A synthesis method for multicomponent distillation sequences with fewer columns

An easy‐to‐use matrix‐based method for the systematic synthesis of distillation configurations using less than n‐1 columns to separate any zeotropic n‐component feed into n product streams is described. The method is easily extended to obtain additional thermally coupled configurations. The only information needed to generate the configurations is the number of components in the feed, or equivalently, the number of distinct composition final product streams. We have successfully enumerated configurations for feeds containing up to eight components. This has resulted in a large number of hitherto unknown configurations even for four‐component separations. Some of the novel configurations generated using the method have substantially lower heat duty than the previously known fewer column configurations for a four‐component feed separation. Therefore, it is essential to include these novel configurations in the search space to find the optimal distillation configuration with fewer columns for a given application. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2479–2494, 2012     

A matrix method for multicomponent distillation sequences

We describe a simple‐to‐use “matrix” method for obtaining all the basic distillation configurations and additional thermally coupled configurations that separate a zeotropic multicomponent feed into essentially pure product streams. This provides an opportunity to rank‐list the configurations for a given application subject to criteria of interest. The only information needed to generate the configurations is the number of components in the feed. We have successfully enumerated all the configurations for feeds containing up to eight components. The method can also be used to generate nondistillation and hybrid separation configurations, and even easy‐to‐retrofit configurations. We illustrate the use of this method by applying it to the highly energy‐intensive problem of petroleum crude distillation. We have identified more than 70 new configurations that could potentially have lower heat duty than the existing configuration. A significant number of these could reduce the heat demand by nearly 50%. © 2009 American Institute of Chemical Engineers AIChE J, 56: 1759–1775, 2010     

外部热耦合复合精馏塔的经济性与可控性评价

外部热耦合复合精馏塔系统是一种新型的精馏塔系统,通过操作在不同压力下的两个精馏塔的精馏段和提馏段之间的热传递来提高热力学效率。根据精馏段和提馏段热耦合的相对位置不同,外部热耦合复合精馏塔系统可分为对称型和非对称型两种结构。为便于设计和实现,可用外部换热器替代外部热耦合得到简化的结构。本文以乙烯乙烷物系分离过程为对象,通过对外部热耦合复合精馏塔系统建立了静动态模型,采用四点控制的方法,对三种结构的外部热耦合复合精馏塔的经济性和可控性两方面做了分析,证明了非对称型优于对称型外部热耦合复合精馏塔。同时,对使用外部换热器简化外部热耦合结构的方法提供了理论依据和参考。     

Thermal coupling links to liquid‐only transfer streams: An enumeration method for new FTC dividing wall columns

Novel dividing wall columns (DWCs) can be obtained by converting thermal couplings to liquid‐only transfer streams. Here, we develop a simple four‐step method to generate a complete set of DWCs containing n ? 2 dividing walls, for a given n‐component fully thermally coupled (FTC) distillation. Among the novel DWCs, some easy‐to‐operate DWCs possess the property that the vapor flow in every section of the DWC can be controlled during operation by means that are external to the column. We develop a simple method to enumerate all such easy‐to‐operate DWCs. We expect that the easy‐to‐operate DWCs can be operated close‐to‐optimality; leading to a successful industrial implementation of the n‐component (n ≥ 3) FTC distillation in the form of a DWC. As an illustration, we show figures of all easy‐to‐operate DWCs with two dividing walls for the four‐component FTC distillation. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1200–1211, 2016     

A novel method for generating distillation configurations

An improved matrix method for generating distillation configurations with (N−1) and less than (N−1) columns was proposed for the separation of an N-component mixture into essentially pure product streams based on the concepts of streams matrix and 0–1 matrixes proposed by Agrawal. In contrast with the matrix method developed by Agrawal, the present method removes the intermediate process centered on the splits, and complex column configurations, allowing the direct generation of multi-feeds and multi-product streams. Furthermore, certain configurations that cannot be generated directly and that are missing in the matrix method are obtained. Through rigorous simulations and optimization, we have demonstrated that these configurations have the potential to outperform certain existing configurations.     

Modified basic distillation configurations with intermediate sections for energy savings

Basic distillation configurations have been well studied, comprising of energy efficient (n ? 1)‐column configurations that can be used to separate an n‐component nonazeotropic feed mixture into its components. In this article, we present new (n ? 1)‐column configurations which differ from the conventional basic configurations due to the introduction of extra intermediate sections and the additional transfer‐streams associated with submixtures of these sections. We demonstrate using four‐component examples that these small differences lead to some interesting nonintuitive physical effects in the new configurations, resulting in large energy savings compared to the basic configurations. The proposed configurations offer more operable and energy efficient options for onsite implementation than the corresponding optimally operated basic configurations. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1091–1097, 2014     

Dynamic modeling and collocation‐based model reduction of cryogenic air separation units

