Reactive Absorption Processes Research Articles

Hydrodynamic and mass transfer parameters for CO2 absorption into amine solutions and its blend with nano heavy metal oxides using a bubble column

ABSTRACT In this work, hydrodynamic and mass transfer of CO2 absorption into metal oxide nanofluid includes TiO2, ZnO, and ZrO2 nanoparticles at Piperazine (Pz) solution in the bubble column was studied. The novel rigorous correlations were extended based on the variable parameters for the calculation of gas holdup, enhancement factor, and mass transfer coefficients in the reactive absorption processes using the π-Buckingham method. The correlations were constructed based on dimensionless numbers, including nanoparticle loading, film parameter, CO2 partial pressure ratio to the total pressure, the film thickness of phases, CO2 loading, and ratio of diffusion coefficients of CO2 in the gas and liquid phases. Experimental data were used to calculate the correlations, consisting of Pz concentration, CO2 partial pressure, nanoparticle loading, and stirrer speed in the range of 0.1–0.5 mol/l, 16.0–35.2 kPa, 0.0–0.1 wt%, and 0–300 rpm, respectively. The film parameter was applied to exert the influence of chemical reactions on the performance of absorption. The mean squared error (RMSEP), the average absolute error (%AARD), and the coefficient of correlation of the results (R2 ) for the correlations were in the range 0.021–2.604, 3.78–18.14, and 0.861–0.980, respectively, which represents the excellent accuracy of the proposed correlation.

Mass Transfer Modeling of CO2 Absorption into Blended Aqueous MDEA–PZ Solution

In this research, the rate of CO2 absorption into methyl diethanolamine–piperazine (MDEA–PZ) solution was investigated. To model the mass transfer flux in the reactive absorption processes, the dimensionless parameters of the process were obtained using the Buckingham Pi theorem and considering the effective parameters in mass transfer. The CO2 mass transfer flux in the reactive absorption process depends on the mass transfer parameters of both the liquid and gas phases. Based on the dimensionless parameters obtained, a correlation is proposed to calculate the mass transfer flux of acidic gases in MDEA–PZ solutions. The mass transfer flux in the reactive absorption process is modeled based on the four laws of chemical equilibrium, phase equilibrium, mass balance, and charge balance. Experimental data from the literature were used to determine the constants of the derived correlation as a function of dimensionless parameters. In the provided correlation, the effects of dimensionless parameters including film parameter, CO2 loading, ratio of diffusion coefficients in the gas–liquid phase, CO2 partial to total pressure, and film thickness ratio as well as factors such as temperature, the number of free amines in the solution, the partial pressure of CO2, on the CO2 mass transfer flux were investigated. According to the results, the absorption rate decreases with increasing CO2 loading and film parameter, and the mean absolute deviation is about 3.6%, which indicates the high accuracy of the correlation.

Pressure drop and flooding in rotating packed beds

Rotating packed beds (RPB) are an intensified alternative to conventional absorption columns. RPBs could substantially reduce costs associated with reactive-absorption processes, such as post-combustion CO2 capture. This study presents internal radial pressure profiles obtained experimentally from inside a rotating packed bed. This approach allows the individual contributions to the overall pressure drop to be investigated in greater detail than previous studies. Experiments examined the effect of rotation speed, gas flow and liquid flow on pressure drop in RPBs. The result illustrated the pressure drop can be described well using a simple 1D pressure drop contribution model. The casing pressure drop is due in part to a free vortex that forms in the RPB eye, along with entrance and exit losses. This result improves understanding of gas pressure drop, and how these factors affect the performance and design of full-scale RPBs.

