1,721,402 research outputs found
A new reaction-separation unit: The simulated moving bed reactor
Full text linkProcess intensification is one of the most promising tools for improving current technologies. This requires dealing in most cases with complex systems where many different and often interacting physicochemical phenomena take place simultaneously. Continuous reactive chromatography, implemented through the simulated moving bed reactor technology, represents a novel example of such an intensified process. In this case, only a detailed description of the selective polymer swelling and reaction kinetics enables the modeling and the understanding of the behavior of the chemical production process. The optimization of the unit reveals its major economic limitation to arise from the sorptive properties of the currently available stationary phase. Based on a proper understanding of the unit operation, it is possible to design new stationary phases, tailor-made for specific reacting systems, which allow the performance of the process, to be significantly improved
Belousov-zhabotinskii Oscillations In A Batch Reactor
No full textThe oscillations of the Belousov-Zhabotinskii reaction in a batch reactor vanish due to the consumption of the main reactants, i.e. bromate and malonic acid. This behavior has been analyzed first using the classical Oregonator model, where reactant concentrations are kept constant, and then introducing a modified model, in which the reactant concentration is allowed to vary according to the stoichiometry of the considered set of chemical reactions. Two different routes to the extinction of oscillations have been found, depending on whether the Oregonator exhibits a subcritical or a supercritical Hopf bifurcation. These theoretical findings are consistent with the experimental observations, that indicate that the disappearance of the oscillations takes place in two rather different ways: either suddenly after a series of practically undamped oscillations or through very slowly damped oscillations
Comparing true countercurrent and Simulated Moving-Bed chromatographic reactors
No full textSimulated Moving Bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single continuous and countercurrent unit. This integration promises substantial improvements in process performance, especially when applied to equilibrium-limited reactions involving such heat-sensitive products as fine chemicals and pharmaceuticals. In this work, the interplay among the relevant process design parameters (dimensionless ratios of the fluid and solid flow rates, and the Damkohler numbers for each section of the unit) is investigated. For this, an analytical solution of differential mass-balance equations for the corresponding true countercurrent process (TCC), using as a model system the reaction A reversible arrow B + C with each species exhibiting linear adsorption behavior, was developed. Based on this solution, criteria were derived for the optimum process design with respect to productivity and solvent consumption. Comparing these results with numerical simulations of an SMBR unit shows that the TCC model does not apply to SMBR units with a finite number of columns per section, that is, units of practical relevance, because the two units exhibit different residence time distributions and, hence, lead to different degrees of conversion
Bifurcation-analysis of the Oregonator Model In the 3-d Space Bromate Malonic-acid Stoichiometric Coefficient
No full textBy considering the classical Oregonator model, the effect of the main reactants, i.e., bromate and malonic acid, on the behavior of the Belousov-Zhabotinskii system is studied. This analysis is a prerequisite for the understanding of the occurrence and disappearance of oscillations in batch reactors. A parametric analysis of the Oregonator in the three-dimensional space spanned by the bromate concentration, the malonic acid concentration, and the stoichiometric coefficient f is performed. Through asymptotic stability analysis the Hopf bifurcation set in the parameter space is determined analytically, whereas its character is defined through normal form analysis. In this study the most plausible values of the kinetic parameters, namely the ''Lo'' values proposed by Tyson (Tyson, J. J. In Oscillations and Traveling Waves in Chemical Systems; Field, R. J., Burger, M., Eds.; Wiley-Interscience: New York, 1985), are used. The system may exhibit both sub- and supercritical Hopf bifurcations, and near the latter the phenomenon of canard explosion has been observed. The results obtained with the ''Hi'' values are also reported for a comparison with previous studies in the literature
Robust design of countercurrent adsorption separation .3. Nonstoichiometric systems
