Conventional Distillation Column Research Articles

Methyl lactate synthesis using batch reactive distillation: Operational challenges and strategy for enhanced performance

Batch reactive distillation is well known for improved conversion and separation of desired reaction products. However, for a number of reactions, the distillation can separate the reactants depending on their boiling points of them and thus not only reduces the benefit of the reactive distillation but also offers operational challenges for keeping the reactants together. Methyl lactate (ML) synthesis via the esterification of lactic acid (LA) with methanol in a reactive distillation falls into this category and perhaps that is why this process has not been explored in the past. The boiling points of the reactants (LA, methanol) are about 490 K and 337 K while those of the products (ML, water) are 417 K and 373 K respectively. Clearly in a conventional reactive distillation (batch or continuous) methanol will be separated from the LA and will reduce the conversion of LA to ML significantly. Here, first the limitations of the use of conventional batch distillation column (CBD) for the synthesis of ML is investigated in detail and a semi-batch reactive distillation (SBD) configuration is studied in detail where LA is the limiting reactant and methanol is continuously fed in excess in the reboiler allowing the reactants to be together for a longer period. However, this poses an operational challenge that the column has to be carefully controlled to avoid overflow of the reboiler at any time of the operation. In this work, the performance of SBD for the synthesis of ML is evaluated using model based optimization in which operational constraints are embedded. The results clearly demonstrate the viability of the system for the synthesis of ML.

Distillation process optimization for styrene production from a styrene-benzene-toluene system in a Petlyuk column

The conventional way to produce styrene is ethyl-benzene dehydrogenation, which generates benzene and toluene as byproducts. In general, the separation of these three components is made through successive reactor product distillation, first removing benzene and, then, separating toluene from benzene. The main problem with the use of conventional distillation columns is excessive energy consumption in comparison with modern separation methods. This work simulates the conventional distillation system and the coupled column system (Petlyuk), in order to compare the benefits obtained on energy economy. The results show that, while the energy consumption to feed the boilers in the conventional distillation system is 2295kW, for the Petlyuk system the energy consumption was 1554kW, representing energy economy of 32.3%. Moreover, with application of this method higher purity was obtained for toluene, from 76.19% to 99.67%, and for styrene, from 99.71% to 99.98%.

Modeling and analysis of conventional and heat‐integrated distillation columns

A generic model that can cover diabatic and adiabatic distillation column configurations is presented, with the aim of providing a consistent basis for comparison of alternative distillation column technologies. Both a static and a dynamic formulation of the model, together with a model catalogue consisting of the conventional, the heat‐integrated and the mechanical vapor recompression distillation columns are presented. The solution procedure of the model is outlined and illustrated in three case studies. One case study being a benchmark study demonstrating the size of the model and the static properties of two different heat‐integrated distillation column (HIDiC) schemes and the mechanical vapor recompression column. The second case study exemplifies the difference between a HIDiC and a conventional distillation column in the composition profiles within a multicomponent separation, whereas the last case study demonstrates the difference in available dynamic models for the HIDiC and the proposed model. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4251–4263, 2015

Energy Efficient Distillation Columns Design for Retrofit NGLs Fractionation Process

Distillation is the primary separation process widely used in the industrial chemical process. Although it has many advantages, the main drawback is its large energy requirement, which may significantly influence the overall plant profitability. However, the large energy requirement of these processes can be systematically reduced by using driving force and energy integration methods. This paper presents a methodology for designing energy efficient distillation columns systems based on those two methods. Generally, the proposed methodology consists of four hierarchical steps. In the first step, the system of distillation columns for multicomponent separation is designed based on the conventional distillation column design (shortcut) method. Then, the conventional distillation columns systems design is improved in terms of energy saving by using driving force method in the second step. It is expected in the third step that the distillation columns systems design can be further improved in terms of energy saving by using energy integration method. Finally, the distillation columns systems design is evaluated in terms of economic performance. The simulation results by using Aspen HYSYS have shown that the driving force sequence by using shortcut method for NGLs has energy savings with 55% reduction compared to a direct method with shortcut method. It can be verified that the proposed methodology has the capability in designing energy efficient distillation columns in an easy, systematic and practical manners.

Energy conserving effects of dividing wall column

The energy-conserving performance of dividing wall column (DWC) is discussed in this paper. The heat transfer through the dividing wall is considered and the results are compared with that of common heat insulation dividing wall column (HIDWC). Based on the thermodynamic analysis of heat transfer dividing wall column (HTDWC) and HIDWC, both computer simulation and experiments are employed to analyze the energy-conserving situation. Mixtures of n-hexane, n-heptane and n-octane are chosen as the example for separation. The results show that the energy consumption of HTDWC is 50.3% less than that of conventional distillation column, while it is 46.4% less than that of HIDWC. It indicates that DWC is efficient on separating three-component mixtures and HTDWC can save more energy than HIDWC. Thus it is necessary to consider the heat transfer while applying DWC to industry.

