Nonequilibrium Stage Model Research Articles

Energy analysis and optimization of heat integrated air separation column based on non-equilibrium stage model

A large scale of nitrogen and oxygen production has been through a cryogenic air separation process. This process takes place in a distillation column at excessive energy consumption. The heat-integrated air separation column (HIASC) can save energy when utilizing minimum pressure while ensuring adequate heat distribution along the portion of the column. However, accurate prediction of thermodynamic and fluid properties requirement is critical for process optimization and potential energy saving. The nonequilibrium stage model is employed to gain insight into the thermodynamic properties. While examining the influence of these properties, essential process operating variables are optimized to save energy. The research result shows that more than 17% of energy is preservable in the HIASC.

A pseudo transient nonequilibrium method for rigorous simulation of multicomponent separation columns

Nonequilibrium stage model is a significant improvement in multicomponent separation process simulation, but more equations are involved and the solution of the model equations, which relies on an adequate initial guess for convergence of the Newton method, is difficult. In this work, based on the concept of pseudo-transient continuation approach, we proposed a new pseudo-transient (PT) nonequilibrium method. The proposed method decouples the strongly coupled model equations by introducing dynamic equations for material and energy conservation, as well as transition equations. Thus, the steady-state solution of the nonequilibrium stage model can be obtained through a robust and fast integration process, and the initial guess issue in Newton method can be effectively avoided. Two simulation cases were used to demonstrate the advantages and applicability of the proposed PT nonequilibrium method.

Determining catalyst loading in reactive distillation column

The widespread industrial application of reactive distillation technology is inseparable from the development of catalytic packing. However, systematic research on the heterogeneous catalyst loading in the reactive distillation column is vacant, and existing studies about catalyst loading on each stage and their distribution ignore the physical structure of the reactive distillation column. In this work, Modular Catalytic Structured Packing combining catalyst bags and structured packing sheets is applied in the modeling and optimization of reactive distillation processes. The catalyst loading on each stage is obtained from three parameters: the catalyst volume fraction of catalytic packing, the catalytic distillation column diameter, and the height of the theoretical stage. Using the effective diffusivity method, a simplified non-equilibrium stage model is chosen to simulate the reactive distillation process, and the bubble-point method for solving the non-equilibrium stage model’s equation system is introduced. The genetic algorithm optimizes both catalyst volume fraction and the number of theoretical stages in the reactive section. Case studies of methyl acetate esterification and ethyl tert-butyl ether (ETBE) synthesis show that greater theoretical stages of the reactive section and enough reflux ratio will reduce the demand of catalysts. Redistribution of catalyst is implemented by dividing the reactive section into several. The non-uniform distribution of catalyst will reduce both energy consumption and catalyst amount for the methyl acetate system, but insignificant for ETBE synthesis, due to different interactions of reaction and separation.

Experimental Studies of a Petlyuk Column and Validation of a Non‐Equilibrium Stage Model

AbstractIntegrated distillation equipment like the Petlyuk column offers enormous energy‐saving potential. However, high complexity and few available experimental data lead to high uncertainty. An integrated approach to reduce this risk is proposed. The first study on the influence of relative volatilities on the process behavior is presented. Comprehensive pilot‐plant experiments are displayed for three different component systems of linear alcohols. The experiments are carried out for a wide range of operating conditions. Results show a stable operation and high product purities. Based on the experimental data, a predictive non‐equilibrium stage model for column design is validated. Experiment and simulation indicate good agreement.

Microwave-assisted continuous reactive distillation process for preparation of ethyl acetate

AbstractAs a novel process intensification technology, microwave-assisted continuous reaction distillation (MRD) was proposed for the esterification reaction and separation of ethyl acetate (EtOAc). The effects of reflux ratio, mole ratio of acetic acid (HOAc) to ethanol (EtOH), reboiler duty, microwave power on EtOH conversions, EtOAc purity and mass ratio of distillate to feed (D/F) were explored. In comparison with conventional heating, the experimental results revealed that the EtOAc purity in the distillate under microwave conditions (MC) was improved. Computer simulations for conventional and MRD systems were performed using the Aspen Plus non-equilibrium stage model to substantiate the experimental results. The model predictions are in good agreement with the experimental data, revealing the accuracy and reliability of the non-equilibrium model. This new MRD process can be an effective and productive method of ester production.

