Coupled Distillation Sequences Research Articles

Synthesis of heat-integrated distillation sequences with mechanical vapor recompression by stochastic optimization

A method of synthesis of heat-integrated distillation sequences with mechanical vapor recompression (MVR-HIDSs) is proposed based on stochastic optimization in this study. The purpose is to introduce heat pump operation, which can be driven by green electricity, to distillation sequencing as a way of altering stream temperatures to increase the chance of heat integrations. Two types of MVR-HIDSs, namely the vapor recompression heat-integrated distillation sequences (VR-HIDSs) and bottom flashing heat-integrated distillation sequences (BF-HIDSs), are considered for the synthesis separately. To illustrate the proposed method, a case for separating five-component alcohol mixture is solved by the proposed method. The results show that the incorporation of the MVR can provide effectively higher economic efficiency compared with the traditional optimal heat-integrated and thermal coupled distillation sequences. The proposed method makes it possible to synthesize distillation sequences that use green electricity to partially replace the fossil fuel based thermal energy to reduce CO2 emission.

An inherently safer development approach for thermally coupled distillation sequences: Application in hazardous chemical separation

Process safety is always a core issue in chemical industry, which is conventionally not taken into consideration during the conceptual design stage of a process flowsheet. Distillation handles more than ninety percent of the separation and purification tasks of hazardous chemicals and accounts for no less than forty percent of the total energy consumption. Therefore, this work presents an inherently safer synthesis framework for thermally coupled distillation sequences with structure constraints, which can efficiently quantify and optimize the inherent safety index of each column configuration in the sequence, based on State-Task Network and Dow’s F&EI. To efficiently quantify the inherent safety index of each column configuration, a series of structure constraints are proposed. Within the superstructure, Hengstebeck-Underwood-Gilliland shortcut method is employed for distillation column design. Finally, two hazardous zeotropic mixture separation examples are performed to demonstrate the feasibility and validity of the proposed synthesis methodology.

Optimal modes of side-section flow in heat-pump-assisted extractive distillation systems for separating allyl alcohol–allyl acetate mixtures with butyl propionate

Objectives.To investigate the influence of side-section flow modes on the energy efficiency of apartially thermally coupled distillation sequence (PTCDS) with a vapor recompression heat pumpfor the extractive distillation of an allyl alcohol–allyl acetate mixture with n-butyl propionate andidentify modes under which the combined use of a PTCDS and heat pump are the most efficient.Methods.Mathematical modeling in the Aspen Plus V10 software package was used as the mainresearch method. The local composition equation of the non-random two-liquid model was usedas a model for describing the vapor–liquid equilibrium, while the Redlich–Kwong model was usedto consider the non-ideal vapor phase. When modeling the conventional extractive distillationscheme and PTCDS, parametric optimization was carried out according to the criterion of the totalenergy costs in the column reboilers. For the economical evaluation, Aspen Process EconomicAnalyzer V10.1 tools were used.Results.For extractive distillation of a mixture of allyl alcohol (30 wt %) and allyl acetate (70 wt %)with n-butyl propionate as an entrainer, the minimum energy consumption was achieved at thesame side-section flow mode for the variants of a PTCDS with and without a heat pump. Thereduction in energy costs relative to the conventional scheme was 20% for the sequence withouta heat pump and 38% for that with a heat pump. An economic assessment was made of thebest options in comparison with the conventional extractive distillation scheme. The PTCDS witha heat pump had an advantage over the sequence without a heat pump only for long periods ofoperation.Conclusions.For the extractive distillation of an allyl alcohol–allyl acetate mixture, the optimalmodes for the combined use of a PTCDS with a vapor recompression heat pump coincide with theoptimal modes for a PTCDS without a heat pump.

A novel stochastic optimization method to efficiently synthesize large‐scale nonsharp distillation systems

AbstractThis study presents a novel stochastic optimization method for the efficient synthesis of large‐scale nonsharp distillation systems, where heat integration and thermal coupling can be involved simultaneously. A new binary tree encoding method was developed to represent distillation sequences with no limits on the number of middle components in nonsharp splits to ensure a complete solution space. Thermally coupled structures were defined by 0–1 binary variables. Evolutionary rules were developed to generate neighboring distillation configurations randomly. Finally, an optimization framework was proposed, where simulated annealing (SA) algorithm was used to optimize distillation configurations; for a certain distillation configuration randomly generated by SA, its continuous variables were optimized using particle swarm optimization algorithm. Four cases—including the synthesis of six‐ and seven‐component nonsharp heat integrated and thermally coupled distillation sequences—were studied to demonstrate that the proposed method was efficient and could obtain optimal and valuable suboptimal solutions with high probabilities.

