Macromixing study for various designs of impellers in a stirred vessel

The effect of the impeller designs and impeller clearance level (C/T) on power consumption, mixing time and air entrainment point in a single liquid phase under turbulent conditions (Re > 104) were investigated. Different impeller designs including conventional and new designs, were used to consider both axial and radial flow impellers. The electric conductivity method, suspended motor system and observation method were employed to determine the mixing time, the power consumption and the air entrainment point, respectively. The reduction in the impeller clearance level form T/3 to T/6 resulted in a decrease in power number values for up-flow pumping impellers while it was increased for down-flow pumping. The same trend was observed for the mixing time results. Moreover, axial flow impellers and specially HE3 are preferable for higher agitation speeds due to the less air entrainment. The results verified that the axial flow impellers and specifically down-flow impellers are more efficient than the radial flow impellers. ANFIS-Fuzzy C–means (ANFIS–FcM) and nonlinear regression were used to develop models to predict the mixing time based on the energy dissipation rate and clearance. The results verified that the model predictions successfully fit the experimental mixing time data

Gas storage

International audienceThe continuous increase of energy demands based on fossil fuels in the last years have lead to an increase of greenhouse gases (GHG) emission which strongly contribute to global warming. The main strategies to limit this phenomenon are related to the efficient capture of these gases and to the development of renewable energies sources with limited environmental impact. Particularly, carbon dioxide (CO2) and methane (CH4) are the main constituents of greenhouse gases while hydrogen (H2) is considered an alternative clean energy source to fossil fuels. Therefore, tremendous research to store these gases has been reported by several approaches and among them the physisorption on activated carbons (AC) have received significant attention. Their abundance, low cost and tunable porous structure and chemical functionalities with an existing wide range of precursors that includes bio-wastes make them ideal candidates for gas applications. This chapter presents the recent developments on CH4, CO2 and H2 storage by activated carbons with focus on biomass as precursor materials. An analysis of the main carbon properties affecting the AC's adsorption capacity (i.e. specific surface area, pore size and surface chemistry) is discussed in detail herein

Fixed-bed adsorption of carbon dioxide onto ammonia-modified activated carbon : experimental and modeling study / Mohammad Saleh Shafeeyan

A commercial granular activated carbon (GAC) adsorbent was modified through an oxidation–amination process in an effort to increase its surface basicity and consequently enhance its CO2 adsorption capacity. To optimize the amination conditions of activated carbon adsorbents the effects of amination temperature, amination time, and the type of starting materials (variables) on the CO2 adsorption/desorption capacities of the adsorbents (responses) were investigated using a central composite design. The use of a pre-oxidized sorbent as a starting material and amination at 425 ºC for 2.1 h were found to be the optimum conditions for obtaining an efficient carbon dioxide adsorbent. The activated carbon modified at optimum conditions (OXA-GAC) exhibited CO2 adsorption and desorption capacity values of 26.47 mg/g and 95.4%, respectively. The promising characteristics of the OXA-GAC in terms of adsorption capacity (exhibiting an increase of 44% in capacity compared with the capacity of the GAC at 1 atm and 105 ºC) and multicycle durability make it suitable for practical applications. The equilibrium adsorption isotherms of CO2 on the GAC and the OXA-GAC were measured using a static volumetric method. CO2 adsorption measurements were performed at three different temperatures (303, 318, and 333 K) and pressures up to 1 atm. The obtained equilibrium data were fitted to the Freundlich, Sips, and Toth isotherms using a semi-empirical approach to differentiate the contributions of physical and chemical adsorption to the total CO2 uptake. The Toth semi-empirical equilibrium model provided the best fit to the experimental data, over the entire analyzed ranges of temperature and pressure. The isosteric heats of CO2 adsorption onto the GAC and OXA-GAC adsorbents were determined using the Clausius–Clapeyron equation. The initial isosteric heats of adsorption of 68 kJ mol-1 and 23 kJ mol-1 corresponded to the chemisorption and physisorption of CO2 on the OXA-GAC adsorbent, respectively, and these values were in excellent agreement with the zero-coverage heats of adsorption obtained using the temperature-dependent parameters of the proposed model. The kinetics of CO2 adsorption on the GAC and OXA-GAC adsorbents over the temperature range of 30–60 °C were studied using the pseudo-first-order, pseudo-second-order, and Avrami kinetic models. The best fit with the experimental kinetic data for both of the studied adsorbents was obtained by applying the Avrami kinetic model. Fixed-bed breakthrough experiments for CO2 adsorption onto the GAC and OXA-GAC adsorbents were performed by changing the adsorption temperature over the range of 30 to 60 °C and the feed flow rate from 50 to 100 ml min-1. The largest values of the CO2 equilibrium dynamic capacity (0.67 mol kg-1) and breakthrough time (10.9 min) over the range of operating conditions investigated were obtained using OXA-GAC adsorbent at 30 °C under a 50 ml min-1 feed flow rate. To predict the breakthrough behavior of the fixed-bed adsorption of CO2, a simple model was developed, including the Toth and Avrami equations to describe the equilibrium and kinetics of adsorption, respectively. The set of coupled differential equations was solved using a numerical approach based on the finite element method implemented in COMSOL Multiphysics software. The validity of the model predictions was evaluated by a comparison with the experimental data. The findings showed that the model predictions successfully fit the experimental data over the studied range of feed gas flow rates and adsorption temperatures

Tailoring photocatalytic performance through Fe-doped TiO2/ZnO for effective remediation of organic contaminants