No full textThe separation of a multicomponent mixture in a countercurrent adsorptive separation unit is analyzed. A procedure for the optimal and robust design of the operating conditions of the unit is developed in the frame of equilibrium theory, where the adsorption equilibria are described through the constant selectivity nonstoichiometric Langmuir model, while mass-transfer resistances and axial dispersion are neglected. This allows for extending the results previously obtained for systems characterized by either the linear or stoichiometric Langmuir isotherm to cases where the adsorbates exhibit rather different loading capacities. Analogies and differences among these three equilibrium models with respect to the criteria for the choice of operating conditions of countercurrent separation units are discussed. A comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical results achieved in this work
Design and operation of a HPLC-SMB laboratory unit for continuous chromatographic separations
No full textOn-line monitoring of enantiomer concentration in chiral simulated moving bed chromatography
No full textOn Comparing Packed Beds and Monoliths for CO2 Capture from Air Through Experiments, Theory, and Modeling
Full text linkThis study compares the performance of amine-functionalized gamma-alumina sorbents in the form of 3 mm gamma-alumina pellets and of a gamma-alumina wash-coated monolith for CO2 capture for direct air capture (DAC). Breakthrough experiments were conducted on the two contactors to analyze the adsorption kinetics and performance for different gas feeds. A constant pattern analysis revealed dominant mass transfer resistances in the gas film and in the pores, with axial dispersion also observed, particularly at higher concentrations. A 1D, physical model was used to fit the experiments and thus to estimate mass transfer and axial dispersion coefficients, which appear to be consistent with the hypotheses derived from constant pattern analysis. A dual kinetic model to describe mass transfer was found to better describe the tail behavior in the monolith, whereas a pseudo-first-order model was sufficient to describe breakthroughs on packed beds. A substantial two-order magnitude decrease in mass transfer coefficients was noted when reducing the feed concentration from 5.6% to 400 ppm CO2, thus underscoring the significant mass transfer limitations observed in DAC. Comparison between the contactors revealed notably higher mass transfer coefficients in the monolith compared to the packed beds, which are attributed to shorter diffusion lengths and lower equilibrium capacity. While the faster mass transfer coefficients observed in the monolith experiments led to reduced specific energy consumption and increased adsorption productivity compared to the packed bed at 400 ppm, no significant improvement was observed for the same process at the higher concentration of 5.6% CO2 in the feed. This finding highlights the need to tailor the contactor design to the specific gas separation requirements. This research contributes to the understanding and quantification of mass transfer kinetics at DAC concentrations in both packed bed and monolith contactors. It demonstrates the crucial role of the contactor in DAC systems and the importance of optimizing the adsorption step to enhance productivity and DAC performance
Temperature gradient operation of a simulated moving bed unit
No full textThe simulated moving bed (SMB) technology has shown great potential for fine chemical separations, particularly for the resolution of the enantiomers of chiral compounds. Further improvements of separation performance are expected when each section of the unit is optimized independently by applying a gradient of temperature, pressure, or solvent composition along the unit. The aim of this work is to extend the design criteria for nonlinear SMBs to the case where a temperature gradient mode is adopted. It is shown how beneficial this can be in terms of productivity and solvent consumption. Finally, the temperature transient in the columns is studied. It is shown that temperature changes in the column of the unit yield a constraint on the maximum fluid velocity. This is analogous to the constraints due to column efficiency and packing stability requirements. The results show that the temperature gradient operation of an SMB unit is feasible and may have significant advantages over the traditional isothermal mode
Adsorption of supercritical carbon dioxide on silica
No full textIn this work, the adsorption isotherms of carbon dioxide on silica have been measured with a gravimetric method at nine different temperature values (from 312 up to 466 K) and at pressure levels up to 366 bar. The adopted technique allows for a direct evaluation of the density that is measured simultaneously to the adsorption load, thus avoiding possible systematic errors arising from the use of an equation of state to calculate density from pressure and temperature data. The enthalpy changes related to the adsorption process have been obtained from a thermodynamic analysis based only on excess properties
- …
