Coupled Distillation Research Articles

Synthesis and Optimal Design of Thermodynamically Equivalent Thermally Coupled Distillation Systems

A thermodynamically equivalent structure (TES) is the distinct feature of a thermally coupled configuration. This paper presents the synthesis and optimal design of thermodynamically equivalent thermally coupled systems for multicomponent distillations. First, the original thermally coupled configurations (OTCs) for traditional distillation configurations with sharp splits are generated. Then, generation of all of the possible TESs for the OTCs is presented. Two rules are developed for synthesizing the possible TESs for any OTC involving both sharp and sloppy splits. Formulas are derived to calculate the number of TESs for an n-component mixture. It is observed that there exist intrinsic uneven distributions of vapor and liquid flows between columns of the OTCs. The TESs provide the opportunities to redistribute the vapor and liquid flows between columns and thus improve column equipment designs of an OTC. Heuristics and a simple procedure have been approached to find an optimal TES.

Thermodynamic Analysis of Thermally Coupled Distillation Sequences

Thermodynamic efficiency calculations have been performed for the separation of ternary and quaternary mixtures of hydrocarbons in both conventional and thermally coupled distillation sequences. When ternary mixtures were considered, energy savings achieved in the thermally coupled distillation sequences were between 10 and 30% in comparison to the two conventional distillation sequences. Regarding thermodynamic efficiency, thermally coupled distillation sequences presented the highest values in almost all of the cases considered. When the analysis was extended to the separation of quaternary mixtures, the thermally coupled distillation sequences show acceptable energy savings and the thermodynamic efficiency of a thermally coupled distillation sequence linked to a side stripper and a side rectifier was better than that obtained in the conventional distillation sequence; however, for the thermally coupled distillation with prefractionator (Petlyuk-type column), the second law efficiency was low when compared to that of the conventional distillation sequence for the separation of a quaternary mixture with a low content of intermediate components.

Energy Efficiency of an Indirect Thermally Coupled Distillation Sequence

AbstractSeveral 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.

Energy-Efficient Designs of Thermally Coupled Distillation Sequences for Four-Component Mixtures

A design procedure for thermally coupled distillation sequences for the separation of four-component mixtures is presented. The schemes analyzed include a sequence with a side rectifier and a side stripper and a sequence with a prefractionator (Petlyuk-type column). Initial designs for the thermally coupled distillation sequences are obtained from the tray distribution of a conventional distillation sequence and then optimized for energy consumption using two interconnecting flows as search variables. Several tray arrangements for each thermally coupled design were analyzed, and the results show that the sequence with side columns can reach a lower energy consumption.

Synthesis of Heat-Integrated Thermally Coupled Distillation Systems for Multicomponent Separations

This paper presents the synthesis of heat-integrated thermally coupled distillation systems for multicomponent separations. The synthesized new distillation systems employ the thermal coupling and heat-integration principles simultaneously. As a consequence, they have the potential to significantly reduce both the energy and capital costs to a bigger magnitude than the traditional simple column configurations as well as the systems employing either heat integration or thermal coupling alone. First, a subspace of the possible heat-integrated partially coupled (HIPC) configurations with sharp splits has been identified for a multicomponent distillation. A formula is derived to calculate the number of possible HIPC configurations for any n-component mixture. A simple procedure is given to obtain the practical HIPC configurations for an n-component mixture. Then, the possible thermodynamically equivalent structures of the identified HIPC configurations are presented. The other possible heat-integrated thermally coupled distillation systems involving sloppy splits for an n-component mixture are also discussed. These heat-integrated thermally coupled distillation systems constitute a specific search space to look for the optimal distillation systems for multicomponent separations.

Distillation of Multicomponent Mixtures of Higher Aliphatic Acids in Thermally Coupled Distillation Systems

A method is proposed to calculate the characteristics of distillation of multicomponent mixtures of higher aliphatic acids in distillation systems with reversible mixing of flows and coupled heat flows. The method is based on the use of the Underwood equations and the Thiele–Geddes method for independently determining the concentrations. Application of the method is illustrated by the example of calculating the characteristics of distillation of a mixture of coconut oil fatty acids into four fractions (C6–C10, C12, C14, and C16–C18 acids) in a system containing three columns, one reflux condenser, and two reboilers.

