Complex Distillation Research Articles

Optimal design of distillation systems with less than N − 1 columns for a class of four component mixtures

The optimal design of complex distillation systems is a highly non-linear and multivariable problem, with several local optimums and subject to different constraints. In addition, some 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 paper, a method for the multiobjective optimization of distillation systems, conventional and thermally coupled, with less than N−1 columns is presented. We use a multiobjective genetic algorithm with restrictions coupled to AspenONE Aspen Plus; so, the complete MESH equations and rigorous phase equilibrium calculations are used. Results show some tendencies in the design of intensified sequences, according to the nature of the mixture and feed compositions.

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.

Response surface optimization of the operating parameters for a complex distillation column based on process simulation

The effects of operating parameters on the performance of a prefractionator in solvent fermentation were studied by process simulation. Effects of feed position, side stream position and flow rate, overhead reflux rate and cooling temperature on the total solvent content in the bottom residue were investigated. These parameters were further optimized by response surface analysis according to the principles of central composite design (CCD), with the minimization of total solvent content in the bottom residue as the objective. A mathematical model for the correlation of total solvent content and operating parameters was established in the optimization, and the optimized results were verified by both simulation and experiments. Results showed that the response surface approach is more advantageous than single-factor analysis in the optimization of complex distillation columns, and the simulated results agreed well with the experimental data.

Improvement of natural gas liquid recovery energy efficiency through thermally coupled distillation arrangements

ABSTRACTThe depropanizing, debutanizing, and deisobutanizing fractionation steps of processing natural gas liquids were improved through studying complex distillation arrangements, including the double prefractionator arrangement (DPA), the double dividing wall column (DDWC) arrangement, and the Agrawal arrangement. The DPA and DDWC arrangements were obtained from an initial conventional column sequence, and both offer benefits by decreasing reboiler and condenser power consumption. Reducing the number of columns and their diameters in the DDWC arrangement can potentially reduce construction costs. Inter‐reboiling of the bottom section of the second dividing wall column improved the performance of the DDWC. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

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.

Evolutionary design of optimum distillation column sequence

Synthesis of the optimum distillation column sequence (DCS), which incorporates a huge search space composed of both conventional and complex arrangements, is a highly complicated combinatorial problem in the field of chemical process design and optimisation. In this study, a novel procedure for the synthesis of optimum DCS proposed by Boozarjomehry et al. [Boozarjomehry et al., Can. J. Chem. Eng. 87, 477–492 (2009)] is expanded to include the complex distillation arrangements. The method is based on evolutionary algorithms, and the total annual cost (TAC) is the main criterion used to screen alternatives. Efficient procedure has been proposed for encoding mechanism to include and classify various complex arrangements together with conventional distillation columns. All columns existing in each DCS alternative are designed using the most recommended short-cut methods to estimate the TAC of the DCS. Four standard benchmark case studies are carried out to clearly demonstrate the excellent performance of the proposed method. The produced results for these problems indicate that the proposed method outperforms the other existing approaches in terms of flexibility, accuracy and comprehensiveness. © 2011 Canadian Society for Chemical Engineering

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.

Short-cut methods for the optimal design of simple and complex distillation columns

New short-cut methods providing optimal design parameters for distillation columns with simple and complex configurations including two-feed and one-feed-one-side-stream columns are presented. The methods assume constant relative volatilities and constant molar flow rates within each distillation section. The design equations are based on the Underwood equations for the calculation of minimum reflux (reboil) ratio, the analytical formulations of the distillation line, the Eigenfunction and the number of theoretical stages for each mass transfer section of the column. Furthermore, the geometrical properties of a given separation are considered. Optimization algorithms based on the minimization of the total number of theoretical stages of the column with taking into account the mass balance at each feed section have been elaborated. In comparison to the boundary value method the new short-cut methods require a minimum number of specifications; they do not need any graphical support, and provide a lower total number of theoretical stages particularly for complex configurations. The new short-cut methods have been extended to the design of columns separating azeotropic mixtures by approximating the latter by appropriate pseudo-ideal mixtures. Several separation examples for azeotropic mixtures, including different types of splits as well as columns with simple and complex configurations were tested and show a very good agreement with the simulation results obtained with Radfrac (Aspenplus).