High purity distillation columns and multi‐stream heat exchangers (MSHXs) are critical units in cryogenic air separation plants. This article focuses on modeling approaches for the primary section of a super‐staged argon plant. A full‐order stage‐wise model for distillation columns in air separation units (ASUs) that considers key process phenomena is presented, followed by a reduced‐order model using a collocation approach. The extent of model reduction that can be achieved without losing significant prediction accuracy is demonstrated. A novel moving boundary model is proposed to handle MSHXs with phase change. Simulation results demonstrate the capability of the proposed model for tracking the phase change occurrence along the length of the heat exchanger. Dynamic simulation studies of the integrated plant show that the thermal integration between the feed and product streams captured in the primary heat exchanger is critical to accurately capture the behavior of ASUs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1602–1615, 2016     

A novel multistage vapor recompression reactive distillation system with intermediate reboilers

Most of the published studies have focused on the thermal integration of nonreactive distillation columns. The key limitation of reactive distillation (RD) technology is that the necessary conditions (such as pressure and temperature) for the reaction must match those of distillation. Owing to this constraint, the reaction conversion may be adversely affected at the elevated pressure in the reactive section of an internally heat integrated distillation column (HIDiC). This fact forces us to adopt an external heat integration approach for an industrial heterogeneously catalyzed ethyl tert‐butyl ether (ETBE) RD column. The direct vapor recompression column (VRC) is an external heat integration scheme that is successfully used as an energy efficient scheme for separating a close‐boiling mixture. Interestingly, there exists a large temperature difference between the two ends of the representative ETBE column, and it makes the external heat integration more challenging. Aiming to improve the thermal efficiency of the ETBE column under the VRC framework, various heat pump arrangements with intermediate reboiler(s) (IR(s)) are explored and analyzed with performing a comparative study in terms of energy consumption and economics. To improve further the thermal efficiency, in this contribution, a novel multistage vapor recompression RD column with IRs is introduced addressing a number of practical concerns. An algorithm for the proposed column is formulated showing the sequential steps involved in heat integration. It is inspected that the proposed multistage vapor recompression RD system appears overwhelmingly superior to the classical vapor recompression RD and its conventional stand alone column providing a significant savings in energy as well as cost. © 2012 American Institute of Chemical Engineers AIChE J, 59: 761–771, 2013     

New multicomponent distillation configurations with simultaneous heat and mass integration

In this work, well known Brugma's configurations are shown to be only a subset of a much larger proposed set of simultaneously heat and mass integrated configurations. Furthermore, a systematic classification that allows the exploration of the entire set of simultaneously heat and mass integrated distillation configurations is described. The classification can be used as an underlying principle of any synthesis algorithm and helps identify several novel simultaneously heat and mass integrated distillation configurations for separating a multicomponent feed. A hitherto unknown class of simultaneously heat and mass integrated configurations characterized by strategic side‐stream withdrawals is also described. These new configurations provide an array of novel energy efficient options for the nonazeotropic distillation processes. © 2012 American Institute of Chemical Engineers AIChE J, 59: 272–282, 2013     

内部热耦合精馏塔构型研究

以丙烯-丙烷分离过程为例,研究了4种内部热耦合精馏塔的性能,并与常规精馏塔和热泵精馏塔进行了比较。结果发现,不同构型的内部热耦合精馏塔之间性能差异很大,其中提馏段与精馏段上端对齐,逐板进行热交换的构型性能最佳,其有效能耗比热泵精馏塔低25%—40%,节能效果显著。还探讨了内部热耦合精馏塔的压缩比与换热面积的关系,压缩比越小,换热面积越大,换热面积的逐板分布越不均匀。     

A fundamental principle and systematic procedures for process intensification in reactive distillation columns

A fundamental principle is developed for process intensification through internal mass and energy integration in reactive distillation columns and three systematic procedures are devised for process synthesis and design. For reactive distillation columns involving reactions with highly thermal effect, process intensification can be achieved with an exclusive consideration of internal energy integration between the reaction operation and separation operation involved. However, in the case of a highly endothermic reaction with an extremely low reaction rate and/or small chemical equilibrium constant, internal mass integration has also to be considered between the reactive section and stripping section. For reactive distillation columns involving reactions with negligibly or no thermal effect, process intensification can be performed with an exclusive consideration of internal mass integration. For reactive distillation columns involving reactions with moderately thermal effect, process intensification must be conducted with a careful trade-off between internal mass and energy integration. Five hypothetical and two real reactive distillation systems are employed to evaluate the principle and procedures proposed. It is demonstrated that intensifying internal mass and energy integration is really effective for process intensification. Not only can the thermodynamic efficiency be improved substantially, but also the capital investment can be further reduced.     

A novel multi-effect methanol distillation process

Crude methanol distillation is an energy-intensive separation process and contributes significantly to the cost of methanol production. Although a number of energy-efficient distillation systems have been proposed, there is potential for energy savings in methanol distillation. To further reduce the energy consumption of methanol distillation, a novel five-column multi-effect distillation process is proposed in this work, which is essentially an improved version of an existing four-column scheme. The four-column scheme is made up of a pre-run column, a higher-pressure column, an atmospheric column and a recovery column. The new five-column scheme adds a medium-pressure column after the original higher-pressure column. In this way, the load of the original higher-pressure and atmospheric columns can be decreased by about 30%. The five-column arrangement creates a multi-effect distillation configuration involving efficient heat integration between higher-pressure and medium-pressure columns, atmospheric and recovery columns, and recovery and pre-run columns. Steady-state process simulation results indicate that temperature differences at two sides of each heat exchanger are appropriate, allowing effective heat transfer. Economic analysis shows that the energy consumption of the five-column scheme can be reduced by 33.6% compared to the four-column scheme. Significant savings in operating costs can therefore be achieved, resulting in an economically viable process for methanol distillation.     