Improving the Performance of Extractive Dividing-Wall Columns with Intermediate Heating

In spite of the fact that using extractive dividing-wall columns (EDWCs) lead to a considerably reduced reboiler heat duty in comparison with conventional extractive distillation column (CEDC) flowsheets, the involvement of a heavy entrainer frequently requires a relatively high-pressure steam for process heating and incurs consequently adverse steady-state economics. To address the potential deficiency, we proposed, in the present study, to facilitate the EDWC with intermediate heating because the great boiling-point difference between the components in the azeotropic mixture and entrainer results in steep temperature profiles and allows effectively recovering sensible heat from the recycled entrainer and using relatively low-pressure steam. When the extractive separation operation dominates the EDWC, intermediate heating should be arranged in the left side of the dividing wall and feed preheating is frequently the most favorable option. On the other hand, when the entrainer recovery operation dominates the EDWC, intermediate heating should be arranged below the lower end of the dividing wall, i.e., in the stripping section, and an intermediate reboiler is the favorable option. Under both circumstances, the heat recovery from the recycled entrainer should be considered prior to the use of available utilities, thereby permitting great improvement in the steady-state economics. The strategy features simplicity in principle and requires a relatively small number of trial and error searches. It is evaluated in terms of extractive separations of two binary azeotropic mixtures: dimethyl carbonate and methanol (with aniline as the entrainer) and acetone and methanol (with dimethyl sulfoxide as the entrainer). It is found that intermediate heating could substantially enhance the performance of the EDWC with the resultant steady-state economics overwhelmingly above that of the CEDC flowsheet. Even compared with the CEDC flowsheet reinforced with intermediate heating, the EDWC is still likely to yield comparable steady-state economics. These outcomes indicate that intermediate heating should be taken into account in the synthesis and design of the EDWC and the EDWC with intermediate heating should be regarded as a potential option for the extractive separations of binary azeotropic mixtures.

Using an internally heat-integrated distillation column for ethanol–water separation for fuel applications

Distillation is widely used as a key process to concentrate ethanol from fermented broth in the Brazilian sugarcane industry. In this study, Aspen Plus® was used to compare the energy demand for a heat-integrated distillation column (HIDiC) both with and without heat panels with the energy demand for a conventional distillation column for a separation process that could be applied to the Brazilian bioethanol market. The overall convective heat transfer coefficient, area, and other parameter values were calculated and combined with the HIDiC simulation. The reboiler duty results showed that by assuming a fixed fraction of the fluids exchanged energy at the panels, energy savings of 77% were possible with the HIDiC column compared to the energy requirements of a conventional column. This approach also allowed a stage-by-stage calculation of the overall heat transfer coefficient instead of assuming a constant value.

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.

Economic optimization of heat pump-assisted distillation columns in methanol-water separation

Finding efficient alternative to CDiC (Conventional Distillation Column) for methanol-water separation has been an attractive field of study in literature. In this work, five heat pump-assisted schemes are proposed and compared to each other to find the optimal one; (1) VRC (Vapor Recompression Column), (2) external HIDiC (Heat-Integrated Distillation Column), (3) intensified HIDiC with feed preheater, (4) double compressor intensified HIDiC-1, and (5) double compressor intensified HIDiC-2. GA (Genetic Algorithm) is then implemented for optimization of the schemes when TAC (Total Annual Cost) is its objective function. During optimization, two new variables are added for using only appropriate amount of the overhead stream in VRC and double compressor intensified HIDiCs, and another new binary variable is also used for considering feed preheating. Although TAC of the intensified HIDiC with feed preheater is found higher than CDiC by 25.0%, all optimal VRC, external HIDiC, double compressor intensified HIDiCs schemes are reached lower optimal TAC by 3.1%, 27.2%, 24.4%, and 34.2%. Introduced for the first time, the optimal scheme is the double compressor intensified HIDiC-2 with 34.2% TAC saving, 70.4% TEC (Total Energy Consumption) reduction with payback period of 3.30 years.

Energy Saving through Modeling of Integrated Distillation Column (Side-Stripper) in Natural Gas Liquid Processing

In Natural Gas Liquid (NGL) recovery, distillation process is the most common and widely used separation method. The productions of ethane, propane, butane and stabilized condensate are achieved through a series of conventional distillation columns which require massive amount of energy consumption, contributing more than 40% of the overall plant energy thus increasing the operating cost. This work focuses on energy saving in natural gas liquid processing by comparing the integrated side-stripper column with the conventional separation trains at steady state using a commercial simulator, Aspen Plus HYSYS. Energy analysis from the study shows that the integrated column arrangement which implement the side-stripper configuration achieved favourable result as it offers the highest percentage in energy reduction which is up to 68% (for reboiler) and 70% (for condenser). Consequently, this can be linked to the reduction of operating cost for hot and cool duties and trim down the overall production cost for existing NGL recovery plant.

