Steady-state design of thermally coupled reactive distillation process for the synthesis of diphenyl carbonate

Diphenyl carbonate, a precursor in the production of polycarbonate, is traditionally synthesized by the transesterification reaction of dimethyl carbonate and phenol. In this study, phenyl acetate was used instead of phenol to react with dimethyl carbonate and yield diphenyl carbonate, due to its higher reaction conversion and the absence of side reactions and azeotropes. A plant-wide process with a reactive distillation (RD) column and a separation column was optimized by minimizing the total annual cost. The performance of the thermal coupling between these two columns was also investigated. RD with thermal coupling was demonstrated to provide better energy efficiency than conventional RD. The remixing phenomenon associated with thermodynamic inefficiency in conventional distillation sequences could be greatly reduced by implementing thermal coupling between columns. Reactant concentrations that were closer to stoichiometric balance in the reaction zone were given for the thermally coupled RD column.     

内部热耦合空分塔的节能优化分析

内部热耦合精馏是迄今为止所提出的节能效果最好,而唯一没有商业化的节能技术。将内部热耦合技术用于空分塔,可以带来良好的节能效果。根据低温空气分离过程以及三组分精馏的特点,提出了一种新的内部热耦合空分精馏塔优化模型。在优化模型基础上,对热耦合塔进行了深入的节能优化与分析,并且与常规空分仿真结果进行了比较分析,压缩机能耗下降20.75%,产值增加17.46%,单位产值能耗下降32.53%。内部热耦合空分塔的提取率以及能耗均优于常规热耦合空分塔,节能效果显著。     

三组元全热耦合精馏过程的模拟计算

本文利用Aspen软件对三组分进料全热耦合精馏的分离流程进行设计和模拟,建立了相应的具体计算步骤。首先利用三塔模型把热耦合精馏过程简化为3个单独的简单清晰分割塔,通过简捷法设计和模拟得到塔板数和回流比等初值后进行严格法模拟,得到热耦合精馏的数据和操作条件。然后采用Aspen软件中的RadFrac模型,将三塔模型的模拟初值代入全热耦合模块进行严格模拟。结果表明,采用全热耦合精馏分离C4三组分比传统精馏的直接序列和间接序列节能约为20%。     

液化天然气分离的节能蒸馏工艺

采用严格模拟计算的方法,对液化天然气分离过程的现有常规蒸馏工艺与热耦蒸馏工艺、侧线提馏工艺、双效蒸馏工艺和热集成工艺进行了模拟,并比较了其能量消耗。模拟计算结果显示,复杂蒸馏工艺都比常规蒸馏工艺减少了能耗和操作成本。其中,热耦蒸馏工艺可比常规蒸馏工艺节能21.4%,侧线提馏工艺节约13.3%,双效蒸馏工艺可节约34.7%,热集成工艺节约则达37.6%左右,表明新工艺在此分离过程中都有较好的应用前景,尤以多效蒸馏和热耦蒸馏最有优势。     

Rapid assessment of thermally coupled side columns

Thermally coupled distillation is well-proven in low temperature gas processing where energy is relatively expensive. Although the thermodynamic efficiency of thermally coupled columns is similar to conventional columns, thermal coupling reduces energy flows. This can lead to attractive savings in both energy and capital cost. This paper shows that the conventional Fenske-Underwood-Gilliland method for short cut distillation design is as appropriate for thermally coupled columns as it is for conventional distillation. This enables comparisons of conventional and thermally coupled columns to be made easily, and should help to make thermal coupling a more accepted means of improving energy efficiency in distillation systems.     

热耦蒸馏及其选用原则

热耦蒸馏与常规蒸馏序列相比可减少能耗和设备投资,但其应用有一定的适用范围。文章通过严格模拟计算研究了热耦蒸馏适用范围,结果表明:该蒸馏技术主要适用于分离指数接近1的混合物,或分离指数小于1但中间组分含量较高的混合物。     

Energy‐efficient complex distillation sequences: Control properties

Four thermally coupled distillation systems were designed for the separation of five‐component mixtures (the light‐ends separation section of a crude distillation plant); their steady‐state design was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The thermally coupled distillation systems were compared to sequence based on conventional columns design. Comparison was based on controllability properties under open and closed loop operation, following the dynamic behaviour after common industrial operating disturbances. Simulation results were analyzed by the singular value decomposition technique and with the performance examination of elimination of feed disturbances using PI controllers. It was found that thermally coupled distillation systems are controllable and, sometimes, they exhibit dynamic responses that are easier to manage than in the case of conventional distillation sequences; this result is innovative in the study of this kind of systems.     