Flowsheet synthesis and complex distillation column design for separating ternary heterogeneous azeotropic mixtures

The boundary value method for design of distillation columns separating ternary heterogeneous azeotropic mixtures is extended to include complex configurations, i.e. columns with integrated decanters and with multiple heterogeneous stages; double-feed columns; columns with intermediate decanters. The methods can be used for establishing product feasibility in a column and evaluating a column design in terms of cost. Multiple feasible designs can be generated for a given set of product specifications, according to the reflux ratio, number of heterogeneous stages and liquid phase ratio; these designs may be evaluated with respect to operating and equipment costs. Case studies illustrate the design method. An algorithmic approach is presented for synthesising novel sequences for separating, by distillation and decanting, ternary heterogeneous azeotropic mixtures. Existing synthesis procedures that consider only simple single-feed columns with decanters and integrated decanters are extended to include more industrially relevant options such as columns with several heterogeneous stages, double-feed columns, columns with intermediate decanters, and those accepting a heterogeneous liquid feed. With these advanced column configurations included in the synthesis method, a wider range of sequences may be considered systematically, allowing sequences that are more economically attractive than conventional designs to be identified. A case study illustrates the approach.

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.

Computer‐Aided Design, Modeling and Simulation of a New Solar Still Design

The clean and pure drinking water is important in today′s life but current water sources are usually brackish with bacteria that cannot be used for drinking. About 78% of water available in the sea is salty, 21% of water is brackish, and only 1% of water is fresh. Distillation is one of the feasible processes applied to water purification, and it requires the energy inputs, such as solar radiation. Water is evaporated in this distillation process and water vapor can be separated and condensed to pure water. Now, with the change from conventional fuels to renewable and environment friendly fuels sources, the modern technology allows to use the abundant energy from the sun. It is better to use solar energy to process the water desalination since it is more economical than the use of conventional energies. The main focus of this paper is applying computer‐aided modeling and simulation to design a less complex solar water distillation system. The prototype of this solar still system is also built to verify its feasibility, functionality, and reliability. The computational simulation and prototype testing show the reliability and proper functionality of this solar water distillation system.

Rigorous synthesis and simulation of complex distillation networks

AbstractRecent insights for better understanding the thermodynamic foundations of separation processes have renewed the interest in exploring energy‐efficient distillation networks. Complex column networks have substantial potential for energy savings over conventional configurations. This article introduces a computational algorithm for synthesizing such complex energy‐efficient networks. A robust feasibility criterion drives the selection of design specification and operating conditions. It will be shown that columns composed of sections whose liquid stage composition profiles have no gaps are realizable. To prove the rigor of design computations, numerous separation networks were synthesized and validated with the Aspen flowsheet simulator. By using our computational results as input, AspenPlus simulations converged in a few iterations. Our method builds on temperature collocation, a thermodynamically motivated search method for determining feasible operating conditions and design details for achieving the desired product targets. Our findings suggest that significant energy savings can be realized with rigorous complex networks synthesis for industrial separation problems. © 2010 American Institute of Chemical Engineers AIChE J, 2011

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.

From the dilute solution to the pure compound: Extraction strategy based on a multi-stage process of phase separation

It is very rare that a one-step process of extraction leads to the pure compound with a high degree of purity specified by an industrial application. The various stages of a synthesis process and possible secondary reactions may lead to the synthesis of more or less complex and highly diluted solutions. In this work, the rationale and strategy for extraction and purification of a high added value compound are discussed. All the thinking is based on the knowledge and the exploitation of phase diagrams and then developed for different unit operations of the process. The most significant research tools are the experimental data and the modelling of phase equilibrium to estimate the yield of each step of extraction. The significant example chosen involves all the basic methods of phase separation, starting with liquid-vapour equilibrium: stripping of high volatility components and then more or less complex distillation are classically employed. The theoretical plateau number can be deduced from the equilibrium equation curves. The second step is based on the study of the liquid-liquid equilibrium and is an intermediate step for enrichment of the solution when distillation is not possible. A final step based on solid-liquid equilibrium consists of the selective crystallization of the pure product at low temperature, in order to satisfy the requirements of purity and safety imposed by industrial use. The conclusion includes all isolation operations in the form of a general extraction and purification scheme.

Graphical algorithms and threshold error rates for the 2d color code

Recent work on fault-tolerant quantum computation making use of topological error correction shows great potential, with the 2d surface code possessing a threshold error rate approaching 1\%. However, the 2d surface code requires the use of a complex state distillation procedure to achieve universal quantum computation. The color code of is a related scheme partially solving the problem, providing a means to perform all Clifford group gates transversally. We review the color code and its error correcting methodology, discussing one approximate technique based on graph matching. We derive an analytic lower bound to the threshold error rate of 6.25\% under error-free syndrome extraction, while numerical simulations indicate it may be as high as 13.3\%. Inclusion of faulty syndrome extraction circuits drops the threshold to approximately 0.10 \pm 0.01\%.

