MILP-based optimization of oxygen distribution system in integrated steel mills

Simultaneous multiperiod optimization is conducted for minimizing the oxygen emission of an oxygen-distribution system, based on the generalized MILP-based model, which covers various configurations of the captive oxygen factory in integrated steel mills. By simultaneously optimizing all of the variables, such as the load of air separation units (ASU), the on-off states of compressors, the load of liquefiers, etc., the model can promptly provide mill managers with responsive solutions for adjusting the variables involved on the supply-side to minimize oxygen emission. The case study in this paper shows that the proposed model performs well in minimizing oxygen emission, and provides a global optimization result covering the entire planning horizon. Moreover, based on the proposed model, the emission amounts can be rapidly and readily calculated for various scheduling scenarios of ASU maintenance, which is helpful to the manager seeking to optimally schedule ASU maintenance in time.     

大型煤化工空分技术与设备发展现状

介绍了国内大型煤化工项目空分设备的发展现状,结合大唐多伦煤化工项目空分装置的实际应用情况和天津荣成钢厂空分装置设计情况,分析了国内大型空分设备的设计能力,概述了国内大型空分装置静设备和动设备的制造能力,并分析了空分系统压缩机组的应用情况,在此基础上给出了煤化工项目选用大型空分设备的建议。     

煤化工配套空分设备的选择

以煤化工项目配套空分设备选择为研究对象,首先介绍了煤化工行业发展以及煤气化技术,接着分析空分设备规模选择,在分析了加压煤气化工艺配套空分设备的流程选择,最后介绍了空分设备关键配套部门的选择,以便使得我国煤化工设备的生产规模可以不断扩大。     

Simultaneous mixed-integer dynamic scheduling of processes and their energy systems

Increasingly volatile electricity prices make simultaneous scheduling optimization desirable for production processes and their energy systems. Simultaneous scheduling needs to account for both process dynamics and binary on/off-decisions in the energy system leading to challenging mixed-integer dynamic optimization problems. We propose an efficient scheduling formulation consisting of three parts: a linear scale-bridging model for the closed-loop process output dynamics, a data-driven model for the process energy demand, and a mixed-integer linear model for the energy system. Process dynamics is discretized by collocation yielding a mixed-integer linear programming (MILP) formulation. We apply the scheduling method to three case studies: a multiproduct reactor, a single-product reactor, and a single-product distillation column, demonstrating the applicability to multiple input multiple output processes. For the first two case studies, we can compare our approach to nonlinear optimization and capture 82% and 95% of the improvement. The MILP formulation achieves optimization runtimes sufficiently fast for real-time scheduling.     

Enriched Air or Pure Oxygen as Oxidant for Gas‐to‐Liquid Process with Microchannel Reactors

The syngas production step is one of the most costly steps in a gas‐to‐liquid plant. Commonly, oxygen is used as an oxidant in the reforming step. However, through the introduction of microchannel reactors, the use of enriched air may be justified. The merits of using enriched air versus pure oxygen are analyzed by utilizing an autothermal reformer, with microchannel reactors in the once‐through Fischer‐Tropsch (FT) step. Pure oxygen is provided by a cryogenic air separation unit (ASU) and enriched air by use of air separation membranes. Pure oxygen requires a smaller FT reactor volume, which means lower reactor costs at the expense of having a costly cryogenic ASU to produce pure oxygen. The operating cost of the ASU is lower than that of the air membrane, but the installed cost is higher.     

A framework for efficient large scale equation-oriented flowsheet optimization

Despite the economic benefits of flowsheet optimization, many commercial tools suffer from long computational times, limited problem formulation flexibility and numerical instabilities. In this study, we address these challenges and present a framework for efficient large scale flowsheet optimization. This framework couples advanced process optimization formulations with state-of-the-art algorithms, and includes several notable features such as (1) an optimization-friendly formulation of cubic equation of state thermodynamic models; (2) a new model for distillation column optimization based on rigorous mass, equilibrium, summation and heat (MESH) equations with a variable number of trays that avoids integer variables; (3) improvements on the Duran–Grossmann formulation for simultaneous heat integration and flowsheet optimization; and (4) a systematic initialization procedure based on model refinements and a tailored multi-start algorithm to improve feasibility and identify high quality local solutions.Capabilities of the framework are demonstrated on a cryogenic air separation unit synthesis study, including two thermally coupled distillation columns and accompanying multistream heat exchangers. A superstructure is formulated that includes several common ASU configurations in literature. As part of the optimization problem the solver selects the best topology in addition to operating conditions (temperatures, flowrates, etc.) for coal oxycombustion applications. The optimization problem includes up to 16,000 variables and 500 degrees of freedom, and predicts specific energy requirement of 0.18 to 0.25 kWh/kg of O₂ depending on design assumptions. These results are compared to literature and plans to extend the framework to an entire coal oxycombustion power plant optimization study are discussed.     

