Rate-based Simulation Research Articles

Model validation and simulation issues for carbon dioxide reactive absorption with mixed amines

Capture of CO2 form point emission sources is a dramatically urgent task to limit the current impact of the power generation sector and of industrial fields classified as “hard to abate”. Many challenges are still present from the design point of view to define correctly the properties (chemical, physical, rheological) of the used solvents. These affect the sizing and rating of absorption devices and the regeneration systems. In turn, this jeopardises the prediction of feasibility of capture plants and the relative economics.In this work, the modeling steps commonly applied to calculate the CO2 absorption into a mixed amine-water solvent have been exemplified starting from ground data. First, several open datasets of equilibrium and rheological quantities have been reviewed. This allowed the selection of the thermodynamic parameters that give the best compromise between physical and chemical equilibria. Then an adsorption tower was dimensioned, checking the impact that different tunable parameters have on the calculation outcome. Furthermore, two absorber-stripper tandem columns have been simulated in order to reproduce available plant data. The performed rate-based simulations find a convergence point only for proper liquid/gas ratios, and are significantly slowed-down as the number of kinetic reactions increases respect to instantaneous equilibrium ones.Results demonstrate that the CO2 loading into a solvent depends crucially on the partial pressures of all the species and on setting the mixture at fixed volume or pressure. This data collection and validation was exemplified following the whole procedure, as a guidance for technicians in the field. An example of gas flowrate up to 160 kg/h, with CO2 content 15 – 20 vol% and for CO2 capture efficiencies up to 90%.

Optimization and experimental validation of an extractive distillation system with solvents mixture as separating agent

This work aims to develop an economic optimization and experimental validation of ethanol separation by extractive distillation using an ethylene glycol - glycerol mixture as separating agent with and without thermal integration. An anhydrous ethanol production plant with a capacity of 300 000 L/day was simulated through a rate-based approach in Aspen Plus V10®. The optimization problem was solved, firstly, through a rate-based simulation, using a Sequential Quadratic Programming (SQP) algorithm available in Aspen Plus® and secondly, through a stochastic algorithm (genetic) available in Matlab R2019a®. US$ 0.1034 was the optimized TAC/kg of anhydrous ethanol, this parameter reduced by US$ 0.0004 to US$ 0.0006 when thermal integration was included and differed by less than US$ 0.0038 between both methodologies, where the lowest value was found using the stochastic algorithm. Experimental validation was conducted on a pilot scale extractive distillation to verify the results from the process optimization. The use of the 60–40 mol% ethylene glycol - glycerol mixture showed a 40.1% reduction in the total energy consumption of the process compared to the case where only ethylene glycol is used as entrainer. This allowed to confirm the synergistic effect of the solvents mixture and the possibility of employing them.

The unexpected weirdness of rate-based simulation of some high-pressure column operations

Diffusion is the process by which molecules move. One way to model multicomponent diffusion is with a version of Fick’s law: (J) = - [D] d(c)/dz where [D] is a matrix of multicomponent diffusion coefficients. [D] is the product of two other matrices, one of which involves partial derivatives of the fugacity or activity coefficients. It is usually the case that all of the elements on the main diagonal of [D] have a positive value. Wambui Mutoru and Firoozabadi (2011, 2012) showed that CO2-rich and N2-rich hydrocarbon systems could have main diagonal elements of [D] that are negative. Using rate-based column simulations, we have found that these negative main diffusion coefficients readily occur in some high-pressure distillation operations. We investigate these processes in more detail and explore the consequences for the design and operation of columns in which this phenomenon is predicted to occur.

