stage strategy for synthesizing flexible multi-stream HEN (FMSHEN), optimized by genetic/simulated annealing algorithm (GA/SA), is presented. First, based on the pseudo-temperature enthalpy (T–H) diagram method, a new nonlinear programming (NLP) formulation involving all of the vertices of the polyhedral uncertainty region in the space of process parameters is proposed, with the supposition that the feasible region defined by the reduced inequality constraints is convex. An over-design FMSHEN is obtained by optimizing the stream heat transfer temperature difference contribution. Secondly, the optimal structure of the over-design FMSHEN is retained and each heat exchanger area is modified in order to make the FMSHEN less costly. The total annual cost of MSHEN, obtained from the simulation of MSHEN according to the vertices of the polyhedral uncertain region, is regarded as an objective function, and GA/SA is adopted for optimizing the heat exchanger areas. The remarkable feature of the strategy is that the size and the complexity of the problem are reduced significantly and with more probability of locating the global solution. Finally, two examples are illustrated to demonstrate the performance of the strategy for the synthesis of flexible multi-stream heat exchanger networks.     

A new multi‐scale model based on CFD and macroscopic calculation for corrugated structured packing column

A multi‐scale approach with the combination of computational fluid dynamic (CFD) and macroscopic calculation methods has been proposed to predict the hydrodynamics behavior in the corrugated structured packing column. On the basis of the concept of the representative unit, the three‐dimensional (3‐D) volume of fluid (VOF) model of the structured packing is applied in the small scale simulation, and the stream split fraction coefficients and effective wetted area ratio are calculated. The unit network model, which is a mechanistic model, is applied in large scale calculation basing on the small scale results. The liquid holdup distribution in the entire column can be available by this multi‐scale method. A comparison between the simulation results and the experimental data of our previous work is given to validate the present model. The multi‐scale model is proved to be prospective to assist the analysis and design of structure packing columns in chemical engineering. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3119–3130, 2013     

Simultaneous synthesis of structural‐constrained heat exchanger networks with and without stream splits

This paper presents a comprehensive simultaneous synthesis approach based on stage‐wise superstructure to design cost‐optimal heat exchanger network (HEN). It is well known that the simultaneous synthesis model has very complicated mixed integer nonlinear programming formulations, which are non‐convex, non‐continuous and have many local optima. Up till now, it cannot be expected that an algorithm can find, in polynomial time, the global solution to the simultaneous synthesis problem of HEN. In order to reduce computational complexity, some simplified assumptions for structures, such as no stream splits, stream splits with isothermal mixing, no stream split flowing through more than one exchanger, etc, are adopted to prune the search space at the expense of neglecting certain important alternatives in the network configuration. In this work, a flexible stage‐wise superstructure is proposed to control the solution performance and search space efficiently. At each stage of the superstructure, with or without stream splits is determined at random or by the experience of designers. In this way, various candidate series and split network designs featuring the lowest annual cost can be found. Moreover, an efficient two‐level optimisation algorithm is employed for solving the presented model utilising genetic algorithm and particle swarm optimisation algorithm. Three case studies are presented to show the applicability of the proposed methodology. In addition, the results show that the new approach is able to find more economical networks than those generated by other methods. © 2012 Canadian Society for Chemical Engineering     

Simultaneous integration of water and energy in heat‐integrated water allocation networks

This article proposes a new methodology for simultaneous integration of water and energy in heat‐integrated water allocation networks (WAHEN). A novel disjunctive model is first developed to determine an optimal water allocation network (WAN) where water and energy are integrated in one step. Based on the optimal WAN, a detailed heat exchanger network (HEN) to satisfy the utility target is then synthesized. Although the final network structure is obtained through two steps, the targets of freshwater and utility are optimized simultaneously. The proposed method has specific advantages. First of all, it can capture a tradeoff among freshwater usage, utility consumption, and direct heat transfer by nonisothermal mixing. Second, it can greatly reduce the complexity of subsequent HEN design. Finally, it is effective for simultaneous water and energy integration in large‐scale WAHEN systems. The advantages and applicability of this new method are illustrated by three examples from literature. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2202–2214, 2015     

Heat exchanger network cost optimization considering multiple utilities and different types of heat exchangers

Supertargeting based on composite curves (CC) is widely used to determine the optimum approach temperature (ΔT_min) that yields the minimum total cost for heat exchange networks (HEN). Supertargeting using CC has two key limitations. Firstly, the HEN area calculations are drastically simplified through the assumption that CC segments may be considered as pseudo-single hot and cold streams exchanging heat via only one exchanger that is governed by a single cost correlation. Secondly, the current Supertargeting approach of considering only one hot and one cold utility level may lead to a crude estimation of the total HEN cost and the optimum ΔT_min. This work presents the stream temperature vs. enthalpy plot supertargeting (STEPS) method that overcomes these limitations. This paper proves that supertargeting based on CC can lead to up to 50% error in the total cost target and poor ΔT_min estimations.     

