Coupled Distillation Research Articles

Thermodynamics fundamentals and energy efficiency for the separation and high-valued utilization of Fischer–Tropsch heavy oil

The development trend of Fischer–Tropsch (F–T) technology is to develop high value-added products. The separation of linear α-olefins with low cost is an effective method. Nevertheless, the lack of thermodynamic data and the huge energy consumption are the two main problems restricting the development of the separation process. The thermodynamic data of the key components (1-dodecene and n-dodecane) in the F–T product were measured. The Wilson binary interaction parameters of the key components were obtained. Next, one traditional distillation column sequence and two dividing wall column (DWC) sequences were designed to separate the F–T heavy oil to obtain narrow fractions with different carbon numbers. Then, the obtained fractions of C10 and C12 were simulated to obtain 1-decene and 1-dodecene, respectively. There was a traditional distillation and a differential pressure thermal coupling distillation process. When separating 95.0% purity 1-decene and 1-octene, the direct DWC process and differential pressure thermal coupled distillation are an excellent combination, which can reduce the energy by 33.1% (i.e., 11,286 kW) and total annual cost by 15.9% (i.e., 3.96 × 106 $) compared with traditional distillation.

Study on the Simulation Control of Neural Network Algorithm in Thermally Coupled Distillation

Thermally coupled distillation is a new energy-saving method, but the traditional thermally coupled distillation simulation calculation process is complicated, and the optimization method based on the traditional simulation process is difficult to obtain a good feasible solution. The neural network algorithm has the advantages of fast learning and can approach nonlinear functions arbitrarily. For the problems in complex process control systems, neural network control does not require cumbersome control structures or precise mathematical models. When training the network, only the input and output samples it needs are given, so that the dynamics of the system can be controlled. In this way, the dynamic performance of the system can be approximated. This method can effectively solve the mathematical model of the thermally coupled distillation process, and quickly obtain the solution of the optimized variables and the objective function. This article summarizes the research progress of artificial neural network and the optimization control of thermally coupled distillation and the application of neural network in thermally coupled distillation.

Model Predictive Control of a High‐Purity Internal Thermally Coupled Distillation Column

AbstractThe energy‐saving potential of the internal thermally coupled air separation column (ITCASC) is well‐established, but distinct dynamic characteristics and control loop interactions make it inflexible to control. To take care of high‐purity ITCASC control complications, a state‐space model predictive control (MPC) was formulated. A direct finite‐horizon control approach was exploited to align the dynamic states with the model predictions. MPC‐I and MPC‐II were developed, and further compared to a previous adaptive multivariable generalized prediction control (AM‐GPC). The results obtained show that the control performance of the proposed MPC‐II is superior to that of MPC‐I and AM‐GPC.

Use of Partially Thermally Coupled Distillation Systems and Heat Pumps for Reducing the Energy Consumption in the Extractive Distillation of an Isobutanol–Isobutyl Acetate Mixture Using Dimethylformamide

The extractive distillation of an isobutanol–isobutyl acetate mixture using dimethylformamide has been considered. Schemes that use vapor recompression heat pumps and partially thermally coupled distillation systems both without and with a heat pump are considered in comparison with the conventional schemes. It is shown that the use of heat pumps in the process under investigation allows for a considerable reduction in the energy consumption and total annual costs.

Energy-Efficient Design of an Ethyl Levulinate Reactive Distillation Process via a Thermally Coupled Distillation with External Heat Integration Arrangement

In this paper we describe an energy-efficient process design using a reactive distillation (RD) of the esterification reaction for the synthesis of ethyl levulinate (LAEE). Two designs, a neat design and an excess design, were examined to find the best configuration for the process. In the neat design, there are two columns with equal molar feeds to produce LAEE. An additional column is required in the excess design to separate LAEE from the unreacted reactant (levulinic acid, LA). Later in the process, this unreacted LA is recycled back into the RD column. Compared to the neat design, the excess design showed superiority for the esterification process in terms of the total annual cost (TAC) and energy requirement. Therefore, three energy integration configurations, external heat integration (HI), thermally coupled distillation (TCD), and a combination of TCD and HI, were investigated in this excess design. A side reboiler was implemented in the water removal column to overcome the limitation of the tempe...

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.

Separations of Different Binary Hydrocarbon Mixtures Using Pressure Swing Thermally Coupled Distillation Process

This paper presents a novel distillation and separation process: Pressure Swing Thermally Coupled Distillation, and investigates the performance of the process by separating 24 different kinds of binary hydrocarbon mixtures with different boiling point differences by this process, and calculates their cost-saving efficiency in comparison with conventional distillation column, respectively. This work focuses on the comparison of cost-saving efficiency, and adopts the comparison methods of standard coal equivalent and total annul cost calculation. Based on the comparison results, this process is optimized and confirmed that it is more cost-saving than conventional distillation and even heat-pump distillation for the separation of binary systems; therefore, this process will increase distillation efficiency greatly and save much energy for all the world, and thus it is worth considering the practical application and this paper will supply valuable guidelines and simulation calculation basis for industrial application.

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.

New Design Method for Fully Thermally Coupled Distillation Column Using Group and Approximate Methods

A new design method for determining the structure of a fully thermally coupled distillation column is proposed. The method involves approximate group method and the Fenske equation. To design the prefractionator, an approximate calculation procedure that relates compositions of multicomponent vapor and liquid streams is developed to formulate the expressions used to compute the number of stages. To investigate the usability and performance of this design method, different feed mixtures are tried. The results indicate that the method works well for a variety of process conditions; the results show that the structure design from group method leads to a process similar to a feasible actual process. The structure design results and operating conditions for the whole column system enable the examination of the optimal internal flows and their effects on the structure design equations. The results show that the proposed design method provides good initial values for rigorous simulation.

