Theoretical Control Properties Research Articles

Open-Loop Analysis for the Stability of Reactive Distillation Columns with Intermediate Condensers for the Production of Silane

The production of cost-effective Silane is paramount for reducing the cost in the solar photovoltaic industry to make it a more competitive renewable energy source. Thus, the production of Silane through reactive distillation (RD) is an alternative that reduces the equipment size, cost, and operation in comparison with a typical reaction-separation process mainly because the chemical reactions are limited by equilibrium. This work presents an open-loop analysis for RD columns with two intermediate condensers (RD-2IC) and different heat distribution in the inter condensers (IC). The open-loop stability of the RD columns analysis is done to understand the relationship between heat distribution at IC and their theoretical control properties to determine which heat distribution conditions result in stable operation. The results showed that RD-2IC columns attained economic savings compared to the RD column. Also, the optimal RD-2IC has better control properties than the RD column in an open-loop analysis.

Dynamic behavior of the intensified alternative configurations for quaternary distillation

Process intensification emerges as an important tool in the synthesis of multicomponent distillation configurations aimed at the reduction of the energy use and capital costs. Operational and fixed costs savings coupled with simplicity and controllability design configurations appear as an essential characteristic for industrial acceptance of intensified configurations. Following the aforementioned principles, two intensified configurations for the separation of quaternary mixture, were considered. These configurations were recognized as a valid alternative to overcome the complexity of the quaternary Petlyuk scheme.The study of the dynamics and control properties represent an important research issue in the analysis of new configurations to prove their effective applicability. The theoretical control properties of the alternative intensified configurations were obtained using the singular value decomposition technique in all frequency domain. In order to complete the control study, the distillation schemes were subjected to closed-loop dynamic simulations. The results show that there are cases in which the intensified sequences do not only provide energy savings, but also may offer dynamic advantages in comparison to the conventional four-component scheme.

A new design method and operation of fully thermally coupled distillation column

From economic point of view, the fully thermally coupled distillation columns have shown benefits in terms of energy and capital cost savings when compared with conventional distillation columns. However, the design of such column systems is a topic of ongoing research because there are more degrees of freedom to deal with during the design phase. In this paper, a design method is presented and used to determine the structures of conventional two-product and Petlyuk columns, and the results show that the proposed design method works well. The steady state models are developed using HYSYS software, and the results show that the proposed design method provides good initial values for rigorous simulation. According to controllability analysis, the steady state models present better theoretical control properties; consistent with controllability results, when the steady state model is studied under closed-loop using PI controllers good dynamic responses for load rejection are obtained.

Multiple Steady States in Thermally Coupled Distillation Sequences: Revisiting the Design, Energy Optimization, and Control

The design, optimization, and control of thermally coupled distillation sequences with side columns have been extensively studied in the literature. Energy savings of up to 30% have been reported for such systems; further, dynamic simulations show that the theoretical control properties and closed-loop dynamic responses of thermally coupled distillation sequences are better than those of the conventional distillation sequences. In this paper, we have extended the optimization search for the energy required on the separation of a ternary mixture, detecting two feasible solutions for the energy required in the reboiler for the same value of the interconnecting stream (multiple steady states), filling the gap about this topic on these systems. The multiple steady states detected in the thermally coupled distillation sequences (TCDS) were subjected to a controllability analysis and a closed-loop dynamic study. According to the controllability analysis, the steady state with higher demand of energy in the reboiler presented better theoretical control properties than those of the steady state with lower energy requirement in the reboiler. Consistent with the controllability results, when the two steady states were studied under closed-loop using PI controllers, the best dynamic responses for set point tracking and load rejection were exhibited by the steady state design with the higher energy requirement in the reboiler.

Effect of Using Adjusted Parameters, Local and Global Optimums, for Phase Equilibrium Prediction on the Synthesis of Azeotropic Distillation Columns

We analyze the effect of using adjusted parameters, corresponding to local and global optimums, in the NRTL thermodynamic model on the complete process synthesis (design, optimization and control) of homogeneous azeotropic distillation columns. The adjusted parameters that correspond to a global optimum were obtained with simulated annealing technique, while the adjusted parameters that correspond to a local optimum were taken from the Dechema Collection. Both sets of parameters were used to design a conventional sequence, a side-stream column, and a Petlyuk sequence. These designs were used as the initial solution to a multiobjective genetic algorithm with constraints handling, coupled to a processes simulator, where the number of stages and heat duty of each column were considered as objectives; as a result, a set of optimal designs, called the Pareto front, was obtained. Then, we chose some designs to analyze their theoretical control properties and the dynamic performance. Results show remarkable differences in structure, energy consumption, control properties, and dynamic performance of these schemes, depending on the use of adjusted parameters. The results show the importance of using the best adjusted parameters available, which in our case correspond to global optimums obtained with the simulated annealing technique.

Process control analysis for intensified bioethanol separation systems

The purification of bioethanol is traditionally performed by extractive distillation using three column sequences. In the present work new arrangements composed of two columns are considered for the analysis of control properties. The control properties study was based on the controllability properties under open loop operation, followed by the dynamic behavior for common industrial operating disturbances. Simulation results were analyzed by the singular value decomposition technique. The results from the theoretical control properties indicate that the presence of a side stream in the extractive distillation sequences does not necessarily provide operational disadvantages. The results also suggest that control properties are ruled by the kind of solvent used. The best performances were obtained when glycerol is used as entrainer.