Modeling and simulation of packed-bed absorbers for post-combustion capture of carbon dioxide by reactive absorption in aqueous monoethanolamine solutions

Capture of carbon dioxide (CO2) from flue gases by reactive absorption in aqueous monoethanolamine (MEA) solutions can be regarded as a mature base-line technology for mitigation of greenhouse gas emissions at fossil fuel-fired power plants. Reliable modeling and simulation of the required packed-bed absorber is important for the design, scale-up, control, and optimization of post-combustion CO2 capture processes. Three types of rate-based models have been used for the simulation of CO2 packed-bed absorbers: the continuous differential contactor model, the nonequilibrium stage model, and the continuous film reaction model. This paper begins with a comparative review of previous works in which these rate-based models have been used for the simulation of the reactive absorption of CO2 in aqueous MEA solutions. The continuous differential absorber model (proposed by Pandya in 1983) stands out as the kind of rate-based model more often used in previous works. However, in order to obtain a consistent formulation of the differential equations that describe the material and energy balances for the liquid phase in the packed-bed absorber, we provide here a revised mathematical derivation of Pandya's model. By making a judicious selection of the most suitable correlations (or methods) currently available for the calculation of the liquid-phase speciation, the reaction rate constant, the mass-transfer enhancement factor, the density, heat capacity, viscosity, and surface tension of CO2-loaded aqueous MEA solutions, the heat of reactive absorption of CO2 in aqueous MEA, the gas-phase and liquid-phase mass transfer coefficients, the effective interfacial area, and the packing liquid holdup, we show from the computer simulation of our revised version of Pandya's model that it is possible to obtain accurate predictions of the absorber temperature profiles that were measured in a recently published and well-documented pilot-plant study.

Integration of Enzymatic Catalysts in a Continuous Reactive Distillation Column: Reaction Kinetics and Process Simulation

This work presents a feasibility study for an enzymatic reaction in a continuously operated reactive distillation column. As a model reaction, the transesterification of ethyl butyrate with n-butanol in the presence of lipase CALB was considered. For use in the distillation column, lipase CALB was immobilized by entrapment in a hydrophobic silica xerogel and introduced as granulate into the catalytic packing Katapak-SP-11. The reaction kinetics was experimentally determined for different concentration and temperature ranges and described by means of the Michaelis–Menten double-substrate kinetic model in combination with the Arrhenius model. With these kinetic data, process simulations were carried out with an Aspen Custom Modeler nonequilibrium-stage model validated for a DN50 pilot-scale column. The concentration of n-butanol in the reactive section was maintained low to decrease the inhibiting effects on the enzyme. For an optimized setup and operating conditions, conversion rates of more than 90% were achieved for n-butanol and 26% for ethyl butyrate. These results clearly demonstrate that lipase CALB can be applied in a continuously operated reactive distillation column.

Non-equilibrium stage based modeling of heat integrated air separation columns

The heat integrated air separation column (HIASC) is a frontier of energy saving distillation column due to its large energy saving potential. However, up to now, most of the related research works are based on an equilibrium stage model, which is of limited value for design and practical application purposes. A novel non-equilibrium stage based model of HIASC, which includes both the mass and energy transfer, is first proposed. The tuning of configuration parameters including the total stage number, the feed position and the sidestream position is then carried out. The characteristic analyses show that the non-equilibrium stage model is more accurate and valuable and thus provides an efficient tool for further design, optimization and control studies. At last, the operation optimization is further carried out, and the optimization results show that the energy consumption of HIASC decreases by about 12.4% compared with the conventional air separation columns (CASC).

Performance Prediction of Structured Packing Column for Cryogenic Air Separation with Hybrid Model

A detailed investigation of a thermodynamic process in a structured packing distillation column is of great importance in prediction of process efficiency. In order to keep the simplicity of an equilibrium stage model and the accuracy of a non-equilibrium stage model, a hybrid model is developed to predict the structured packing column in cryogenic air separation. A general solution process for the equilibrium stage model is developed to solve the set of equations of the hybrid model, in which a separation efficiency function is introduced to obtain the resulting tri-diagonal matrix and its solution by the Thomas algorithm. As an example, the algorithm is applied to analyze an upper column of a cryogenic air separation plant with the capacity of 17000m3·h-1. Rigorous simulations are conducted using Aspen RATEFRAC module to validate the approach. The temperature and composition distributions are in a good agreement with the two methods. The effects of inlet/outlet position and flow rate on the temperature and composition distributions in the column are analyzed. The results demonstrate that the hybrid model and the solution algorithms are effective in analyzing the distillation process for a cryogenic structured packing column.

Optimal design of catalytic distillation columns: A case study on synthesis of TAEE

The design of catalytic distillation (CD) columns is a challenging task because of the superposition of chemical reaction and distillation in one apparatus. In this work, a method to design a cost-optimal CD column for chemical systems with large number of components and chemical reactions is presented. The method is based on the following steps: (1) estimation of the number of theoretical stages and catalyst volume by the decomposition of the CD column into a sequence of chemical reactors and non-reactive distillation columns, (2) estimation of the column diameter and operating conditions using an equilibrium stage model, and (3) design of the column applying an optimisation algorithm and using a rigorous non-equilibrium stage model to represent the CD process. The method is applied to determine the optimal column configuration and operating conditions for the synthesis of tert-amyl ethyl ether from ethanol and isoamylenes. Eight components and four chemical reactions were selected to represent the chemical system in the simulations.

