Advances and trends in two-zone fluidized-bed reactors

Two-zone fluidized bed reactors, devices to carry on two reactions in a single reactor, have been widely studied in our group. This work summarizes the developments achieved after a previous review published in 2005 and shows how this technology can be applied in a wide variety of reactions. The advantages for each kind of process are discussed, as well as several proposals for improvement of the reactor aimed to achieve new functions or to improve the process performance. Finally, the possibilities of this kind of reactors at industrial scale are discussed

Production and purification of hydrogen by biogas combined reforming and steam-iron process

Cobalt ferrite and hematite with minor additives have been tested for production and purification of high purity hydrogen from a synthetic biogas by steam-iron process (SIP) in a fixed bed reactor. A catalyst based in nickel aluminate has been included in the bed of solids to enhance the rate of the reaction of methane dry reforming (MDR). The reductants resulting from MDR are responsible for reducing the oxides based on iron that will, in the following stage, be oxidized by steam to release hydrogen with less than 50 ppm of CO. Coke minimization along reduction stages forces to operate such reactors above 700 °C for reductions, and as low as 500 °C for oxidations to avoid coke gasification. To avoid problems such as reactor clogging by coke in reductions and/or contamination of hydrogen by gasification of coke along oxidations, steam in small proportions has been included in the feed with the aim of minimizing or even avoiding formation of carbonaceous depositions along the reduction stage of SIP. Since steam is an oxidant, it exerts an inhibiting effect upon reduction of the oxide, that slows down the efficiency of the process. It has been proved that co-feeding low proportions of steam with an equimolar mixture of CH4 and CO2 (simulating a poor heating value desulphurized biogas) is able to avoid coke deposition, allowing the operation of both, reductions and oxidations, in isothermal regime (700 °C). Empirical results have been contrasted with data found in literature for similar processes based in MDR and combined (or mixed) reforming process (CMR), concluding that the combination of MDR + SIP proposed in this work, taking apart economic aspects and complex engineering, shows similar yields towards hydrogen, but with the advantage of not requiring a subsequent purification process

Glycerol steam reforming with low steam/glycerol ratio in a two-zone fluidized bed reactor

The production of hydrogen from glycerol steam reforming has been studied in several reactors. In conventional reactors the catalyst is deactivated by coke: in fixed bed reactors plugging was observed if a low steam/glycerol ratio was employed, while in fluidized bed reactors it was possible to operate for a longer time-on-stream. The use of a two-zone fluidized bed reactor is studied in this work, as a method to counteract the problem of catalyst deactivation by coke. The glycerol reforming takes place in the upper part of this reactor while the catalyst is simultaneously regenerated in the lower part, where a stream of a regenerating gas is introduced. It has been found that CO2, O2 or H2O can act as regenerating gas in a two-zone-fluidized bed reactor, allowing steady state operation at a water:glycerol molar ratio as low as 1.25. The effect of the operating conditions has been studied and the yield to the main products was compared with the calculated values assuming thermodynamic equilibrium

Particle tracking in a Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR). Experimental technique and CCBM model validation

The Two Zone Fluidized Bed Reactor (TZFBR) provides a high level of process integration, allowing reaction and, in situ, continuous catalyst regeneration, in a single fluidized bed. Reactive and regenerative atmospheres are induced simultaneously inside the single vessel due to a separated gas inlet and particles fluidization provides reactor continuous operation. Therefore, the fluid dynamic key factor in a TZFBR is the mixing rate between solids inside the two bed zones (a good mixture provides continuous catalyst regeneration, while bad contact between bed zones leads to deactivation and loss of catalytic surface). In the present study, phosphorescent particles have been used as optical tracers to measure solids axial mixing between reactor zones in a catalytic TZFBR. Additionally, a different cross-sectional area between zones has been studied to get a certain fluidization regime, allowing small flowrates in the regeneration zone. This geometry implied a transition angle α between zones to be implemented (Two-Section TZFBR). In line with it, a modified counter-current backmixing model (CCBM) without fitting parameters was developed to predict mixture rates inside the bed for different TS-TZFBR geometries. Modifications carried out in the CCBM basis model involved reactor geometry and the presence of two simultaneous gas feed points along the bed. Model parameters related to bubble/solid fraction and wake-emulsion mass transfer were fully correlated to operational conditions (gas velocity, feed point location and section change). The adapted CCBM model predictions were further validated with optical tracers experimental fluid dynamic data, resulting in a high agreement

Kinetic Study of Dry Reforming of Methane Over Ni–Ce/Al 2 O 3 Catalyst with Deactivation

