Reactive Distillation Model Research Articles

Rigorous design and economic optimization of reactive distillation column considering real liquid hold-up and hydraulic conditions of industrial device

The liquid hold-up in a reactive distillation (RD) column not only has a significant impact on the extent of reactions, but also affects the pressure drop and hydraulic conditions in the column. Therefore, the liquid hold-up would be a critical design factor for RD columns. However, the existing design methods for RD columns typically neglect the influence of considerable amount of liquid hold-up in downcomers owing to the difficulties of solving a large-scale nonlinear model system by considering downcomer hydraulics, resulting in significant deviations from actual situation and even operation infeasibility of the designed column. In this paper, a pseudo-transient (PT) RD model based on equilibrium model considering tray hydraulics was established for rigorous simulation and optimization of RD plate columns considering the liquid hold-up both in downcomers and column trays, and a steady-state optimization algorithm assisted by the PT model was adopted to robustly solve the optimization problem. The optimization results of either ethylene glycol RD or methyl acetate RD demonstrated that assuming all the liquid hold-up of a stage belonged to the tray will cause significant deviations in the column diameter, weir height, and the number of stages, which leads to not meeting the separation requirements and even operation hydraulic infeasibility. The rigorous model proposed in this study which considers the liquid hold-up both on trays and in downcomers as well as hydraulic constraints can be applied to systematically design industrial RD plate columns to simultaneously obtain optimal operating variables and equipment structure variables.

1,3-Dioxolane production via reactive distillation combined with vapor permeation: Experiments and modelling

To overcome the problem of difficult separation of the 1,3-dioxane (DOL)–H2O azeotrope in the production of DOL, a new process via reactive distillation (RD) combined with vapor permeation (VP) was established. The reliability of the RD model has been reported in the literature. The VP membrane used an Aspen Custom Modeler (ACM) self-built model, with the permeation parameters of H2O, DOL, and formaldehyde (FA) in the membrane flux equation fitted through the H2O-DOL-FA dehydration experiments by NaA zeolite membrane. The results showed that the relative error between the mass of H2O in a series of permeates in experiments and simulated by the VP model was mainly within 20%, and R2 is 0.94. For DOL and FA, which have very small permeation, the value was within 50%, implying the reliability of the VP model. On this basis, a two-dimensional VP model was established. Finally, a new RD-VP-D process for producing DOL was designed. The VP was used to overcome the azeotropic point of DOL and H2O by dehydration. Then 99.9 wt% DOL can be obtained at the bottom of the subsequent DOL purification column, with DOL- H2O azeotrope at the top recycled back to the VP membrane. The effects of the degree of VP dehydration and column pressure on membrane area, reboiler duty, and TAC were investigated, meanwhile introducing a new concept: logarithmic mean partial-pressure difference (LMPD). Under the optimal parameter, TAC was reduced by 20.0% compared with the process reported in the literature, and CO2 emissions were reduced by 35.8%.

Entrainer assisted production of high purity 2-phenyl ethyl acetate by reactive distillation

Reactive distillation (RD) is a promising alternative for the processes involving reversible reactions. In this work, we examine its potential for the esterification of a heavy alcohol (2-phenyl ethyl alcohol) with acetic acid in the presence of an ion exchange resin (Amberlyst-15) as the catalyst. The proposed RD process facilitates separation of the desired product, 2-phenyl ethyl acetate, in the pure form and provides a cost-effective and compact alternative to the conventional batch process. We determine the feasibility of this approach by conducting experiments in a laboratory-scale hybrid reactive distillation column having structured packings filled with Amberlyst-15. The experimental results are compared with the predicted values from an equilibrium-staged RD model. The model uses reaction kinetics developed in our earlier studies and the vapor–liquid equilibrium model is based on the experimental data generated in the present work and data available in literature. The experimentally validated model is further used to identify the best possible operating and design parameters to achieve high purity, high conversion and longer catalyst life. All in all, entrainer-based reactive distillation was found to be feasible in this case. The methodology followed here can be easily extended to similar reactions of heavy alcohols with lower carboxylic acids like acetic acid.