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.

Design and Optimization of Fully Thermally Coupled Distillation Scheme for the Naphtha Splitter Process

Naphtha feed stock is separated into light naphtha, heavy naphtha and light kerosene in sequence. Systems and methods of naphtha feed stock are destined for a conventional distillation sequence and the other applications with less energy consumption than conventionally possible, while producing less greenhouse gas emissions. In this work, the comparison between the conventional sequence and the fully thermally coupled distillation column or so-called dividing wall column was carried out based on the minimum vapor flow rate at infinite number of stages. The evaluation results showed that the dividing wall column can be effectively used to improve the performance of the naphtha splitter process. In this paper, the pseudo-components for defining a liquid fraction have been described through using a commercial process simulator. A response surface methodology was applied to the design of the dividing wall column. A less number of facilities are provided and correspondingly less capital investment as well as reduced operating cost can be achieved. Reducing CO2 emission is another benefit which is derived from the proposed sequence.

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

Dynamic Simulation of Petlyuk Column for Separation of Ternary Mixtures

The Petlyuk distillation configuration has received considerable attention because of its efficiency to reduce the energy required for the separation of ternary mixtures. The dynamic properties of fully thermal coupling distillation column, which separates ternary mixtures of H2O, ethylene glycol (EG), diethylene glycol (DEG), are analyzed. The possible control configurations were investigated using Relative Gain Array (RGA) Analysis and dynamic simulation. The results indicate that the provided control scheme is able to produce a stable control response for thermally coupled distillation systems and shows better dynamic responses than the conventional distillation sequence.

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...

Nonlinear Model Predictive Control Based on Wave Model of High-Purity Internal Thermally Coupled Distillation Columns

Although the internal thermally coupled distillation column (ITCDIC) is much more energy-efficient than the conventional distillation column, the complex dynamics stemming from thermal coupling between the rectifying column and the stripping column have restrained the achievement of control strategies and application in chemical industry. To reduce the difficulties, a novel nonlinear wave model of ITCDIC process is proposed based on the wave theory in the separation processes to simplify the mechanism model of ITCDIC. A nonlinear model predictive control based on the proposed wave model (NWMPC) of the ITCDIC process is further established to handle the multivariable interactions and improve the control performance. A dynamic optimization scheme characterized by an improved control parametrization method is introduced to solve the problem generated by the NWMPC model. An IMC controller is also explored as a comparison research. In the example of a benzene–toluene system, NWMPC presents great advantage over...

The Study of Energy Savings of Separation Quaternary Mixture with Thermally Coupled Distillation

The thermally coupled distillation configuration (TCDS) could reduce energy consumption and capital investment compared with conventional distillation sequences. But their use was still limited. The five kinds of conventional distillation sequences and five kinds of thermally coupled distillation sequences were performed in separation of quaternary mixtures with distillation rigorous simulations. The energy savings of a series of thermally coupled distillation sequences were compared. The results showed that the feed composition and ease of separation of the composition have great effect on the choice of the thermally coupled sequences. In the most cases, the TCDS with side rectifier or striper had the more energy saving effect.

Shortcut Design of Fully Thermally Coupled Distillation Systems with Postfractionator

A modification to the fully thermally coupled distillation system, or Petlyuk column, in which a postfractionator is added has recently been considered. The modified structure incorporates the post...

Reducing Energy Consumption in Propylene-Propane Separation with Heat Coupled Distillation

The heat coupled distillation column which applies the principle of internal heat integration between the rectifying section and the stripping section of a distillation column by increasing the temperature of the rectifying section with a compressor attracts extensive attention worldwide for its high thermodynamic efficiency. In this study, the economic features of a heat coupled distillation column (HCDC) are investigated and compared to a conventional column and a commercial column with vapor recompression system (VRC) for the separation of propylene-propane mixture. The economic analysis indicates that HCDC has 10 – 20 % total annual cost (TAC) saving associated with VRC, which appeared to be strongly sensitive to the pressure ratio.

Sensitivity Analysis of an Energy-Efficient Heat Coupled Distillation Column

In this paper, an energy-efficient heat coupled distillation column (HCDC) which is industrially viable is constructed for propylene–propane mixture separation and the sensitivity analysis was performed. Some comparative studies are made with conventional distillation column (CC) and a column with a vapor recompression system (VRC). The results show that the thermal efficiency of the HCDC appears to be strongly sensitive to the compression ratio between rectification section and stripping section.

Design and Control of a Fully Thermally Coupled Distillation Column Modified from a Conventional System

AbstractA modified fully thermally coupled distillation column is proposed, with utilization of the existing distillation columns of the conventional system, and its control scheme is suggested here. The proposed distillation system is applied to a benzene‐toluene‐xylene (BTX) separation process, of which the system design and control performance evaluation are conducted using the HYSYS software. The performance of the suggested 3 × 3 control is examined in the set‐point tracking of product specification and the regulation for the changes of feed flow and composition. The pairings of three proportional‐integral control loops are the reflux flow and the specification of overhead product, the prefractionator vapor flow and that of the side product, and the vapor boil‐up rate and that of the bottom product. The multi‐variable controllability using various indices is investigated for the proposed control scheme, and the controllability is compared with that of the cross‐pairing between the control loops of the side and bottom products.