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。     

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。     

基于全周期节能的有旁路换热网络裕量优化设计

换热网络全周期运行期间,由于设备老化等因素,换热性能逐步下降,能耗逐步提高。而当前工业换热网络的裕量设计并没有有效地解决此问题。因此提出了一种基于有旁路换热网络全周期节能优化的裕量优化设计方法,通过旁路调节,逐步释放有效面积裕量,达到全周期节能的目的。该优化方法以换热网络运行全周期内总累积费用为目标函数,同时考虑结垢热阻对换热器换热效果的影响以及旁路对换热网络的调节作用,在满足工艺条件的基础上求解换热网络的最优裕量,最终实现换热网络的持续节能。最后,以某炼油厂的常减压脱盐前换热网络为具体的研究对象,说明所提方法的有效性和使用前景。     

换热网络旁路优化设计

提出了一种可以降低已有换热网络运行费用的旁路优化设计方法,使得网络控制自由度增大,运行费用明显降低。该优化设计方法将网络运行总费用作为目标函数,同时考虑结垢热阻对换热器换热效果的影响,经过多次优化计算确定旁路位置和开度,并根据现场具体情况选择合适的旁路开度下限,确定旁路数目,最终实现网络旁路优化设计。通过示例说明了本文所提方法的有效性和实用前景。     

基于全周期持续节能的换热网络滑动窗口分析与裕量缓释优化控制

换热网络是过程系统节能降耗的关键,对于给定换热网络其操作条件及生产要求等常发生变化,尤其是换热器普遍存在缓慢结垢现象,导致换热网络的运行参数偏离设计值,甚至难以满足生产操作要求。为此设计时一般留出一定的面积裕量,并通过旁路控制实现对面积裕量的在线调节。本文针对换热网络运行的全周期,以持续节能为目标,提出一种裕量缓释的优化控制方法。首先,针对换热网络整个运行周期,结合滑动窗口算法分析缓慢结垢现象与优化控制的关系,得出一种换热网络全周期持续节能的控制策略。然后,在划分的每个滑动窗口内以提高控制性能为目标,实现换热网络的在线优化控制。进而针对换热网络的全运行周期,以持续节能为目标,研究一种裕量缓释优化方法,优化求解每个窗口内可利用的换热网络面积裕量。最后,提出一种以全周期持续可控为目标的换热网络裕量缓释优化控制方法。结合实例说明采用本文提出的方法实现了全周期持续可控同时兼顾了各时间窗口内系统的动态控制性能。     

多管程换热器网络的最小温差分析与夹点设计

在实际生产过程中,多管程换热器广泛应用,其中既存在逆流换热也存在并流换热。针对多管程换热器网络中并流换热与逆流换热时换热温差对夹点特性尤其是换热网络综合的影响,定量分析最小温差的选择与夹点特性及其总费用的关系。兼顾管程数目的选择以及换热网络总费用两方面因素,以年度化总费用最低为目标,优化求解多管程换热器网络的最小温差。然后,将求得的最小温差作为夹点温差,遵循夹点设计原则综合换热网络。同时,计算得出多管程换热器网络的F_T修正因子,并对结果进行分析。最后,结合示例对比分析求解结果,证明方法的有效性。     

有旁路换热网络全周期节能的动态优化控制实现方法

为了实现换热网络的全周期持续节能,在网络上设置旁路从而增加其控制自由度,同时设计一定的裕量来提供优化控制的操作空间。为了较好地利用旁路调节和裕量空间,提出一种基于换热网络动态模型的在线优化控制方法,巧妙地结合原有常规控制回路,不但扩大了优化控制的可行域,并且满足原常规控制回路的精度要求。该方法以换热网络一定周期内的累积费用最小为目标函数,同时考虑扰动对换热网络的影响,在满足工艺条件的基础上,求解最佳旁路开度,以实现换热网络的持续节能。由于采用闭环校正、迭代计算和滚动优化的实施方法,始终把优化建立在实际的基础上,尽管它每次不一定能得到全局最优解,然而使得实际控制结果达到最优。最后,以某炼油厂的常减压脱盐前换热网络为具体的研究对象,说明所提方法的有效性和使用前景。     

换热器因应结垢慢时变的控制系统重构分析

换热器作为化工过程中重要的热量传递设备,一般需要持续运行较长时间,期间由于结垢热阻的不断累积,换热器的换热效率将随时间逐渐下降,直至无法满足工艺要求。在实际的工业换热过程中,工艺人员通常会对换热器进行裕量设计,然而,当换热器需要运行的时间增长或者裕量设计不足时,面对不可避免的结垢增长,常用的流量控制策略在换热器运行末期的调节效果逐渐变差,旁路控制的方案无法实现全周期的持续控制。因此,本文首先在渐近增长模型的基础上建立了换热器结垢累积的积分渐近模型,该模型考虑了过程参数对结垢速率的影响;其次,以一个用于水循环的小面积换热器为例,分析了考虑结垢增长的条件下流量控制和旁路控制的调节效果;最后,针对结垢过程的慢时变和持续性等特点,设计了基于流量与旁路开度的控制系统重构方案。示例的运行结果表明,对于同样面积裕量有限的换热器,这一控制方案能延长其使用寿命,实现其全周期持续可控的目标。     

