Structural design of fully thermally coupled distillation column using approximate group methods

The thermally coupled distillation column systems can save energy and capital cost compared with traditional distillation columns. Since the thermally coupled column system was introduced, the design concerns have been peeped out due to more degrees of freedom. This paper introduces a new design method that can be used to determine the structure of thermally coupled distillation column systems, namely the number of stages in all sections of the column system. The design method employs the approximate group methods. To explore the design performance of the proposed design method, three feed systems are analyzed to investigate its usefulness. The design procedure gives the optimum structure for a given ternary separation; with given product specifications various design methods can yield approximately the same results. Like structure designs, the optimal internal flow distributions are examined. The results indicate that the method works well for a variety of process conditions.     

基于随机优化的热耦合复杂精馏系统的综合(Ⅰ)模型化方法

基于随机型最优化策略,针对包含简单塔、带有侧线蒸出及侧线汽提塔的复杂塔、全热耦合（或Petlyuk）塔的热耦合复杂精馏塔系统的综合问题,提出一种模型化方法.针对热耦合复杂精馏流程系统所需塔段数目以及冷凝器和再沸器数目的不确定性,提出了一种分解求解策略,将原问题分解成一系列具有不同塔段数的子问题分别求解;针对流程结构的优化提出一种流程结构的编码表达法,该方法将问题的分离序列结构和热耦合方式分别用两组编码表示,对分离序列的编码采用了数据结构理论中的二叉树排序方法,使流程结构的描述变得更加简便;最后以预分馏塔组分回收率及回流比为连续变量,建立了热耦合复杂精馏系统优化的[JP+1]混合整数非线性规划（MINLP）模型,该模型用改进的模拟退火算法求解,可同时得到优化的流程结构和操作参数.     

Design and Control of a Fully Thermally Coupled Distillation Column Modified from a Conventional System

A modified fully thermally coupled distillation column is proposed, with utilization of the existing distillation columns of the conventional system, and its control scheme is suggested here. The proposed distillation system is applied to a benzene‐toluene‐xylene (BTX) separation process, of which the system design and control performance evaluation are conducted using the HYSYS software. The performance of the suggested 3 × 3 control is examined in the set‐point tracking of product specification and the regulation for the changes of feed flow and composition. The pairings of three proportional‐integral control loops are the reflux flow and the specification of overhead product, the prefractionator vapor flow and that of the side product, and the vapor boil‐up rate and that of the bottom product. The multi‐variable controllability using various indices is investigated for the proposed control scheme, and the controllability is compared with that of the cross‐pairing between the control loops of the side and bottom products.     

Design of a fully thermally coupled distillation column based on dynamic simulations

A dynamic simulation of a fully thermally coupled distillation column is conducted for the design of a possible operation scheme, and its performance is examined with an example process of butanol isomer ternary system. The outcome of the dynamic simulation indicates that the column can be operated by using a 3 × 3 control structure. The structure consists of three controlled variables of the compositions of overhead, bottom and side products and three manipulated variables of the flow rates of reflux and steam and liquid split ratio between a main column and a prefractionator. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Optimum design of Petlyuk and divided-wall distillation systems using a shortcut model

An optimization approach for fully thermally coupled distillation systems for the separation of ternary mixtures is presented. The thermally coupled schemes considered here can be implemented in the form of the original Petlyuk arrangement (with an external prefractionator) or the modified Kaibel arrangement (in the form of a divided-wall column). The approach uses a shortcut design method that allows the system to be modeled as a nonlinear programming problem. Given the practical interest in divided-wall column arrangements, special attention is given in this work to this type of structure. Several cases of study are presented to show the applicability of the proposed approach.     

Industrial application of an extended fully thermally coupled distillation column to BTX separation in a naphtha reforming plant

Aromatic compounds are yielded from naphtha reforming in a petrochemical plant, and the products are separated with binary distillation columns for benzene, toluene, xylene and heavy components in sequence. In this study, the first three columns of the fractionation process in the naphtha reforming unit are replaced with an extended fully thermally coupled distillation column (EFTCDC) also known as the extended Petlyuk column. An industrial-sized application of the EFTCDC is examined to compare the performance of the column with a conventional system. From a structural design giving the optimum structure of the column, a practical column structure is derived and used in the HYSYS simulation to find the optimal operation condition for a given set of product specifications. The EFTCDC gives an energy saving of 9.7% over a conventional three-column process. In addition, it is proved that the design procedure is good for an industrial process of 18 components.     