Effect of Recycle Streams on the Closed-Loop Dynamics of Thermally Coupled Distillation Sequences

The effects of thermal links on the closed-loop dynamics of thermally coupled distillation sequences for the separation of quaternary mixtures have been studied in this work by using rigorous dynamic simulations. The incorporation of thermal links into conventional distillation sequences can lower the energy consumption up to 40% without introducing additional control problems to the resulting thermally coupled distillation sequences. In some cases, the thermally coupled distillation sequences outperformed the dynamic behavior of the conventional distillation sequences for set point tracking. This result is important to establish that thermally coupled distillation sequences not only can have significant energy savings but also good dynamic properties.     

差压热耦合反应精馏塔的模拟研究

为了降低反应精馏塔能耗,将差压热耦合技术与反应精馏技术结合,提出了一种新型的差压热耦合反应精馏的流程,并将其应用于乙酸正丁酯的合成中。应用Aspen Plus模拟软件对新工艺流程以及常规反应精馏流程进行了模拟,通过考察压缩比、进料位置、进料醇酸摩尔比等因素对差压热耦合反应精馏合成乙酸正丁酯工艺的影响,得到最优条件。同时,将该工艺与常规反应精馏工艺进行比较,结果显示,新工艺能够大幅度降低能耗,与常规反应精馏装置相比可节能40%左右。     

Energy Efficiency of an Indirect Thermally Coupled Distillation Sequence

Several thermally coupled distillation sequences have been proposed to improve the thermal inefficiency of conventional distillation sequences. Particularly, for the separation of ternary mixtures, structures that perform a lateral extraction in one of the columns of the integrated arrangement have been shown to provide significant energy savings. The structure of existing sequences, based on conventional distillation columns, might provide the basis for alternate thermally coupled designs. In this paper, it is shown how a thermally coupled system derived from an indirect conventional sequence can provide energy savings through a proper optimization of the interconnecting streams.     

Controllability analysis of alternate schemes to complex column arrangements with thermal coupling for the separation of ternary mixtures

Thermally coupled distillation systems (TCDS) have been proposed to perform distillation separation tasks with the incentive of achieving lower energy consumption levels with respect to conventional distillation sequences. In particular, the presence of recycle streams for TCDS schemes has influenced the notion that control problems might be expected during the operation of those systems with respect to the rather well-known behavior of conventional distillation sequences. That has been one of the main reasons for the lack of industrial implementation of thermally coupled distillation schemes. Recently, some alternatives to thermally coupled distillation arrangements that might provide better operational properties than the complex columns have been proposed. In this work, we analyze the control properties of two alternatives to the coupled systems. The results indicate that a reduction in the number of interconnections in alternate configurations does not necessarily provide an improvement of controllability properties.     

Optimization of the Design and Operation of Extractive Distillation Processes

The achievement of the optimal operating point of extractive distillation systems involves determining the values of the process variables, such as the solvent flowrate, the reflux ratio of the extractive, and recovery columns. From the point of view of design, the optimum involves defining the number of stages of extractive and recovery columns, as well as the feed stage positions of these columns. The above-mentioned columns are coupled through a recycle stream, which makes obtaining the optimal operating and design points a more complex task. This study arose from a new procedure for the analysis of extractive distillation columns, in which the solvent mole fraction in the solvent feed stage is the primary variable to be analyzed. The procedure allows for determining the values of the process and design variables that provide the global minimum for the total annual cost and the specific energy consumption of the extractive distillation processes (extractive and recovery columns). Furthermore, it is possible to determine the minimum solvent flowrate and the minimum reflux ratio for separation. Obtaining anhydrous ethanol using ethylene glycol as solvent is the case study of this work.     

Economic evaluation of energy saving alternatives in extractive distillation process

Until now, there has not been consensus about the superiority of thermally coupled sequence over the conventional sequence in the extractive distillation process. In this sense, the main goal of this paper is to analyze three approaches for saving energy in the extractive distillation process: optimization, thermal integration and thermal coupling. Three azeotropic mixtures were investigated: ethanol and water (M1); tetrahydrofuran and water (M2); and acetone and methanol (M3). The solvents were ethylene glycol for M1 and M2, and water for M3. The results are shown in terms of the total annual cost (TAC) and specific energy consumption (SEC), and revealed that a thermally coupled extractive distillation sequence with a side rectifier did not always present the best results. Taking the case studies from literature as a starting point (without thermal integration), the optimization procedure used in this work found that TACs are always lower. The inclusion of thermal integration in configurations led to reducing TAC for all mixtures under investigation when compared to the sequences without this integration. When comparing two modifications in the layout of extractive distillation, it can be seen that it is more advantageous to use the preheating of the azeotropic feed with the recycle stream from the recovery column of the conventional sequence than using a thermally coupled sequence.     