Hybrid Membrane-Distillation Separation Processes

A novel synthesis approach is developed to evaluate and optimise various hybrid membrane-distillation schemes. The problem is formulated as an optimisation problem, where the objective is to minimise the total operating cost of the separation scheme. Interactions between the separation and refrigeration systems are considered and opportunities for heat integration are exploited. A case study, ethylene/ethane separation, is presented to demonstrate the synthesis approach. Schemes that apply a membrane in parallel to and in series with a distillation column are explored. Shortcut models that account for multiple feeds and products are used to represent the distillation column. The distillation model predictions are shown to be in good agreement with results of more rigorous simulations carried out using HYSYS. Established membrane models (Shindo et al., 1985) are applied and shown to be valid for the system of interest. A systematic approach is developed to account for heat recovery between: i) column feeds and products; ii) the membrane feed and products and iii) the associated refrigeration system. The optimisation results reveal that a facilitated transport membrane reported in the literature (Pinnau and Toy, 2001) used in parallel with a distillation column can reduce the condenser duty by about 33 %, compared to a conventional distillation column operating at the same pressure (20 bar). However, recompression and sub-ambient cooling are required for the permeate stream, incurring operating costs. The total operating cost of the heat-integrated parallel hybrid scheme may be reduced by 11 %, compared to a conventional distillation column..

Some operational aspects and applications of dividing wall columns: energy requirements and carbon dioxide emissions

High prices of oil and biofuels as well as tight environmental regulations on gas emissions have promoted theoretical and industrial developments in distillation; in particular, in complex distillation columns. The dividing wall distillation column (DWDC) has been adopted in industrial practice in order to achieve savings in both energy and cooling water requirements, reductions in green house gases emissions, and production and purification of biofuels. Simulation studies and practical implementations in the area of process control have shown that the control properties of DWDC are similar to those of conventional distillation columns. This work provides a discussion about the operational issues and a review of the practical applications of DWDC’s.

Feasibility study of heat-integrated distillation columns using rigorous optimization

In this work, rigorous optimization of HIDiC (Heat-Integrated Distillation Column) and VRC (Vapor Recompression Column) is implemented by GA (Genetic Algorithm) to find an alternative for CDiC (Conventional Distillation Column). The objective function is TAC (Total Annual Cost). Three different case studies are investigated, being composed of benzene-toluene, propane-propylene, and methanol-water. A novel strategy is proposed to consider all the heat integration possibilities resulting in more efficient search space than our previous attempt. It is observed that the heat exchangers arrangement of optimum HIDiCs are very similar to VRCs in ideal case studies. Although CDiC is the optimum configuration in the benzene-toluene separation, 6.6% reduction is achieved for the presented HIDiC compared to previous work. For propane-propylene splitter, VRC is the economical alternative with a 44.1% decrease in the TAC of CDiC. Moreover, VRC and HIDiC optimizations leads to 25.5% and 4.4% reductions in TAC compared to previous work, respectively. However, for the non-ideal methanol-water separation, which has a wide boiling point range, the TAC of optimum HIDiC is surprisingly lower than CDiC and optimum VRC by 3.4% and 31.2%, respectively.

A new process for ternary separations: Semicontinuous distillation without a middle vessel

In this work, a novel semicontinuous process for the separation of ternary mixtures is presented. The new semicontinuous without middle vessel (SwoMV) configuration is a process intensification technique that makes separation of three components to desired purities possible in a single distillation column without the necessity of the middle vessel as opposed to the conventional semicontinuous processes. The elimination of the middle vessel omits the charging and discharging modes of conventional semicontinuous processes, reduces the direct costs of plant and facilitates the retrofit of available distillation columns for purification of new products. Furthermore, SwoMV configuration improves the conventional semicontinuous system by reducing the energy consumption in most cases and expanding the range of system capacities at which it is economically optimal. The separation of benzene, toluene, and o-xylene is used as a case study to show the feasibility of the SwoMV configuration. An economic analysis is performed, and conventional continuous distillation, conventional semicontinuous, SwoMV and single side stream column configurations are compared by calculating the total annualized cost of these systems over a range of production rates. The results show the feasibility, applicability and profitability of the novel proposed configuration.