Rate-based column simulation in the complex domain

Several organizations provide both equilibrium stage and rate-based column models in commercially available process simulation software. Many years of troubleshooting rate-based simulations tells us that they are more likely to suffer convergence issues than is an equivalent equilibrium stage model. The increased difficulty of converging rate-based simulations has to do with ties to actual equipment size that does not exist in the classic equilibrium stage model. Some years ago, Lucia and coworkers proposed solving chemical process models using computations in the complex-domain. They found that some hard-to-converge equilibrium stage column simulations were much easier to solve when carried out in the complex domain. We wondered if Lucia’s approach could help converge very challenging rate-based simulations. To answer that question, we have built a rate-based complex-domain code. Our most important result is that the complex-domain code can easily solve-rate based column simulations that are too difficult for the real-domain code.

Improvement of CO2 capture processes by tailoring the reaction enthalpy of Aprotic N‑Heterocyclic anion-based ionic liquids

A wide variety of Aprotic N‑Heterocyclic Anion-based Ionic Liquids (AHA-ILs) were designed using DFT/COSMO-RS quantum chemical calculations to optimize their performance as chemical absorbents on industrial-scale CO2 capture processes by tailoring the enthalpy of reaction. Five different substituents located at multiple positions on eight different heterocyclic rings were considered to cover a wide range of CO2-IL reaction enthalpies. Twelve representative AHA-ILs based on [P66614] cation with an enthalpy of reaction from -30 to -64 kJ/mol were evaluated on post-combustion, biogas and pre-combustion CO2 chemical capture processes through Aspen Plus Rate-based simulations. AHA-ILs presenting a CO2 reaction enthalpy between -43 and -54 kJ/mol were found to minimize both solvent and energy consumptions and thus equipment and operating costs in all studied CO2 capture systems, since cyclic capacity is successfully optimized by a correct selection of the reaction enthalpy. Some non-synthesized AHA-ILs, as [P66614][4-BrPyra] (-49.3 kJ/mol), are proposed as promising CO2 chemical absorbents based on their enhanced performance in post-combustion, biogas and pre-combustion capture processes from current techno-economic analysis.

Extractive distillation to produce anhydrous bioethanol with choline chloride with urea (1:2) as a solvent: a comparative evaluation of the equilibrium and the rate-based models

The purification of bioethanol to achieve anhydrous purity standards requires separation processes other than conventional distillation, such as extractive distillation. Different solvents have been studied to find alternatives to ethylene glycol, which is widely used in this application. In this study, the aim is to find the column configuration that offers the minimum total annual cost (TAC), considering a more realistic modeling than the equilibrium stage approach for the extractive distillation with ChCl:Urea (1:2) under atmospheric pressure. This evaluation uses the rate-based modeling in the Aspen Plus® simulator. The equilibrium stage model results show that, for the same number of distillation stages and solvent flow rate, ChCl:Urea (1:2) offers from 27 to 43 % lower energy requirement for the extractive distillation process than ethylene glycol. In addition, for the same number of distillation stages it is required lower solvent amount of ChCl:Urea (1:2) in comparison with ethylene glycol. Considering the configurations of each mole flow evaluated of ChCl:Urea (1:2), on average, the rate-based simulation required columns with 62 % more stages and 8.4 % higher TAC than the equilibrium simulation. These results demonstrate the importance of considering mass and heat transfer effects to model the real process.

Simulation and validation of carbon dioxide removal from the ethane stream in the south pars phase 19 gas plant by different amine solutions using rate-based model

The high pressure absorption of carbon dioxide (CO2) from a distilled ethane stream at Phase 19 of South Pars Gas Complex (in Iran) has been simulated by Aspen Plus using non-random two-liquid and Peng-Robinson models in addition to the technical and operational data. The plant employs 40 wt% diethanolamine (DEA) solution as the actual absorbent in a conventional column. The potential single and blended absorbents including methyldiethanolamine (MDEA), piperazine (PZ) + MDEA, and DEA + MDEA have been modelled to estimate the outlet CO2 and sulfur compounds concentrations, CO2 loading, reboiler power supply and regeneration heat duty to determine the most efficient substitute. The model is in good agreement with the real data of the plant. The rate-based simulation results reveal that CO2 and H2S concentrations have nearly reached zero while the total sulfur content is below the demanded specification (10 ppm wt) by the application of a blended amine solution with a low PZ concentration as 1 wt% PZ + 39 wt% MDEA in the operational limits of 110–120 ton/hr and 40–55 °C, suggesting that the PZ + MDEA solution can be used as the most appropriate alternative to the 40 wt% DEA solution due to the higher CO2 concentration (1.950% mole) and CO2 loading (0.216) of the rich solution as well as the lower required regeneration heat duty (10.590 GJ).

