Extractive Dividing-wall Column Research Articles

Novel Heat-Pump-Assisted Extractive Distillation for Bioethanol Purification

The purification of bioethanol fuel involves an energy-intensive separation process to concentrate the diluted streams obtained in the fermentation stage and to overcome the azeotropic behavior of the ethanol–water mixture. The conventional separation sequence employs three distillation columns that carry out several tasks, penalized by high-energy requirements: preconcentration of ethanol, extractive distillation, and solvent recovery. To solve this problem, we propose here a novel heat-pump-assisted extractive distillation process taking place in a dividing-wall column (DWC). In this configuration, the ethanol top vapor stream of the extractive DWC is recompressed from atmospheric pressure to over 3.1 bar (thus to a higher temperature) and used to drive the side reboiler of the DWC, which is responsible for the water vaporization. For a fair comparison with the previously reported studies, we consider here a mixture of 10 wt % ethanol (100 ktpy plant capacity) that is concentrated and dehydrated using ethylene glycol as mass-separating agent. Rigorous process simulations of the proposed vapor recompression (VRC) heat-pump-assisted extractive DWC were carried out in AspenTech Aspen Plus. The results show that the specific energy requirements drop from 2.07 kWh/kg (classic sequence) to only 1.24 kWh/kg ethanol (VRC-assisted extractive DWC); thus, energy savings of over 40% are possible. Considering the requirements for a compressor and use of electricity in the case of the heat-pump-assisted alternative, it is possible to reduce the total annual cost by approximately 24%, despite the 29% increase of the capital expenditures, for the novel process as compared to the optimized conventional separation process.

Control analysis of an extractive dividing-wall column used for ethanol dehydration

This paper deals with design and control of an extractive dividing-wall distillation column (EDWC) for ethanol dehydration using ethylene glycol as entrainer. An initial design, based on a section analogy procedure for a conventional extractive distillation sequence, was obtained and then used in an optimization process to minimize the total annual cost. It was shown that the EDWC can result in significant savings over the conventional process. As these savings sometimes go along with a decrease in the control properties, an investigation of two control structures for the EDWC and one for the conventional column configuration was performed next.It was observed in closed-loop simulations that the EDWC with an appropriate structure exhibited good control properties and that its closed-loop responses were similar to those obtained for the operation of a conventional extractive distillation system.

Design and Control of Extractive Dividing Wall Column for Separating Benzene/Cyclohexane Mixtures

This paper considers the design and control of the separation of benzene and cyclohexane process via extractive distillation in a dividing wall column. To aid the separation, furfural is used as the heavy boiling entrainer. The optimal flow sheet with minimum energy requirements has been established using the multiobjective genetic algorithm with constrains. Three control strategies are proposed: the basic control strategy uses four composition controllers, and two improved control strategies with and without vapor split ratio use temperature controllers that are more practical in application than the basic control strategy. The dynamic simulations reveal that the three control strategies can object to the feed disturbances well. The dynamic responses of two improved control structures show that vapor split ratio as a manipulated variable can maintain the product purities at their set points when a feed disturbance is introduced.

Design and simulation of ethane recovery process in an extractive dividing wall column

Abstract Separation of CO2 from hydrocarbons in the natural gas is complicated due to the existence of an azeotrope between ethane and CO2 at the cryogenic temperatures. The key issues to break this azeotrope are high investment costs for the unit equipments and the associated high energy requirements. Accordingly, an innovative process based on the dividing-wall column (DWC) technology is designed using short-cut methods and relevant rigorous simulations. The energy demand and some environmental factors such as CO2 removal efficiency and CO2 emission reduction are studied for the conventional and DWC processes. It is found that the process including DWC is a better choice than the conventional one from economical and environmental point of views. Remarkably, this technology reduces the energy demand up to 51.6%.

Energy optimization and comparative study of pre- and post-fractionator extractive dividing wall column for the CO2–ethane azeotropic process

Two possible extractive dividing-wall column (DWC) arrangements are explored to find the potential benefits derived from thermally coupled distillations in separation of a mixture including the CO2–ethane azeotrope with a low boiling point. It is shown that the process including pre-fractionator in the DWC design in its optimized state leads to 51.6% reduction in total duties in comparison with the conventional process. Furthermore, a comparison between conventional extractive distillation columns and the new DWC process is made in terms of cost estimation, CO2 removal efficiency and CO2 emission reduction. Remarkable, the results clearly show that DWC process is interesting/feasible and the novel proposed DWC alternative reduces the steam requirements by 41% and the equipment costs by 31%.

Design and Control of Extractive Dividing-Wall Column for Separating Ethyl Acetate–Isopropyl Alcohol Mixture

An extractive dividing-wall column (EDWC) is simulated as a three-column system in this work. The design and control of the EDWC to separate ethyl acetate and isopropyl alcohol using ethylene glycol as the entrainer is studied in Aspen Plus and Aspen Dynamics. The optimum EDWC design with minimal total annual cost is screened first; then sensitivity analysis is conducted to investigate whether the entrainer flow rate and vapor split ratio can be used as control variables to hold the purity specifications; finally, three control structures for the EDWC are established, and the dynamic results of these control structures are compared. It is found that two improved control structures achieve satisfactory performance with much smaller deviation and shorter settling time by comparison to the basic control structure. Dynamic results also revealed that it is useful to adjust the entrainer flow rate or vapor split ratio to hold the purity specifications.