Heat Integrated Distillation System Design

Distillation is the largest single energy consumer in the chemical process industries. However, distillation does not consume energy but degrades the heat input to the reboiler that is subsequently rejected in the condenser. The most effective way to reduce the energy consumption of distillation is by effective heat integration. However, the distillation design and operation must be considered simultaneously with its heat integration. For single distillation columns it is straightforward to identify appropriate heat integration opportunities. For complex distillation systems, the most appropriate combination of distillation system design and operation and heat integration are far from straightforward. The whole separation system together with its heat integration and utility system must be considered simultaneously. This presentation will explain new approaches to be design of heat integrated distillation systems. Examples will be developed from crude oil distillation and from low-temperature separation in chemicals production. Copyright ? 2010, AIDIC Servizi S.r.l.

Modelling and control of a continuous distillation tower through fuzzy techniques

This paper presents a methodology for the design of a fuzzy controller applicable to continuous processes based on local fuzzy models and velocity linearizations. It has been applied to the implementation of a fuzzy controller for a continuous distillation tower. Continuous distillation towers can be subjected to variations in feed characteristics that cause loss of product quality or excessive energy consumption. Therefore, the use of a fuzzy controller is interesting to control process performance. A dynamic model for continuous distillation was implemented and used to obtain data to develop the fuzzy controller at different operating points. The fuzzy controller was built by integration of linear controllers obtained for each linearization of the system. Simulation of the model with controller was used to validate the controller effectiveness under different scenarios, including a study of the sensibility of some parameters to the control. The results showed that the fuzzy controller was able to keep the target output in the desired range for different inputs disturbances, changing smoothly from a predefined target output to another. The developed techniques are applicable to more complex distillation systems including more operating variables.

Aggregate models based on improved group methods for simulation and optimization of distillation systems

This note deals with aggregate models for complex distillation systems in large-scale flowsheets. Group methods were originally devised for simple absorber and stripper calculations with no major extensions for handling distillation. In this work, group methods are systematically analyzed and further improved by modifying some of the previously proposed approximations. As a result, the improved group method exhibits accurate predictions and this is demonstrated using simulation and optimization case studies for a variety of chemical systems and operating conditions. It is observed that the prediction of output variables is in close agreement with that of the rigorous equilibrium stage model. In case of optimization problems, the optimal number of trays and feed locations differ by only one or two trays. The aggregate model can be applied in a sequence of steps in order to improve the reliability and robustness of the solution procedure. A rounding heuristic is also proposed which can provide near-optimal solutions with a significant reduction in computational time.

Complex Column Design by Application of Column Profile Map Techniques: Sharp-Split Petlyuk Column Design

Currently employed short-cut design techniques have a tendency to be configuration-specific. Few can be applied to complex distillation configurations. In this work, we will demonstrate, in detail, the use of column profile maps (CPMs) for the comprehensive analysis and design of complex distillation systems by applying the CPM technique to the design of the fully thermally coupled (Petlyuk) distillation column at sharp-split conditions. It is shown that, for set product composition specifications and a set reflux ratio, only a small region of key parameters (vapor and liquid split ratios) result in feasible separations. These results—and, hence, the CPM design procedure—are validated by the work of Halvorsen in his 2001 Ph.D. thesis. It is also shown that the minimum reflux solution can be found using this methodology. The results are valid for all zeotropic separation syntheses.

Synthesis of Distillation Column Sequences for Nonsharp Separations

In this paper an algorithm to generate all possible distillation column sequences for a nonsharp separation of c components to m multicomponent as well as pure desired products, based on the product specifications, is presented. Each desired product is defined by the minimum and maximum acceptable values of product rates as well as the concentration of each component. The possible sequences are generated by the introduction of a separation matrix. In the proposed algorithm each sequence can include between 1 and c − 1 simple and/or complex distillation columns and bypass streams. The sequences with less than c − 1 distillation columns can be generated systematically by the proposed algorithm. Also, a design strategy has been proposed for the optimization of the sequences. This algorithm has been used for a nonsharp separation example. A sequence with less than c − 1 number of distillation columns and bypass streams is selected as the optimum structure for this example. This optimum sequence can cause sign...