Flexible Operation of the Potassium Taurate Solvent Absorption Section for CO2 Removal in a Coal-Fired Power Plant

To reduce global warming effects due to the increase of the carbon dioxide concentration in the atmosphere, CO2 can be removed from large emitting industrial sources as power production stations with several available technologies. Among these, the most common one is chemical absorption with aqueous amines. The benchmark solvent is MonoEthanolAmine (MEA), though presenting a few disadvantages such as being corrosive and toxic and requiring large amounts of energy for its regeneration. An aqueous solution of potassium taurate has the potential to replace MEA because of degradation resistance, low toxicity and low energy requirements. In addition, at specific conditions, it can enhance the chemical absorption of carbon dioxide by precipitating and forming a slurry with presence of solid taurine. The considered process scheme is similar to the one for the MEA system (basically composed of absorption column and stripping or distillation column), with the addition of a heat exchanger for the dissolution of any solid particle present in the system before being fed to the regeneration section. When applied to a power plant, CO2 removal decreases the revenues from the sale of electricity. Indeed, the requirement of thermal energy for running the reboiler of the regeneration section and of electrical energy for compressing the separated CO2 are some of the main drawbacks of the application of Carbon Capture and Storage (CCS) and may result in a significant decrease in the power output and profit. Operating the CO2 removal section in flexible mode significantly reduces effects on the profit and CO2 emissions. The research reported here focuses on determining the best operating conditions for the potassium taurate solvent absorption section with the aim of limiting the impact of the CO2 capture operation and maintaining a substantial reduction of CO2 emissions in a coal-fired power plant. The process of post-combustion scrubbing is well-suited for operation in flexible mode because it is located downstream of the power production system. For simulating the process, a tool previously developed and based on the commercial software ASPEN Plus® has been employed. The process simulator had been user customized for the representation of the chemical reacting system of carbon dioxide with potassium taurate, which is not present by default in the database. The Vapor-Liquid-Solid Equilibrium (VLSE) is considered in the thermodynamic model and rate-based simulations, also taking into account the kinetics, have been performed. An in-house tool, taking as input the data for electricity demand and price and the result of simulations in ASPEN Plus®, has been then used for analyzing the flexible operation of the plant. The study of a flexible solution for carbon dioxide removal has been carried out in this work, considering the amount of electricity sold during the day for which data are available from the Italian national service institution.

Simulation Comparison Between Equilibrium and Rate-Based Approach for CO2 Removal Via Promoted K2CO3 with Glycine

The main limitation of rate-based approach in Aspen Plus is the unavailability of the method under dynamic analysis. Hence, to support the development of the integrated process of natural gas treatment at a higher scale, a comprehensive equilibrium and rate-based simulations of CO2 removal via potassium carbonate promoted with glycine (PCGly) is conducted. The purpose of this study is to observe the efficiency of the equilibrium-based model to represent the CO2 removal system before the dynamic simulation can be developed. The validation is conducted based on published experimental data and the result shows that at steady state, the equilibrium-based method is able to predict the CO2 removal as satisfactorily as the rate-based approach. The error deviation between both methods is 2.11 % and through this study, it is shown that the equilibrium-based method is able to simulate the CO2-PGly system efficiently. Thus, this would enable further investigation of the CO2-PGly system under dynamic simulation using the equilibrium-based method.

