基于相对增益和优先级的化工过程协调优化裕量设计

化工过程设计裕量一般是通过设计经验或经济优化给出的,设计经验无法保证经济性能的优化,而经济优化需要求解大规模非线性优化问题,计算复杂,容易陷入局部极值点,设计结果有时与设计经验违背。本文用非方相对增益阵和非方相对能量增益阵描述化工过程设计的自变量和因变量的灵敏度关系,将自变量划分为操作变量和设计变量,将因变量划分为经济指标和约束变量。相对增益具有无量纲化和归一化的优点,因此可根据经济指标和约束变量的相对增益对操作变量和设计变量划分优先级,针对过程不确定性的大小按照优先级依次调整各个操作变量和设计变量,找到对过程经济性能影响最小并有效移动操作点、远离约束边界的裕量设计方案。以串联反应釜为例对该设计方法进行了验证,结果表明,与求解经济最优化问题的裕量设计方法相比,本设计方法得到了经济性能与之接近的设计结果,计算简单,无须求解最优化问题。     

化工过程总体裕量与控制性能的权衡优化

化工过程的总体裕量可以用操作优化的经济效益进行评价,根据稳态优化和动态优化的经济效益可进一步划分为服务于操作控制的控制裕量和表征过程可实现经济效益的工艺裕量,二者都与化工过程的控制性能有关。针对具有一定裕量的化工过程进行多目标动态优化,优化目标分别为操作点的经济效益与动态过程中的控制性能指标,采用0-1变量描述控制结构,将控制结构和控制器参数也作为优化变量进行混合整数动态优化,采用Constrained NSGA-Ⅱ算法求解非劣解集,根据非劣解集分析总体工艺裕量、总体控制裕量与控制性能指标的关系。通过催化裂化装置的实例分析发现,对于具有一定裕量的化工过程,控制性能越高,所需的总体控制裕量越多,表征操作优化可实现经济效益的总体工艺裕量越少,只有通过对总体控制裕量和总体工艺裕量进行权衡,才能找到兼顾工艺要求和控制性能的工艺操作点和控制设计方案。     

化工过程总体裕量与控制性能的权衡优化

化工过程的总体裕量可以用操作优化的经济效益进行评价,根据稳态优化和动态优化的经济效益可进一步划分为服务于操作控制的控制裕量和表征过程可实现经济效益的工艺裕量,二者都与化工过程的控制性能有关。针对具有一定裕量的化工过程进行多目标动态优化,优化目标分别为操作点的经济效益与动态过程中的控制性能指标,采用0-1变量描述控制结构,将控制结构和控制器参数也作为优化变量进行混合整数动态优化,采用Constrained NSGA-Ⅱ算法求解非劣解集,根据非劣解集分析总体工艺裕量、总体控制裕量与控制性能指标的关系。通过催化裂化装置的实例分析发现,对于具有一定裕量的化工过程,控制性能越高,所需的总体控制裕量越多,表征操作优化可实现经济效益的总体工艺裕量越少,只有通过对总体控制裕量和总体工艺裕量进行权衡,才能找到兼顾工艺要求和控制性能的工艺操作点和控制设计方案。     

先进控制条件下化工过程操作裕量与控制性能分析

引言化工过程的设计裕量可以定义为考虑过程不确定参数(包括工艺条件、设备条件、外来扰动)发生变化时为满足生产和操作要求需要在正常操作的标称设计值上增加的量。设计裕量根据其属性可以     

基于全周期缓慢结垢的多效蒸发海水淡化慢时变系统优化设计

多效蒸发（MED）是最主要的海水淡化方法之一,作为典型的慢时变系统,该系统在长期运行的过程中,往往会由于结垢导致蒸发器传热效率降低,造成减产甚至停工。为避免出现这种问题,工艺设计者会采取冗余设计,增大传热面积,这会导致设备投资的显著增加。为保证MED系统能够全周期运行,且尽可能减少总传热面积,提出了一种全周期优化设计方法。该方法以总传热面积最小为目标,对决策变量在整个周期内进行分段优化,同时考虑结垢过程、工艺变化以及控制方面的需求,对各效传热面积进行裕量设计,通过一步优化求解得到最优操作条件与最小传热面积,实现对慢时变系统的优化设计。最后,以八效MED海水淡化装置为例,同时用等面积法、等温差法、稳态优化设计方法以及全周期优化设计方法对系统进行设计。结果表明,全周期优化设计方法能够最大限度减少传热面积,大大降低了系统的设备投资,是一种有着良好应用前景的多效蒸发海水淡化系统优化设计方法。     