Thermodynamic analysis, simulationand optimization on energy savings of ideal internal thermally coupleddistillation columns

Internal thermally coupled distillation columns (ITCDIC) are the frontier of distillation energy saving research. In this paper, a novel energy saving model of ideal ITCDIC and a simulation algorithm are presented,upon which a series of comparative studies on energy savings with conventional distillation columns are carried out. Furthermore, we present an optimization model of ideal ITCDIC, which can be used to achieve the maximum energy saving and find the optimal design parameters directly. The binary system of benzene-toluene is adopted for the illustrative example of simulation and optimization. The results show that the maximum energy saving of ITCDIC is 52.25% (compared with energy consumption of conventional distillation under the minimum reflux ratio operation); the optimal design parameters are obtained, where the rectifying section pressure and the feed thermal condition are Pr=0.3006 MPa and q=0.5107 respectively.     

热耦蒸馏技术进展

热耦蒸馏作为一种新型蒸馏技术,与常规蒸馏相比,热耦蒸馏平均可节省能耗30％左右,同时还可降低设备投资。本文介绍热耦蒸馏塔的构造、分类、特点以及设计和应用。     

Steady state analysis of snowball effects for reaction–separation–recycle systems with thermally coupled distillation sequences

It is known that the steady and dynamic characteristics of processes with recycles can be significantly different from those without recycle streams. Particular attention has been given to the reactor–separator–recycle (RSR) system; because of the existence of snowball effects, a small load change can translate into a large change in the recycle flowrate, which propagates throughout the whole system. Previous works have analyzed the influence of separators on snowball effects using flash units and conventional distillation columns. This work explores the existence of snowball effects in steady state conditions under the implementation of thermally coupled distillation sequences in RSR systems. In particular, the implementation of coupled systems with side columns (sequences with side rectifiers or side strippers) is considered. A simplified process for ethylbenzene production is taken as a basis, and the effects of implementing thermally coupled systems on snowball effects are compared to those generated by conventional distillation systems.     

Analysis of control properties of intensified distillation sequences: Reactive and extractive cases

In this work, we obtain and compare the control properties of thermally coupled reactive distillation sequences and thermally coupled extractive distillation sequences with those of conventional reactive and extractive distillation configurations. All sequences have been designed using a multiobjective genetic algorithm with restrictions. The theoretical control properties of those schemes were obtained using the singular value decomposition technique in all frequency domain. In order to complete the control study, the distillation options were subjected to closed-loop dynamic simulations. The effects of total stages, reactive stages, and extractant/feed ratio on the energy consumption and control properties are obtained for the intensified distillation options. The results show that there are cases in which integrated reactive and extractive sequences do not only provide significant energy savings with respect to the conventional reactive and extractive arrangements, but also may offer dynamic advantages in high energy consumption conditions.     

进料组成改变对隔板塔经济性影响的模拟

完全热耦合精馏相比传统精馏可以减少设备投资和操作费用,可代替传统精馏分离多组分混合物。对隔板塔（完全热耦合精馏塔）用于分离三组分混合物时的可操作性和经济性进行了研究。采用严格模拟方法,针对4种不同的进料组成设计了4种不同的隔板塔,并得出各个隔板塔气相和液相分割比对隔板塔年度总费用（TAC）的关系曲线,研究了当进料组成改变时4种隔板塔的经济性。     

Controllability analysis of modified Petlyuk structures

Fully thermally coupled distillation columns (Petlyuk‐type columns) represent an interesting alternative to conventional distillation sequences used in multicomponent mixture separation processes, due to potential savings in both energy and capital costs. However, possible operational difficulties have limited the industrial applications of Petlyuk systems. Some of the control challenges result from the transfer of vapour stream back and forth between columns. This means that those columns do not display a uniform lower or higher pressure with respect to the other. Recently, some alternative Petlyuk‐type schemes that might provide better operation properties than the traditional Petlyuk column have been proposed. In this work, the theoretical control properties of six alternative schemes to the Petlyuk system were obtained and compared. This was performed by using the singular value decomposition (SVD) technique in the frequency domain. Also, dynamic closed‐loop responses for set point tracking and disturbance rejection were obtained to support the theoretical control properties. The results showed that the reduction in the number of interconnections and the use of unidirectional flows affected the dynamic properties of the complex schemes leading to potential operational advantages in thermally coupled distillation sequences.     

Reducing energy consumption and CO2 emissions in extractive distillation: Part II. Dynamic behavior

The structure of thermally coupled distillation systems offers several control challenges arising from the transfer of vapor (or liquid) streams between columns. In particular, the presence of recycle streams for coupled schemes has led to the notion that control problems might be expected during the operation of these systems, in contrast to the rather well-known behavior of conventional distillation sequences. In this work, we analyze the control properties of thermally coupled extractive distillation schemes studied previously (Gutiérrez-Guerra, R., Segovia-Hernández, J.G. and Hernández, S., 2009, Reducing energy consumption and CO₂ emissions in extractive distillation. Chem Eng Res Des, 87: 145–152). Control properties are analyzed with the application of the singular value decomposition technique and a closed-loop analysis. The results showed that the energy savings predicted in the complex extractive distillation sequence can be achieved along with good dynamic behavior and reductions in greenhouse gas emissions.