Sensor network design for maximizing process efficiency: An algorithm and its application

Sensor network design (SND) is a constrained optimization problem requiring systematic and effective solution algorithms for determining where best to locate sensors. A SND algorithm is developed for maximizing plant efficiency for an estimator‐based control system while simultaneously satisfying accuracy requirements for the desired process measurements. The SND problem formulation leads to a mixed integer nonlinear programming (MINLP) optimization that is difficult to solve for large‐scale system applications. Therefore, a sequential approach is developed to solve the MINLP problem, where the integer problem for sensor selection is solved using the genetic algorithm while the nonlinear programming problem including convergence of the “tear stream” in the estimator‐based control system is solved using the direct substitution method. The SND algorithm is then successfully applied to a large scale, highly integrated chemical process. © 2014 American Institute of Chemical Engineers AIChE J, 61: 464–476, 2015     

An area targeting algorithm for the synthesis of heat exchanger networks

A new algorithm for the prediction of area requirements in heat exchanger networks (HEN) is presented. The method uses a diverse pinch diagram as a basis, similar to the one suggested by Rev and Fonyo (Chemical Engineering Science, 46 (7), 1623). A numerical application for a problem involving streams with significant differences in their heat transfer coefficient values is included to show how the proposed algorithm provides better estimates for minimum area requirements in HEN than the widely-used Bath formula and the algorithm by Rev and Fonyo.     

考虑系统可靠性分析的柔性换热网络综合

基于虚拟温-焓图法与遗传/模拟退火算法,提出了一种考虑系统可靠性水平的柔性换热器网络综合方法。为简化求解问题的复杂度,首先通过虚拟温-焓图法得到了柔性网络匹配,对于得到的网络结构,通过网络结构的连接矩阵分析网络之中的连接状况,搜索出换热网络中存在关联的换热器单元,以关联的换热器作为主线识别出网络之中不相关的子系统,根据包含换热器单元个数最多的不相关的子系统含有换热器的数量,计算出换热网络系统的可靠性。对于未能满足网络系统可靠性要求的网络结构,通过去掉与其它换热器相关联总数最多的换热器,进行网络解耦,改变换热网络的关联状况,提高了系统的可靠性。换热网络结构改变后,采用遗传/模拟退火算法对热负荷回路中相关换热器的热量分配和换热面积进行计算,得出满足系统可靠性需求,并且总费用最为节约的网络结构。本文所提出的方法可以将需要改善系统可靠性的网络结构系统可靠性提高至设计的容许范围,适用于包含系统可靠性分析的柔性换热网络综合。计算实例表明了所提方法的实用性与可行性。     

Minimizing investment cost for multi-period heat exchanger network retrofit by matching heat transfer areas with different strategies

Multi-period heat exchanger network (HEN) retrofit is usually performed by targeting and matching heat trans-fer areas. In this paper, based on the reverse order matching method we proposed previously, three strategies of matching heat transfer areas are proposed to minimize the investment cost for the retrofit of HEN in multi-period, in which replacement of heat exchangers, addition of heat exchangers and addition of heat transfer areas are performed. We demonstrate the procedures through three scenarios, including maximum number of substituted heat exchangers after retrofit, minimum additional heat transfer areas in the retrofitted HEN, and minimum investment cost for retrofit. The strategies are extended to a single period HEN retrofit problem. The results of multi-period and single period HEN retrofit problems indicate the effectiveness of the strategies. More-over, these results are better than those reported in literature. The strategies are simple and easy to implement, which are of great benefit to large-scale HEN retrofit in practice.     

基于温度和热容流率同时变化的有分流换热网络弹性设计的研究

A new superstructure model of heat exchanger networks (HEN) with stream splits based on rangers of streams supply temperatures and heat capacity flow rates is presented. The simultaneous optimal mathematical model of flexible HEN synthesis is established too. Firstly, the streams with rangers of supply temperatures and/or the streams with the rangers of heat capacity flow rates are pretreated; Secondly, several rules are proposed to establish the superstructure model of HEN with splits and the simultaneous optimal mathematical model of flexible HEN; Thirdly, the improving genetic algorithm is applied to solve the mathematical model established at the second step effectively, and the original optimal structure of HEN based on the maximum operation limiting condition can be obtained easily; Finally, the rules of heat exchange unit merged and the heat load of heat exchanger relaxed are presented, the flexible configuration of HEN satisfied the operation condition between the upper and down bounds of supply temperature and heat capacity flow rates can be obtained based on the original optimal structure of HEN by means of these rules. A case study demonstrates the method presented in this paper is effective     

Floating pinch method for utility targeting in heat exchanger network (HEN)

Most of the established methods for utility targeting in a heat exchanger network (HEN) are mainly focusing on fixed stream conditions, where the flow rate, heat capacity, supply and target temperatures are fixed. However, in the process industries, the stream conditions (flow rates and temperatures) are not fixed. Therefore, the established HEN targeting methods cannot be directly applied to locate the hot and cold utility targets for HEN problem with varying flow rates and temperatures. To address this issue, a revised floating pinch method which uses binary variables to parameterise the stream locations on the composite curves, is presented in this work to identify the minimum utilities targets. The revised method simplify the earlier version of floating pinch method presented by Duran and Grossmann (1986) by avoiding the non-differentiability in the mathematical program. Two cases, one with fixed parameters while another with temperature-dependent properties and varying operating parameters are solved to illustrate the revised model.     