Optimal configuration of ternary distillation columns using heat integration with external heat exchangers

The subject of energy saving in distillation column sequencing is of critical importance. Heat integration in a multicomponent separation can be industrialized by saving considerable energy and cost. In this work, external heat-integrated distillation column with external heat exchanger has been studied and the annual cost function has been optimized using Genetic Algorithm. Introducing the layout and binary matrices enabled the investigation of all possible locations for the heat exchanger arrangement successfully. Moreover, exchangers heat loads and compressors pressure, have also been considered as optimization variables. Benzene, toluene, xylene and n-alkanes, separations have been studied as case studies. It has been demonstrated that the proposed optimization method in the alkane separation case decreased the total annual cost by 22.6% in comparison with the proposed thermally coupled distillation sequence columns. The heat integration of external heat exchangers in an external heat-integrated distillation column configuration and the proposed dived-wall column resulted in decreasing the total annual cost by 17% and 39% respectively, in comparison with conventional distillation columns.

Synthesis of simultaneously heat integrated and thermally coupled nonsharp distillation sequences based on stochastic optimization

Synthesizing simultaneously heat integrated and thermally coupled distillation sequences (HITCDSs) is a complex combinatorial optimization problem and it is difficult to solve this problem, especially when considering nonsharp splits. Based on stochastic optimization, we propose an efficient approach to synthesize nonsharp HITCDSs. First, an implicit mathematical formulation for the HITCDS optimization is presented. Next, encoding strategies are used tactfully to represent HITCDS structures and evolving rules are developed to generate neighboring HITCDS structures. Finally, simultaneously optimizing the distillation sequences, thermal coupling structures, heat integrated networks, and operating parameters of the columns is easily attained through an improved simulated annealing algorithm. Three cases are studied to validate the high solution efficiency of the approach. The optimization results show that nonsharp HITCDSs are more energy efficient than the sharp ones. Besides, HITCDSs have lower total annual cost than conventional distillation configurations, configurations only employing heat integration, and configurations only employing thermal coupling.

Sustainable production of renewable aviation fuel through intensification strategies

In the transportation industry, aviation is one of the most dynamic sectors. The forecast indicates that aviation will grow at 4.8% until 2036, which implies that fuel requirements will double along with CO2 emissions. Therefore, the sustainable development of aviation sector faces great challenges and relies on a four-pillar strategy, which includes increasing engine efficiency, optimization of flight trajectories, and alternative fuels. Renewable jet fuel, also known as biojet fuel, consists of hydrocarbons in the range of boiling point of fossil jet fuel, C8-C16, obtained from the biomass processing. There are different pathways to produce biojet fuel, being the hydroprocessing of triglyceride feedstock one of the most reliable and advanced. The hydrotreating process consists of three main steps: hydrodeoxygenation, cracking and isomerizing, and finally purification; also, it allows obtaining biojet fuel with the required properties from different renewable oils. Nevertheless, it has several opportunity areas to decrease energy consumption and environmental impact.Therefore, in this work an analysis of hydrotreating processes to produce biojet fuel is presented in terms of energy consumption, total annual costs, and CO2 emissions. Different renewable raw materials of high productive potential in Mexico are considered; along with the implementation of process intensification strategies for the separation train. Results show that the use of thermally coupled distillation sequences allows designing sustainable processes for the production of biojet fuel economically competitive with fossil jet fuel.

Multiplicities in dividing wall distillation columns in the purification of bioethanol: energy considerations

In this paper, we present results about the energy requirements in the reboilers of an extractive dividing wall distillation column and a thermally coupled distillation sequence with a side rectifier for bioethanol dehydration using glycerol as entrainer, and the study generates the appearance of multiple steady states. During the energy optimization of the dividing wall distillation column, using glycerol as entrainer, the recycle streams were varied until the energy consumption in the reboiler was obtained and the multiple steady states can present a difference of up to 400%. This finding is important since we are interested in detecting the optimal energy consumption in order to reduce the environmental impact caused by the usage and production of energy from petroleum. This information is being used in a current pilot plant in order to detect the optimal operation conditions to dehydrate bioethanol.

