Transversal patterns in three‐dimensional packed bed reactors: Oscillatory kinetics

Formation of transversal patterns in a 3D cylindrical reactor is studied with a catalytic reactor model in which an exothermic first‐order reaction of Arrhenius kinetics occurs with a variable catalytic activity. Under these oscillatory kinetics, the system exhibits a planar front (1D) solution with the front position oscillating in the axial direction. Three types of patterns were simulated in the 3D system: rotating fronts, oscillating fronts with superimposed transversal (nonrotating) oscillations, and mixed rotating–oscillating fronts. These solutions coexist with the planar front solution and require special initial conditions. We map bifurcation diagrams showing domains of different modes using the reactor radius as a bifurcation parameter. The possible reduction of the 3D model to the 2D cylindrical shell model is discussed. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Are 3‐D models necessary to simulate packed bed reactors? analysis and 3‐D simulations of adiabatic and cooled reactors

Simulations and analysis of transversal patterns in a homogeneous three‐dimensional (3‐D) model of adiabatic or cooled packed bed reactors (PBRs) catalyzing a first‐order exothermic reaction were presented. In the adiabatic case the simulation verify previous criteria, claiming the emergence of such patterns when (ΔT_ad/ΔT_m)/(Pe_C/Pe_T) surpasses a critical value larger than unity, where ΔT_ad and ΔT_m are adiabatic and maximal temperature rise, respectively. The reactor radius required for such patterns should be larger than a bifurcation value, calculated here from the linear analysis. With increasing radius new patterned branches, corresponding to eigenfunction of the problem emerge, whereas other branches become unstable. The maximal temperature of the 3‐D simulations may exceed the 1‐D prediction, which may affect design procedures. Cooled reactor may exhibit patterns, usually axisymmetric ones that can be characterized by two anomalies: the peak temperature may exceed the corresponding value of an adiabatic reactor and may increase with wall heat‐transfer coefficient, and the peak temperature in a sufficiently wide reactor need not lie at the center but rather on a ring away from it. In conclusions, we argue that transversal patterns are highly unlikely to emerge in practical adiabatic PBRs with a single exothermic reaction, as in practice Pe_C/Pe_T > 1. That eliminates patterns in stationary and downstream‐moving fronts, whereas patterns may emerge in upstream‐moving fronts, as shown here. This conclusion may not hold for microkinetic models, for which stationary modes may be established over a domain of parameters. This suggests that a 1‐D model may be sufficient to analyze a single reaction in an adiabatic reactor and a 2‐D axisymmetric model is sufficient for a cooled reactor. The predictions of a 2‐D cylindrical thin reactor with those of a 3‐D reactor were compared, to show many similarities but some notable differences. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Transversal moving-front patterns: Criteria and simulations for two-bed and cylindrical shell packed-bed reactors

We show that a moving-front solution in a cylindrical shell packed-bed catalyzing a first-order activated reaction may bifurcate into transversal patterns when Pe_C/Pe_T<ΔT_ad/ΔT_m, i.e. when the ratio of the mass to heat Pe numbers is smaller than the ratio of the adiabatic to maximal temperature rises. This coincides with the previous condition of transversal patterns to emerge in stationary fronts [Pe_C/Pe_T<1 [Viswanathan, G., Bindal, A., Khinast, J., Luss, D., 2005. Stationary transversal hot zones in adiabatic packed-bed reactors. A.I.Ch.E. Journal 51, 3028-3038]] and extends the bifurcations condition to the case of moving fronts. The novel condition cannot be satisfied in a downstream propagating front (ΔT_m/ΔT_ad>1), but for an upstream propagating front (toward the cold reactor inlet) ΔT_m/ΔT_ad<1 and the symmetry breaking can be obtained within a feasible domain of operating conditions (Pe_C/Pe_T>1). It was also assumed that the axial and the transversal Pe numbers vary consistently, i.e. κ_C=Pe_C⊥/Pe_C=κ_T=Pe_T⊥/Pe_T. A similar condition was also obtained using a simplified model composed of two 1-D beds with heat and mass exchange between them.Bifurcation diagram showing domains of transversal patterns is constructed using a learning two-bed model. These predictions are verified by direct numerical simulations of the continuous 2-D cylindrical shell model showing various types of moving transversal patterns within a feasible domain of the state parameters with Pe_C>Pe_T. In the case of varying ratio (κ_C≠κ_T) the pattern domain can be significantly extended toward larger Pe_C/Pe_T.     