Novel intensified process for di-isopropyl ether production from isopropanol etherification using reactive distillation technology

The preparation of di-isopropyl ether (DIPE), as an important chemical solvent, by isopropanol (IPA) etherification is limited by chemical equilibrium and azeotropic formation in its production processes. A novel sustainable etherification process based on reactive distillation (RD) coupled with extractive distillation is proposed. The reaction of IPA etherification was carried out by cation exchange resin NKC-9 in a 250 mL autoclave and fitted by a pseudo-homogeneous model. According to the reaction kinetic, the IPA etherification RD model was established. To confirm the viability of RD and the accuracy of the model, pilot-scale RD experiments for generating DIPE were investigated. Based on the established model matching the pilot scale experiments, the operating conditions of the etherification RD column were optimized by sensitivity analysis. The simulation results show that the conversion of IPA in the RD column can be increased from 18.2 % to 77.9 % compared with autoclave and the yield of DIPE in the whole process flowsheet can reach 99.9 %, which demonstrates that combining reactive distillation with extractive distillation technology is a feasible and advantageous solution for DIPE production.

Pilot-scale validation and novel design of reactive distillation with HY@SiC foam catalytic packing for cyclohexyl acetate production

For reactive distillation (RD) process limited by the separation efficiency of gas–liquid mass transfer, if the physical size of the alternating operation in reaction and separation can be further reduced, it would greatly promote the efficiency of the RD process. For this purpose, a structured HY@SiC catalytic packing was developed in which the gas–liquid mass transfer separation process and the catalytic reaction occurred simultaneously on its surface. The HY@SiC catalytic packing was used in a pilot-scale column for experimental study on production of cyclohexyl acetate (CA) by RD to verify the performance of the catalytic packing for the first time. Furthermore, a RD model based on the HY@SiC catalytic packing was established based on the reaction kinetics of HY zeolite catalyst obtained in this work. Finally, a novel RD process with double recycle stream for CA production was proposed, which will be optimized design with TAC minimize as the optimization objective based on sequential iterative algorithm.

Investigation of sustainable reactive distillation process to produce malonic acid from dimethyl malonate

The new energy battery is currently a hot research topic, leading to an increase in the demand for malonic acid (MA) as a raw material. The objective of the present work is to find a potential reactive distillation (RD) process to solve the problem of low conversion and high energy consumption in hydrolytic system. In the meantime, the RD technology still faces the problem of how to distribute water in the RD column. Aiming at the problem, four different RD processes are proposed: RD combined water separation process (RDWS), RD combined with MA concentration process (RDMC), RD combined methanol separation process (RDMS), and single RD process (SRD). Beforehand, reaction kinetic of dimethyl malonate (DM) hydrolysis was investigated using the ion exchange resin catalysts, and the developed RD model was verified via pilot-scale RD experiments. Finally, the proposed RD process is analyzed from the aspects of economic and environmental and compared with the traditional reaction + distillation (R + D) process. Consequently, the proposed SRD process is a better economy-saving and environment-friendly technology, and has certain directive significance for the industrial production of MA.

Tributyl citrate production via reactive distillation: Model reconciliation, optimization, scale up and sustainability indicators

• Reconciliation of a reactive distillation (RD) model for tributyl citrate (TBC) production. • A RD Industrial process for TBC production is designed and optimized. • Homogeneous and heterogenous catalysts are assessed. • Comparison with current industrial technology is presented. • Economic and sustainability indicators are presented. This work describes a model reconciliation of a reactive distillation (RD) process for the production of tributyl citrate based upon pilot scale experimental data. Steady state modeling of the process was constructed using previously validated thermodynamics and reaction kinetics that included catalytic and self-catalytic contributions. The set of pilot scale experimental data included input–output flow rates, and composition and temperature profiles of 18 runs under continuous operation. In order to reconcile the RD model, heat losses, catalyst effectiveness, Murphree efficiencies, and reactive and non-reactive liquid holdup were adjusted to fit the measured data. After reconciliation, the obtained model was in good agreement with pilot scale results, with average mass fractions and temperatures deviations < 2%. Once reconciled, the RD model was used to design and optimize the performance of a 5000 ton/yr industrial scale facility. The optimization procedure included decision, design and operating variables of continuous or discrete nature. The best configurations involved a single RD column for the heterogeneous catalyst, or a pre-reactor-RD column scheme for the homogeneously catalyzed process. Reflux ratios ranged from 0.01 to 0.5 and pre-reactor residence times were around 1 h. The optimized configuration for heterogeneous catalyzed process exhibited up to 30% energy savings with respect to the homogeneous case. The unitary TBC production costs were calculated to be 1.66 and 1.69 USD/kg for the heterogeneous and homogeneous case respectively. Finally, the optimized configurations were compared using sustainability indicators.