NMR spectroscopic method for studying homogenous liquid phase reaction kinetics in systems used in reactive gas absorption and application to monoethanolamine–water–carbon dioxide

An NMR spectroscopic method for measuring homogenous liquid phase reaction kinetics in systems that are used in reactive gas absorption processes is presented. In the kinetic experiment, carbon dioxide loaded and unloaded aqueous amine solutions are mixed such that no gas phase is involved. This procedure enables studying liquid phase reaction kinetics without the influence of the kinetics of the physical absorption process. A rapid-mixing NMR flow cell is used for the measurements, which are carried out in stopped-flow mode. 1H NMR spectra are taken at short intervals to monitor the kinetic process. The cell is liquid thermostatted and pressure resistant, such that a wide range of conditions can be studied. The new method was used for studying reaction kinetics in the system monoethanolamine (MEA)–water (H2O)–carbon dioxide (CO2) at temperatures between 293 and 333 K. From the data, information on the kinetics of the reaction of MEA with bicarbonate (HCO3-) to form MEA carbamate was obtained. That reaction was investigated for the first time in the present work without using bicarbonate salts. An Arrhenius model was fitted to the new data. The formation of MEA carbamate from MEA and HCO3- is often neglected in models for describing the reactive absorption of CO2 with aqueous MEA solutions. A process simulation study carried out in the present work reveals that it should be taken into account in predictions of reactive absorption processes that are operated under elevated pressure with a high mole fraction of CO2 in the gaseous phase.

Different Strategies for Accelerated CO2 Absorption in Packed Columns by Application of the Biocatalyst Carbonic Anhydrase

Within this work, the combination of an energetically favorable aqueous N-Methyldiethanolamine (MDEA) solution and the enzyme carbonic anhydrase is investigated in a packed column pilot plant. The use of aqueous MDEA solution for CO2 separation is already known from natural gas applications, for which an increased driving force due to the higher partial pressures of CO2 overcompensates reaction kinetic limitations. The objective of the addition of carbonic anhydrase is to countervail the loss of separation efficiency caused by the lower driving force in CO2 capture from power plant flue gases. Hence, carbonic anhydrase acts as a key to harness the energetic advantage of these solvent systems. However, application of the enzyme also poses restrictions on the process. Especially compliance with the enzyme stability limits is challenging for desorption, which is generally performed at high temperatures.In order to determine an optimal implementation of the enzyme into the process the current work presents different strategies how to apply CA as a biocatalyst in reactive absorption processes and shows how absorption efficiency is influenced. For introducing the enzyme to a packed column two approaches are investigated in this study. The simplest way of application is to dissolve the enzyme in the solvent. This allows the enzyme to work exactly where the reaction kinetic limitation can be found, in the liquid boundary layer. However, due to the temperature sensitivity of the enzyme an additional enzyme recovery step prior to the desorber might be necessary if desorption is to be performed at high temperatures. The immobilization of the enzyme inside the absorption column presents an alternative to prevent this additional separation, but may cause additional mass transfer limitations at the solid particles in which the enzyme is immobilized. The immobilization and the necessity of a suitable packing in which the enzyme particles can be filled, also makes this strategy more complicated but allows placing the enzyme at a location of most suitable process conditions in the column. But most importantly it completely avoids that the enzymes experience high temperature in the desorber. Systematic investigations of the influence of specific liquid load, liquid inlet temperature, MDEA-concentration and enzyme immobilization on absorption performance are conducted. Dissolved enzyme showed a nearly three times higher absorption performance than the immobilized enzyme under equivalent operating conditions, however the immobilized enzyme concentration used was effectively 50 times lower, meaning the result is actually quite promising for immobilized CA. From the investigated operating conditions, a liquid inlet temperature of 20°C, a MDEA concentration of 30wt.-% and a liquid flow rate of 24 m3 m-2 h-1 showed the best absorption performance with the dissolved enzyme. The measured absorption rate was 7.57 times higher than without enzyme added.

