Modeling Fischer–Tropsch kinetics and product distribution over a cobalt catalyst

A detailed kinetic model describing the consumption of key components and productdistribution in the Fischer–Tropsch synthesis (FTS) over a 20%Co/0.5Re γ-Al2O3commercial catalyst is developed. The developed model incorporates the H2O-assisted CO dissociation mechanism developed by Rytter and Holmen and a novelapproach to product distribution modeling. The model parameters are optimizedagainst an experimental dataset comprising a range of process conditions: total pres-sure 2.0–2.2 MPa, temperature 210–230C, CO conversion range of 10%–75% andfeed with and without added water. The quality of the model fit measured in termsof mean absolute relative residuals (MARR) value is 23.1%, which is comparable to lit-erature reported values. The developed model can accurately describe both positiveand negative effects of water on the rate kinetics, the positive effect of water on thegrowth factor, temperature and syngas composition on the kinetics and product dis-tribution over a wide range of process conditions, which is critical for the design andoptimization of the Fisher–Tropsch reactors.publishedVersio

Applicability of enhancement factor models for CO2 absorption into aqueous MEA solutions

In many chemical industrial processes, mass transfer across gas-liquid interfaces accompanied by chemical reaction is the governing phenomena. In case of mass transfer accompanied by a chemical reaction in the liquid phase, the reaction will enhance mass transfer and generally the mass transfer enhancement is quantified in terms of an enhancement factor. Large number of enhancement factor models have been developed in literature and used without critical analysis for analyzing pilot data for CO2 absorption into aqueous amines. In order to perform such a critical analysis, 24 models are tested using lab-scale experimental data from four independent apparatuses for CO2 absorption into MEA solutions covering a range of different conditions such as short and long contact times, with and without gas phase resistance, high and low CO2 loadings and temperatures. Of the 24 enhancement factor models tested only six models were found to satisfactorily predict the experimental CO2 fluxes. These were the models based on the simple pseudo-first order reaction assumption, Emodels 1, 2 and 3 by Hatta [2] and Dankwerts [4] respectively, Emodel 20, the deCoursey and Thring [44] model based on Danckwert’s surface renewal theory with unequal diffusivities, Emodel 24, the recently published generalized model by Gaspar and Fosbøl [51] and Emodel 21, the Tufano et al. [67] model based surface renewal theory. All these models were found to work equally well to the discretized penetration model. No significant difference was found between Emodels 1, 2 and 3, indicating that whether one uses as basis a film, penetration or surface renewal model, is of insignificant importance. The success of the simple models is attributed to the short contact times in the experiments used as basis and the accuracy of the kinetic model. Contact times of the same magnitude between mixing points is also encountered in industrial packings and it is believed that the simple enhancement factor models may work well also in these cases if an accurate kinetic model is used.submittedVersio

Study of the effect of condensation and evaporation of water on heat and mass transfer in CO2 absorption column

A rate-based combined heat and mass transfer model developed based on penetration theory is used to study the effect of water evaporation and condensation on the CO2 absorption process using six different cases with real pilot-scale plants flue gas conditions. The effect of water evaporation and condensation on the concentrations, temperature profiles and reaction rates are studied in detail. The model predicted reasonable profiles as one would expect for water condensation and evaporation. The degree of liquid temperature rise depends mainly on the gas water saturation level and the temperature difference between the gas and liquid. Temperature profiles are flat in the liquid, whereas the transferring components create steep concentration gradients close to the interface making the interface concentrations change rapidly with position in absorber. This is in line with the thermal and mass diffusivities. Concentration build-up or depletion of species takes place in the liquid phase close to the gas-liquid interface up to 10 μm distance from the interface. For the case with absorber bottom pinch conditions, it was found that the CO2 flux sign changes and desorption occurred when taking the evaporation and condensation effects into account, whereas, without these effects, only absorption was predicted. For most of the cases, absorption rate of CO2 was not affected significantly even though concentration gradients and temperature changes were found. However, for the extreme case of warm unsaturated exhaust from an NG fired plant, case C1, and for the near pinch situation, case C6, significant changes to the CO2 absorption rates were found

CO2 absorption into loaded aqueous MEA solutions: Impact of different model parameter correlations and thermodynamic models on the absorption rate model predictions

A two-dimensional discretized rate-based model was used for assessing the impact of correlations and/or models for VLE (thermodynamics), solubility (CO2 henry’s law constant), reaction rate kinetic models and diffusivity of CO2 in aqueous MEA solutions on the absorber model predictions for CO2 capture. Experimental data from four different set-ups covering a wide range of conditions were utilized for the assessment. Four different thermodynamic models and eight different Henry’s constant correlations/models including the Aspen Plus V8.6 e-NRTL-RK model was used in the study. Even though the individual sub-models, e.g. solubility, physical properties, were validated with independent experimental data, the use of a random selection of these models will give different predictions when used in a rate-based simulation. It was seen that using different Henry’s law constant correlations had a huge effect on model predictions. Seven different reaction rate kinetic models were used and it was found that no single kinetic model was able to predict the experimental data from all the sources better than the chosen base case kinetic model. It was also seen that transport property (CO2 diffusivity in MEA) correlations could have a large impact on the outcome of model predictions and correlations based on the N2O analogy were better than the ones based on a modified Stokes-Einstein correlation. As a special case, frequently used kinetic models were used with the thermodynamic model and transport properties taken from Aspen Plus V8.6. It was found that two of the kinetic models predicted the experimental data with acceptable accuracy

