Coupled Distillation Sequences Research Articles

Editorial

This fall issue of 2012 is rich in new developments in IT. Thus, it is a real pleasure for a geologist to write the editorial of a number of Acta Universitaria beginning with a research in Petrography. In Petrografía y distribución de óxidos de Fe, Al y Si en fragipanes blancos de origen volcanico, Otilio Acevedo-Sandoval and contributors present very interesting results about the analysis of the distribution of iron, alumi­nium and silicon in three white soil profiles of hardened volcanic horizons (know as “fragipanes”) in the NW of the State of Mexico.The following two articles show the performance of Compute Unified Device Ar­chitecture using GPUs (Graphics Processing Units) as compared to CPU, applied to a large set of biological data. In Autómata Celular Estocástico paralelizado por GPU aplicado a la simulación de enfermedades infecciosas en grandes poblaciones, Hector Cuesta-Arvizu and co-workers explain how GPU parallelized software helps in simulating large-scale infection disease events with Stochastic Cellular Automata (SCA). On the other hand, Eduardo Romero-Vivas and contributors, in their article Implementación en GPU del Estadístico t para análisis de expresión genética en microarreglos, propose the use of t-paired parametric analysis with GPU implemen­tation for evaluating the gene expression levels for microarray hybridization.The paper of Emmanuel Ortiz-Lopez and contributors (Control de acceso usando FPGA y RFID) presents the design and implementation of an access control system using Radio frequency Identification (RFID) controlled by a field Programmable Gate Array. This system allows supervising, managing and reporting access or movement of individuals or equipment. Beside the control, it can help in avoiding large number of human errors.Finally, the study of Alejandro Tzompantzi Sanchez and co-workers, named Alterna­tivas de control para la secuencia de destilación térmicamente acoplada Petlyuk, shows that a closed loop dynamic response of thermally coupled distillation sequences for the separation of ternary mixture is significantly improved with an Internal Model Control Non Linear, when compared with the flow in the bottom and the top.Autumn is still well installed in Belgium. The rain and the leaves are falling. The ideal weather for taking a good chair, reading interesting papers and enjoying science development. I wish you to do so with this issue of Acta Universitaria, wherever you are in the world.

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

Thermally coupled distillation sequences: Steady state simulation of the esterification of fatty organic acids

Thermally coupled distillation columns have been used for the separation of multicomponent mixtures achieving energy savings in contrast to the well known conventional distillation sequences. In this work, the reaction between fatty organic acids (a mixture similar to Jatropha curcas L. seed oil) and methanol to produce esters in reactive thermally coupled distillation options is studied. The results showed that it is possible to obtain esters as bottoms products with high purity suitable for use as biodiesel. Some additional benefits were found, for instance, recovering of the excess of methanol and removing of the water produced, achieving the total consumption of the acid. Also, energy savings of around 30% were achieved in the case of thermally coupled reactive distillation systems in contrast to the classical two feeds reactive distillation column.

Optimization and Controllability Analysis of Thermally Coupled Reactive Distillation Arrangements with Minimum Use of Reboilers

The optimal design of reactive complex distillation systems is a highly nonlinear and multivariable problem, and the objective function used as optimization criterion is generally nonconvex with several local optimums and subject to several constraints. The esterification of lauric acid and methanol is explored using thermally coupled distillation sequences with side columns and with a minimum number of reboilers. The product of the esterification can be used as biodiesel. This is a major step forward since thermally coupled reactive distillation sequences with side columns and with a minimum number of reboilers offer significant benefits, such as the following: reductions on both capital investment and operating costs due to the absence of the reboilers and higher conversion and selectivity since products are removed as they are produced as well as no occurrence of thermal degradation of the products due to a lower temperature profile in the column. In this work, we have studied the design of reactive distillation with thermal coupling with a minimum number of reboilers, using differential evolution with restrictions coupled to AspenONE Aspen Plus. Also, we have analyzed the control properties of the reactive distillation schemes studied, and the results indicate that the energy requirements and the total annual cost of the complex distillation sequences with a minimum number of reboilers are reduced significantly in comparison with the conventional reactive distillation process and can be also achieved without significant control problems.