The development of highly efficient photocatalysts holds significant promise for addressing contemporary environmental challenges. This study focuses on the synthesis and characterization of a novel photocatalyst, iron-doped TiO2/ZnO particles, created through the sol-gel technique. The incorporation of iron ions into the crystalline structure of TiO2/ZnO aims to enhance the photocatalytic efficiency of TiO2. The findings revealed that Fe–ZnO/TiO2 exhibits a more thermally stable lattice compared to TiO2/ZnO, thereby retarding the phase transformation from anatase to rutile under higher calcination temperature. The photocatalytic performance of the synthesized photocatalyst was evaluated through the degradation of methyl orange under visible light irradiation. A statistical response surface methodology based on a central composite design was employed to develop a predictive model for color removal (as the dependent variable) considering variations in irradiation intensity, initial color concentration, catalyst concentration, reaction time, and pH (as independent variables). The analysis of variance identified the initial color concentration as the most influential factor, negatively affecting the predicted response. The experimental values of color removal exhibited good agreement with the predicted values from the regression model with a coefficient of determination of 0.949, indicating the accuracy of this model in predicting the mentioned outcomes. The presented model indicated that, for a sample of methyl orange with an initial concentration of 21.97 ppm, a reaction time of 117.69 min under direct irradiation of 21.63 W, a catalyst concentration of 0.61 g/L, and a pH of 4.74, the optimal color removal efficiency of 78.99 % was achieved

A review of mathematical modeling of fixed-bed columns for carbon dioxide adsorption

Carbon dioxide emissions must be stabilized to mitigate the unfettered release of greenhouse gases into the atmo-sphere. The removal of carbon dioxide from flue gases, an important first step in addressing the problem of CO2emissions, can be achieved through adsorption separation technologies. In most adsorption processes, the adsor-bent is in contact with fluid in a fixed bed. Fixed-bed column mathematical models are required to predict theperformance of the adsorptive separation of carbon dioxide for optimizing design and operating conditions. A comprehensive mathematical model consists of coupled partial differential equations distributed over time and spacethat describe material, energy, and the momentum balances together with transport rates and equilibrium equa-tions. Due to the complexities associated with the solution of a coupled stiff partial differential equation system, theuse of accurate and efficient simplified models is desirable to decrease the required computational time. The simpli-fied model is primarily established based on the description of mass transfer within adsorption systems. This paperpresents a review of efforts over the last three decades toward mathematical modeling of the fixed-bed adsorptionof carbon dioxide. The nature of various gas–solid equilibrium relationships as well as different descriptions of themass transfer mechanisms within the adsorbent particle are reviewed. In addition to mass transfer, other aspects ofadsorption in a fixed bed, such as heat and momentum transfer, are also studied. Both single- and multi-componentCO2adsorption systems are discussed in the review

Modeling of carbon dioxide adsorption onto ammonia-modified activated carbon: Kinetic analysis and breakthrough behavior

The removal of carbon dioxide from the flue gas of fossil-fueled power plants can be achieved using adsorption separation technologies. In this study, the breakthrough adsorption of CO2 on fixed beds of commercial granular activated carbon (GAC) and ammonia-modified GAC (OXA-GAC) adsorbents was measured. The breakthrough curves were acquired from dynamic column measurements at temperatures ranging from 30 to 60 °C with a feed gas flow rate that varied from 50 to 100 mL min–1 and a total pressure of 1.0 atm. An earlier breakthrough time and lower dynamic adsorption capacity were observed with increasing temperature, increasing feed flow rate, and the use of the GAC adsorbent. The largest CO2 equilibrium dynamic capacity (0.67 mol kg–1) and breakthrough time (10.9 min) over the range of operating conditions investigated were obtained using OXA-GAC adsorbent at 30 °C under a 50 mL min–1 feed flow rate. To predict the breakthrough behavior of the fixed-bed adsorption of CO2, a simple model based on mass balance was developed. This model consists of an Avrami equation to describe the kinetics of adsorption and a semiempirical Toth equation to represent the gas–solid equilibrium isotherm. The Avrami equation was selected because it provided the best fit with the experimental kinetic curves for both adsorbents, with average relative errors of less than 2% over the temperature range of 30–60 °C. The resultant set of coupled differential equations was solved using a numerical approach based on the finite element method implemented in COMSOL Multiphysics software. The findings showed that the model predictions successfully fit the experimental data over the studied range of feed gas flow rates and adsorption temperatures

Adsorption equilibrium of carbon dioxide on ammonia-modified activated carbon

The equilibrium adsorption isotherms of carbon dioxide on a commercial granular activated carbon (GAC) and an ammonia-modified GAC (OXA-GAC) were measured using a static volumetric method. CO2 adsorption measurements were performed at three different temperatures (303, 318, and 333 K), and pressures up to 1 atm. The obtained equilibrium data were fitted to the Freundlich, Sips, and Toth isotherms using a semi-empirical approach to differentiate the contributions of physical and chemical adsorption to the total CO2 uptake. The isotherm parameters were determined independently for each mechanism by non-linear regression. To evaluate the adequacy of the fit of the isotherm models, two different error functions (i.e., the average relative error and the nonlinear regression coefficient) were calculated. The Toth semi-empirical equilibrium model provided the best fit to the experimental data, with average relative errors of less than 3% observed at all temperatures. The isosteric heats of CO2 adsorption onto the ammonia-modified adsorbent and onto the untreated adsorbent were determined using the Clausius–Clapeyron equation. The loading dependence of the isosteric enthalpy of CO2 adsorption over the OXA-GAC reflected an energetic heterogeneity of the adsorbent surface. The initial isosteric heats of adsorption of 70.5 kJ mol−1 and 25.5 kJ mol−1 correspond to the adsorption of CO2 on the modified and untreated adsorbents, respectively, and these values were in excellent agreement with the zero-coverage heats of adsorption obtained using the temperature-dependent parameters of the proposed model