Structure of operating domains of loop reactors

A loop reactor (LR), composed as an N-unit loop with step-wise shifted inlet and outlet ports, is one of suggested technological solutions for low-concentration volatile organic compounds (VOC) combustion. Such a scheme ensures a sufficiently high temperature with autothermal behavior and nearly uniform catalytic utilization. The main drawback of the LR is a very narrow window of switching velocities that sustain a stable “frozen” solution that exists if the switching and the pulse velocity are synchronized. In the present work we show the existence of many “finger”-like domains of complex frequency-locked solutions that allow to significantly extend the operation domain, rendering the LR scheme more attractive for practical implementation. A brief comparison with the other heat recuperation technologies (reverse flow and circular loop reactors) is presented. © 2008 American Institute of Chemical Engineers AIChE J, 2008     

CO2 reduction with Zn particles in a packed‐bed reactor

A two‐step solar thermochemical cycle for splitting CO₂ with Zn/ZnO redox reactions is considered, consisting of: (1) the endothermic dissociation of ZnO with concentrated solar radiation as the heat source and (2) the non‐solar, exothermic, reduction of CO₂ to CO by oxidizing Zn to ZnO; the latter is recycled to the first step. The second step of the cycle is investigated using a packed‐bed reactor where micron‐sized Zn particles were immobilized in mixtures with submicron‐sized ZnO particles. Experimental runs were performed for Zn mass fractions in the range 67–100 wt % and CO₂ concentration in the range 25–100%, yielding Zn‐to‐ZnO conversions up to 71% because of sintering prevention, as corroborated by SEM analysis. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Operational stability of Thermomyces lanuginosa lipase during interesterification of fat in continuous packed‐bed reactors

The operational stability of a commercial immobilized lipase from Thermomyces lanuginosa (“Lipozyme TL IM”) during the interesterification of two fat blends, in solvent‐free media, in a continuous packed‐bed reactor, was investigated. Blend A was a mixture of palm stearin (POS), palm kernel oil (PK) and sunflower oil (55 : 25 : 20, wt‐%) and blend B was formed by POS, PK and a concentrate of triacylglycerols rich in n‐3 polyunsaturated fatty acids (PUFA) (55 : 35 : 10, wt‐%). The bioreactor operated continuously at 70 °C, for 580 h (blend A) and 390 h (blend B), at a residence time of 15 min. Biocatalyst activity was evaluated in terms of the decrease of the solid fat content at 35 °C of the blends, which is a key parameter in margarine manufacture. The inactivation profile of the biocatalyst could be well described by the first‐order deactivation model: Half‐lives of 135 h and 77 h were estimated when fat blends A and B, respectively, were used. Higher levels of PUFA in blend B, which are rather prone to oxidation, may explain the lower lipase stability when this mixture was used. The free fatty acid content of the interesterified blends decreased to about 1% during the first day of operation, remaining constant thereafter.     

Syngas production from H2O and CO2 over Zn particles in a packed‐bed reactor

The solar thermochemical production of H₂ and CO (syngas) from H₂O and CO₂ is examined via a two‐step cycle based on Zn/ZnO redox reactions. The first, endothermic step is the thermolysis of the ZnO driven by concentrated solar energy. The second, nonsolar step is the exothermic reaction of Zn with a mixture of H₂O and CO₂ yielding syngas and ZnO; the latter is recycled to the first step. A series of experimental runs of the second step was carried out in a packed‐bed reactor where ZnO particles provided an effective inert support for preventing sintering and enabling simple and complete recycling to the first, solar step. Experimentation was performed for Zn mass fractions in the range of 33–67 wt % Zn‐ZnO, and inlet gas concentrations in the range 0–75% H₂O–CO₂, yielding molar Zn‐to‐ZnO conversions up to 91%. A 25 wt % Zn‐ZnO sample mixture produced from the solar thermolysis of ZnO was tested in the same reactor setup and exhibited high reactivity and conversions up to 96%. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

