萃取精馏分离苯/环己烷共沸体系的控制策略

将常规萃取精馏、差压热耦合萃取精馏以及隔壁塔萃取精馏技术应用于以糠醛为萃取剂的苯和环己烷共沸物分离过程。在稳态模型的基础上,利用Aspen Dynamics软件进行控制研究,对三工艺流程提出了若干控制策略。结果表明,对于常规萃取精馏过程,再沸器热负荷与进料量比值控制结构在降低控制过程超调量方面表现出明显优势;对于差压热耦合萃取精馏过程,带有压力-补偿控温策略的方案控制效果更佳;而对于隔壁塔,则选择了无隔板下方气液分离比控制的结构来作为较优的控制策略。     

Concentration of squalene from shark liver oil by short‐path distillation

Investigations on the concentration of squalene from shark liver oil using short‐path distillation are presented and compared to alternative techniques. Aspects of chemical composition analysis are discussed as well as physical and sensory properties. A product is obtained with squalene purities above 97% at feed flows of up to 7 kg/h and temperatures between 185 and 230 °C. In order to assure product quality, a simple refractive method is applied. Based on the presented study, scale‐up to industrial production is performed taking into account the complete process for obtaining an acceptable product with a neutral odor.     

不同热集成变压精馏工艺分离乙酸乙酯/正己烷共沸物的优化与控制

基于变压精馏分离乙酸乙酯/正己烷共沸体系两塔的温差,利用Aspen Plus软件,以年度总成本最小为目标函数,对部分及完全热集成变压精馏工艺进行了稳态模拟及优化。在此基础上,利用Aspen Dynamics软件开发了多种控制结构,通过引入不同进料流量及组成的扰动测试控制结构的有效性。结果表明,完全热集成变压精馏工艺比部分热集成变压精馏工艺的经济性稍好。动态响应结果表明,部分热集成变压精馏工艺的压力?补偿温度控制结构可有效处理不同程度的干扰,能有效提高控制结构对干扰的响应速度,缩短达到新稳态的时间,保证乙酸乙酯和正己烷产品纯度在99.9wt%之上;而完全热集成变压精馏工艺的组分?温度串级控制结构仅能处理较小的组分和流量干扰,实现稳健控制,无法处理较大的干扰。综合比较两种工艺的经济性和可控性,认为部分热集成变压精馏工艺分离乙酸乙酯/正己烷共沸体系优于完全热集成变压精馏工艺。     

Control structure selection of increased-pressure extractive distillation process for DMC-MeOH azeotropic mixture

Producing dimethyl carbonate (DMC) as a green chemical with the desired purity is important in the industry. Although studies on the steady-state design of energy-efficient extractive distillation processes are important for the purification of DMC-methanol (DMC-MeOH) azeotropic mixtures, the dynamic controllability of these processes is also critical in the case of feed condition changes, and it should be investigated carefully. Results of the limited studies in the literature show that changing the operating pressures in extractive distillation processes might have different effects on the dynamic controllability of different systems. Thus, in this study, alternative control strategies are developed for a recently proposed increased-pressure extractive distillation process to separate DMC-MeOH mixture. All control structures are designed using inferential temperature controllers, which have a general acceptance in industrial applications. Effects of different ratio controllers are investigated by evaluating the dynamic responses of control structures for disturbances in feed flowrate and composition. Two metrics including integral absolute error and steady-state deviation of purities are used in the evaluation of alternatives. Results of dynamic simulations show that a control structure including reflux ratio controller is not a suitable strategy for this process. It is demonstrated that a control structure including reflux to feed ratio controller for both distillation columns is necessary for the robust and efficient control of a pressure-increased extractive distillation process. These efficient dynamic results support the economic advantage of increased-pressure extractive distillation process separating DMC-MeOH azeotropic mixtures.     

Design and Control of Heterogeneous Azeotropic Distillation for Separating 2‐Methylpyridine/Water

In industrial processes, 2‐methylpyridine/water mixtures can be separated via the conventional heterogeneous azeotropic distillation (HAD) using benzene as entrainer. 2‐Methylpyridine and water can form a heterogeneous azeotrope by themselves, based on which an improved HAD process is proposed. This allows for reducing the total operating cost and total annual cost (TAC) by more than one‐tenth compared with the conventional HAD process. Two different control structures were established for the enhanced HAD process. The results indicate that applying the feedforward ratio controllers to the control structure can handle the feed disturbances and maintain product purities with smaller transient deviations and shorter settling times.     