Simulation and optimization of the thermally coupled reactive distillation column for producing toluene diisocynate

AbstractToluene diisocyanate is an important chemical intermediate prepared by phosgenation reaction with two steps named cold and thermal phosgenation, respectively. The thermal phosgenation reaction is a complex reactive distillation process with high energy consumption and multiple targets of reaction and separation, so it is of great significance to optimize the step. The reactive distillation model of the thermal phosgenation was established for an industrial installation by aspen plus, and the effects of the different parameters of temperatures, residence times, etc., were investigated firstly; then a thermally coupled reactive distillation column was presented to perform the thermal phosgenation process, and the optimal operation and configuration parameters were obtained by simulation. The results showed that the proposed process can save the heat and cold energy with 7.29% and 32.78%, respectively, and reduce the total annual cost by about 17.11%.

Reactive distillation for sustainable synthesis of bio-ethyl lactate: Kinetics, pilot-scale experiments and process analysis

Bio-ethyl lactate (EL), an alternative green solvent to the traditional petroleum-based solvent, has a distinct advantage in the chemical industry. However, to solve the problem of high energy consumption and low yield in the production of EL is the premise of large-scale production and application. This work proposes an economical and energy-efficient reactive distillation (RD) process through excess lactic acid (LA). As the primary data of RD process design, the kinetics of esterification reaction between LA and EL were investigated using the ion exchange resins KRD001 catalyst. Besides, the developed RD model has been validated utilizing five pilot-scale RD experimental data for process design. Furthermore, the proposed process is optimized with the objective of the minimum heat duty per kilogram product (HDP) of EL, taking into account energy consumption, reaction conversion, economic and environmental impacts. The proposed process has obvious economic and environmental advantages, which can cause a reduction of 52.25% for the total annual cost per kilogram product (TACK) of EL and 4.63% for the gas emissions compared with the excess ethanol (EtOH) process. This provides guidance and reference for the green technology development and production of EL.

Intensification of 2-Methyl-1,3-Dioxolane hydrolysis for recovery of ethylene glycol through reactive distillation: Kinetics and process design

Chemical Looping technology, which consist of aldolization and hydrolysis processes, is proved as a potential and effectively method to separate ethylene glycol (EG) in the coal-based EG production process. However, the problem of acetal hydrolysis is that this reaction is hindered by the boiling points ranking and the chemical equilibrium limitations, which is not the most favorable. In this paper, a potentially sustainable reactive distillation (RD) process is presented for intensification of hydrolysis of 2-Methyl-1,3-Dioxolane (2MD). To accurate design the RD process, the hydrolysis kinetics was measured and a regressed pseudo homogeneous model showed good agreement with experiments. Furthermore, an applicable reactive distillation model was proposed to analyzing the effect of pressure, feed ratio, feed position, reflux ratio, number of reaction plates, rectification plates and stripping plates on the energy consumption and 2MD conversion. Results show that the designed RD process will shift the conventional hydrolysis process towards completion with a 2MD conversion higher than 99.9 %, even without high excess water. The obtained results could give a guide for the pilot scale of recovery EG in coal-based EG production process.

A multi-objective reactive distillation optimization model for Fischer–Tropsch synthesis

In the design of a reactive distillation column, aspects such as column configuration, catalyst loading, tray temperature, and side extraction rates should be well considered. Though preferences in Fischer–Tropsch (FT) synthesis may vary, it is acknowledged that the final product contains a wide range of hydrocarbons including fuel gas, gasoline, diesel, and linear wax. Zhang et al. (2018) previously developed an equation-oriented framework for optimal synthesis of integrated reactive distillation systems for FT processes.Here, we extend the mass, equilibrium, summation, and heat equations to a mathematical program with complementarity constraints to deal with possible dry trays in the non-reactive sections. The purpose of describing disappearing phases is to avoid infeasibilities due to multiple bilinear terms in the model for complicated model structures. The model is implemented by solving initialization steps and a sequence of nonlinear programming problems to determine an optimal structure and operating conditions.Design specifications for multiple products could be set as individual objectives to determine design limits. Moreover, a balance of multiple objectives could be reached by formulating the reactive distillation model as a multi-objective optimization problem. In this work, we employ the augmented ϵ-constraint method. The results show that significant design insights can be gained from the Pareto-optimal front regarding acceptable trade-offs among various objectives.