Dynamic optimization of the methylmethacrylate cell‐cast process for plastic sheet production

Traditionally, the methylmethacrylate (MMA) polymerization reaction process for plastic sheet production has been carried out using warming baths. However, it has been observed that the manufactured polymer tends to feature poor homogeneity characteristics measured in terms of properties like molecular weight distribution. Nonhomogeneous polymer properties should be avoided because they give rise to a product with undesired wide quality characteristics. To improve homogeneity properties force‐circulated warm air reactors have been proposed, such reactors are normally operated under isothermal air temperature conditions. However, we demonstrate that dynamic optimal warming temperature profiles lead to a polymer sheet with better homogeneity characteristics, especially when compared against simple isothermal operating policies. In this work, the dynamic optimization of a heating and polymerization reaction process for plastic sheet production in a force‐circulated warm air reactor is addressed. The optimization formulation is based on the dynamic representation of the two‐directional heating and reaction process taking place within the system, and includes kinetic equations for the bulk free radical polymerization reactions of MMA. The mathematical model is cast as a time dependent partial differential equation (PDE) system, the optimal heating profile calculation turns out to be a dynamic optimization problem embedded in a distributed parameter system. A simultaneous optimization approach is selected to solve the dynamic optimization problem. Trough full discretization of all decision variables, a nonlinear programming (NLP) model is obtained and solved by using the IPOPT optimization solver. The results are presented about the dynamic optimization for two plastic sheets of different thickness and compared them against simple operating policies. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

催化裂化装置再生器主风裕量的动态分析

以某催化裂化装置为基础,建立反应-再生系统的动态机理数学模型。从生产操作和闭环控制的角度,用动态优化的方法对再生器的主风裕量进行了计算,指出过程动态特性对主风设计裕量的选取有着很大影响。在考虑工艺和设备条件变化留出的稳态裕量之外,需要留出一定的动态裕量以满足过程操作和控制的要求,其大小与控制系统设计有关。系统对控制器性能的要求愈高,所需要的主风裕量应愈大。故在进行工艺设计时需要对系统的控制性能和主风设计裕量进行综合考虑,使设计结果既能满足工艺要求,又能实现良好的自动控制。     

Investigation of a Model-Based Deep Reinforcement Learning Controller Applied to an Air Separation Unit in a Production Environment

The need for load flexibility and increased efficiency of energy-intensive processes has become more and more important in recent years. Control of the process variables plays a decisive role in maximizing the efficiency of a plant. The widely used control models of linear model predictive controllers (LMPC) are only partly suitable for nonlinear processes. One possibility for improvement is machine learning. In this work, one approach for a purely data-driven controller based on reinforcement learning is explored at an air separation plant (ASU) in productive use. The approach combines the model predictive controller with a data-generated nonlinear control model. The resulting controller and its control performance are examined in more detail on an ASU in real operation and compared with the previous LMPC solution. During the tests, stable behavior of the new control concept could be observed for several weeks in productive operation.     

空分短期停车时间阈值对氧气生产调度的影响

针对钢铁企业高炉休风场景下的氧气生产调度问题,提出以空分短期计划性停车为主要手段的调度策略,并基于MILP方法建立了氧气系统的优化调度模型。调度模型的优化目标为整个规划周期内氧气高压管网的综合压力最小化。模型包含了空分和部分氧气压缩机短期停车再启动操作的约束条件,并结合实际情况考虑了前述设备的停车时间阈值和运行时间阈值。以国内某大型钢铁企业为实际案例,验证了调度模型的合理性与可行性,然后基于模型计算分析了空分停车时间阈值对调度目标的影响规律。分析结果表明,减小空分停车时间阈值有利于获得更优的调度目标,但空分停车时间阈值对优化目标的影响规律具有阶跃特性,而非简单的比例关系。     