Design and control of a p-xylene oxidation process

The p-xylene (PX) oxidation process is of great industrial importance because of the strong demand of the global polyester fiber. A steady-state model of the PX oxidation has been studied by many researchers. In our previous work, a novel industrial p-xylene oxidation reactor model using the free radical mechanism based kinetics has been developed. However, the disturbances such as production rate change, feed composition variability and reactor temperature changes widely exist in the industry process. In this paper, dynamic simulation of the PX oxidation reactorwas designed by Aspen Dynamics and used to develop an effective plantwide control structure, which was capable of effectively handling the disturbances in the load and the temperature of the reactor. Step responses of the control structure to the disturbances were shown and served as the foundation of the smooth operation and advanced control strategy of this process in our future work.     

Controllability and energy efficiency of a high-purity divided wall column

The divided wall column system is a promising energy-saving alternative for separating multi-component mixtures. However, high energy efficiency and stable operations can only be achieved with careful design of steady state operation and control scheme. In this study, the effects of liquid split and vapor split ratios on the energy efficiency and controllability of a divided wall column system for separating ethanol, n-propanol, and n-butanol were investigated. A region with high energy efficiency was identified. However, relative gain analysis found that the performance of multi-loops composition control would be very poor. Dynamic tests showed that multi-loop temperature control cannot return the product compositions to the desired values in case of feed composition disturbances. Outside this region, composition control can compensate for external disturbances such as feed flow rate and feed composition changes but not changes in operating region caused by internal variations such as liquid and vapor splits. Offsets in the product purities were found if temperature controls were used and there are disturbances in feed composition or changes in operating region caused by upsets in liquid and vapor splits. There is a trade-off between energy efficiency and controllability. A composition + temperature cascade scheme was proposed to stabilize the operation with high energy efficiency. The proposed scheme was able to maintain high product purity and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

Understanding the dynamic behaviour of middle-vessel continuous distillation columns

The fundamental dynamic behaviour of a middle-vessel continuous distillation column is studied in this paper. The interaction between the middle vessel (MV) design and control parameters is identified by means of a linear analysis. Transfer functions relating the dynamics of the product composition responses to changes in the disturbance inputs and manipulated inputs are derived. The role of the MV holdup and MV level controller gain in altering the product composition responses is identified. It is shown that the MV level controller gain can significantly affect the control performance when feed flow disturbances must be compensated for. However, it has a much lower impact in the control performance when feed composition disturbances need to be rejected; in this case, exploiting the “buffering” effect of the MV is more important for control. The analysis is carried out in detail for a LV control configuration, and is subsequently extended to an “on-demand” DB configuration. Finally, simple guidelines for the choice of the MV level controller gain and MV holdup are provided.     

Design and control of reactive distillation for ethyl and isopropyl acetates production with azeotropic feeds

Reactive distillations for the production of ethyl acetate (EtAc) and isopropyl acetate (IPAc) are classified as the type-II process where the first column consists of a reactive zone and a rectifying section followed by a stripper [Tang et al., 2005. Design of reactive distillations for acetic acid esterification with different alcohols. A.I.Ch.E. Journal 51, 1683-1699]. Instead of using pure alcohols and acetic acid as reactants, this paper studies the effects of reactant purity on the design and control of reactive distillation. This offers significant economical incentives (by reducing raw materials costs), because ethanol forms an azeotrope with water at 90 mol% and isopropanol/water has an azeotrope at 68%. The purities of the acid is set to 95% for acetic acid (industrial grade), 87% for ethanol, and 65% for isopropanol. The results show that the total annual costs (TAC) increase by a factor of 5% for EtAc and 8% for IPAc production using reactive distillation. Next, the operability of the reactive distillations with azeotrope feeds is explored. Three disturbances, feed flow, acid feed purity, and alcohol feed composition, are introduced to assess control performance using dual-temperature control and one-temperature-one-composition control. Simulation results indicate good control performance can be achieved for reactive distillation with azeotropic feeds.     