Optimisation of thermally coupled distillation sequences with side columns

An approach for the optimal design of thermally coupled distillation sequences (TCDS) with side columns is presented. Both systems with side strippers and side rectifiers are considered. The design task is formulated as a non-linear programming problem that uses shortcut methods for the thermally coupled systems. The application of the optimisation model provides the tray structure and operating conditions that minimise the total annual cost of the system, which includes capital investment and utilities costs. When the shortcut designs were validated through rigorous simulations, one major feature of the resulting TCDS structures was that they properly located the interconnecting tray for the thermal coupling, thus providing energy-efficient designs by minimising remixing effects. A comparison with reported results for the design of TCDS with side columns from the use of a mixed-integer non-linear programming model is also presented.

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.

Simulation study of the production of biodiesel using feedstock mixtures of fatty acids in complex reactive distillation columns

Biodiesel can be produced from a number of natural, renewable sources, but vegetable oils are the main feedstocks. The current manufacturing biodiesel processes, however, have several disadvantages: expensive separation of products from the reaction mixture, and high costs due to relatively complex processes involving one to two reactors and several separation units. Therefore, to solve these problems, in recent years several researchers have developed a sustainable biodiesel production process based on reactive distillation. In this paper the production of biodiesel using feedstock mixtures of fatty acids is explored using reactive distillation sequences with thermal coupling. The results indicate that the complex reactive distillation sequences can produce a mixture of esters as bottoms product that can be used as biodiesel. In particular, the thermally coupled distillation sequence involving a side rectifier can handle the reaction and complete separation in accordance with process intensification principles.

The Study of Energy Savings of Separation Quaternary Mixture with Thermally Coupled Distillation

The thermally coupled distillation configuration (TCDS) could reduce energy consumption and capital investment compared with conventional distillation sequences. But their use was still limited. The five kinds of conventional distillation sequences and five kinds of thermally coupled distillation sequences were performed in separation of quaternary mixtures with distillation rigorous simulations. The energy savings of a series of thermally coupled distillation sequences were compared. The results showed that the feed composition and ease of separation of the composition have great effect on the choice of the thermally coupled sequences. In the most cases, the TCDS with side rectifier or striper had the more energy saving effect.

Strategies for the robust simulation of thermally coupled distillation sequences

This paper presents a novel strategy for the simulation of thermally coupled distillation sequences using process simulators. First, we show that the two side stream connections involved in a ‘thermal couple’ can be accurately substituted by a combination of a material stream and heat flow; enabling a sequence of thermally coupled distillation columns to be simulated without recycle streams, similar to conventional simulations of zeotropic distillation sequences. In fact, using this method, a sequence of thermally coupled distillation columns is not more difficult to converge than other distillation systems without recycles. Furthermore, in most cases, this approach introduces negligible errors, and provides excellent starting points for rigorous simulations of actual thermally coupled systems with recycle streams. Different examples, including mixtures of hydrocarbons (C4s–C5s–C6s), aromatics (BTX), alcohols, non-ideal azeotropic systems (acetone, benzene, chloroform) and systems involving 4 or 5 components are presented. In addition, various thermodynamically equivalent configurations, corresponding to different alternatives for implementing this approach, are discussed.

Using thermally coupled reactive distillation columns in biodiesel production

Production of methyl dodecanoate (biodiesel) using lauric acid and methanol with a solid acid catalyst of sulfated zirconia is studied by using two distillation sequences. In the first sequence, the methanol recovery column follows the reactive distillation column. In the second sequence, the reactive distillation and methanol recovery columns are thermally coupled. Thermally coupled distillation sequences may consume less energy by allowing interconnecting vapor and liquid streams between the two columns to eliminate reboiler or condenser or both. Here we study the thermally coupled side-stripper reactive distillation and eliminate the condenser of the reactive distillation column. Both the sequences are optimized by using the thermal and hydraulic analyses of the Column Targeting Tools of Aspen Plus simulator. Comparisons of the optimized sequences show that in the thermally coupled sequence, the energy consumption is reduced by 13.1% in the reactive distillation column and 50.0% in the methanol recovery column. The total exergy losses for the columns are reduced by 281.35 kW corresponding to 21.7% available energy saving in the thermally coupled sequence. In addition, the composition profiles indicate that the thermally coupled reactive distillation column operates with the lower concentration of water in the reaction zone which reduces catalytic deactivation.