An Alternative Structure of a Fully Thermally Coupled Distillation Column for Improved Operability.

The operation of a fully thermally coupled distillation column (FTCDC) is known to be difficult, which prevents wide application of the column. By separating the main column of the FTCDC into two sections the transfer of material between the two sections is disconnected, while heat transfer is sustained by installing a heat exchanger between the sections as a substitute of a reboiler of the upper section and a condenser of the lower section. It is expected that the modification of the FTCDC improves column operability with the maintained benefit of high thermodynamic efficiency of the original FTCDC.The structural analysis and design detail of the modified column are addressed here, and the design outcome is compared with the original column. In addition, the operability of the new column is examined from the evaluation of various multivariable controllability indices. The comparison of the indices with those of the original column indicates the improved operability.

Control Behaviour of Thermally Coupled Distillation Sequences

The controllability properties of thermally coupled distillation sequences for the separation of ternary mixtures are compared with those of the conventional direct and indirect sequences. Closed loop responses to set point changes were performed, and controllers were tuned to minimize their ISE values. The results indicate that the integrated systems exhibit better control properties than sequences based on conventional distillation columns. This result provides a further incentive for the use of those integrated systems.

Design and Optimization of Fully Thermally Coupled Distillation Columns: Part 2: Application of Dividing Wall Columns in Retrofit

This paper addresses the application of dividing wall columns in retrofit. It emphasizes the need to take maximum advantage of the existing hardware with minimum capital outlay. Based on this study, several practical issues associated with the application of the dividing wall column in retrofit have been identified and as a result, its thermodynamically equivalent arrangements, such as the prefractionator arrangement and the Petyluk column, are often recommended instead. A case study involving the improvement of energy efficiency and capacity expansion of the NGL separation train has been illustrated to demonstrate the analysis involved.

Design and Optimization of Fully Thermally Coupled Distillation Columns: Part 1: Preliminary Design and Optimization Methodology

The design of a fully thermally coupled distillation column, or its thermodynamically equivalent arrangement, the dividing wall distillation column, is more complex than conventional arrangements because of the greater number of degrees of freedom. All of these degrees of freedom must be initialized before rigorous simulation can be performed. The distribution of stages in the various sections of the column, the reflux ratio, vapour and liquid splits on either side of the fully thermally coupled columns and feed condition must all be initialized. Yet these are important degrees of freedom that all interact with each other in the design. A new approach to the design of fully thermally coupled columns is proposed in this paper. The procedure uses the equilibrium stage composition concept developed for the design of azeotropic distillation systems1. The method is semi-rigorous in nature, providing an initial design that is very close to the results of rigorous simulation. The approach then allows the degrees of freedom to be optimized simultaneously and an optimized initial design established for rigorous simulation. A case study has been used to demonstrate the application of the new method.

New Concepts for the Production of High-purity Isobutene in Coupled Reactive Distillation Columns

New Concepts for the Production of High-purity Isobutene in Coupled Reactive Distillation Columns

Structural Design of Extended Fully Thermally Coupled Distillation Columns

A structural design procedure for extended fully thermally coupled distillation columns is proposed and applied to the example quaternary systems of several combinations of relative volatility in order to show the design performance. The procedure gives the optimum structure for a given quaternary separation, and therefore iterative computation encountered in the design using commercial packages is not required. Because of extra degrees of freedom in the design of the columns, various designs can yield the same products for a given specification. The structural design of this study leads to the most efficient design thermodynamically. With the outcome of the structural design, thermodynamic efficiency, arrangement of main and satellite columns, and location of interlinking streams are also examined.

Operational Optimization of Ideal Internal Thermally Coupled Distillation Columns

AbstractLack of the optimal operation parameters in operation is one of major difficulties associated with the use of advanced energy saving distillation methods. In this paper, the operational optimization of the ideal Internal Thermally Coupled Distillation Column (ITCDIC) is considered. An optimization model and the related simulation algorithm are proposed. An optimization and the related result analysis are carried out, which pave the way for further design studies and its practical application.