Controllability Analysis of Distillation Sequences for the Separation of Bio‐Jet Fuel and Green Diesel Fractions

AbstractThe aviation sector is nowadays looking for a biofuel to partially substitute the fossil fuel used for flights, i.e., jet fuel. Among the proposed processes for the production of the renewable jet fuel, the hydrotreating process has, to date, the best performance. At the final stage of the hydrotreating process, a distillation train is required to obtain hydrocarbon fractions useful as fuels. Conventional or thermally coupled distillation sequences can be used in this step, and it can be expected that thermally coupled sequences require less energy to perform the separation. Nevertheless, in the selection of the best sequence for a given separation, it is important to analyze its control properties. A controllability analysis was performed here for different distillation sequences used to obtain biofuels from a hydrocarbon renewable mixture, in order to detect the best sequence in terms of control properties.

Simultaneous energy integration and intensification of the hydrotreating process to produce biojet fuel from jatropha curcas

The sustainable development of aviation sector relies on four strategies, being the use of biojet fuel as the most promising. There are different processes to produce biojet fuel; however, the hydrotreating process is certified by ASTM for its use in commercial and passenger flights. This process has several opportunity areas to decrease energy consumption and environmental impact. In this work, we propose the simultaneous energy integration and intensification of the hydrotreating process to produce biojet fuel from jatropha curcas oil. The released energy by the hydrodeoxygenation reactor is used to perform the energy integration of the process. Moreover, the separation zone of the process is intensified through thermally coupled distillation sequences, which are optimized with a multiobjective stochastic strategy. Results show that it is possible to reduce significantly the energy requirements of the process when energy integration is performed; however, the decreasing in the service costs is accompanied by an increasing in the equipment costs. On the other hand, the intensification of the separation zone does not lead to a decreasing in energy consumption. Therefore, net effect of both strategies on the total annual cost and biojet fuel prices is small, but significant decreasing in CO2 emissions is achieved.

Energy Recovery in a Naphtha Splitter Process through Debottlenecking of Retrofitted Thermally Coupled and Double-Effect Distillation Sequences

A thermally coupled distillation sequence (TCDS) and a double-effect sequence (DES) were proposed to retrofit an existing naphtha splitter process in the refinery plant for enhanced energy efficiency. This, however, can create a bottleneck in the columns. A side reboiler was proposed to debottleneck the retrofitted columns. The concept of minimum vapor flow rate was used as a preliminary calculation to determine the benefits of these proposed arrangements. A practical optimization using the response surface methodology was applied to the design of TCDS and DES. In addition, a column grand composite curve was used to highlight the thermodynamic feasibility of the implementation of a side reboiler into the retrofitted sequences. As a result, the DES was shown as the best alternative through combination of a side reboiler, and this achieved 32.7 % savings on operating costs with a maximum reuse of the existing equipment.

A hybrid technology combining heat pump and thermally coupled distillation sequence for retrofit and debottlenecking

Increasing the capacity of an existing distillation process has been a major focus of the chemical process industry. On the other hand, entrainment flooding can occur as a result, which can create a bottleneck in the distillation process. This paper reports the results of a techno-economic feasibility study to debottleneck the distillation column using a proposed hybrid process combining a heat pump and thermally coupled distillation sequence. Fractional utilization of the area was used to identify flooding problems in the column as well as how much area is available for vapor flow on an existing stage. A heat pump aided thermally coupled distillation sequence (HPTCDS) was designed and optimized using a response surface methodology. Two cases were examined to test the proposed sequence. The results showed that the proposed sequence can achieve significant energy savings and remove the bottleneck problem.

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.

Retrofit and Debottlenecking of Naphtha Splitter Process to Thermally Coupled Distillation Sequence with a Side Reboiler

The naphtha splitter process is a representative distillation processused in the refinery industry. To improve the fractionation of the naphtha boiling range derived from hydro-treatment, naphtha feedstock is sequentially separated into light naphtha, heavy naphtha, and light kerosene. Conventional naphtha splitting columns can be retrofitted to an advanced configuration requiring less energy consumption with consequently less CO2 emission. A systematic design for retrofitting naphtha splitting columns for the thermally coupled distillation sequence (TCDS) that addresses the bottleneck phenomenon by integration of existing shells was developed in this study. The results showed that the operating cost could be reduced drastically by novel combination of the TCDS and a side reboiler with minimal process modification.