Simulation of Reactive Distillation: Comparison of Equilibrium and Nonequilibrium Stage Models

In the present study, two distinctly different approaches are followed for modeling of reactive distillation column, the equilibrium stage model and the nonequilibrium stage model. These models are simulated with a computer code developed in the present study using MATLAB programming. In the equilibrium stage models, the vapor and liquid phases are assumed to be in equilibrium and allowance is made for finite reaction rates, where as in the nonequilibrium stage models simultaneous mass transfer and reaction rates are considered. These simulated model results are validated from the experimental data reported in the literature. The simulated results of equilibrium and nonequilibrium models are compared for concentration, temperature and reaction rate profiles in a reactive distillation column for Methyl Tert Butyle Ether (MTBE) production. Both the models show similar trend for the concentration, temperature and reaction rate profiles but the nonequilibrium model predictions are higher and closer to the experimental values reported in the literature. Keywords—Reactive Distillation, Equilibrium model, Nonequilibrium model, Methyl Tert-Butyl Ether

Synthesis of dimethyl carbonate and propylene glycol in a pilot-scale reactive distillation column: Experimental investigation, modeling and process analysis

The transesterification of propylene carbonate with methanol to produce the two valuable products dimethyl carbonate and propylene glycol is associated with an unfavorable chemical equilibrium and complex thermodynamic behavior. This results in a challenging and cost-intensive process currently used for their production. The application of reactive distillation, which integrates chemical reaction and distillation into one single column is considered as potential candidate to improve efficiencies and product yields within chemical-equilibrium-limited systems. However, the design of such a column for industrial-scale applications is challenging as it needs the integration of model-based approaches with reliable experimental data. This paper presents an experimental and theoretical investigation of the simultaneous production of dimethyl carbonate and propylene glycol in a pilot-scale reactive distillation column. Experiments varying decisive operating parameters were successfully performed showing the feasibility of simultaneously producing dimethyl carbonate and propylene glycol in a reactive distillation column. The experimental results were used to select and validate a modeling approach for the simulation of the reactive distillation process. The successfully validated non-equilibrium stage model was applied to perform a process analysis pointing out the trends of both the reactant conversions and the product purities. The simulation results showed the option of achieving high propylene carbonate conversions while recovering an azeotropic mixture of dimethyl carbonate and methanol in the distillate. These results define the operating range for the economic optimization and establishment of an industrial-scale reactive distillation process for this chemical system.

Optimisation of industrial-scale n-butyl acrylate production using reactive distillation

The current production process for the synthesis of n-butyl acrylate is challenging and cost intensive, due to a complex thermodynamic behaviour and a high risk of polymerisation. An integration of chemical reaction and distillation into one unit at the same time can be done by reactive distillation (RD). RD is one of the best known examples of process intensification. In order to facilitate the industrial application of this concept for the production of n-butyl acrylate, a process optimisation is presented in this article. A nonequilibrium-stage model is used, which was validated using experimental data in an earlier study. The packing characteristics are adapted to an industrial scale column to allow a reliable optimisation. The validated model is used to perform a process optimisation of two process configurations which are a single RD column and an RD column coupled with a decanter at the top to allow a recycling of unused reactants. Finally, an economic evaluation of both introduced processes and a comparison to the conventional process will be shown which identifies the coupling of reactive distillation with a decanter as the most economic process for the production of n-butyl acrylate.

Esterification of Acrylic Acid and n-Butanol in a Pilot-Scale Reactive Distillation Column—Experimental Investigation, Model Validation, and Process Analysis

Due to a high risk of polymerization and complex thermodynamic behavior of the chemical system, the current production process for n-butyl acrylate synthesis is cost-intensive and challenging. Reactive distillation integrates chemical reactions and distillation into one unit at the same time and is one of the best-known examples of process intensification. To facilitate industrial application of this concept for the production of n-butyl acrylate, reliable experimental data are required. This article presents an experimental and theoretical investigation of the synthesis of n-butyl acrylate using reactive distillation. Experiments were conducted in a pilot-scale reactive distillation column, and the decisive operational parameters were varied. To predict the experimental results, a nonequilibrium-stage model was applied and the model was validated using the experimental data. The validated model was then used to perform a process analysis, showing trends in the conversion of acrylic acid and n-butanol and the purity of n-butyl acrylate that can be used for prospective optimization studies.