Rigorous correlation for CO2 mass transfer flux in reactive absorption processes

Mass transfer flux of CO2 in reactive absorption processes depends on mass transfer variables of both liquid and gas phases. In this work, a new rigorous correlation for calculation of CO2 mass transfer flux in reactive absorption processes was developed based on the variables. The correlation parameters including film parameter, CO2 loading, concentration ratio, pressure ratio, film thickness ratio and diffusion coefficient ratio were determined using Buckingham Pi-theorem. Experimental data of CO2 reactive absorption in piperazine solution were used to evaluate the correlation. The data used in calculating the correlation constants are temperature in range of 40–100°C, partial pressure of CO2 in range of 18–66,330Pa, CO2 loading in range of 0.226–0.412mole of CO2 per mole of amine and amine concentration of 2–8molal (Dugass, 2009). The average absolute relative error for the presented correlation is 5.8% which infers high accuracy of this correlation compared to the previous correlations reported based on enhancement factors. The results showed that increasing the temperature and CO2 loading has negative effect on CO2 mass transfer flux. Also the results indicated that on increasing partial pressure of CO2 and piperazine concentration, the rate of mass transfer was increased.

NONEQUILIBRIUM MODELING OF REACTIVE ABSORPTION PROCESSES

A nonequilibrium stage model of reactive absorption processes has been developed, considering the interactions between reaction and diffusion in the film region. Dynamic and steady-state film models have been incorporated into the process model for low and highly soluble gases respectively. The model was validated against experimental data. The data were obtained from a pilot-plant absorption column in which simultaneous reactive absorption of ammonia and carbon dioxide in aqueous partially carbonated ammonia solution occurred. A comparison between simulation results and the experimental data showed that the model could reasonably predict the process. The model predictions were also compared with those of a process model including enhancement factor. The results indicate that using a film model leads to a more accurate process model than applying enhancement factor.

The applicability of activities in kinetic expressions: A more fundamental approach to represent the kinetics of the system [formula omitted]– [formula omitted]–salt in terms of activities

The applicability of utilizing activities instead of concentrations in kinetic expressions has been investigated using the reaction of CO 2 in sodium hydroxide solutions also containing different neutral salts (LiCl, KCl and NaCl) as model system. For hydroxide systems it is known that when the reaction rate constant is based on the use of concentrations in the kinetic expression, this “constant” depends both on the counter-ion in the solution and the ionic strength which is probably caused by the strong non-ideal behavior of various components in the solution. In this study absorption rate experiments have been carried out in the pseudo-first-order absorption rate regime. The experiments have been interpreted using a new activity based kinetic rate expression instead of the traditional concentration-based rate expression. A series of CO 2 absorption experiments in different NaOH (1, 1.5, 2.0 mol l - 1 )–salt (LiCl, NaCl or KCl)–water mixtures has been carried out, using salt concentrations of 0.5 and 1.5 mol l - 1 all at a temperature of 298 K. Interpretation of the data additionally required the use of an appropriate equilibrium model (needed for the calculation of the activity coefficients), for which, in this case, the Pitzer model was used. The additions of the salts proved to have a major effect on the observed absorption rate. The experiments were evaluated with the traditional concentration based-approach and the “new” approach utilizing activity coefficients. With the traditional approach, there is a significant influence of the counter-ion and the hydroxide concentration on the reaction rate. The evaluation of the experiments with the “new” approach—i.e. incorporating activity coefficients in the reaction rate expressions—reduced the influence of the counter-ion and the hydroxide concentration on the reaction rate constant considerably. The absolute value of the activity based-reaction rate constant k OH - m ( γ ) for sodium hydroxide solutions containing either LiCl, KCl or NaCl is in the range between 10 000 and 15 000 kg ( kmol - 1 s - 1 ) compared to the traditional approach where the value of the lumped reaction rate constant k OH - is between 7000 and 34 000 m 3 ( kmol - 1 s - 1 ) . Therefore, it can be concluded that the application of the new methodology is thought to be very beneficial especially in processes where “the thermodynamics meet the kinetics”. Based on this it is anticipated that the new kinetic approach will first find its major application in the modeling of integrated processes like Reactive Distillation, Reactive Absorption and Reactive Extraction processes where both, thermodynamics and kinetics, are of essential importance and, additionally, activity coefficients deviate substantially from unity.