基于经济目标和分解协调策略的换热网络最优设计

分析了换热网络在过程系统中的功能 ,从火用分析和火用经济学的角度提出了换热网络最优合成设计的实用火用经济目标函数 ,它包含了网络的流动火用损费 ,且可分解为各个换热单元的总费用之和 .在换热网络目标函数可分解性的基础上 ,本文又进一步提出了换热网络分解协调优化的研究策略 ,它是在各个匹配单元 (子系统 )和整个换热网络结构 (复杂系统 )之间进行优化协调 ,并获得换热网络的最优结构和各匹配单元的最优操作参数 .最后 ,通过一换热网络的最优合成 ,说明了该方法的应用前景     

基于经济目标和分解协调策略的换热网络最优设计

分析了换热网络在过程系统中的功能 ,从火用分析和火用经济学的角度提出了换热网络最优合成设计的实用火用经济目标函数 ,它包含了网络的流动火用损费 ,且可分解为各个换热单元的总费用之和 .在换热网络目标函数可分解性的基础上 ,本文又进一步提出了换热网络分解协调优化的研究策略 ,它是在各个匹配单元 (子系统 )和整个换热网络结构 (复杂系统 )之间进行优化协调 ,并获得换热网络的最优结构和各匹配单元的最优操作参数 .最后 ,通过一换热网络的最优合成 ,说明了该方法的应用前景     

换热网络的优化改进和控制

通过对换热网络的定性分析提出了换热网络优化改进的4条规则：①控制变量应对调节变量反应灵敏；②选择的调节变量数应小于或等于控制变量数；③一组控制变量应有一组调节变量相匹配；④如果在标准操作条件下现有换热网络是最佳的，而且改进过程中对它做最小的改变就使换热网络适合当前的或多周期操作条件, 那么该改进费用一定是最小的，因此这种改进就是最佳的.根据上述规则，结合换热网络的灵敏度分析建立了换热网络优化改进及控制的数学模型和求解方法，给出了换热网络优化改进的步骤，通过实例计算及应用证明本文所提方法正确、可靠、有效，可在解决实际工程问题中使用.     

Synthesis of multipass heat exchanger networks based on pinch technology

The multipass heat exchanger is the most common type of heat transfer equipment used in heat exchanger networks (HENs) by the chemical process industries. There are many methods that have been proposed for the synthesis of HENs with multipass heat exchangers, which are mostly derived from the FT design method. In this paper, an alternative new method to synthesis multipass HENs is presented based on the classical pinch technology. In the multipass heat exchanger, both countercurrent and co-current flow are involved. For the co-current flow, composite curves and problem tables are modified, and compared with that of the countercurrent flow. A proper minimum temperature difference is also selected considering the energy-capital cost trade-offs, and then a multipass HEN is synthesized. Results of the case study demonstrate that the new approach meets operating requirements and minimizes the total cost successfully.     

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.     

Heat exchanger network synthesis integrated with flexibility and controllability

Over the last three decades, flexibility and controllability considerations for heat exchanger networks(HENs) have received great attention, respectively. However, they should be simultaneously incorporated in HEN synthesis to allow the economic performance to be achievable in a practical operating environment. This paper proposes a method for simultaneous synthesis of flexible and controllable HEN by considering their coupling. The key idea is to add the bypasses with optimized initial fractions and positions to explore such coupling, and consequently enabling HENs to be operated successfully over a range of disturbance variations. These are implemented by identifying and quantifying disturbance propagations,and then examining the sensitivity of bypasses to the entire HEN. In this way, the superstructurebased mixed integer non-linear programming(MINLP) with objective function of minimizing the total annual cost is formulated. A case study is used to demonstrate the application of the proposed method.Quantitative measures and dynamic simulation show the ability to provide the satisfactory flexibility and controllability of the obtained HEN.     

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.     

Heat Exchanger Network synthesis with Detailed Heat Exchanger Design

Pressure drop and fouling are important issues in heat exchanger network synthesis, which are usually neglected. Heat exchangers were designed in detail during the heat exchanger network synthesis, and pressure drop and fouling effects were taken into account. Pinch analysis combined with exergoeconomic analysis was used for determining optimal minimum approach temperature (ΔT_min) for heat exchanger network synthesis. Exergy consumption of heat transfer in HENs was calculated using a subsection integral on balanced composite curves. The heat transfer coefficients of all heat exchangers in the network were calculated iteratively to meet the requirements of optimal area and allowable pressure drops. The proposed method was applied to an industrial case. Numerical results indicate the importance of the detailed design of heat exchangers in HENs synthesis.