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.

Implementation and Operation of a Dividing‐Wall Distillation Column

AbstractThe 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.

Reactive Thermally Coupled Distillation Sequences: Pareto Front

Design of reactive distillation sequences is a major computer-aided design challenge. The optimal design of reactive complex distillation systems is a highly nonlinear and multivariable problem, and the objective function used as optimization criterion is generally nonconvex with several local optimums and subject to several constraints. In addition, several attributes for the design of these separation schemes are often conflicting objectives, and the design problem should be represented from a multiple objective perspective. As a result, solving with traditional optimization methods is not reliable because they generally converge to local optimums and often fail to capture the full Pareto optimal front. In this work, we have studied the design of reactive distillation with thermal coupling (using as study case the production of fatty esters), generalizing the use of a multiobjective genetic algorithm with restrictions coupled to Aspen ONE Aspen Plus, previously used in the design and optimization of intensified distillation systems. The results obtained in the Pareto front indicate that the energy consumption of the complex distillation sequence can be reduced significantly by varying operational conditions. Trends in the energy consumption, total annual cost, and greenhouse gas emissions of the thermally coupled reactive distillation sequences can be obtained.

DESIGN STUDY OF THE CONTROL OF A REACTIVE THERMALLY COUPLED DISTILLATION SEQUENCE FOR THE ESTERIFICATION OF FATTY ORGANIC ACIDS

This article reports the esterification of lauric acid and methanol studied using a thermally coupled distillation sequence with a side rectifier and the Petlyuk distillation column. The product of the esterification can be used as biodiesel. It was found that the thermally coupled distillation sequence with a side rectifier can produce ester with a high purity (around 0.999) and also pure water, and the excess of methanol is recovered in the side rectifier. The results indicate that the energy requirement of the complex distillation sequence with a side rectifier can be reduced significantly by varying operational conditions. These reductions in energy requirements can be interpreted as reductions in carbon dioxide emissions. Moreover, dynamic tests for control of the composition of the ester and control of two temperatures for the thermally coupled distillation sequence with a side rectifier indicate that it is possible to eliminate disturbances in the feed composition, while the composition of the biodiesel remains at the desired value.

Experimental study on pressure drops in a dividing wall distillation column

Previous studies in the fields of process design and process control [1] have shown the potential benefits that can be achieved through the implementation of thermally coupled distillation sequences, in particular, the dividing wall distillation column. The dividing wall distillation column meets important goals of process intensification, including energy savings, reduction in carbon dioxide emissions and miniaturization. In this paper, an experimental study on the hydrodynamic behavior of a dividing wall distillation column is presented. Several different values for gas and liquid velocities were tested in order to measure pressure drops and identify operational regions; the air/water system was used as the basis for the experimental setup. Results regarding pressure drops (fitted to the model of Stichlmair et al.) provide operational limits for the operation of the packed dividing wall distillation column. According to the results, the experimental dividing wall column can be operated at turbulent regime that is associated to proper mass transfer.

A Note on the Controllability of Two Short-Cut Designs for a Class of Thermally Coupled Distillation Sequence

The separation of multicomponent mixtures with conventional distillation sequences is energy-intensive. The use of thermal coupling sequences provides a more-efficient alternative, although their industrial usage has been quite limited, because of the complexity of design methods and potential difficulties during operation. In this work, we study a direct thermally coupled sequence for the separation of ideal ternary mixtures and perform a comparative study of two different designs, one based on the tray structure of an equivalent conventional sequence and the other specifically considering the effect of the thermal coupling on tray distribution and operational flows. Using the singular value decomposition technique on open-loop transfer function matrixes, controllability indexes were obtained. The controllability indexes of both designs are of the same order of magnitude, which means that their dynamic responses are basically the same. Although additional studies of dynamic simulation under closed-loop conditions are necessary, our results indicate that the controllability properties are similar, and that the design that provides the smallest energy consumption should be favored.