A theory of ultradeep hydrodesulfurization of diesel in stacked‐bed reactors

Hydrodesulfurization catalysts have two types of active sites for hydrogenation and hydrogenolysis reactions. While hydrogenation sites are more active for desulfurizing refractory sulfur species, they are more susceptible to organonitrogen inhibition than hydrogenolysis sites. In contrast, hydrogenolysis sites are more resistant to organonitrogen inhibition but are less active for desulfurizing refractory sulfur species. This dichotomy is exploited to develop an ultradeep hydrodesulfurization stacked‐bed reactor comprising two catalysts of different characteristics. The performance of such a catalyst system can be superior or inferior to that of either catalyst alone. A mathematical model is constructed to predict the optimum stacking configuration for maximum synergies between the two catalysts. The best configuration provides the precise environment for the catalysts to reach their full potentials, resulting in the smallest reactor and minimum hydrogen consumption. Model predictions are consistent with experimental results. A selectivity‐activity diagram is developed for guiding the development of stacked‐bed catalyst systems. © 2017 American Institute of Chemical Engineers AIChE J, 64: 595–605, 2018     

CO2 abatement in oscillating packed‐bed scrubbers: Hydrodynamics and reaction performances for marine applications

Gas‐liquid flow dynamics and CO₂‐monoethanolamine absorption performances of an oscillating countercurrent packed bed were analyzed by means of a transient 3D nonisothermal two‐fluid flow model with a goal to understand the behavior of scrubbing units on‐board floating production, storage and offloading platforms. Gas‐liquid flow deviation from axial symmetry was significant at larger vessel inclinations prompting noticeable liquid accumulation in the column lowermost area. Conversely, in static vertical and slightly inclined columns only a reduced fraction of the liquid was subject to transverse segregation. Externally‐generated column oscillations brought about complex secondary flows in radial and tangential directions resulting in oscillatory patterns with amplitude and propagation frequency affected by the packed bed oscillations. CO₂ abatement in inclined and asymmetrically oscillating columns suffered perceptible deviations with respect to vertical configuration while symmetrically oscillating columns gave rise to CO₂ performances oscillating around the steady‐state solutions of the vertical column. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1064–1076, 2017     

Simultaneous phenol removal and biological reduction of hexavalent chromium in a packed‐bed reactor

BACKGROUND: Phenol and hexavalent chromium are considered industrial pollutants that pose severe threats to human health and the environment. The two pollutants can be found together in aquatic environments originating from mixed discharges of many industrial processes, or from a single industry discharge. The main objective of this work was to study the feasibility of using phenol as an electron donor for Cr(VI) reduction, thus achieving the simultaneous biological removal/reduction of the two pollutants in a packed‐bed reactor. RESULTS: A pilot‐scale packed‐bed reactor was used to estimate phenol removal with simultaneous Cr(VI) reduction through biological mechanisms, using a new mixed bacterial culture originated from Cr(VI)‐reducing and phenol‐degrading bacteria, operated in draw–fill mode with recirculation. Experiments were performed for feed Cr(VI) concentration of about 5.5 mg L^?1, while phenol concentration ranged from 350 to 1500 mg L^?1. The maximum reduction/removal rates achieved were 0.062 g Cr(VI) L^?1 d^?1 and 3.574 g phenol L^?1 d^?1, for a phenol concentration of 500 mg L^?1. CONCLUSION: Phenol removal with simultaneous biological Cr(VI) reduction is feasible in a packed‐bed attached growth bioreactor. Phenol was found to inhibit Cr(VI) reduction, while phenol removal was rather unaffected by Cr(VI) concentration increase. However, the recorded removal rates of phenol and Cr(VI) were found to be much lower than those obtained from previous research, where the two pollutants were examined separately. Copyright © 2008 Society of Chemical Industry     

Analysis of a nonisothermal simulated moving‐bed reactor

Simulated moving‐bed reactor (SMBR) is a multifunctional reactor wherein in situ separation of the products facilitates the reversible reaction to completion beyond thermodynamic equilibrium and at the same time obtaining products of high purity. In this work, we investigate the feasibility of introducing variances in adsorption strength, which has recently been proven to effectively improve SMB performance for pure separation, into an SMB system to include reactions. Synthesis of methyl acetate catalyzed by amberlyst 15 is considered as model system. Numerical simulations were carried out for an SMBR unit consisting of four columns and operated with various temperature distributions in the range of 308–323 K. SMBR productivities were evaluated and compared under the constraints of complete conversion and complete product separation. The effects of kinetics, heat transfer efficiency and adsorption strength of reactant were systematically investigated. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4705–4714, 2013     