Design of Industrial Reactive Absorption Processes in Sour Gas Treatment Using Rigorous Modelling and Accurate Experimentation

In this paper a rigorous rate-based model for the selective absorption and desorption of sour gases in packed towers within a coke oven gas purification process (VACASULF ®-process) is presented. It considers multi-component mass transfer of CO 2, H 2S, NH 3 and HCN in aqueous potassium hydroxide (KOH) or potash (K 2CO 3) solutions and the influence of chemical reactions on mass transfer using enhancement factors. A modified Pitzer model is used to estimate activity coefficients in the electrolyte solution for the calculation of the chemical and the phase equilibria as well as the reaction kinetics (Brönsted-Bjerrum equation). Using data from the literature this thermodynamically consistent approach was successfully validated for the phase equilibria and the reaction kinetic calculation. Since no experimental data for the above-mentioned chemical system in packed towers is available in the literature, own experiments were carried out in a pilot plant packed tower. A newly developed experimental method using redundant measurements and a Data Reconciliation procedure improved the accuracy of concentration profiles for the gas and the liquid phase. Using eight absorption and eight desorption experiments, as well as industrial on-site measurements of the entire plant, the model was successfully validated. In addition, the model was used for global optimization using evolutionary algorithms, revealing an optimization potential of 30% in operating costs.

Design sensitivity of reactive absorption units for improved dynamic performance and cleaner production: the NO x removal process

The dynamic characteristics of reactive absorption processes are of great importance for the smooth operation of the unit and the overall performance of the implemented control system under the influence of process disturbances and the presence of tight environmental and safety constraints. In the present study, the effect of the major design parameters and column configurations on the dynamic behaviour of the environmentally sensitive NO x removal processes, through the use of rigorous rate-based dynamic models, is investigated. Static and dynamic disturbance rejection properties are evaluated for the screening and assessment of alternative design decisions.

04/01075 Dynamic simulation of industrial reactive absorption processes: Schneider, R. et al. Chemical Engineering and Processing, 2003, 42, (12), 955–964

04/01075 Dynamic simulation of industrial reactive absorption processes: Schneider, R. et al. Chemical Engineering and Processing, 2003, 42, (12), 955–964

Rate-based modelling of SO 2 absorption into aqueous NaHCO 3/Na 2CO 3 solutions accompanied by the desorption of CO 2

A rate-based model of a counter-current reactive absorption/desorption process has been developed for the absorption of SO 2 into NaHCO 3/Na 2CO 3 in a packed column. The model adopts the film theory, includes diffusion and reaction processes, and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. Model predictions were compared to experimental data from literature. For the calculation of the absorption rate of SO 2 into NaHCO 3/Na 2CO 3 solutions and concomitant CO 2-desorption, it is important to take into account all reversible reactions simultaneously. It is clear that the approximate analytical based model cannot be expected to predict the absorption rates under practical conditions because of the complicated nature of the reactive absorption processes. The rigorous numerical approach described here only requires definition of the individual reactions in the system, and subsequent solution is independent of specific assumptions made, or operational variables like pH or compound concentrations. As an example of the flexibility of this approach, additional calculations were conducted for SO 2 absorption in a phosphate-based buffer system.

Rigorous Modeling of Reactive Absorption Processes

AbstractAbsorption of gases in liquid solutions accompanied by chemical reactions, or reactive absorption, represents one of the most important industrial operations. The advantages of this process are enhanced solution capacity, low impurity concentrations up to the ppm range, moderate operation pressure, and selective removal of contaminants. Reactive absorption is based on a combination of various kinetically controlled phenomena, the focus being on the coupling of chemical reactions and mass transfer in multicomponent mixtures. Therefore, the process description using the equilibrium concept is often insufficient and kinetic modeling is required. This paper describes the basics of rigorous modeling of reactive absorption processes. Several case studies are used in order to validate the models. In addition, a discussion of necessary model parameters, like diffusion coefficients and reaction kinetics, is given. A reduced model with lower computation time is obtained via detailed sensitivity analysis. This model is successfully used in dynamic simulations. In this respect, a proper consideration of the film reactions appears to be crucial.