A novel method for determining the optimal operating points of reactive distillation processes

Reactive distillation (RD) allows reaction and separation to take place simultaneously in the same unit, thus giving major benefits especially to equilibrium limited reactions. Although the application of RD in chemical industries is attractive, it is considerably challenging. Unlike classic distillation, the optimal configuration of RD from an economical perspective is hardly identified quickly. Usually, any specific reaction system may need extensive studies and rigorous simulations to develop a RD model. This study aims to determine the optimal operating points of a RD application in a quick and reliable way. A novel method is employed for a clear visualization of the RD applicability area (i.e. a plot of reflux ratio vs number of stages). Using this method, an economic analysis can be performed resulting in essential insights into the optimal configurations. The production of amyl acetate by esterification of amyl alcohol and acetic acid is selected as case study, since this reaction sufficiently represents non-ideal behaviours in real systems. The outcome of the analysis reveals that the boundary line of its RD applicability graph consists of the optimal points of RD configurations which generate the lowest total annual cost in the RD operation. Furthermore, it is observed that the additional cost for the reactive section (relative to a separation section) is marginal, which means that the rules of thumb for the optimal configurations in classic distillation could also be applied.

Equation-oriented optimization of reactive distillation systems using pseudo-transient models

Reactive distillation has significant economic and environmental advantages compared to conventional processes. However, the optimal design of a reactive distillation system is far from mature. This work proposed a novel equation-oriented optimization framework based on a rigorous equilibrium stage model for the optimization of reactive distillation systems. A pseudo-transient continuation (PTC) reactive distillation model was developed to solve the convergence problem of simulations. The bypass efficiency method was incorporated into the PTC model to avoid the intractable mixed integer optimization. The developed model can be integrated well with a steady-state optimization algorithm assisted by PTC models. Two cases demonstrated that the proposed model and optimization framework were robust and fast. However, the bypass efficiency method caused physically meaningless solutions, but they were found to be convertible to physically meaningful solutions without influencing the accurate optimization of number of stages according to the intrinsic properties of the bypass efficiency method introduced in the current work.

Analysis of direct synthesis of dimethyl carbonate from methanol and CO2 intensified by in-situ hydration-assisted reactive distillation with side reactor

A reactive distillation (RD) process with a gas-phase side reactor potentially promoted by in-situ hydration is proposed to overcome the thermodynamic restriction of direct synthesis of dimethyl carbonate (DMC) from methanol and CO2. A homogeneous RD model with fixed bed reactor is established in gPROMS platform. The feasibility of the process for direct DMC synthesis is analyzed. The important configuration parameters (e.g., side withdrawal stage and recycle stage) and operating conditions (e.g., reflux ratio and feed ratio) of the RD process are systematically studied. In-situ hydration of ethylene oxide occurring only in the column is introduced for further enhancement. With the process intensification, the conversion of methanol is both improved from approximately 10% in a one-step reactor to 99.5% of RD with and without EO hydration.

Equation-Oriented Framework for Optimal Synthesis of Integrated Reactive Distillation Systems for Fischer–Tropsch Processes

To explore reactive distillation (RD) for Fischer–Tropsch synthesis (FTS), we develop a steady-state adiabatic RD model for this system. Here, the calculation of the vapor–liquid equilibrium (VLE) through cubic equation of state is used to describe the phase behavior. Rate expressions for the FT and the water–gas shift reactions are expressed in terms of fugacities and product selectivity of catalysts are based on Anderson–Schulz–Flory distribution and experimental data. Next, a mass, equilibrium, summation, and heat model is extended by considering bypass streams for nonreactive trays and is used to integrate column structure blocks. A step-by-step initialization procedure is proposed to accommodate the complexity of the RD column and the high nonlinearity. This includes case studies of operating variables in a reactive flash model prepared for performance optimization of RD. Finally, a typical low-temperature FT process, which favors diesel production is implemented in the RD model. Comparing the RD for...