Slamming wave impact of a composite buoy for wave energy applications: Design and large‐scale testing

In the European FP6 project SEEWEC, the design of a wave energy converter has been studied. The floating point absorbers that extract the wave energy were a crucial structure for the efficient functioning of this device and were manufactured from filament wound composite. Their mechanical design was to a large extent dominated by the survivability requirement in extreme storm conditions. During such storms, the structure is hit by very large waves. The associated mechanical loading by the waves is called “slamming,” which is characterized by very high local water pressures (several tens of bar) with short duration (ms) that act on the structure. This study discusses the mechanical design of the filament wound composite floating point absorbers, with the focus on the storm survivability conditions. The finite element calculations have been supported by small‐scale and large‐scale experimental tests, where the large‐scale tests simulate the repeated wave impacts under yearly storm conditions and the survival under extreme storm conditions. It has been proved that a design that takes into account the deformability of the structure can withstand the slamming loads, although it does not comply with the international design codes which assume rigid behavior of the impacted structure. Further, the experimentally measured slamming peak pressures strongly depend on the rigidity of the composite structure. Given the broad context of the covered research work and the imposed page limitations, this study does not pretend to give all the details of all the individual steps in the design and testing processes, but rather gives a comprehensive overview of the complete chain of design decisions and validations between finite element simulations and experiments. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Thermodynamic study of a pressure‐temperature phase diagram for poly(N‐isopropylacrylamide) gels

The volume change, ΔV_h,_, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N‐isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative ΔV_h′?2.5 cm³/mol. The P‐T phase diagram, the coexistence curve, for the gels was determined from the swelling ratio‐pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P‐T plain, which is a feature peculiar to the reversible pressure‐temperature denaturation of a protein. The thermodynamic analysis of the Clausius–Clapeyron relation for the measured ΔV_h elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, ΔH, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume‐dependent free energy, Δv_mi · P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio‐pressure curve of PNIPA gels. The value of Δv_mi changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 405–412, 2005     

结构可观的换热网络温度测点分析及控制系统设计

复杂换热网络中间温度状态变量众多且变量间耦合严重,若不加以控制与观测,往往不利于物流间的热量传递与安全监测。换热网络的温度测点设置是实施控制的基础,为保证全部状态的可观性,凭经验设置的温度测点往往过多,经济性差,也缺乏理论论证与分析。因此从控制理论的角度,基于结构可观性理论分析换热网络的结构特性。分析旁路设置对换热网络结构可观性的影响,权衡可观性与经济性两方面因素,为实现设置最少的温度测点以满足全部状态的可观性要求,提出了换热网络全部状态可观的温度测点优化设计法。然后,基于相对增益分析,将已有旁路与优化得出的温度测点进行配对,构成换热网络的常规控制系统。最后,以某常减压蒸馏装置脱盐前换热网络为例验证了方法的可行性。     

Refrigeration system synthesis based on de-redundant model by particle swarm optimization algorithm

Simultaneous optimization of refrigeration system (RS) and its heat exchanger network (HEN) leads to a large-scale non-convex mixed-integer non-linear programming (MINLP) problem. Conventionally, researchers usually adopted simplifications to confine problem scale from being too large at the cost of reducing solution space. This study established an optimization framework for the simultaneous optimization of RS and HEN. Firstly, A more comprehensive and compact model was developed to guarantee a relatively complete solution space while reducing model scale as well as its solving difficulty. In this model, a tandem arrangement of connecting sub-coolers and expansion valves was considered in the superstructure; and the pressure/temperature levels were optimized as continuous variables. On this basis, we proposed a “two-step transformation method” to equivalently transform the cross-level structure into a non-cross-level structure, and the de-redundant superstructure was established with ensuring comprehensiveness and rigor. Furthermore, the MINLP model was developed and solved by Particle Swarm Optimization algorithm. Finally, our methodology was validated to get better optimal results with less CPU time in two case studies, an ethylene RS in an existing plant and a reported propylene RS.     

采用换热器负荷图指导换热网络改造的新方法

目前大部分图形工具仅适用于以节能为目标的换热网络(HEN)设计或者改造。能量回收量的增加往往伴随换热单元数的增多,而换热单元数对设备投资费有较大影响。采用换热器负荷图(HELD)提出一种系统化的换热网络改造新方法。新方法基于夹点分析,在改造区间中选择匹配目标,重新构建改造用HELD,从而简化问题;通过在水平方向上平移热流股曲线实现跨夹点负荷的重新分配,完成节能目标,并结合经验规则,尽可能减少改造后换热网络的换热器数目。以一个工业造纸厂为例,对其进行节能改造方案设计,相较于文献报道结果,得到两个节能目标值相近且换热器改动数目更少的新方案,验证了新方法的有效性。