Development of a mass transfer model for the rate-based simulation of a batch distillation column

The rate-based model for distillation computes mass and heat transfer rates, allowing better predictions of the performance of the columns. Nevertheless, one of the fundamental issues with the rate-based model is the necessity of reliable mass transfer models, which may vary in terms of the kind of mixture and the trays/packing geometry. In this work, a mass transfer model is proposed to represent the separation of a methanol/ethanol mixture in the trays of a batch column. The model assumes a bubbling regime. The mass transfer model is incorporated to the rate-based model of the column, obtaining the composition and temperature profiles along the column. The model is validated with experimental data. The results indicate that rate-based model with the mass transfer model has a reduction of 3.5% in the error of the compositions profile compared with the equilibrium model.

Rate-based simulation study of boric acid promoted potassium carbonate capture system

This study presents parametric analysis and process modification on a post-combustion carbon dioxide (CO2) capture system using boric acid promoted potassium carbonate (K2CO3) solution. The model was simulated in Aspen Plus® V10 and ELECNRTL property package was employed. Un-promoted K2CO3 system was adopted as a base case study and its simulation was validated using literature data. The K2CO3 concentration in the lean solvent makeup was then varied from 25 wt% to 40 wt% to examine the effects of lean solvent concentration on key operating variables such as stripper reboiler duty, stripper condenser duty and CO2 capture rate. A 2 wt%, 4 wt% and 6 wt% aqueous boric acid solutions were then added as promoter to the hot K2CO3 solvent and the simulation results indicated that capture rate efficiency can be enhanced by 0.7%, 0.9% and 1.2% respectively. Energy saving process modification was also carried out to the stripper column. Compared to the conventional unmodified H3BO3/K2CO3 system, the modified promoted system is able to achieve 21.09% savings in reboiler duty.

Surrogate-Assisted Modeling and Robust Optimization of a Micro Gas Turbine Plant With Carbon Capture

Abstract The growing share of wind and solar power in the total energy mix has caused severe problems in balancing the electrical power production. Consequently, in the future, all fossil fuel-based electricity generation will need to be run on a completely flexible basis. Microgas turbines (mGTs) constitute a mature technology which can offer such flexibility. Even though their greenhouse gas emissions are already very low, stringent carbon reduction targets will require them to be completely carbon neutral: this constraint implies the adoption of postcombustion carbon capture (CC) on these energy systems. Despite this attractive solution, an in-depth study along with a robust optimization of this system has not yet been carried out. Hence, in this paper, a typical mGT with exhaust gas recirculation has been coupled with an amine-based CC plant and simulated using the software aspenplus. A rigorous rate-based simulation of the CO2 absorption and desorption in the CC unit offers an accurate prediction; however, its time complexity and convergence difficulty are severe limitations for a stochastic optimization. Therefore, a surrogate-based optimization approach has been used, which makes use of a Gaussian process regression (GPR) model, trained using the aspenplus data, to quickly find operating points of the plant at a very low computational cost. Using the validated surrogate model, a stochastic optimization has been carried out. As a general result, the analyzed power plant proves to be intrinsically very robust, even when the input variables are affected by strong uncertainties.

Rate-based simulation and techno-economic analysis of coal-fired power plants with aqueous ammonia carbon capture

Ammonia is a promising solvent for the chemical absorption of carbon dioxide owing to its characteristics of low toxicity, low degradation, low cost, and low heat of desorption. The present work evaluates the energy and economic performances of the aqueous ammonia technology applied to a coal-fired power plant. First, the kinetics of absorption are characterized in the Aspen Plus software and are calibrated against the experimental data from the Munmorah pilot plant. Then, the validated model is used for sizing the full-scale plant columns and for simulating the overall capture plant. After determining the sizes of the plant components and their energy consumptions, the capture plant is integrated with the coal-fired power plant, and the economic analysis is assessed with both retrofitted and green-field approaches as described in reports from the European Benchmarking Task Force and the United States National Energy Technology Laboratory, respectively. The novelties presented here are the new approach in the absorption reactions implemented in the rate-based model, which changes the dependency of the ammonia concentration in the absorption process, as well as the full economic analysis with both retrofitted and green-field assumptions, which enable a sound comparison of the outcomes. The results with the retrofitted approach compared with the monoethanolamine case of the European Benchmarking Task Force report returned higher electric efficiency (36.3% versus 33.50%), lower levelized cost of electricity (87.66 €/MWhel versus 92.27 €/MWhel), and lower cost of carbon dioxide avoided (47.03 €/tonCO2 versus 51.62 €/tonCO2). The results using the green-field approach compared against the Cansolv case of the National Energy Technology Laboratory report returned higher electric efficiency (33.06% versus 32.50%), lower cost of electricity (124.3 $/MWhel versus 133.2 $/MWhel) and lower cost of carbon dioxide avoided (66.5 $/tonCO2 versus 75.25 $/tonCO2). In both comparisons, aqueous ammonia had appreciably better performance than the ammine reference cases.