Thermal coupling links to liquid‐only transfer streams: A path for new dividing wall columns

We propose new dividing wall columns (DWCs) that are equivalent to the fully thermally coupled (FTC) configurations. While our method can draw such configurations for any given n‐component mixture (n ≥ 3), we discuss in detail the DWCs for ternary and quaternary feed mixtures. A special feature of all the new DWCs is that during operation, they allow independent control of the vapor flow rate in each partitioned zone of the DWC by means that are external to the column. Because of this feature, we believe that the new arrangements presented in this work will enable the FTC configuration to be successfully implemented and optimally operated as a DWC in an industrial setting for any number of components. Also, interesting column arrangements result when a new DWC drawn for an n‐component mixture is adapted for the distillation of a mixture containing more than n components. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2949–2961, 2014     

无冷凝器及再沸器的热集成蒸馏塔技术进展

蒸馏塔内部热集成技术系指同一塔内通过精馏段和提馏段之间的热量集成。介绍了内部热集成蒸馏塔的结构、特点、节能原理以及发展概况。该项技术被认为是最具节能潜力的新型蒸馏技术之一,与传统蒸馏塔相比,内部热集成蒸馏塔节能可达到30%—60%。近年来,塔内部热集成技术研究呈现加速发展的势态,国外学者已成功地完成了中试,并开始工业化应用的研究。     

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.     

Falling film distillation column with heat transfer by means of a vapor chamber. Part II: operation with a temperature profile

Based on previous results and a new proposal for the configuration of falling film distillation columns heated by a vapor chamber operating at atmospheric pressure, the present study analyzed a different application for heat transfer by means of an the axial temperature profile in the same vapor chamber, generated by the presence of non-condensable gases. Experimental tests were performed with an ethanol–water mixture by varying the feed flow rate and vapor chamber temperature. This new heat transfer proposal allowed the formation of a temperature gradient inside the distillation tube (similarly to the isothermal proposal previously reported), thus indicating that this heat transfer method is feasible for a distillation unit. The results demonstrated that higher flow rates are suitable for the distillation of ethanol and water in this new proposal; the temperature profile operation shows the feasibility of this new heat transfer proposal in falling film distillation columns and encourages further studies on the optimization of the process and the evaluation of the energy consumption.     

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     

A novel stochastic optimization method to efficiently synthesize large-scale nonsharp distillation systems

This study presents a novel stochastic optimization method for the efficient synthesis of large-scale nonsharp distillation systems, where heat integration and thermal coupling can be involved simultaneously. A new binary tree encoding method was developed to represent distillation sequences with no limits on the number of middle components in nonsharp splits to ensure a complete solution space. Thermally coupled structures were defined by 0–1 binary variables. Evolutionary rules were developed to generate neighboring distillation configurations randomly. Finally, an optimization framework was proposed, where simulated annealing (SA) algorithm was used to optimize distillation configurations; for a certain distillation configuration randomly generated by SA, its continuous variables were optimized using particle swarm optimization algorithm. Four cases—including the synthesis of six- and seven-component nonsharp heat integrated and thermally coupled distillation sequences—were studied to demonstrate that the proposed method was efficient and could obtain optimal and valuable suboptimal solutions with high probabilities.     

A simple synthesis method for studying thermally integrated distillation sequences

In this paper an improved method for studying processes for separation of multicomponent mixtures is presented. A thermodynamically oriented procedure was developed for the analysis and synthesis of the best heat‐integrated distillation system. Systematic ranking of possible schemes was used in order to verify the best structure of a heat‐integrated distillation system. An integrability criterion was introduced for the selection of the most promising heat‐integrated sequences. Distillation columns were studied according to their ability for heat integration. The proposed selection of possible distillation columns sequences using rising integrability criteria gave a good approximation of the best heat‐integrated distillation column sequences according to their total annual cost. Computational simulation of the process and pinch analysis were used for thermodynamic optimization of energy consumption. The method is illustrated with five‐ and six‐component example problems.     

Global optimization of multicomponent distillation configurations: 2. Enumeration based global minimization algorithm

We present a general Global Minimization Algorithm (GMA) to identify basic or thermally coupled distillation configurations that require the least vapor duty under minimum reflux conditions for separating any ideal or near‐ideal multicomponent mixture into a desired number of product streams. In this algorithm, global optimality is guaranteed by modeling the system using Underwood equations and reformulating the resulting constraints to bilinear inequalities. The speed of convergence to the globally optimal solution is increased by using appropriate feasibility and optimality based variable‐range reduction techniques and by developing valid inequalities. The GMA can be coupled with already developed techniques that enumerate basic and thermally coupled distillation configurations, to provide for the first time, a global optimization based rank‐list of distillation configurations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2071–2086, 2016