Implementation and Operation of a Dividing‐Wall Distillation Column

The design and construction of a prototype of a dividing‐wall distillation column was possible by integrating previous knowledge in process intensification, energy savings, theoretical control properties, and closed‐loop dynamics of thermally coupled distillation sequences. In order to achieve the predicted energy savings for this class of complex distillation column, a dividing wall and a side tank were implemented in order to manipulate the internal flows associated with energy consumption. The reaction between ethanol and acetic acid was conducted within the prototype, and the experimental results indicate that a heterogeneous mixture of ethyl acetate and water is obtained as the top product. The temperature profile measured during the experimental run can be used for controlling the batch distillation column in cyclic operation mode.     

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.     

Sidestream Analysis and Optimization of Full Tower Internal Thermally Coupled Air Separation Columns

An appropriate sidestream is the key to improve the yields and purities of products in the cryogenic air separation process. The sidestream of a full tower internal thermally coupled air separation column is characterized. Sensitivity analysis indicates that heat duty distribution, purities, and yields of the products are strongly influenced by the withdrawn position and the flow rate of the sidestream. Optimization models are proposed by adopting the minimum flow rate of the low‐purity liquid nitrogen product and with maximum flow rates of the oxygen product or nitrogen product as optimization target. The optimization research allows selecting the optimum operating conditions with different production requirements.     

Synthesis of complex thermally coupled distillation systems including divided wall columns

The design of thermally coupled distillation sequences explicitly including the possibility of divided wall columns (DWC) is described. A DWC with a single wall can be considered thermodynamically equivalent to a fully thermally coupled (FTC) subsystem formed by three separation tasks (a Petlyuk configuration in the case of three‐component mixtures). It is shown how to systematically identify all the sequences of separation tasks that can produce configurations that include at least a DWC. Feasible sequences that explicitly include DWCs are enforced through a set of logical relationships in terms of Boolean variables. These logical relationships include as feasible alternatives from conventional columns (each column must have a condenser and a reboiler) to FTC systems (only one reboiler and one condenser in the entire system). A comprehensive disjunctive programming formulation for finding the optimal solution is presented. The model is based on the Fenske, Underwood Gilliland equations. However, the disjunctive formulation allows easily the use of any other shortcut, aggregated or even rigorous model without modifying much the structure of the model. Two illustrative examples illustrate the procedure. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1139–1159, 2013     

Response surface methodology in optimization of a divided wall column

dividing wall column (DWC) is a thermally coupled distillation system with a high energy efficiency that requires lower space and investment compared to the conventional column system. The design of a DWC involves a number of structural and process parameters that need to be optimized simultaneously to improve energetic and economic potential and reduce space requirement. We used response surface methodology (RSM) to optimize DWC nonlinearly and to figure out the effect of parameters and their interactions on energy consumption, product quality, and dimensions of a DWC. Results demonstrate that process variables have significant effects on the energy efficiency of a DWC as compared to the effect of structural variables. The optimum DWC parameters can be found by RSM with minimal simulation runs and the prediction results of RSM agree well with the rigorous simulation results.     

Logic hybrid simulation-optimization algorithm for distillation design

In this paper, we propose a novel algorithm for the rigorous design of distillation columns that integrates a process simulator in a generalized disjunctive programming formulation. The optimal distillation column, or column sequence, is obtained by selecting, for each column section, among a set of column sections with different number of theoretical trays. The selection of thermodynamic models, properties estimation, etc. is all in the simulation environment. All the numerical issues related to the convergence of distillation columns (or column sections) are also maintained in the simulation environment. The model is formulated as a Generalized Disjunctive Programming (GDP) problem and solved using the logic based outer approximation algorithm without MINLP reformulation. Some examples involving from a single column to thermally coupled sequence or extractive distillation shows the performance of the new algorithm.     

Feasibility study of a thermally coupled reactive distillation process for biodiesel production

Biodiesel fuel represents an interesting alternative as a clean and renewable substitute of fossil fuels. A typical biodiesel production process involves the use of a catalyst, which implies high energy consumptions for the separation of the catalyst and the by-products of the reaction, including those of undesirable side reactions (such as the saponification reaction). A recently proposed process involves the use of short-chain alcohols at supercritical conditions, avoiding the use of a catalyst and the occurrence of the saponification reaction. This process requires fewer pieces of equipment than the conventional one, but its high energy requirements and the need of special materials that support the reaction conditions makes the main product, biodiesel fuel, more expensive than petroleum diesel. In this work, a modification of the supercritical process for the production of biodiesel fuel is proposed. Two alternatives are proposed. The process involves the use of either reactive distillation or thermally coupled reactive distillation. Simulations have been carried out by using the Aspen One™ process simulator to demonstrate the feasibility of such alternatives to produce biodiesel with methanol at high pressure conditions. A design method for the thermally coupled system is also proposed. Both systems have been tested and the results indicate favorable energy performance when compared to the original scheme. Furthermore, the thermally coupled system shows lower energy consumptions than the reactive distillation column.     