Modeling the Self-Assembly of Protein Complexes through a Rigid-Body Rotational Reaction-Diffusion Algorithm.

The reaction-diffusion equations provide a powerful framework for modeling nonequilibrium, cell-scale dynamics over the long time scales that are inaccessible by traditional molecular modeling approaches. Single-particle reaction-diffusion offers the highest resolution technique for tracking such dynamics, but it has not been applied to the study of protein self-assembly due to its treatment of reactive species as single-point particles. Here, we develop a relatively simple but accurate approach for building rigid structure and rotation into single-particle reaction-diffusion methods, providing a rate-based method for studying protein self-assembly. Our simplifying assumption is that reactive collisions can be evaluated purely on the basis of the separations between the sites, and not their orientations. The challenge of evaluating reaction probabilities can then be performed using well-known equations based on translational diffusion in both 3D and 2D, by employing an effective diffusion constant we derive here. We show how our approach reproduces both the kinetics of association, which is altered by rotational diffusion, and the equilibrium of reversible association, which is not. Importantly, the macroscopic kinetics of association can be predicted on the basis of the microscopic parameters of our structurally resolved model, allowing for critical comparisons with theory and other rate-based simulations. We demonstrate this method for efficient, rate-based simulations of self-assembly of clathrin trimers, highlighting how formation of regular lattices impacts the kinetics of association.

Thermodynamic modelling using e-UNIQUAC model for CO2 absorption by novel amine solutions: 1-Dimethylamino- 2-propanol (1DMA2P), 3-dimethylamino-1-propanol (3DMA1P) and 4-diethylamino-2-butanol (DEAB)

The proper selection of thermodynamic models is an important step in the design and simulation of CO2 removal processes using amine solutions. In this study, we aim to study the thermodynamic behaviour of the CO2 removal process by employing the extended-universal quasichemical (e-UNIQUAC) model for three novel amine systems, namely CO2+1DMA2P + H2O, CO2+3DMA1P + H2O, and CO2+DEAB + H2O. The thermodynamic behaviour was studied in terms of the CO2 loading of amines, the ion speciation profiles, the isothermal pressure-composition (Pxy) profiles, and the pH of the amine solutions. Adjustable parameters of the model include binary interaction parameters, and the volume and surface area parameters of the amine and protonated amine were determined using different objective functions based on the experimental data available in the literature. The ion and molecular speciation profiles are obtained and compared with nuclear magnetic resonance (NMR) data for CO2+1DMA2P + H2O and CO2+DEAB + H2O systems. The model predicted the experimental data with average absolute relative deviations (AARDs) of 8.06%, 13.39% and 16.50% for CO2 loading values of DEAB, 1DMA2P, and 3DMA1P, respectively. In addition, the results of Pxy profiles at different temperatures show that the azeotrope does not form in the 3DMA1P + H2O system. The adjustable parameters and predicted data of the applied e-UNIQUAC model may contribute to the rate-based simulation of CO2 absorption processes using novel amine systems.