Design and Control of Ionic Liquid‐Catalyzed Reactive Distillation for n‐Butyl Acetate Production

A simulation study of n‐butyl acetate production with the [Hpy][HSO₄] ionic liquid catalyst was performed. Due to the lack of phase equilibrium data, the binary interaction parameters of the NRTL model for ionic liquid and reactive species were calculated by the COSMO‐RS technique. A reactive distillation process with recycled ionic liquid stream was proposed, and the column configuration was optimized by minimization of the total annual capital. The novel process is considerably efficient and economic compared to the traditional reactive distillation process of nonionic liquids. With the steady‐state parameters, a plant‐wide control structure was further developed to evaluate the robustness of the control system by exerting the disturbances of feed flow rate and feed composition. Dynamic simulation results suggest that the control scheme with a composition controller is timely and effective.     

Design and control of acetonitrile/N-propanol separation system via extractive distillation using N-methyl pyrrolidone as entrainer

The extractive distillation process for separating a binary azeotrope of acetonitrile and n-propanol is investigated using N-methyl pyrrolidone as entrainer. On the basis of multiparameter optimization, a design with optimized operation conditions for the process is presented. Two dynamic control structures are proposed. Feed disturbances are used to evaluate the dynamic performance. The response results reveal that the control structure CS1 with temperature–composition cascade control loop can handle all the disturbances and maintain product purities, except one large transient deviation, while the improved control structure CS2 with average temperature control loop can overcome this obstacle and handle disturbances effectively.     

乙酸戊酯酯化反应精馏过程系统控制模拟及分析

反应精馏技术是将反应和精馏两个单元操作耦合在一个设备中同时进行,其非线性给反应精馏过程的平稳操作和过程控制提出了更高的要求。目前,反应精馏过程控制研究都是针对特定反应体系进行的有针对性的研究,对于乙酸戊酯反应体系的控制研究少有报道。本文利用Aspen Plus和Aspen Dynamic 软件对乙酸戊酯的反应精馏过程进行动态模拟,针对塔釜产品组成要求提出温度控制和组成控制两种不同的控制方案,对两个控制系统分别进行±10%的进料流量扰动测试和酸醇比为1:1、1.2:1的进料组成扰动测试。研究了不同扰动对产品质量的影响并比较了两种方案在不同扰动干扰下的控制效果。结果表明:进料组成扰动会对两个控制方案造成较大影响;温度控制方案的控制稳定性要好于组成控制方案。     

带有中间储罐的间歇精馏工艺动态控制模拟优化

甲酸甲酯-甲醇-水是化工生产过程中最常见的三元混合物之一。目前,间歇精馏工艺分离该三元混合物的研究较少,在动态控制方面也少有报道。本文研究了分离甲酸甲酯-甲醇-水的带有中间储罐的间歇精馏工艺动态控制模拟优化。利用Aspen Plus和Aspen Plus Dynamics软件,在稳态模拟的基础上,分别考察了液位控制结构和组分控制结构两种控制方案。结果表明,液位控制结构控制性能较差,达到稳定后甲醇和水的纯度较低。组分控制结构虽能提高产品纯度,但出现了较为严重的振荡现象。根据对组分控制结构的动态响应分析,本文提出了一种改进的组分控制结构,该控制结构能实现带有中间储罐的间歇精馏工艺的稳健控制,使各产品纯度得到提高。     

甲醇-苯共沸体系变压精馏分离工艺的动态控制

基于甲醇和苯共沸体系的压敏性,利用Aspen Plus和Aspen Dynamics软件对变压精馏分离该体系的稳态工艺进行了模拟和优化,研究了该工艺的动态特性,提出了控制产品纯度的3种控制结构：基础控制结构、比例控制结构和双比例与温度?组分联合控制结构,通过对控制结构添加±20%的组分和流量干扰测试控制结构的稳定性. 结果表明,基础控制结构基本能实现稳健控制,但不能解决组分干扰引起的产品纯度偏差过大等问题;比例控制结构可实现相对稳健的控制,但改进效果不显著;双比例与温度?组分联合控制结构在受到20%进料和组分干扰后,产品纯度能较快恢复至设定值的99.90%,实现稳健控制.     

Investigation of the operability for four-product dividing wall column with two partition walls

For separating some specific four component mixtures into four products, the four-product dividing wall column (FPDWC) with two partition walls can provide the same utility consumption with the extended Petlyuk configuration, although with structure simplicity. However, the reluctance to implement this kind of four product dividing wall column industrially also consists in the two uncontrollable vapor splits associated with it. The vapor split ratios are set at the design stage and might not be the optimal value for changed feed composition, thus minimum energy consumption could not be ensured. In the present work, a sequential iterative optimization approach was initially employed to determine the parameters of cost-effective FPDWC. Then the effect of maintaining the vapor split ratios at their nominal value on the energy penalty was investigated for the FPDWC with two partition walls, in case of feed composition disturbance. The result shows that no more than + 2% above the optimal energy requirements could be ensured for 20% feed composition disturbances, which is encouraging for industrial implementation.     