Rigorous Design of Fully Thermally Coupled Distillation Column.

A rigorous design procedure for a fully thermally coupled distillation column is proposed and applied to an example system of butanol isomer ternary mixture. The design procedure is composed of the calculation of limiting requirements and a rigorous simulation using material and energy balances. The result of the proposed design is compared with the design of a conventional two-column system. It is found that the fully thermally coupled distillation requires less investment and energy cost than conventional distillation, even if higher reboiler temperature is required. It is also pointed out that the dividing wall structure gives less efficient performance than the Petlyuk column having a smaller number of trays of a prefractionator than that of the mid-section of a main column.

A Method to Draw Fully Thermally Coupled Distillation Column Configurations for Multicomponent Distillation

A simple and easy to use stepwise procedure is presented to draw all possible fully thermally coupled distillation column configurations for the separation of an n-component feed mixture into n product streams, each enriched in one of the components. Several new configurations are found for feed mixtures containing four or more components. A recursive formula is derived to calculate the total number of feasible fully thermally coupled configurations for an n-component mixture.

Modeling, Control, and Optimization of Ideal Internal Thermally Coupled Distillation Columns

Internal Thermally Coupled Distillation Columns (ITCDIC) are the frontier of energy saving distillation research. In this paper, the ideal ITCDIC is considered. A novel mathematical model and a related simulation algorithm are proposed. The dynamic responses of open-loop, PID controllers and the responses of closed-loops are carried out. The results show that the ITCDIC is a self-balance process and could be operated smoothly with two PID controllers; the steady-state optimization met the need of ITCDIC optimization. Furthermore, a steady-state optimization model of the operation parameters is presented, which can be used to directly obtain the optimal operation parameters simultaneously guaranteeing not only the product quality and the maximum energy savings but also the dynamic operability and controllability. The benzene-toluene system is studied as an illustrative example.

Modeling, Control, and Optimization of Ideal Internal Thermally Coupled Distillation Columns

Internal Thermally Coupled Distillation Columns (ITCDIC) are the frontier of energy saving distillation research. In this paper, the ideal ITCDIC is considered. A novel mathematical model and a related simulation algorithm are proposed. The dynamic responses of open-loop, PID controllers and the responses of closed-loops are carried out. The results show that the ITCDIC is a self-balance process and could be operated smoothly with two PID controllers; the steady-state optimization met the need of ITCDIC optimization. Furthermore, a steady-state optimization model of the operation parameters is presented, which can be used to directly obtain the optimal operation parameters simultaneously guaranteeing not only the product quality and the maximum energy savings but also the dynamic operability and controllability. The benzene-toluene system is studied as an illustrative example.

Controllability Analysis of Thermally Coupled Distillation Systems

A comparison of the controllability properties of three thermally coupled distillation sequences (Petlyuk, sequence with side rectifier, and sequence with side stripper) using singular value decomposition is developed. Those properties are also compared to the energy consumption required for separating ternary mixtures. The parameters obtained via singular value decomposition show that sequences with a side rectifier or a side stripper have better control properties than the Petlyuk system, although the Petlyuk scheme has lower energy requirements than the systems with side columns.

More Operable Fully Thermally Coupled Distillation Column Configurations for Multicomponent Distillation

More operable fully thermally coupled distillation column configurations do not have the classical problem of controlling vapour transfers from one column to another. A simple and easy to use procedure is presented to draw all possible such distillation column configurations for the separation of an n-component feed mixture into n product streams, each enriched in one of the components. All the more operable fully thermally coupled configurations that can be derived from the known sequential and satellite column configurations are illustrated for four and five component feed mixtures. A brief discussion for the distillation of an n-component feed mixture in a single column shell is also presented.

Optimal Design of Thermally Coupled Distillation Columns

This paper considers the optimal design of thermally coupled distillation columns and dividing wall columns using detailed column models and mathematical optimization. The column model used is capable of describing both conventional and thermally coupled columns, which allows comparisons of different structural alternatives to be made. Possible savings in both operating and capital costs of up to 30% are illustrated using two case studies.