Rate-based simulation of a high pressure counter-current packed column for supercritical CO2 extraction of alcohol from dilute aqueous mixtures

In this work, the modeling and simulation of a fractionation packed column for the recovery of isopropanol from dilute aqueous mixtures using supercritical CO2 is presented. The model is based on the numerical resolution of differential mass balances for each component over the column height. The multicomponent mass transfer between phases is described using a “rate-based” approach and the concept of local mass transfer coefficients. The model was validated by reproduction of experimental steady-state results for the fractionation of 5% isopropanol aqueous solutions obtained in a bench scale counter-current column. The effect of process conditions on the separation performance was satisfactorily described by the model calculations, showing that operation pressure and CO2 flow rate enhance IPA recovery and extract purity, while operation temperature has a negative effect. Model deviations (AARD) were in all cases lower than 20%.

Evaluation and comparison of aqueous ChCl/Urea and other physical absorbents for biogas upgrading process

Biogas has been considered as an alternative renewable energy, and CO2 removal from raw biogas (i.e. biogas upgrading) is needed for producing biomethane used as vehicle fuels or injected into the natural gas grid. Biogas upgrading using physical absorbents is a simple and efficient technology with low energy requirements for regeneration. In this work, the conceptual process for biogas upgrading using 4 kinds of physical solvents, i.e. water, dimethyl ether of polyethylene glycol (DEPG), propylene carbonate (PC) and aqueous choline chloride (ChCl) /urea (AQDES) was developed and simulated with Aspen Plus. The energy utilization, the amount of recirculated solvent and the diameters of absorber and desorber were analyzed based on equilibrium approach. After that, the rate-based simulation was established to evaluate the specific cost of the process using different solvents. Based on equilibrium approach the comparison between the solvents in respect to the energy utilization for biogas upgrading using different solvents shows the following order: DEPG > water > AQDES > PC, whereas the amount of recirculated solvent and the diameters of absorber and desorber follow another order: water > DEPG > AQDES > PC. The rate-based results show that the process using PC has the lowest total specific cost, followed by AQDES, water and DEPG.

Comparison of Mass Transfer Models on Rate-Based Simulation of CO2 Absorption and Desorption Processes

The effective technology for capturing CO2 at the low concentration is chemical absorption, due to the high reactivity between CO2 and aqueous amine solutions. To capture CO2, the process involves complex reactive separations. The accurate calculation of hydrodynamic properties, and mass and energy transfer are of importance for the design of the absorber and desorber columns. This paper performs the rate-based simulations of CO2 absorption by Monoethanolamine in Aspen Plus. In the calculation of the mass transfer coefficients, different mass transfer models were implemented. In comparison with the desorber, the impacts of mass transfer models were more significant in the simulation of the absorber. For both columns, the impacts of the mass transfer models on the concentration profiles were more significant than those on the temperature profiles. For the absorber, the maximum deviations occur at the bottom of the column for both the concentration and the temperature profiles. Different from the absorber, for the desorber, the maximum deviations occur close to the top of the column.

Rate-based modelling and simulation of large-scale co2 capture using a piperazine-promoted aqueous ammonia solution

Rate-based modelling and simulation of large-scale co2 capture using a piperazine-promoted aqueous ammonia solution

Analysis on Regeneration Energy of NH3-based CO2 Capture with Equilibrium-based Simulation Method

In this paper, an equilibrium-based simulation method for CO2 capture using aqueous ammonia was investigated. Quantitative comparisons of the equilibrium- and rate-based methods were performed. Effects of the stage number on the equilibrium- and rate-based simulations were obtained. Analysis on regeneration energy of NH3-based CO2 capture with the equilibrium-based simulation method was carried out. As the stage number for the equilibrium-based simulations increases, the regeneration energy decreases gradually to a constant value. For the equilibrium-based simulations of the NH3-based CO2 regeneration process, with the increase of the reboiler heat duty, the regeneration energy decreases to a minimum value firstly, and then begins to increase. In addition, effects of the temperature, NH3 concentration and CO2 loading of the rich solvent flowing into the stripper on the regeneration energy were obtained using the equilibrium-based simulation method.