Applicability of enhancement factor models for CO2 absorption into aqueous MEA solutions

In many chemical industrial processes, mass transfer across gas-liquid interfaces accompanied by chemical reaction is the governing phenomena. In case of mass transfer accompanied by a chemical reaction in the liquid phase, the reaction will enhance mass transfer and generally the mass transfer enhancement is quantified in terms of an enhancement factor. Large number of enhancement factor models have been developed in literature and used without critical analysis for analyzing pilot data for CO2 absorption into aqueous amines. In order to perform such a critical analysis, 24 models are tested using lab-scale experimental data from four independent apparatuses for CO2 absorption into MEA solutions covering a range of different conditions such as short and long contact times, with and without gas phase resistance, high and low CO2 loadings and temperatures. Of the 24 enhancement factor models tested only six models were found to satisfactorily predict the experimental CO2 fluxes. These were the models based on the simple pseudo-first order reaction assumption, Emodels 1, 2 and 3 by Hatta [2] and Dankwerts [4] respectively, Emodel 20, the deCoursey and Thring [44] model based on Danckwert’s surface renewal theory with unequal diffusivities, Emodel 24, the recently published generalized model by Gaspar and Fosbøl [51] and Emodel 21, the Tufano et al. [67] model based surface renewal theory. All these models were found to work equally well to the discretized penetration model. No significant difference was found between Emodels 1, 2 and 3, indicating that whether one uses as basis a film, penetration or surface renewal model, is of insignificant importance. The success of the simple models is attributed to the short contact times in the experiments used as basis and the accuracy of the kinetic model. Contact times of the same magnitude between mixing points is also encountered in industrial packings and it is believed that the simple enhancement factor models may work well also in these cases if an accurate kinetic model is used

Applicability of enhancement factor models for CO2 absorption into aqueous MEA solutions

In many chemical industrial processes, mass transfer across gas-liquid interfaces accompanied by chemical reaction is the governing phenomena. In case of mass transfer accompanied by a chemical reaction in the liquid phase, the reaction will enhance mass transfer and generally the mass transfer enhancement is quantified in terms of an enhancement factor. Large number of enhancement factor models have been developed in literature and used without critical analysis for analyzing pilot data for CO2 absorption into aqueous amines. In order to perform such a critical analysis, 24 models are tested using lab-scale experimental data from four independent apparatuses for CO2 absorption into MEA solutions covering a range of different conditions such as short and long contact times, with and without gas phase resistance, high and low CO2 loadings and temperatures. Of the 24 enhancement factor models tested only six models were found to satisfactorily predict the experimental CO2 fluxes. These were the models based on the simple pseudo-first order reaction assumption, Emodels 1, 2 and 3 by Hatta [2] and Dankwerts [4] respectively, Emodel 20, the deCoursey and Thring [44] model based on Danckwert’s surface renewal theory with unequal diffusivities, Emodel 24, the recently published generalized model by Gaspar and Fosbøl [51] and Emodel 21, the Tufano et al. [67] model based surface renewal theory. All these models were found to work equally well to the discretized penetration model. No significant difference was found between Emodels 1, 2 and 3, indicating that whether one uses as basis a film, penetration or surface renewal model, is of insignificant importance. The success of the simple models is attributed to the short contact times in the experiments used as basis and the accuracy of the kinetic model. Contact times of the same magnitude between mixing points is also encountered in industrial packings and it is believed that the simple enhancement factor models may work well also in these cases if an accurate kinetic model is used

Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst in a fixed bed reactor – Effect of process parameters

Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al2O3) by varying the H2/CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H2/CO ratio higher than 1.7. Water was found to always increase the C5+ selectivity regardless of the H2/CO ratio. Increasing conversion led to increased C5+ selectivity. The selectivity to CO2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role.publishedVersio

Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst in a fixed bed reactor – Effect of process parameters

Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al2O3) by varying the H2/CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H2/CO ratio higher than 1.7. Water was found to always increase the C5+ selectivity regardless of the H2/CO ratio. Increasing conversion led to increased C5+ selectivity. The selectivity to CO2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role

Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst in a fixed bed reactor – Effect of process parameters

Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al2O3) by varying the H2/CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H2/CO ratio higher than 1.7. Water was found to always increase the C5+ selectivity regardless of the H2/CO ratio. Increasing conversion led to increased C5+ selectivity. The selectivity to CO2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role

Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst in a fixed bed reactor – Effect of process parameters

Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al2O3) by varying the H2/CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H2/CO ratio higher than 1.7. Water was found to always increase the C5+ selectivity regardless of the H2/CO ratio. Increasing conversion led to increased C5+ selectivity. The selectivity to CO2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role