Dynamic simulation of industrial reactive absorption processes

Accurate dynamic models for industrial reactive absorption processes have to be both rigorous enough in order to reflect the process complexity and simple enough in order to ensure feasibility of process simulations. A comparison of different model approaches revealed that traditional equilibrium stage models and efficiency approaches are inadequate in this context. Therefore, we have developed a new rigorous dynamic two-phase model based on the two-film theory, which serves as a reference description and takes into account the influence of chemical reactions and additional driving forces in electrolyte systems on mass transfer, thermodynamic non-idealities and the impact of structured packings and liquid distributors on the process hydrodynamics. For the model-based control and dynamic on-line simulation some model reductions are required to limit the calculation time for the numerical solution. According to sensitivity study results, simplifications for some physical parameters and a linearisation of the film concentration profiles have been accomplished leading to a reduction of the total number of equations by half. The model has been applied to an industrial sour gas purification process with many parallel and consecutive reversible chemical reactions within a multicomponent system of aqueous electrolytes and validated by pilot plant experiments.

Rigorose Modellierung von Reaktivabsorptionsprozessen

Die Absorption von Gasen in flüssigen Waschmitteln mit begleitenden chemischen Reaktionen (reaktive Absorption) stellt eines der wichtigsten industriellen Verfahren dar. Vorteilhaft erweisen sich dabei Eigenschaften wie hohe Beladbarkeit der Waschflüssigkeit, Entfernung von gasförmigen Komponenten bis in den ppm-Bereich, moderate Betriebsdrücke und selektive Entfernung von Schadkomponenten. Die reaktive Absorption beruht auf einer Kombination verschiedener kinetisch kontrollierter Phänomene, wobei im Mittelpunkt die Wechselwirkungen zwischen chemischen Reaktionen und Stofftransport in Mehrkomponentengemischen stehen. Daher ist eine Beschreibung auf der Basis des Gleichgewichtsmodells häufig unzutreffend und eine kinetisch basierte Modellierung von entscheidender Bedeutung. In diesem Beitrag werden die Grundlagen der rigorosen Modellierung von Reaktivabsorptionsprozessen erläutert. Anhand mehrerer Fallbeispiele werden detaillierte Modelle validiert. Darüber hinaus werden für die Simulation unerlässliche Modellparameter wie Diffusionskoeffizienten oder Reaktionskinetiken diskutiert. Basierend auf gezielten Sensitivitätsanalysen kann eine Modellreduktion mit einhergehender Verkürzung der Rechenzeiten vorgenommen werden. Das somit erhaltene Modell wird erfolgreich bei der dynamischen Simulation angewendet. Dabei erscheint die Berücksichtigung von Filmreaktionen als unerlässlich. Rigorous Modeling of Reactive Absorption Processes Absorption of gases in liquid solutions accompanied by chemical reactions or reactive absorption, represents one of the most important industrial operations. The advantages of this process are enhanced solution capacity, low impurity concentrations up to the ppm-range, moderate operation pressure, and selective removal of contaminants. Reactive absorption is based on a combination of several kinetically controlled phenomena, the focus being on the coupling of chemical reactions and mass transfer in multicomponent mixtures. Therefore, a process description using the equilibrium concept is often insufficient and kinetic modeling is required. This paper describes the basics of rigorous modeling of reactive absorption processes. Several case studies are used in order to validate the models. In addition, a discussion of necessary model parameters, such as diffusion coefficients and reaction kinetics, is given. A reduced model with lower computation time is obtained via detailed sensitivity analysis. This model is successfully used in dynamic simulations. In this respect a proper consideration of the film reactions appears to be crucial.