Review of Retrofitting Distillation Columns Using Thermally Coupled Distillation Sequences and Dividing Wall Columns to Improve Energy Efficiency

Review of Retrofitting Distillation Columns Using Thermally Coupled Distillation Sequences and Dividing Wall Columns to Improve Energy Efficiency

A novel distillation design for vapor phase dimethyl ether production process

ABSTRACTThe vapor phase dimethyl ether (DME) synthesis is simulated by hysys process simulation software, and relevant cost analysis is also conducted. Based on cost estimation results, it is found that capital investment of the classic DME process is greatly influenced by the distillation towers and operating costs. Accordingly, to solve these problems, an innovative DME process based on a thermally coupled distillation sequence in vapor phase is proposed in this work. The results clearly demonstrated that the novel proposed process leads to 47.9% and 39.7% reductions in operating and capital costs as compared with the conventional DME process, respectively. © 2013 Curtin University of Technology and John Wiley & Sons, Ltd.

Optimal synthesis of thermally coupled distillation sequences using a novel MILP approach

This paper introduces a novel MILP approach for the design of distillation columns sequences of zeotropic mixtures that explicitly include from conventional to fully thermally coupled sequences and divided wall columns with a single wall. The model is based on the use of two superstructure levels. In the upper level a superstructure that includes all the basic sequences of separation tasks is postulated. The lower level is an extended tree that explicitly includes different thermal states and compositions of the feed to a given separation task. In that way, it is possible to a priori optimize all the possible separation tasks involved in the superstructure. A set of logical relationships relates the feasible sequences with the optimized tasks in the extended tree resulting in a MILP to select the optimal sequence. The performance of the model in terms of robustness and computational time is illustrated with several examples.

Debottlenecking the Retrofitted Thermally Coupled Distillation Sequence

The thermally coupled distillation sequence (TCDS) has attracted increasing interest in a range of chemical processes because of its potential energy and capital cost savings in multicomponent separations. In addition, the TCDS has attracted particular attention in retrofit projects because of its lower energy requirements compared to the existing conventional column sequence, easy design, and small modification. This, however, can create a bottleneck in the column. The main aim of this study was to identify bottlenecks in a retrofitted TCDS systematically using a hydraulic performance indicator and fractional utilization of area (FUA), and to propose a novel strategy for the use of a side reboiler or a side condenser for debottlenecking of the column. A practical method using the response surface methodology (RSM) is proposed for TCDS design and optimization. The optimum TCDS structure can be observed in a practical manner while minimizing the simulation runs. The use of a side reboiler or a side condens...

Hybrid Distillation/Melt Crystallization Process Using Thermally Coupled Arrangements: Optimization with evolutive algorithms

Innovative hybrid processes offer significant cost savings, particularly for azeotropic or close-boiling mixtures. Hybrid separation processes are characterized by the combination of two or more different unit operations, which contribute to the separation task by different physical separation principles. Despite of the inherent advantages of hybrid separation processes, they are not systematically exploited in industrial applications due to the complexity of the design and optimization of these highly integrated processes. In this work we study a hybrid distillation/melt crystallization process, using conventional and thermally coupled distillation sequences. The design and optimization were carried out using, as a design tool, a multi-objective genetic algorithm with restrictions coupled with the process simulator Aspen Plus™, for the evaluation of the objective function. The results show that this hybrid configuration with thermally coupled arrangements is a feasible option in terms of energy savings, capital investment and control properties.

Alternativas de control para la secuencia de destilación térmicamente acoplada Petlyuk

El estudio presenta el análisis de la dinámica a lazo cerrado de la secuencia de destilación térmicamente acoplada (sistema Petlyuk) para la separación de mezclas ternarias. Una configuración de tres puntos de control basados en mecanismos clásicos Proporcional- Integral (PI) y su contraparte, Controladores de Modelado Interno No Lineales (IMCNL, por sus siglas en inglés), se implementaron en el sistema Petlyuk. La comparación entre los controladores clásicos PI y los IMCNL se llevó a cabo para analizar el desempeño de una nueva propuesta de control con base en cambios en la composición de los diferentes flujos de productos. Los resultados muestran que la respuesta a lazo cerrado del sistema Petlyuk mejora considerablemente con los controladores IMCNL, tanto para el flujo en la parte inferior como en la superior.