工艺调度对乙炔加氢反应器优化运行策略的影响分析

乙炔加氢反应器是乙烯工业中用于除去高浓度乙烯流中的少量乙炔的重要装置,该装置一般持续运行较长时间,期间反应器内催化剂活性逐渐降低,直至活性难以满足工艺要求。乙炔加氢反应器全周期操作优化一般是针对装置的一个再生周期进行的,在装置运行周期内应按照操作优化方案进行。但是,在实际工业过程中,为了满足临时的工艺调度需求,乙炔加氢反应器在按照操作优化方案运行一定时间后,需要在剩余的运行周期内临时改变操作方案,这给操作优化问题带来了更多变化和挑战。基于裕量估计和慢时变系统的控制优化框架,研究了这类在运行周期中临时改变操作优化方案的全周期动态优化问题。改变操作优化方案的方式包括：变更运行周期、追求经济效益最大化和变更优化目标、追求运行周期最大化。通过对这两种改变操作优化方案的分析,发现前者变更的运行周期越接近原定运行周期,全周期总经济效益越高,后者切换时刻越早,反应器能维持的运行周期越长,但二者的全周期经济效益均不及原操作优化方案,临时的工艺调度对乙炔加氢反应器的全周期优化运行总体上是不利的。     

乙炔加氢串联反应器全周期乙炔转化率最优分配研究

在乙炔加氢反应器的实际生产运行过程中,乙炔加氢反应大部分在第一床层,加氢反应放出的大量热量使得床层内温度高于最佳反应温度范围,致使乙烯选择性降低,乙烯产量下降,而在进行全周期操作优化时并未考虑到此问题。因此,首先考虑温度对绿油累积的影响,修正了催化剂失活动力学方程;其次,为保证反应器各床层内温度都在最佳反应温度范围,从化学反应工程理论和实际生产过程中的安全性两个角度出发,给出两种反应器各床层乙炔转化率分配方案;最后,在常规全周期操作优化模型中添加乙炔转化率约束,建立全周期乙炔转化率分配操作优化模型,并对两种乙炔转化率分配方案进行全周期操作优化。优化结果表明,两种乙炔转化率分配方案操作优化的乙烯产量要远远高于常规操作优化,且乙炔转化率方案为33∶33∶33时,乙烯产量最高,而考虑实际生产过程中的安全性,乙炔转化率分配方案为43∶47∶10时具有更好的效果。     

乙炔加氢串联反应器全周期乙炔转化率最优分配研究

在乙炔加氢反应器的实际生产运行过程中,乙炔加氢反应大部分在第一床层,加氢反应放出的大量热量使得床层内温度高于最佳反应温度范围,致使乙烯选择性降低,乙烯产量下降,而在进行全周期操作优化时并未考虑到此问题。因此,首先考虑温度对绿油累积的影响,修正了催化剂失活动力学方程;其次,为保证反应器各床层内温度都在最佳反应温度范围,从化学反应工程理论和实际生产过程中的安全性两个角度出发,给出两种反应器各床层乙炔转化率分配方案;最后,在常规全周期操作优化模型中添加乙炔转化率约束,建立全周期乙炔转化率分配操作优化模型,并对两种乙炔转化率分配方案进行全周期操作优化。优化结果表明,两种乙炔转化率分配方案操作优化的乙烯产量要远远高于常规操作优化,且乙炔转化率方案为33∶33∶33时,乙烯产量最高,而考虑实际生产过程中的安全性,乙炔转化率分配方案为43∶47∶10时具有更好的效果。     