Model Optimization for the Dynamic Simulation of Reactive Absorption Processes

The optimal design of reactive separations is impossible without reliable process models. Especially for the dynamic simulation and the model-based control of complex reactive absorption processes the model development leads to a contradiction between the required model accuracy to reflect the process complexity and the feasibility of process simulations regarding the computation time. In this respect, we have developed a new rigorous dynamic two-phase model based on the two-film theory as a first step, which takes into account the influence of chemical reactions and additional driving forces in electrolyte systems on mass transfer considering thermodynamic nonidealities as well as the impact of column internals on the process hydrodynamics. For a model optimization, we have performed an analysis of different model approaches for complicated industrial absorption processes and determined an appropriate model complexity. Based on results of sensitivity studies, we have accomplished different model modifications leading to a stabilization of the numerical solution without affecting the good agreement between simulation results and the experimental data. This time-optimized model can be considered superior as compared to previous approaches and facilitates for the first time a rigorous dynamic simulation of entire reactive absorption columns and the application within an on-line process control system.

Model Optimization for the Dynamic Simulation of Reactive Absorption Processes

Model Optimization for the Dynamic Simulation of Reactive Absorption Processes

Rigorous dynamic modelling of complex reactive absorption processes

Reactive absorption processes usually represent kinetically controlled operations in which complex reactions are combined with multicomponent mass transfer. For this reason, an adequate theoretical description of reactive absorption should be based on the rate-based approach, with the direct consideration of the process rates. Along these lines, a rigorous dynamic model allowing for multicomponent two-phase mass transfer, film and bulk reaction mechanisms, thermodynamic nonidealities, fluid dynamics and heat effects is developed. For systems including electrolytes, an additional account of the electrical potential gradient is involved. The whole dynamic rate-based model represents a system of partial differential equations which has to be solved numerically. To validate the model, the reactive absorption of sour gases in an air purification process with packed columns is simulated. Several simulation results including different sensitivity studies are presented. The model is also tested against pilot-plant steady-state experiments. The simulation results are in good agreement with the experimental data.

Modeling of homogeneous reactive separation processes in packed columns

The coupling of physical separation and chemical reactions in one unit operation, the so-called reactive separation processes (RSPs), are of increasing interest for scientific investigation and industrial application. RSPs tend to be very complex due to strong physico-chemical interactions, and a number of the model assumptions that are adequate for traditional unit operations may not be suitable in this new context. Models used to simulate RSPs should therefore be highly accurate in order to reflect this complexity. On the other hand, it is not always economical nor feasible to use the most sophisticated models for a range of different applications, such as process and equipment design, the determination of optimal operating policies, model-based control, etc., and the model complexity has to be adapted to the purpose. In many cases however, it is not evident which simplifications are appropriate, so that the uncertainty concerning adequate ways to model RSPs is still significant. Kreul et al. (1996a, b) have published a very detailed rate-based model and first simulation results for RSPs including reaction kinetic terms in the bulk and the film areas. In this paper, a detailed model investigation is presented, as well as a systematic deduction of possible model simplifications. The completely kinetic model and the corresponding equilibrium stage model are applied to various homogeneously catalyzed reactive distillation and reactive absorption processes. A number of simulation results are presented for four very different test systems. It is concluded that equilibrium and rate-based approaches can lead to significantly larger differences in calculated concentrations profiles for RSPs than for non-reactive operations, so that the additional effort of more complex modeling may be justified. In addition, it is demonstrated that if the kinetics of the mass and energy transfer between the phases are calculated explicitly, in most cases the consideration of interaction phenomena such as diffusional and direct reaction-transfer interaction is not necessary.

Modelling reactive absorption processes via film-renewal theory: Numerical schemes and simulation results

Even though film-renewal theory for gas-liquid mass transfer represents the most realistic mass transfer model under turbulent conditions, it is rarely used in practice since this leads to a complex mathematical description and evaluation of reacting systems. However, in contrast with common opinion numerical, evaluation of this class of models is possible nowadays within reasonable time on normal workstations. Therefore, we present a numerical scheme employing this theory which is capable of calculating mass transfer rates, conversion and remaining gas holdup from system and operating parameters. In contrast with the traditional calculation of mass transfer rates based on the enhancement factor, there are no simplifying assymptions concerning the hydrodynamics of operation, and non-linear phase equilibria can be considered as well as more complex reaction schemes. Simulations for reversible and irreversible second order reactions show the parametric sensitivity as well as the influence of different mass transfer models on the predicted mass transfer rates.