A reactive distillation column with double reactive sections for the separations of two-stage consecutive reversible reactions

In this paper, a novel reactive distillation column with double reactive sections (RDC-DRS) is proposed for the separations of two-stage consecutive reversible reactions. The arrangement of two reactive sections not only allows the careful coordination of the two reaction operations involved, but also provides additional degrees of freedom for the reinforcement of internal mass integration and/or internal energy integration between the reaction operations and the separation operation involved, which could facilitate the RDC-DRS to be more advantageous than the conventional reactive distillation column with a single reactive section (RDC-SRS) in operating cost and capital investment. A representative hypothetical and real two-stage consecutive reversible reactions are chosen to evaluate the steady-state performance of the RDC-DRS. With the constraints of the same total number of stages and the same total number of reactive stages, the RDC-DRS is demonstrated to require less utility consumption than the RDC-SRS and this outcome indicates that the former could be a competitive alternative to the latter in the separations of the two-stage consecutive reversible reactions. The number of reactive sections should therefore be viewed as an important decision variable for the synthesis and design of reactive distillation columns especially in the separations of complicated reacting mixtures involving multiple reversible reactions.

Transesterification of Methyl Acetate with n-Propanol: Reaction Kinetics and Simulation in Reactive Distillation Process

The reaction kinetics of the transesterification of methyl acetate with n-propanol is described by pseudohomogeneous (PH) kinetic model, which can successfully correlate with the experimental data. The influence of temperature as well as catalyst loading and initial molar ratio of n-propanol to methyl acetate on this reaction is studied under the condition of eliminating both internal and external mass transfer resistances. On the basis of the PH kinetic model, the influences of design parameters are studied, such as recycle flow rate, the number of stages of the reactive distillation column (RD), and theoretical stages of the conventional distillation column. These parameters are further optimized to minimize total annual cost in the RD process.

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%.

Effective Arrangement of an External Recycle in Reactive Distillation Columns

For reactive distillation columns separating reacting mixtures with somewhat unfavorable relative volatilities (e.g., the reactants are the light and heaviest components and the products are the lightest and heavy ones), a novel process configuration with an external recycle from the bottom to a certain stage of the reactive section is proposed in this article. The flow rate and location of the external recycle are important decision variables for process development and can help to enhance internal mass integration and internal energy integration between the reaction operation and the separation operation involved. The separations of one hypothetical ideal exothermic reaction mixture, , and two real nonideal reaction mixtures, the esterification of acetic acid with methanol and the transesterification of butyl acetate with ethanol, were chosen to evaluate the impact of such an external recycle, and parametric analysis was also conducted with respect to the relative volatility, chemical equilibrium constant, product purity, total amount of catalyst employed, and heat of reaction in terms of the separation of the ideal reacting mixture. Through strict comparison with conventional reactive distillation columns, it is demonstrated that the inclusion of such an external recycle is beneficial to system performance and should be taken into account in process synthesis and design. The potential applications of an external recycle to other reactive distillation columns separating reacting mixtures with somewhat unfavorable relative volatilities (e.g., the reactants are the lightest and heavy components and the products are the light and heaviest ones) are also discussed.

Methodology Development for Designing Energy Efficient Distillation Column Systems

Distillation is the primary separation process widely used in the industrial chemical processing. Although it has many advantages, the main drawback is its large energy requirement, which can significantly influence the overall plant profitability. However, the large energy requirement of these processes can be systematically reduced by using driving force and energy integration methods. This paper presents a methodology development for designing energy efficient distillation column systems based on those two methods. Accordingly, the proposed methodology consists of four hierarchical steps. In the first step, the system of distillation column for multicomponent separation is designed based on the conventional distillation column design method. Then, the conventional distillation columns systems design is improved in terms of energy saving by using driving force method in the second step. It is expected in the third step that the distillation columns systems design can be further improved in terms of energy saving by using energy integration method. Finally, the distillation column systems design is evaluated in in terms of economic performance. By applying the proposed methodology, it is possible to make an early assumption on sequence of distillation column systems that is the best in terms of energy saving.

Energy Efficient Optimal Design of Waste Solvent Recovery Process in Semiconductor Industry Using Enhanced Vacuum Distillation

Semiconductor is one of the biggest industries in the world because it is the key driver of economic growth and it is continuously growing in a cyclical pattern. Huge amount of various chemicals have been used in the process, but it is not followed by a good waste treatment. This results in high production cost and inflicts on environment because of most of the valuable organic solvents discharged from the process has been incinerated at high temperature. In this work, optimal distillation process configuration was examined to recover valuable organic solvents and to cut down the production-related costs as well as to protect the environment against the industrial wastes. In this distillation process, some columns in a conventional sequence were optimized by using a box optimization algorithm to obtain minimum energy consumption without violence some constraints and then, main product columns were optimized in the economical point of view by a Particle Swarm Optimization (PSO) method. To achieve further objective of energy improvement with high profit, thermally coupled distillation and heat pump assisted distillation were implemented as alternatives of conventional distillation column with low energy efficiency. The results showed that the proposed advanced column configuration could save energy and utility consumption significantly.