集散控制系统在空分工艺中的应用

山东新能滕州能源有限公司36万t/a甲醇项目配套建立了KDON42000/20000型空分设备,利用空气分离出氧、氮气供合成甲醇使用。空分采用先进的分子筛纯化器和增压透平膨胀机,整个工艺流程有空气过滤、压缩、预冷、纯化、膨胀制冷、储存等工序组成。测量控制系统选用Honeywell公司PKS系统,对各工艺参数以及振动,转速,位移等进行检测和联锁控制。系统运行安全、可靠、自动化程度高,达到了预期的目的。     

A novel adaptive surrogate modeling‐based algorithm for simultaneous optimization of sequential batch process scheduling and dynamic operations

A novel adaptive surrogate modeling‐based algorithm is proposed to solve the integrated scheduling and dynamic optimization problem for sequential batch processes. The integrated optimization problem is formulated as a large scale mixed‐integer nonlinear programming (MINLP) problem. To overcome the computational challenge of solving the integrated MINLP problem, an efficient solution algorithm based on the bilevel structure of the integrated problem is proposed. Because processing times and costs of each batch are the only linking variables between the scheduling and dynamic optimization problems, surrogate models based on piece‐wise linear functions are built for the dynamic optimization problems of each batch. These surrogate models are then updated adaptively, either by adding a new sampling point based on the solution of the previous iteration, or by doubling the upper bound of total processing time for the current surrogate model. The performance of the proposed method is demonstrated through the optimization of a multiproduct sequential batch process with seven units and up to five tasks. The results show that the proposed algorithm leads to a 31% higher profit than the sequential method. The proposed method also outperforms the full space simultaneous method by reducing the computational time by more than four orders of magnitude and returning a 9.59% higher profit. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4191–4209, 2015     

Preemptive dynamic operation of cryogenic air separation units

As markets become more competitive and dynamic, manufacturing plants are undergoing transitions toward flexible, agile, and low costs operations. Appropriate coordination within the supply chain is an important factor in manufacturing systems' performance. In this study, the impact of preemptive control action in advance of an upcoming demand change on the economic performance of a cryogenic air separation unit is investigated. The effects of various factors are explored through optimization formulations utilizing a high‐fidelity collocation based dynamic process model. This includes the amount of lead time, choice of manipulated inputs, direction of demand change, and liquid product market conditions. Plant performance is evaluated and analyzed through a comprehensive multipart case study. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3845–3859, 2017     

Optimal design and integration of an air separation unit (ASU) for an integrated gasification combined cycle (IGCC) power plant with CO2 capture

The air separation unit (ASU) plays a key role in improving the efficiency, availability, and operability of an oxygen-fed integrated gasification combined cycle (IGCC) power plant. An optimal integration between the ASU and the balance of the plant, especially the gasifier and the gas turbine (GT), has significant potential for enhancing the overall plant efficiency. Considering the higher operating pressure of the GT, an elevated-pressure air separation unit (EP-ASU) is usually favored instead of the conventional low-pressure air separation units (LP-ASU). In addition, a pumped liquid oxygen (PLOX) cycle is usually chosen if the operating pressure of the gasifier is high. A PLOX cycle helps to improve plant safety and availability and to decrease the capital cost by reducing the size of the oxygen compressor or by eliminating it completely. However, the refrigeration lost in withdrawn liquid oxygen must be efficiently recovered. This paper considers five different configurations of an ASU with PLOX cycle and compares their power consumptions with an EP-ASU with a traditional gaseous oxygen (GOX) cycle. The study shows that an optimally designed EP-ASU with a PLOX cycle can have similar power consumption to that of an EP-ASU with GOX cycle in the case of 100% nitrogen integration. In the case of an IGCC with pre-combustion CO₂ capture, the lower heating value (LHV) of the shifted syngas, both on a mass and volumetric basis, is in between the LHV of the unshifted syngas from an IGCC plant and the LHV of natural gas, for which the GTs are generally designed. The optimal air integration in the case of a shifted syngas is found to be much lower than that of an unshifted syngas. This paper concurs with the existing literature that the optimal integration occurs when air extracted from the GT can be replaced with the nitrogen from the ASU without exceeding mass/volumetric flow limitations of the GT. Considering nitrogen and air integration between the ASU and the GT, this paper compares the power savings in an LP-ASU with a PLOX cycle to the power savings in an EP-ASU with GOX cycle and EP-ASU with PLOX cycle. The results show that an LP-ASU with a PLOX cycle has less power consumption if the nitrogen integration levels are less than 50-60%. In addition, a study is carried out by varying the concentration of nitrogen and steam in the fuel diluents to the GT while the NOx level was maintained constant. The study shows that when the nitrogen injection rate exceeds 50%, an EP-ASU with a PLOX cycle is a better option than an LP-ASU with a PLOX cycle. This paper shows that an optimal design and integration of an ASU with the balance of the plant can help to increase the net power generation from an IGCC plant with CO₂ capture.     