Dynamic Matrix Control of a Bubble‐Column Reactor for Microbial Synthesis Gas Fermentation

A spatiotemporal metabolic model of a representative syngas bubble‐column reactor was applied to design and evaluate dynamic matrix control (DMC) schemes for regulation of the desired by‐product ethanol and the undesired by‐product acetate. This model was used to develop linear step response models for controller design and also served as the process in closed‐loop simulations. A 2 × 2 DMC scheme with manipulation of the liquid and gas feed flows to the column provided a superior performance to proportional integral (PI) control due to slow process dynamics combining the multivariable and constrained nature of the control problem. Ethanol concentration control for large disturbances was further improved by adding the flow of a pure hydrogen stream as a third manipulated variable. The advantages of DMC for syngas bubble‐column reactor control are demonstrated and a design strategy for future industrial applications is provided.     

Kaibel分壁精馏塔的压力补偿-温度控制

Kaibel分壁精馏塔（Kaibel divided-wall column,KDWC）是分离三组分混合物DWC的进一步强化形式,可在一个塔内实现四组分混合物的高纯度分离。建立有效的控制结构是KDWC的研究关键,研究了KDWC分离苯、甲苯、邻二甲苯和均三甲苯四组分体系的动态过程,首次将压力补偿机制引入KDWC的控制,建立了压力补偿-温度控制结构（PTC）,并考察了该结构与温度控制结构（TC）的控制效果。研究结果表明：TC可实现进料流量或组分发生±15%扰动后的平稳控制,少部分产品纯度不合格,最大余差为-0.007;PTC可实现进料流量或组分发生±20%扰动后的平稳控制,产品纯度均合格,最大余差约为-0.003,且调节时间缩短为5~8 h。压力补偿-温度控制结构通过对下侧线温度控制回路设定值的修正,有效地改善了控制效果。     

On-line multivariable adaptive control of a binary distillation column

An experimental evaluation of the simultaneous control of top and bottom product compositions of a binary distillation column utilizing multi-variable self-tuning control algorithms is presented. The study was carried out using an 8-tray, 22.86 cm diameter methanol–water pilot scale column with continuous capacitance analysis of top product composition and bottom composition analyzed on a 3 minute cycle, by a gas chromatograph. Column control is studied for ±25% step disturbances in feed flow rate. Terminal composition regulation using both ‘positional’ and ‘incremental’ forms of a generalized minimum variance self-tuning control algorithm is compared with that achieved using a conventional digital PI/PID multiloop control strategy.     

反应精馏隔壁塔生产乙酸正丁酯的优化与控制

对反应精馏隔壁塔生产乙酸正丁酯过程进行了模拟、优化与控制的系统研究。利用Aspen Plus软件模拟乙酸甲酯与正丁醇的酯交换反应过程,以年总费用（TAC）为目标函数进行过程优化,通过稳态敏感性分析及相对增益矩阵（RGA）判据得到不同的操纵变量与控制变量匹配关系,以此为基础,在Aspen Dynamics平台建立了若干控制结构并进行分析对比。结果表明,利用两股反应物呈比例进料可较为有效地抵抗进料扰动,最后提出的无再沸器热负荷与混合物进料量比值（Q_r/F）控制的改进控制结构CS3,在降低反应精馏隔壁塔控制过程超调量方面有较大的优越性。     

Optimal two point control of a binary distillation column

Optimal two point control and operating policies have been developed for a semi-works scale 10 tray distillation column separating methanol and water. Two controllers, based on different control philosophies were synthesized. They are:

• 1 The controller that will bring the product compositions to their desired values while keeping off-specification products to a minimum.
• 2 The controller that will bring the product compositions to their desired values while maintaining the average product composition of the control period at the desired level.

Two types of disturbances were studied, namely disturbances in the feed stream and changes in product specifications. The column operation was represented by a linearized dynamic mathematical model based on a component material balance. The solution of the problem was obtained by application of the minimum principle of Fontryagin and the modified Riccati transformation technique. Simplified optimal control schemes can be developed from the results of this work. These control schemes are simple enough to be used for direct digital computer control.