Modelling of homogeneously catalysed reactive distillation processes in packed columns: Experimental model validation

The design of reactive distillation processes requires reliable and accurate models to significantly decrease the expensive and time consuming experimental work. Different modelling approaches of varying complexity are available in the open literature. However, only few publications exist in which the question of the optimal modelling depth is discussed for homogeneously catalysed processes. Unlike these publications, we used experimental data in the present study to compare them with simulation results using different modelling depths for homogeneous reactive distillation processes. The nonequilibrium-stage model using the Maxwell–Stefan equations, the nonequilibrium-stage model using effective diffusion coefficients, the equilibrium-stage model including reaction kinetics, and the equilibrium-stage model assuming chemical equilibrium were investigated. The homogeneously catalysed transesterification of dimethyl carbonate with ethanol, for which pilot-scale experimental data were available, was used as a test system.

TAEE synthesis from isoamylenes and ethanol by catalytic distillation: Pilot plant experiments and model validation

In this work, the synthesis of tert-amyl ethyl ether (TAEE) from isoamylenes and ethanol by catalytic distillation (CD) is experimentally and theoretically investigated. As the isoamylenes feedstock for pilot-scale experiments, an industrial light gasoline fraction from a fluid catalytic cracking unit was used. Consequently, the feed stream to the CD column consisted of almost 100 components and approximately 20 chemical reactions took place in the column. For modelling purposes, six chemical reactions and 18 components were selected to represent the chemical system. Despite the simplifications carried out in the chemical system, a good agreement between experimental data and simulation results using a non-equilibrium stage model was found. The results presented in this paper show the technical feasibility of TAEE production by CD. However, the mole fraction of TAEE in the bottom stream of the column was only 24% due to the presence of heavy inert components in the column feed. The experimental conditions must be carefully selected, because TAEE decomposition and a high by-products formation occurred if high pressures or D/F ratios were used during the column operation.

Evaluation of Barros and Wolf Efficiency Correlations for Conventional and Extractive Distillation Columns in Bioethanol Production Process

In view of the increasing environmental concern, the use of renewable and less pollutant energy sources has been encouraged. In this context, bioethanol has been used as fuel in the anhydrous form, blended with gasoline. In this work, simulations of the extractive distillation process in anhydrous bioethanol production were carried out in Aspen Plus® using nonequilibrium stage model and equilibrium stage model with Barros and Wolf efficiency correlations for plate and component. Equilibrium stage model with plate efficiency determined through Barros and Wolf efficiency correlation showed a satisfactory agreement with results predicted by the nonequilibrium stage model for anhydrous bioethanol production process.

Experimental model validation for n-propyl propionate synthesis in a reactive distillation column coupled with a liquid–liquid phase separator

In the synthesis of some organic esters, reactive distillation coupled with a liquid–liquid phase separator is often used to increase the product purity or to recover the reactants. In this article, we present a comprehensive experimental and theoretical study on the heterogeneously catalysed synthesis of n-propyl propionate by reactive distillation and a subsequent liquid–liquid phase separator. The experiments were performed in a pilot-scale reactive distillation column. Data-reconciliation tests proved that the experimental results obtained comprise a complete, reliable set of composition and temperature profiles along the pilot-scale reactive distillation column and can be used for further model validation. A nonequilibrium-stage model was applied to predict the experimental results. Simulation studies demonstrated that the composition and temperature profiles in the rectifying section of the column were highly sensitive to the composition of the reflux stream entering the column. Deviations between the experimental and predicted composition profiles in the rectifying section were identified. An explanation for the deviations is given in this article.

Nonequilibrium stage modelling of dividing wall columns and experimental validation

Dealing with complex process units like dividing wall columns pushes the focus on the determination of suitable modelling approaches. For this purpose a nonequilibrium stage model is developed. The successful validation is achieved by an experimental investigation of fatty alcohol mixtures under vacuum condition at pilot scale. Aim is the recovery of high purity products. The proposed model predicts the product qualities and temperature profiles very well.

Simultaneous Momentum, Mass, and Energy Transfer Analysis of a Distillation Sieve Tray Using CFD Techniques: Prediction of Efficiencies

Conventional models for distillation columns are based on equilibrium and nonequilibrium stage concepts. Although equilibrium and nonequilibrium stage models provide useful results, they neglect the fluid dynamics phenomena by assuming a perfect mixture on the plates in each phase. However, the flow pattern on a distillation tray is of great importance in terms of the mass and energy transfer mechanisms, and this influence can only be analyzed by carrying out a fluid dynamics study. The main objective of this study is to obtain experimental data of clear liquid height in a distillation sieve tray apparatus operating with air and water, and to apply a computational fluid dynamics (CFD) model in a Eulerian−Eulerian framework for gas−liquid flows that is able to predict the momentum, mass, and thermal phenomena of multiphase flows. A two-phase, three-dimensional and transient model with chemical species, energy, and momentum conservation balances was applied to predict the volume fraction, velocity, pressure...