Applicability of DFT model in reactive distillation

AbstractThe density functional theory (DFT) applicability to reactive distillation is discussed. Brief modeling techniques description of distillation and rectification with chemical reaction is provided as a background for quantum method usage description. The equilibrium and nonequilibrium distillation models are described for that purpose. The DFT quantum theory is concisely described. The usage of DFT in the modeling of reactive distillation is described in two parts. One of the fundamental and very important component of distillation modeling is vapor-liquid equilibrium description for which the DFT quantum approach can be used. The representative DFT models, namely COSMO-RS (Conductor like Screening Model for Real Solvents), COSMOSPACE (COSMO Surface Pair Activity Coefficient) and COSMO-SAC (SAC – segment activity coefficient) approaches are described. The second part treats the way in which the chemical reaction is described by means of quantum DFT method. The intrinsic reaction coordinate (IRC) method is described which is used to find minimum energy path of substrates to products transition. The DFT is one of the methods which can be used for that purpose. The literature data examples are provided which proves that IRC method is applicable for chemical reaction kinetics description.

The Effect of Feed Location of a Semi-Batch Reactive Distillation via Esterification Reaction of Acetic Acid and Methanol: Simulation Study

This paper studied the effect of feed location of semi-batch reactive distillation via esterification reaction of acetic acid and methanol producing methyl acetate using Aspen Batch Distillation model. The reactive distillation model used was developed following a design of an in-house made reactive distillation column, which comprised of seven stages: five possible feed stages, where solid catalyst for the reaction was contained, a reboiler and a condenser. The main objectives of this semi-batch reactive distillation operation were to obtain a high yield of methyl acetate product with 95wt% purity. In the operation, methanol was charged to the reboiler and acetic acid was fed to the column continuously at five different feed stages. From the simulation results, it showed that the feed location of acetic acid affecting on the yield and the purity of the main product at the top stage of the column. It was found that the optimum feed stage of acetic acid was at stage four giving the maximum yield and purity of methyl acetate at 74.74% by mole and 97.36wt%, respectively.

Neural - Wiener Model for Multivariable Methyl Tertiary Butyl Ether (MTBE) Reactive Distillation

MTBE is a chemical that can be used as anti-knock additive to replace lead additive (tetra ethyl lead) which can be efficiently produced using reactive distillation process. It has been established in the literature that MTBE reactive distillation poses a highly nonlinear behavior due to the combination of reaction and separation processes. A reliable model for predicting the behavior is required especially for the control purposes. In this work, a Neural Wiener model which is one of the available types of oriented block model was utilised to develop the MTBE reactive distillation model. The required data for the Neural Wiener model were generated using a validated Aspen dynamics model for the MTBE reactive distillation process. It is found that the Neural Wiener model is capable to predict the MTBE purity and isobutene conversion with accuracy of 98.55% and 96.95%, respectively. Those values are quantitatively better in comparison to the state space model which gives lower values for prediction accuracy of 87.86% and 82.90%, respectively.

Process Parameters Optimization of the MTBE Reactive Distillation by Orthogonal Numberical Test and Least Square Method

Because of the large number of operating and equipment parameters of the reactive distillation(RD) and a strong coupling between them, it is difficult to find the optimal process parameters. A novel hybrid method on process parameters optimization of methyl tertiary butyl ether (MTBE) RD was developed in the paper. MTBE RD process was firstly simulated with Aspen Plus. Then based on the MTBE RD model, sensitivity analysis of various parameters was accomplished to determine the key decision parameters. After that, the orthogonal numerical tests were performed in feasible fields to obtain nearly optimal parameters. Finally, with the orthogonal numerical test results, the least square method was used to regress equation, which showed the relationship between objective function and the key decision parameters, thus determining the optimal operating parameters and equipment parameters. Results of analysis indicated that the combination of orthogonal numerical test and the least square method can be attractive since the number of tests was reduced substantially while the specification of the products can be met.

Influence of liquid back mixing on a kinetically controlled reactive distillation process

The state of the art equilibrium model and the rate-based models for reactive distillation (RD) are very well known and have been used since a couple of decades. However, these models are not sufficient to accurately represent a slow reaction process that is kinetically controlled. The shortcoming is due to neglecting the effect of liquid back mixing (LBM) on the whole process. This work presents a review of the available modeling approaches for RD, then discusses the applicability of various models and finally, it applies the findings to an industrially relevant case study—polyester synthesis. The main focus is on extending the dynamic rate-based model to take into account the liquid back mixing. We also show how the axial dispersion can be introduced into the RD model, without adopting the axial dispersion model. The comparison of the results of the rate-based model, with and without axial dispersion, clearly demonstrates that the extended model predicts more accurately the kinetically controlled process as compared to the conventional rate-based model.