Application of chitosan‐entrapped β‐galactosidase in a packed‐bed reactor system

The model enzyme β‐galactosidase was entrapped in chitosan gel beads and tested for hydrolytic activity and its potential for application in a packed‐bed reactor. The chitosan beads had an enzyme entrapment efficiency of 59% and retained 56% of the enzyme activity of the free enzyme. The Michaelis constant (K_m) was 0.0086 and 0.011 μmol/mL for the free and immobilized enzymes, respectively. The maximum velocity of the reaction (V_max) was 285.7 and 55.25 μmol mL^?1 min^?1 for the free and immobilized enzymes, respectively. In pH stability tests, the immobilized enzyme exhibited a greater range of pH stability and shifted to include a more acidic pH optimum, compared to that of the free enzyme. A 2.54 × 16.51‐cm tubular reactor was constructed to hold 300 mL of chitosan‐immobilized enzyme. A full‐factorial test design was implemented to test the effect of substrate flow (20 and 100 mL/min), concentration (0.0015 and 0.003M), and repeated use of the test bed on efficiency of the system. Parameters were analyzed using repeated‐measures analysis of variance. Flow (p < 0.05) and concentration (p < 0.05) significantly affected substrate conversion, as did the interaction progressing from Run 1 to Run 2 on a bed (p < 0.05). Reactor stability tests indicated that the packed‐bed reactor continued to convert substrate for more than 12 h with a minimal reduction in conversion efficiency. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1294–1299, 2004     

Temperature wave‐trains of periodically forced networks of catalytic reactors

Networks of N identical catalytic reactors with periodically switched inlet and outlet sections are studied for first‐order irreversible exothermic reactions. Switching strategies with inlet and outlet sections periodically jumping a fixed number n_s of reactors are considered and the mechanisms governing the formation of traveling temperature wave‐trains are analyzed as n_s and N are varied. To this aim, a geometric approach to the analysis of the network energy balance is developed. Based on this approach, infinite domains of traveling temperature wave‐trains are predicted for any n_s and N. Analytical approximations are derived for the stability limits and the spatiotemporal patterns of these regimes. Stability boundaries predicted analytically include for any solution the largest part of the stability region computed by numerical simulation. Moreover, good agreement is found between the structure of the spatiotemporal patterns computed numerically and that predicted based on the proposed approach. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Solar gasification of carbonaceous waste feedstocks in a packed‐bed reactor—Dynamic modeling and experimental validation

Thermochemical gasification of carbonaceous waste feedstocks (specifically: scrap tire powder, industrial sludge, and sewage sludge) for high‐quality syngas production is numerically modeled and experimentally validated using concentrated solar process heat. The solar reactor consists of two cavities separated by a radiant emitter, with the upper one serving as the solar radiative absorber and the lower one containing the reacting packed bed. The reactor is modeled by considering combined heat transfer coupled to the reaction kinetics, driven by the applied solar flux at the reactor's aperture. Model validation is accomplished in terms of converted mass, reactor temperatures, efficiency, and solar upgrade based on experiments with an 8‐kW reactor subjected to solar fluxes up to 2560 suns and packed bed temperatures up to 1490 K. The transient operation of a 200‐kW pilot‐scale reactor for gasifying industrial sludge is simulated for a solar day, yielding a maximum solar‐to‐fuel energy conversion efficiency of 89%. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

A thermal‐flywheel approach to distributed temperature control in microchannel reactors

Microchannel reactors are a promising route for monetizing distributed natural gas resources. However, intensification and miniaturization represent a significant challenge for reactor control. Focusing on autothermal methane‐steam reforming reactors, a novel microchannel reactor temperature control strategy based on confining a layer of phase‐change material (PCM) between the reactor plates is introduced. Melting‐solidification cycles, which occur with latent heat exchange at constant temperature, allow the PCM layer to act as an energy storage buffer—a “thermal flywheel”—constituting a distributed controller that mitigates temperature excursions caused by fluctuations in feedstock quality. A novel stochastic optimization algorithm for selecting the PCM layer thickness (i.e., distributed controller “tuning”) is introduced. Furthermore, a hierarchical control structure, whereby the PCM layer is complemented by a supervisory controller that addresses persistent disturbances, is proposed. The proposed concepts are illustrated in a comprehensive case study using a detailed two‐dimensional reactor model. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2051–2061, 2013     

Model‐based optimization of hydrogen generation by methane steam reforming in autothermal packed‐bed membrane reformer