Design, optimization and controllability of an alternative process based on extractive distillation for an ethane–carbon dioxide mixture

Alternative configurations based on cryogenic extractive distillation were proposed and simulated by using Aspen Plus 7.0® coupled to a multi-objective stochastic optimization procedure (differential evolution, DE). The evaluation of the performances of the proposed configurations was focused on the ethane–carbon dioxide azeotrope separation considering different liquefied hydrocarbon fractions as entrainers. The design alternatives were compared to the conventional chemical absorption system.The proposed sequences were simultaneously Pareto optimized by minimizing the total annual cost (TAC) and maximizing the acid gas removal. Complementary studies regarding the theoretical control properties, the thermodynamic efficiency and the greenhouse gases generation were conducted for several representative operating conditions obtained from the Pareto optimized fronts. The proposed cryogenic extractive distillation sequences realized the higher carbon dioxide removal together with the lower TAC compared to the conventional chemical absorption system.

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.

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

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

Design and optimization, using genetic algorithms, of intensified distillation systems for a class of quaternary mixtures

The design and optimization of a coupled multicomponent distillation system is a non-linear and multivariable problem. The complexity of this kind of problem results in high solving difficulty. This paper addresses the application of genetic algorithms to the optimization of intensified distillation systems for quaternary distillations. We used a multiobjective genetic algorithm with restrictions coupled to the Aspen Plus™ process simulator for the evaluation of the objective function. Several mixtures to test the effect of relative volatilities of feed mixtures on energy consumption, second law efficiency, total annual cost and theoretical control properties have been studied. Numerical performance shows that this design tool is robust and suitable for the design of coupled multicomponent distillation sequences.

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.

Reducing energy consumption and CO 2 emissions in extractive distillation

In this paper, design and optimization procedures are developed for a conventional extractive distillation sequence and a thermally coupled extractive distillation scheme. The proposed methodologies detect the optimal values of the design variables in order to guarantee the minimum energy consumption. It was found that the optimum energy consumption can be related to the minimum total annual operating cost, minimum greenhouse gas emissions, higher thermodynamic efficiencies and good theoretical control properties. The methodologies were applied to the study of the separation of close boiling point mixtures and azeotropic mixtures using the two distillation sequences. Regarding energy consumption, total annual cost, thermodynamic efficiency, greenhouse gas emissions and theoretical control properties, the thermally coupled extractive distillation sequence outperformed the conventional extractive distillation sequence. The results showed that the energy savings predicted in the complex extractive distillation sequence can be achieved along with good operational properties and reductions in greenhouse gas emissions.

Dividing Wall Distillation Columns: Optimization and Control Properties

AbstractThe optimal design of dividing wall columns is a non‐linear and multivariable problem, and the objective function used as optimization criterion is generally non‐convex with several local optimums. Considering this fact, in this paper, we studied the design of dividing wall columns using as a design tool, a multi‐objective genetic algorithm with restrictions, written in MatlabTM and using the process simulator Aspen PlusTM for the evaluation of the objective function. Numerical performance of this method has been tested in the design of columns with one or two dividing walls and with several mixtures to test the effect of the relative volatilities of the feed mixtures on energy consumption, second law efficiency, total annual cost, and theoretical control properties. In general, the numerical performance shows that this method appears to be robust and suitable for the design of sequences with dividing walls.

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.

Controllability Analysis of Thermally Coupled Distillation Systems: Five-Component Mixtures

The theoretical control properties and dynamic responses under closed-loop operation of thermally coupled distillation sequences for the separation of five-component mixtures of hydrocarbons were c...

Analysis of dynamic properties of alternative sequences to the Petlyuk column

The Petlyuk distillation configuration has received considerable attention because of its efficiency to reduce the energy required for the separation of ternary mixtures. The structure of the Petlyuk system offers some control challenges arising from the transfer of vapor streams back and forth between the columns. This means that neither of the columns can have a uniform lower nor higher pressure. Recently, some alternate Petlyuk-like schemes that might provide better operational properties than the Petlyuk column have been proposed. In this work, we analyze the control properties of six alternative thermally coupled distillation schemes to the Petlyuk system. The theoretical control properties are analyzed with the application of the singular value decomposition technique. Rigorous closed loop simulations are used to supplement the theoretical analysis. The results indicate that a reduction in the number of interconnections of the Petlyuk configuration does not necessarily provide an improvement of its controllability properties.

Analysis of Control Properties of Thermally Coupled Distillation Sequences for Four-Component Mixtures

The control properties of thermally coupled distillation sequences for the separation of quaternary mixtures of hydrocarbons were compared to those of conventional distillation sequences. Two thermally coupled systems were investigated: a thermally coupled distillation sequence with side columns and a Petlyuk-type column. The preliminary steady-state design of the thermally coupled distillation sequences was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The control properties of the sequences considered were obtained by using the singular value decomposition technique and closed-loop dynamic simulations under feedback control. It was found that, for some frequencies, the thermally coupled distillation sequences present theoretical control properties similar to those of conventional distillation sequences, i.e., similar minimum singular values and condition numbers. Also, both types of distillation sequences show interactions when relative gain arrays were obtained. Then, when both types of distillation sequences were studied under the action of proportional integral controllers, the thermally coupled sequences also presented optimum dynamic responses similar to those obtained in the conventional distillation scheme. As a result, it can be concluded that thermally coupled distillation sequences can present theoretical control properties similar to those obtained in conventional distillation sequences. This result is significant because it lets one establish that the energy savings predicted for thermally coupled distillation sequences are achieved without introducing additional control problems.