Synthesis of dividing-wall columns (DWC) for multicomponent distillations—A systematic approach

Dividing-wall columns (DWC) are intensified distillation systems for multicomponent separations. They have the potential to save significantly both energy and capital costs than conventional simple column configurations. In this paper, it is shown that the DWC columns can be systematically generated from the conventional simple column configurations. Because of the simple column sequences with sharp splits are the simple and widely studied conventional schemes for multicomponent distillation, the purpose of this work is to formulate a procedure for systematic synthesis of DWC columns for such simple conventional schemes. A four-step procedure is formulated which systematically generates all the possible DWC columns from the simple column sequences. First, the subspace of the original thermally coupled configurations corresponding to the simple column configurations is generated. Then, the subspace of the thermodynamically equivalent structures corresponding to the original thermally coupled configurations is produced. Finally, the subspace of the DWC columns corresponding to the thermodynamically equivalent structures is achieved. An example of quaternary distillation is used to illustrate the synthesis procedure which is applicable to a mixture with any number of components.     

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。     

Analysis of Energy‐Efficient Complex Distillation Options to Purify Bioethanol

Three complex extractive distillation options were studied for the purification of a dilute mixture of ethanol and water. The first option uses an extractive distillation column and the other two options use thermally coupled extractive distillation sequences. The results indicate that the fully thermally coupled extractive option can reduce energy consumption by ca. 30 % compared to the scheme that uses an extractive distillation column. This fully thermally coupled extractive distillation sequence can produce ethanol as distillate with a mass fraction of 0.995, the entrainer as the bottoms product and a mixture of ethanol and water as the sidestream.     

From simulation studies to experimental tests in a reactive dividing wall distillation column

The design and control of thermally coupled distillation sequences have been studied since many years, but the real implementation occurred until middle of the 1980s using a single shell divided by a wall named dividing wall distillation column. In this work, experimental results for the production of ethyl acetate in a reactive dividing wall distillation column are presented for first time. The experimental results are in agreement with those obtained using steady state simulations with AspenONE Aspen plus. These experimental results are important since it is possible to validate most of the previous results generated using simulations.     

Modeling and Behavior Analyses of Internal Thermally Coupled Air Separation Columns

The internal thermally coupled air separation column (ITCASC) is a frontier of air separation energy savings research. The first principles model of the ITCASC is proposed and is based on the material balance, energy balance, and the related thermal coupling relationship which can be used for further control and design studies. The related solution procedures are given. Detailed behavior analyses are carried out. The research results reveal that the ITCASC process has the perfect separation effect and exhibits different behaviors compared to the conventional air separation column (CASC). The optimization model and the related operation optimization are further explored to gain insight on the energy saving potential of the ITCASC process. Optimization research results show that the ITCASC has about 47 % energy saving potential compared to the CASC process.     

Approximate design of fully thermally coupled distillation columns

An approximate design procedure for fully thermally coupled distillation columns (FTCDCs) is proposed and applied to example ternary systems. The procedure gives a fast solution of structural and operation design for a preliminary study of the FTCDC. The structural information resolves the design difficulty, caused from the interlinking streams of the column, which is encountered when a conventional design procedure is implemented. The design outcome explains that how the thermodynamic efficiency of the FTCDC is higher than that of a conventional two-column system and how the system of a separate prefractionator is different from a dividing wall structure. From the design result of three example systems with three different feed compositions, the useful performance of the proposed scheme is proved. In addition, the structural design of the FTCDC gives better understanding of the system and leads to high efficiency design of the column.     

Experimental and Numerical Investigation of a Reactive Absorption Column for Chlorine Gas Mitigation

Chlorine gas is extensively used in a variety of chemical processes as an end product or reactant, for instance, in the conversion of phthalocyanine dye. Reactive absorption (RA) columns can be applied for the treatment of the waste gas streams at the outlet of the reaction equipment, in order to satisfy the environmental regulations, and these should be prepared for potential accidents. The objective of this study was to evaluate an industrial-scale RA column for the treatment of chlorine gas stream in a phthalocyanine plant in the case of an accident. A coupled experimental and numerical study was performed. A factorial design was applied for the definition of a set of experimental conditions to be evaluated in a pilot-scale plant operated in the laboratory. The data collected were treated by means of the response surface methodology, and coupled numerical schemes allowed the determination of the mass transfer coefficient and the reaction rate constant. Finally, numerical studies were carried out to assess the industrial-scale column behavior. It was found that the industrial absorption column was able to treat a stream of chlorine at a flow rate of 900 kg/h with an efficiency of approximately 79%.