Reduction of Energy Consumption and Greenhouse Gas Emissions in a Plant for the Separation of Amines

AbstractThe chemical industry comprises of the companies that produce industrial chemicals. It is central to the modern world economy, converting raw materials into more than 70 000 different products. However, environmental regulations and the risk of climate change are putting pressure on the chemical industry to minimize greenhouse gas emissions. In this work, we use the concept of process intensification (using thermally coupled distillation) to reduce energy consumption and CO2 emissions in a plant for the separation of amines. The results show that the use of thermally coupled distillation sequences can be related to a reduction in energy consumption, greenhouse gas emissions, and good theoretical control properties in the re‐designed plant.

A Short Note About Energy-Efficiency Performance of Thermally Coupled Distillation Sequences

In this work, we present a comparative study of the energy-efficiency performance between conventional distillation sequences and thermally coupled distillation arrangements (TCDS) for the separation of ternary mixtures of hydrocarbons under the action of feedback control loops. The influence of the relative ease of separation of the feed mixture and its composition was analyzed. The feedback analysis was conducted through servo tests with individual changes in the set points for each of the three product streams. Standard PI controllers were used for each loop. The results show an apparent trend regarding the sequence with a better dynamic performance. Generally, TCDS options performed better for the control of the extreme components of the ternary mixture (A and C), while the conventional sequences offered a better dynamic behaviour for the control of the intermediate component (B). The only case in which there was a dominant structure for all control loops was when the feed contained low amounts of the intermediate component and the mixture had similar relative volatilities. The Petlyuk column provided the optimal choice in such case, which contradicts the general expectations regarding its control behaviour. In addition, the energy demands during the dynamic responses were significantly lower than those observed for the other distillation sequences. TCDS options, therefore, are not only more energy efficient than the conventional sequences, but there are cases in which they also offer better feedback control properties. On présente dans ce travail une étude comparative de la performance d'efficacité d'énergétique entre les séquences de distillation conventionnelles et les configurations de distillation couplées thermiquement (TCDS) pour la séparation de mélanges ternaires d'hydrocarbures sous l'action de boucles de contrôle d'asservissement. L'influence de la facilité relative de séparation du mélange d'alimentation et de sa composition est analysée. L'analyse de rétroalimentation est réalisée grâce à des tests d'asservissement avec des changements individuels dans les points de consigne pour chacun des trois courants de produits. Des contrôleurs PI standards ont été utilisés pour chaque boucle. Les résultats montrent une tendance apparente pour la séquence ayant une meilleure performance dynamique. Généralement, les options TCDS sont meilleures pour le contrôle des composantes extrêmes du mélange ternaire (A et C), tandis que les séquences conventionnelles offrent un meilleur contrôle dynamique pour le contrôle de la composante intermédiaire (B). Le seul cas où il y a une structure dominante pour toutes les boucles de contrôle, c'est lorsque l'alimentation contenant de faibles quantités de la composante intermédiaire et le mélange ont la même volatilité relative. La colonne Petlyuk est le choix optimal dans un tel cas, ce qui contredit les attentes générales concernant son comportement de contrôle. En outre, les demandes d'énergie pendant les réponses dynamiques sont significativement plus faibles que celles observées pour les autres séquences de distillation. Ainsi, non seulement les options TCDS sont plus efficaces que les séquences conventionnelles, mais il y a des cas où elles offrent également de meilleures propriétés de contrôle d'asservissement.

Dynamic Behaviour of Thermally Coupled Distillation Arrangements: Effect of the Interconnection Flowrate

Thermally coupled distillation sequences can provide significant energy savings with respect to the operation of conventional distillation columns. Coupled sequences exhibit a complex structure, with recycle streams, that appear to affect their controllability properties. One potential solution to this problem has been suggested through the operation of complex schemes under conditions that do not provide minimum energy consumption. The basic idea is that if one changes the value of the interconnection flowrate, the control properties might change as well. In this work, we analyze the dynamic behavior of two coupled structures under different operating points, including the one with minimum energy consumption. The control analysis properties are analyzed with the application of the singular value decomposition technique in all frequency domains. The results show that the controllability properties of distillation sequences may change significantly depending on the selected value of interconnection flowrate.

Controllability analysis of modified Petlyuk structures

AbstractFully 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.