乙炔加氢反应器全周期操作优化

乙炔加氢反应器作为乙烯工业流程的重要环节,其运行会很大程度上影响到乙烯产品的产量和纯度。在一个运行周期内,乙炔加氢反应器内催化剂活性会随时间推移而缓慢降低,使操作点偏移,乙烯产量会随之降低。为了实现全周期操作优化,通过研究催化剂的失活机理,提出了考虑绿油累积效果的催化剂失活动力学模型,进而改进了乙炔加氢反应器二维非均相模型。通过在gPROMS平台模拟反应器全周期运行验证了改进模型的正确性,在上层运用Matlab优化器与gPROMS平台交互求解一个运行周期的操作优化问题。优化结果表明,与定值温度补偿方案相比,全周期操作优化在经济效益和反应器再生周期两方面都要优于定值温度补偿方案,且同时优化入口温度与入口加氢量的全周期操作优化方案具有更大的优势。     

Release mechanism analysis of design margin for slowly-time-varying chemical processes

Slowly-time-varying characteristics are common in chemical processes, and the changes of slowly-time-varying parameters in an operating cycle gradually decrease the performance of chemical process. So, enough margins must be added for design variables during the phase of process design according to the possible worst-case influence of slowly-time-varying parameters. The design margins will be released gradually compensating the worse influence of slowly-time-varying parameters in an operating cycle. It can be called as a perfect operation that the operating point is on the boundary of process constraints when an operating cycle is ending. In this paper, the margin release mechanism of slowly-time-varying chemical processes is analyzed. Based on the universal dynamic model containing slowly-time-varying parameters, the full cycle operation optimization is solved by minimum principle of optimal control. It is found that the optimal margin release trajectory is related to the curve of slowly-time-varying parameter, ensuring that the optimal margin release is only dependent on the operating cycle. This mechanism is verified by the example of acetylene hydrogenation reactor. For slowly-time-varying chemical processes, the shorter the operating cycle is set, the faster the design margin is released, the higher temporary economic benefit is obtained; otherwise, the longer the operating cycle is set, the more integrated economic benefit is accomplished.     

Integration of scheduling,design, and control of multiproduct chemical processes under uncertainty

The development of a methodology that addresses the simultaneous design, scheduling, and control of multiproduct processes is focused. The proposed methodology takes into account the influence of disturbances by the identification of their critical frequency, which is used to quantify the worst‐case variability in the controlled variables via frequency response analysis. The uncertainty in the demands of products has also been addressed by creating critical demand scenarios with different probabilities of occurrence, while the nominal stability of the system has been ensured. Two case studies have been developed as applications of the methodology. The first case study focuses on the comparison of classical semisequential approach against the simultaneous methodology developed, while the second case study demonstrates the capability of the methodology in application to a large‐scale nonlinear system. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2456–2470, 2015     

An overview on controllability analysis of chemical processes

Controllability is one of the most important aspects of chemical process operability, because it can be used to assess the attainable operation of a given process and improve its dynamic performance. The purpose of this article is to outline the main methodologies that have been developed to deal with the assessment of process controllability and the improvement of its controllability characteristics. Several existing controllability assessment methods are reviewed and discussed. For improving the controllability characteristic of a process, there are two main design methods: the optimization‐based method and the controllability indices‐based anticipating sequential method. Advantages and disadvantages of these techniques are discussed. It has been emphasized that bifurcation analysis, as a powerful nonlinear analysis tool, could provide important guidance for making processes more controllable by eliminating or avoiding some undesirable behaviors of processes. Further challenges and developments in the field of process controllability are identified. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A survey on applications of optimization-based integrating process design and control for chemical processes

Process design and control are closely related to each other in chemical engineering activities. Traditionally, process design and control system design are carried out in sequence. However, the integration of process design and control (IPDC) can bring greater economic benefits and process dynamic performance than traditional sequential design methods. This is true, particularly for modern chemical processes, in which various process units become more interacting and compact owing to the widespread use of heat integration and recycled streams, and the resulted impacts between process design and control begin to significantly influence both the capital and operational costs. Recently, considerable studies about the IPDC for chemical processes have been reported in published literature. The purpose of the paper is to survey the applications of optimization-based integrating process design and control for chemical processes. Firstly, attention has been focused on the applications of IPDC to different process units, for example, chemical reactors and separation columns. Then, the survey is extended to the applications of IPDC to plant-wide chemical processes. Finally, the future research challenges in the application of IPDC to chemical processes have been briefly discussed.