An autothermal membrane reformer comprising two separated compartments, a methane oxidation catalytic bed and a methane steam reforming bed, which hosts hydrogen separation membranes, is optimized for hydrogen production by steam reforming of methane to power a polymer electrolyte membrane fuel cell (PEMFC) stack. Capitalizing on recent experimental demonstrations of hydrogen production in such a reactor, we develop here an appropriate model, validate it with experimental data and then use it for the hydrogen generation optimization in terms of the reformer efficiency and power output. The optimized reformer, with adequate hydrogen separation area, optimized exothermic‐to‐endothermic feed ratio and reduced heat losses, is shown to be capable to fuel kW‐range PEMFC stacks, with a methane‐to‐hydrogen conversion efficiency of up to 0.8. This is expected to provide an overall methane‐to‐electric power efficiency of a combined reformer‐fuel cell unit of ～0.5. Recycling of steam reforming effluent to the oxidation bed for combustion of unreacted and unseparated compounds is expected to provide an additional efficiency gain. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

A packed‐bed solar reactor for the carbothermal zinc production – dynamic modelling and experimental validation

Integration of concentrated solar energy into the pyrometallurgical Zn production process as clean source of high‐temperature process heat could significantly reduce fossil fuels consumption and its concomitant CO₂ emissions. The solar‐driven carbothermal reduction of ZnO is investigated using a 10‐kW_th solar reactor featuring two cavities, the upper one serving as the solar absorber and the lower one containing a packed‐bed of ZnO and beech charcoal as the biogenic reducing agent. Experimentation in a high‐flux solar simulator is carried out under radiative fluxes of 2300–2890 suns, yielding a peak solar‐to‐chemical energy conversion efficiency of 18.4%. The reactor performance under variable operating conditions is analyzed via a dynamic numerical model coupling heat transfer with chemical kinetics. The model is validated by comparison to the experimental data obtained with the 10‐kW_th packed‐bed solar reactor and further applied to predict the effect of incorporating semi‐continuous feeding of reactants on the process efficiency. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4586–4594, 2016     

Modelling of trickle‐bed reactors in foaming regime

Hydrogenation of α‐methyl styrene (AMS) to cumene at 40°C in a downflow trickle bed was used as a test reaction for an investigation of the prediction of the performance of trickle‐bed reactors. Dynamic and static saturation are well predicted by several models but pressure drop cannot be accurately estimated by correlations now in the literature. This raises doubts about the use of mass transfer models which employ pressure drop taken from the literature. The simple film model for predicting conversion fails in the high‐interaction regime at least with foaming liquids. A correction to the model to allow for the effect of foam on mass transfer characteristics is proposed but was not independently tested.     

Production of structured triacylglycerols in an immobilised lipase packed‐bed reactor: batch mode operation

Structured triacylglycerols with caprylic acid at the sn‐1 and sn‐3 positions of the glycerol backbone and eicosapentaenoic acid (EPA) at the position sn‐2 were synthesised by acidolysis of a commercially available EPA‐rich oil (EPAX4510, Pronova Biocare) and caprylic acid catalysed by the 1,3‐specific immobilised lipase Lipozyme IM. The reaction was carried out in an immobilised lipase packed‐bed reactor by recirculating the reaction mixture through the bed. The exchange equilibrium constants between caprylic acid and the native fatty acids of EPAX4510 were determined. The n‐3 polyunsaturated fatty acids (PUFAs), EPA and docosohexaenoic acid (DHA), were the most easily displaced by the caprylic acid. The exchange equilibrium constants were 3.68 and 3.06 for EPA and DHA, respectively. The influence of the flow rate of the reaction mixture through the packed‐bed and the substrate concentration in the reaction rate were studied. For flow rates between 74 and 196 cm³ h^?1 (bed of 6.6 mm internal diameter and 0.46 porosity) and triacylglycerol concentrations between 0.036 and 0.108 M , the data fitted well to an empirical kinetic model which allowed representative values of the apparent kinetic constant to be obtained. Hence, the average reaction rates and kinetic constants of exchange of caprylic acid and native fatty acids of EPAX4510 could be calculated. In the conditions indicated, the parameter (lipase mass × time/triacylglycerol mass, m_Lt/V[TG]₀) constituted the intensive variable of the process for use in predicting the composition of structured triacylglycerols at different reaction times. At equilibrium, the structured triacylglycerol produced had the following composition: caprylic acid 59.5%, EPA 9.6%, DHA 2.2% and oleic acid 11.8%. Copyright © 2004 Society of Chemical Industry