Integrated design of diesel hydrotreating processes

This work presents an integrated approach for the design of diesel hydrotreating processes employing a simulated annealing optimisation algorithm. The modelling of reactor, separation and heat recovery system for diesel hydrotreating processes is discussed, and a novel optimisation framework is developed for the design of complex refinery processes. A comparison with conventional approach to process design, i.e. sequential evolution of design, is given to illustrate the ability of proposed approach to obtain overall hydrotreating process designs with minimum total annualised costs. The proposed integrated approach takes into account the trade-offs between capital and operating costs, as well as interactions between the hydrotreater, distillation column, and the associated heat exchanger network.     

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。     

Design of flexible heat exchanger network for multi-period operation

Heat exchanger networks (HENs) increase heat recovery from industrial processes by matching hot and cold streams to exchange heat and reducing utility consumption. The design of HENs is a very complex task which generally involves mixed-integer non-linear programming (MINLP).This work evaluates and critically compares existing HEN design methods. It then presents a systematic methodology in the design of HENs under multiple periods of operation. The model presented in this work is a superstructure-based MINLP model which minimises the total annualised cost containing heat exchanger area cost and utility costs. The model is based on the superstructure by Yee and Grossmann [1990. Simultaneous optimisation models for heat integration—II, heat exchanger network synthesis. Computer & Chemical Engineering 14(10), 1165-1184], which was later formulated for multiple periods by Aaltola [2002. Simultaneous synthesis of flexible heat exchanger network. Applied Thermal Engineering 22, 907-918]. It includes a multi-period simultaneous MINLP model to design the HEN structure, and an NLP model to improve the solution and allow for non-isothermal mixing. Modifications to Aaltola's model include the use of maximum area per period in the area cost calculation of the MINLP objective function, and the removal of slack variables and weighed parameters from the existing NLP improvement model.The new model has been applied to one industrial case study, demonstrating that the new combined MINLP-NLP model can obtain better solutions by not relying on the average area assumption in the MINLP stage.     

用伪并行遗传/模拟退火算法进行多流股换热器网络综合的研究

The multi-stream heat exchanger network synthesis (HENS) problem can be formulated as a mixed integer nonlinear programming model according to Yee et al. Its nonconvexity nature leads to existence of more than one optimum and computational difficulty for traditional algorithms to find the global optimum. Compared with deterministic algorithms, evolutionary computation provides a promising approach to tackle this problem. In this paper, a mathematical model of multi-stream heat exchangers network synthesis problem is setup. Different from the assumption of isothermal mixing of stream splits and thus linearity constraints of Yee et al., non-isothermal mixing is supported. As a consequence, nonlinear constraints are resulted and nonconvexity of the objective function is added. To solve the mathematical model, an algorithm named GA/SA (parallel genetic/simulated annealing algorithm) is detailed for application to the multi-stream heat exchanger network synthesis problem. The performance of the proposed approach is demonstrated with three examples and the obtained solutions indicate the presented approach is effective for multi-stream HENS.     

用遗传算法进行多流股换热器网络综合的研究

建立了多流股换热器网络综合数学模型，该模型改进了文献中等温混合的不合理假设。多流股换热器网络综合问题本质上是一个混合整数非线性规划问题(MINLP)，这类问题的非凸非线性的特性使得目标函数存在多个局部最优解。传统的基于梯度的搜索方法在处理这类问题时由于计算规模庞大且极易陷于局部最优解而不再适用，而遗传算法却为解决这类问题提供了很有希望的一个方向。因而对遗传算法求解多流股换热器网络综合问题进行了研究，提出了可以自动产生可行的多流股换热器网络的方法策略，最后通过两个例题说明所提方法是可行的。     

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。     

基于持续节能的多周期换热网络优化设计

在石油化工工业中,换热网络的操作条件时常发生改变,尤其是结垢热阻随运行时间不断增大。针对操作条件的改变,有研究者提出多周期换热网络最优综合的设计与改造方法。但是,这些方法不涉及结垢热阻对设计结果的影响。然而,换热器结垢不可避免,传热系数随之改变,将导致操作工况偏离最优值,难以实现持续节能优化。以多周期换热网络优化为基础,提出一种基于持续节能的多周期换热网络综合设计方法,以换热网络累积总费用为目标函数,考虑换热器结垢对多周期换热网络优化结果的影响,进而实现多周期换热网络的最优综合。首先,结合示例分析换热器结垢对多周期换热网络优化结果的影响,得出考虑到结垢热阻的影响多周期换热网络在各个周期内的优化结果将改变;然后,基于持续节能优化,以换热网络累积总费用为目标函数,提出一种多周期换热网络优化方法;最后,结合示例验证了该设计方法的有效性。     