Temperature Control for Extractive Dividing-Wall Column with an Adjustable Vapor Split: Methylal/Methanol Azeotrope Separation

In our previous work, the extractive dividing-wall column (EDWC) was proved to be a promising energy-saving method for methylal/methanol azeotrope separation because it offered about an 8.3% energy saving over the corresponding conventional extractive distillation columns. We also have explored two effective composition control structures for the stabilization of the EDWC previously. However, the composition control needs online composition analyzers which are usually expensive, require high maintenance, and introduce large lags. Therefore, it is highly desirable to avoid the use of composition analyzers to economically gain this energy-saving potential. In this work, the use of temperature control instead of composition control for the EDWC process is investigated to overcome the above drawbacks. Several control structures are developed on the basis of singular value decomposition (SVD) analysis. Two temperature control structures with an adjustable vapor split are proposed first. Dynamic simulations sho...

Critical Assessment of the Energy-Saving Potential of an Extractive Dividing-Wall Column

In this paper, the energy-saving potential of an extractive dividing-wall column (EDWC) is investigated. One potential drawback on the EDWC design is that two reboilers in the original design need to be combined into one reboiler. Since a heavy entrainer is used in the extractive distillation system, often cases show that the total reboiler duty is reduced but with adversely increasing the total steam cost. Three industrial separation systems of isopropyl alcohol-water, dimethyl carbonate-methanol, and acetone-methanol have been used as demonstrating examples for critical assessment of the energy-saving potential of the EDWC design. It is found that although the savings of the overall reboiler duty can be made by using the EDWC design, only the acetone-methanol system using water as an entrainer actually saves on the steam cost. The control performance of the EDWC design is also hampered because of losing one important control degree-of-freedom.

Energy saving methods for the separation of a minimum boiling point azeotrope using an intermediate entrainer

In this work two energy saving methods are presented for the separation of a minimum boiling point azeotrope by using an intermediate entrainer. Extractive distillation column configurations with conventional, thermally integrated and dividing-wall columns are used for comparison. These configurations are designed, rigorously simulated and optimized to minimize the Total Annual Cost and to reduce the energy requirement. A novel simple method is proposed based on a Genetic Algorithm to determine the main design and operation parameters of the dividing-wall column. The optimization algorithm is written in Excel Macro interconnected with the ChemCad process flow-sheet simulator. In terms of total annual costs, the extractive dividing-wall column was found to be the most economical scheme, followed by the thermally integrated and conventional extractive distillation methods.

Design and Control of Extractive Dividing-Wall Column for Separating Methylal–Methanol Mixture

The design and control of extractive dividing-wall columns (EDWCs) have not yet been investigated, whereas those of dividing-wall columns (DWCs) and conventional extractive distillation columns (CEDCs) have been studied by many researchers. The control of EDWCs is more difficult than that of CEDCs and somewhat different from that of DWCs because of differences in the inner structure and interactions among control loops in EDWCs. In this work, the design and control of an EDWC for methylal/methanol separation are investigated using commercial chemical simulators (Aspen Plus and Aspen Dynamics). The optimum EDWC design based on the total annual cost (TAC) is screened first; then, the interaction between design and control is observed; and finally, two control structures for the EDWC are presented. The first proposed basic control structure featuring four composition controllers and an adjustable vapor split ratio αV can handle all disturbances well with small offsets in product purities, except for a 20% de...

Innovative single step bioethanol dehydration in an extractive dividing-wall column

The large-scale production of bioethanol fuel requires energy demanding distillation steps to concentrate the diluted streams from the fermentation step and to overcome the azeotropic behavior of the ethanol–water mixture. The conventional separation sequence consists of three distillation columns performing several tasks with high energy penalties: pre-concentration of ethanol, extractive distillation and solvent recovery. Despite the novel recent developments in pervaporation and adsorption with molecular sieves, the industrial production of anhydrous bioethanol is still dominated by extractive distillation as the separation method of choice.This study proposes an innovative distillation setup – based on a novel extractive dividing-wall column (E-DWC) – that is able to concentrate and dehydrate bioethanol in a single step, by integrating all units of the conventional sequence into only one distillation column. In this work, a mixture of 10wt% ethanol (100ktpy plant) is concentrated and dehydrated using ethylene glycol as mass separating agent. Rigorous simulations were carried out in Aspen Plus, and for a fair comparison all alternatives were optimized using the reliable sequential quadratic programming (SQP) method. The results show that energy savings of 17%, and a similar decrease in CapEx, are possible for the novel E-DWC alternative, while using a significantly reduced footprint as compared to the conventional separation process.

Extractive Dividing Wall Column: Design and Optimization

This paper proposes a novel extractive dividing wall distillation column, which has been designed using a constrained stochastic multiobjective optimization technique. The approach is based on the use of genetic algorithms to determine the design that minimizes energy consumption and total annualized cost. Several case studies are used to show the feasibility of performing extractive separations in dividing wall distillation columns. The simulation results show the effect of the main variables on the complex extractive distillation process.