A model predictive control strategy for supply chain optimization

This paper describes a model predictive control strategy to find the optimal decision variables to maximize profit in supply chains with multiproduct, multiechelon distribution networks with multiproduct batch plants. The key features of this paper are: (1) a discrete time MILP dynamic model that considers the flow of material and information within the system; (2) a general dynamic optimization framework that simultaneously considers all the elements of the supply chain and their interactions; and (3), a rolling horizon approach to update the decision variables whenever changes affecting the supply chain arise. The paper compares the behavior of a supply chain under centralized and decentralized management approaches, and shows that the former yields better results, with profit increases of up to 15% as shown in an example problem.     

动态模拟在芳烃抽提装置设计中的应用

运用动态模拟解决工程问题在国外已是一项成熟的应用技术,但在国内的工程设计领域很少采用,国内文献也鲜有报道。文中运用Dynsim4.4动态模拟软件对广西石化90万t/a芳烃抽提装置在设计、开车和标定等阶段进行了动态模拟和分析,协助解决了开车阶段装置压力不稳的问题,优化选择控制系统和安全泄放系统的设计,取得了良好的效果,而实际的装置运行结果也验证了动态模拟的准确性。文中对动态模拟建模的基本步骤、模型调试方法和模拟结果的分析进行了详细阐述,可供设计人员、业主和EPC总承包商参考。     

Nonsmooth model for dynamic simulation of phase changes

Dynamic modeling of processes involving phase changes can be challenging due to changes in the model equations caused by appearance and disappearance of equilibrium phases. Dynamic simulation of these processes requires the ability to detect the change in the number of phases and adapt the model to the new phase regime on the fly. In this work, an easy‐to‐use nonsmooth model for dynamic simulation of processes with vapor‐liquid equilibrium is presented. The presented model does not introduce any auxiliary variables or equations, nor does it require solution of an optimization problem to determine the new phase regime during the dynamic simulation. It can therefore be used for comprehensive simulation of, e.g., distillation columns, where the number of phases present can change during startup and shutdown. The nonsmooth model is illustrated through examples of an evaporator and a distillation column. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3334–3351, 2016     

考虑环境影响的多产品间歇化工过程优化排序

提出一种用于多产品厂排序的多目标优化模型 ,其目标函数同时考虑了总生产时间和废物对环境的影响 ,定义了关于废物的环境影响因子及决策因子 ,用以对总生产时间和废物及其对环境的影响进行权衡 .采用改进的模拟退火算法对具有不同决策因子和环境影响因子情况下的算例进行了求解 ,结果表明 ,该模型能够较好地反映环境因素在多产品厂排序问题中的影响 ,使排序结果达到生产时间和环境影响的综合最优     

空分装置三吸附器TSA纯化系统及其节能效果分析

为了回收现有的双吸附器空气净化系统余热,提出了采用三吸附器空气净化系统回收利用余热,并分析了可行性. 以两工业企业20000和21000 Nm3/h制氧机为例进行初步分析计算,结果表明,若把现有系统改造为三吸附器纯化系统,可节约氮气加热电耗分别达51.3%和42.7%. 基于Aranovich-Donohue吸附等温线方程和线性驱动力传质假设的非绝热吸附模型,模型参数通过匹配生产现场监控数据确定,开发了空分变温吸附纯化系统模拟器. 基于此模拟器的数值模拟,对宝钢分公司6号制氧机双吸附器TSA系统进行三吸附器系统改造的安全性和节能效果进行了预测,安全达标情况下,节能率为45%.     

水煤浆装置氧泵因"晃电"跳停原因分析及对策

简单介绍了水煤浆装置空分氧泵变频器的电气控制原理.分析了氧泵变频器因系统"晃电"而导致空分高压液氧泵跳停的情况.通过采取一些有针对性的措施,确保空分氧泉稳定运行.