基于热损失的换热网络夹点设计法

现行的换热网络目标方法,在进行换热网络价格估算时,未考虑热物流的热损失。在真实的设计中,换热器壳体保温后仍与环境温度相差较大,则其热损失不可忽略。文章提出了一个新的基于热损失的换热网络夹点设计法,该方法首先以综合费用最小为目标确定出最小温差,然后建立问题表格确定出夹点位置及最小公用工程消耗,最后再进行换热网络设计。文中采用某石油常减压换热网络系统为典型算例对该方法的前两步进行了分析研究,论证了该方法的必要性及可行性。结果表明:该方法与基本Linnhoff夹点技术法估算的投资费用有较大的差距之外,在一定的最小温差下,其与基本Linnhoff夹点技术法确定的夹点位置不同,公用工程消耗也有较大的差距。     

Design of heat exchanger network based on entransy theory

The heat exchanger network (HEN) synthesis problem based on entransy theory is analyzed. According to the characteristics of entransy representation of thermal potential energy, the entransy dissipation represents the irreversibility of the heat transfer process, the temperature difference determines the entransy dissipation, and four HEN design steps based on entransy theory are put forward. The present study shows how it is possible to set energy targets based on entransy and achieve them with a network of heat exchangers by an example of heat exchanger network design for four streams. In order to verify the correctness of the heat exchanger networks design method based on entransy theory, the synthesis of the HEN for the diesel hydrogenation unit is studied. Using the heat exchange networks design method based on entransy theory, the HEN obtained is consistent with energy targets. The entransy transfer efficiency of HEN based on entransy theory is 92.29%, higher than the entransy transfer efficiency of the maximum heat recovery network based on pinch technology.     

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.     

换热器网络设备面积与清洗时序同步优化

表面结垢会严重影响换热器的传热效率,定期清洗是解决该问题的主要方式.针对以往换热器网络清洗时序优化方法中用于决策的整型变量较多而难以求解的问题,提出以换热器清洗的最大允许污垢热阻为优化变量,取代表示换热器是否清洗的二进制变量,将混合整数非线性规划问题转化成非线性规划问题,能够有效地减小问题规模,降低求解难度.优化过程中兼顾换热器网络的设计型与操作型问题,采用遗传/模拟退火算法同步优化换热器的面积与清洗时序.将该方法用于一个实例,所得年度总费用与文献基本一致,验证了该方法的有效性.     

Synthesis of multipass heat exchanger networks using genetic algorithms

In this work, a methodology based on genetic algorithms (GAs) is developed for the optimal synthesis of multipass heat exchanger networks (HENs). The network model is based on a stagewise superstructure, and the problem of finding the optimum number of 1–2 shells in series of multipass heat exchangers is aided by an efficient optimization model that uses the standard F_T design method. The proposed methodology allows for proper handling of the trade-offs involving energy consumption, number of units, number of 1–2 shells and network area to provide a network with the minimum total annual cost. The results of the examples show that the new approach is able to find more economical networks than those generated by other methods.     

Interval based MINLP superstructure synthesis of heat exchanger networks for multi-period operations

The interval based MINLP superstructure (IBMS) presented by Isafiade and Fraser (2008) is applied to the design of heat exchanger networks which can handle multiple period of operations. In multi-period operations, parameters such as supply temperature, target temperature and the flowrates of streams can vary over a specified range. In this paper the IBMS is formulated for multi-period operations involving unequal period durations. In order to ensure that each exchanger connecting the same pair of streams in two or more periods is able to transfer heat in such streams for all the periods, the maximum area per period approach of Verheyen and Zhang (2006) is used in the objective function. This method helps to simplify the model. The new method is applied to three examples from the literature.     

随机搜索算法在换热网络优化中的应用进展

随着对能源利用率要求的不断提高,换热网络的优化已引起了人们的高度重视。本文综述了国内外应用随机搜索算法处理换热网络优化问题的最新研究进展,分析了当前应用较多的4种随机搜索算法：遗传算法、模拟退火算法、粒子群算法、禁忌搜索算法。阐述了这些算法各自的优势和尚待改进的问题,指出将不同算法结合起来解决大规模的换热网络优化问题是今后的研究方向。     

Design of resilient processing plants—II Design and control of energy management systems

The “standard heat exchanger network problem” has been surprisingly difficult to solve. It is only now that simple and reliable methods have evolved to synthesize the most efficient network of heat exchangers to heat and cool known process streams between stated temperature bounds. This has taken over a decade of research and scores of publications.The “resilient heat exchanger network problem” requires a solution that can cope with the uncertainties of industrial design. Fixed flow rates and temperature bounds rarely occur industrially. Rather, an industrial heat exchanger network problem necessarily involves ranges of flow rates and ranges of temperature bounds, and must include ease of operation and control.In this paper we make several fundamental advances in the design of resilient heat exchanger networks. (1) An efficient design procedure is developed to yield resilient designs which handle fluctuations within the condition of maximum energy efficiency. (2) A control structure and operating policy are developed to adjust flow distributions in the network to meet temperature constraints with minimum utility usage.These developments are based on several new theorems concerning resiliency in network design.     

用于超结构换热器网络热计算和网络综合的显式解

For the optimal design of a heat exchanger network, the inlet and outlet stream temperatures of each heat exchanger in the network should be known. An explicit analytical solution of stream temperatures of an arbitrary connected heat exchanger network was introduced, which is suitable for the thermal calculation of heat exchanger networks. For the heat exchanger network synthesis, this solution was further developed and coupled with the stage-wise superstructure heat exchanger networks. The new calculation procedure reduced the computer memory requirement dramatically. On the basis of this solution, a mathematical model for synthesis of heat exchanger networks with genetic algorithm was formulated, which is always feasible and no iteration is needed. Two examples were calculated with the proposed approach and better results were obtained.     

基于遗传/模拟退火算法考虑压降的换热网络优化改造

工艺系统中需要加入新物流、现存的换热网络结构需要改动、换热网络有较大的能量回收潜力,这些都是换热网络需要进行改造的原因.本文基于非等温混合分级超结构,建立了换热网络优化改造的MINLP数学模型,同时考虑了改造后的公用工程费用、新增换热器的费用、原有换热器新增面积的费用、新增动力设备的费用.将用于换热网络综合的分级超结构模型进行了简要的改进,增加了一些和现存设备相关的限制条件和二元变量.通过对现存换热网络进行分析,充分的利用原有的换热设备和换热网络结构,基于GA/SA算法求解可 得到优化改造的换热网络结构.实际算例的计算结果和文献结果对比表明,本文得到的改造方案用较少的投 资,实现了较多的公用工程节省,新增设备费用的静态投资回收期为0.53年,为换热网络改造提供了好的可行方案.     

基于分解算法的功热交换网络多目标优化

功热网络设计问题指在流程设计中对变压和换热过程进行耦合优化设计的问题,以此来提高整体系统的能效并降低成本。前人工作中一般采用数学规划法对功热网络建模优化。然而,由于存在变压过程和换热器面积计算的非线性约束,以及换热匹配的二元变量,整体模型往往是一个高度非凸的混合整数非线性规划模型,难以求解。本文提出一种高效的功热网络优化方法。模型中分别用透平压缩机和换热器实现功热网络中轴功和热的交换。求解过程采用分解算法,主问题中用随机算法对关键变量优化,功和热两个子网络问题中用确定性算法求解。目标函数考虑了经济和环境影响。案例测试对比了不同优化目标得到的结果以及多目标Pareto曲线,验证了所提出方法的高效性。     

换热器结垢与换热网络裕量设计方法研究进展

从结垢现象影响换热系统正常运行的角度出发,介绍了近几年换热网络裕量设计以及针对结垢问题不同学者研究出的换热网络优化设计。总结了在换热网络设计中对换热器清洗时序、清洗周期优化或者增加换热网络的裕量设计的不同换热网络设计最优方法,但这些方法都是在换热网络设计之初,考虑换热器结垢最严重的情况即换热网络在“最差”工况下进行的优化,因此优化得到的换热网络难以保证换热网络全运行周期的持续节能优化。本文结合现有换热网络设计方法的利弊,针对结垢过程的慢时变、持续特点,提出一种基于长周期持续节能的换热网络设计优化方法,在换热网络设计之初,定量分析结垢对网络结构的影响,以换热网络全周期累积总费用为目标函数,实现换热网络的最优综合。     

考虑冗余控制的多周期换热网络设计

实际多周期换热网络的设计不但要满足多个离散周期的操作条件要求,还应该满足单个周期下的柔性要求,以补偿实际操作中单周期下可能出现的参数波动。而根据工程经验增加换热面积裕量和采用文献中的最大面积思想进行设计,都会造成多周期换热网络中换热面积的过度冗余,导致资源的浪费和投资成本的增加。针对以上问题,在综合考虑多周期操作条件要求和单周期柔性要求的基础上,提出了一种控制面积冗余的多周期换热网络设计方法。该方法首先在单周期换热网络集成的基础上获取一个初始的多周期换热网络设计方案。然后以此方案为基础,构建一个基于单周期换热网络模型和柔性分析模型的换热网络修正模型,并通过求解该模型获得初始多周期换热网络在各周期下的最优修正方案。通过综合初始多周期换热网络设计和修正方案,即可完成最终的多周期换热网络设计。最后,通过将所得多周期换热网络与文献中所得结果进行对比,验证了方法的有效性。