A kinetic study for dry reforming of methane over Ni–Ce/Al 2 O 3 catalyst was performed, taking into account both the main reactions and the catalyst deactivation. The catalyst was prepared by a sequential wet impregnation process, with loadings of 5 wt.% Ni and 10 wt.% Ce. Experimental tests were carried out in a fixed bed reactor between 475 and 550 °C and several spatial times, using nitrogen as diluent. Several kinetic equations were compared. The best fit of experimental data was achieved using a Langmuir–Hinshelwood mechanism which takes into account the presence of two active sites. Pre-exponential factor and activation energy were calculated. the kinetics of deactivation was also determined. The relationship between catalyst activity and coke concentration was also studied. Several deactivation equations were considered in order to choose the best fit with experimental data

Gas permeation effect on the Two-Section Two-Zone Fluidized Bed Membrane Reactor (TS-TZFBMR) fluid dynamics: A CFD simulation study

Two-Fluid Model simulations were conducted using the commercial software Ansys CFX and Fluent to study the effect of the gas extraction on the fluid dynamic behavior of a membrane-assisted Two-Section Two-Zone Fluidized Bed Membrane Reactor (TS-TZFBMR). Simulated bubble properties and bed dynamics were analyzed and compared among different membrane reactor configurations, including reactor-wall (RWM) and immersed tubular (ITM) membranes, for their future use in catalytic reactions, e.g., alkane dehydrogenation or methane steam reforming. According to the solids hold-up distribution at different fluidization regimes and permeation fluxes, the ITM configuration is the most suitable to enhance the gas-particle contact and to favor the solids axial mixing for in-situ catalyst regeneration purposes. However, the RWM configuration provides a greater permeation area for selective gas removal and is preferred to enhance purification. It was found that relative permeation fluxes above 20% of the total feed gas have a significant impact on the fluid dynamic regime within the TS-TZFBMR, concerning the appearance of local defluidized regions, gas channeling and solids axial mixing

Conventional and improved fluidized bed reactors for dry reforming of methane: Mathematical models

Dry reforming of methane is a potentially useful reaction, but has some drawbacks: catalyst deactivation by coke and yield limited by thermodynamic equilibrium. New improved fluidized bed reactors may compensate these disadvantages. Mathematical models for the dry reforming of methane in three types of fluidized bed reactors have been developed. These reactors include: a) conventional fluidized bed reactor, b) two zone fluidized bed reactor, which provides simultaneous reaction and catalyst regeneration in a single fluidized bed, and c) two-zone fluidized bed reactor with hydrogen selective membranes, which in addition to the previous one provides increased yield to hydrogen, because the selective removal of hydrogen through the membrane. The situations where these reactors counteract the two main drawbacks of dry reforming of methane are shown. Comparison with previous experimental results shows that the models predict well the effect of operating conditions

Iron oxide ores as carriers for the production of high purity hydrogen from biogas by steam–iron process

Production of high purity hydrogen (<50 ppm CO) by steam–iron process (SIP) from a synthetic sweetened biogas has been investigated making use of a natural iron ore containing up to 81 wt% of hematite (Fe2O3) as oxygen carrier. The presence of a lab-made catalyst (NiAl2O4 with NiO excess above its stoichiometric composition) was required to carry out the significant transformation of mixtures of methane and carbon dioxide in hydrogen and carbon monoxide by methane dry reforming reaction. Three consecutive sub-stages have been identified along the reduction stage that comprise A) the combustion of CH4 by lattice oxygen of NiO and Fe2O3, B) catalyzed methane dry reforming and C) G–G equilibrium described by the Water-Gas-Shift reaction. Oxidation stages were carried out with steam completing the cycle. Oxidation temperature was always kept constant at 500 °C regardless of the temperature used in the previous reduction to minimize the gasification of eventual carbon deposits formed along the previous reduction stage. The presence of other oxides different from hematite in minor proportions (SiO2, Al2O3, CaO and MgO to name the most significant) confers it an increased thermal resistance against sintering respecting pure hematite at the expense of slowing down the reduction and oxidation rates. A “tailor made” hematite with additives (Al2O3 and CeO2) in minor proportions (2 wt%) has been used to stablish comparisons in performance between natural and synthetic iron oxides. It has been investigated the effect of the reduction temperature, the proportion of methane to carbon dioxide in the feed (CH4:CO2 ratio) and the number of repetitive redox cycles

Use of a-shapes for the measurement of 3D bubbles in fluidized beds from two-fluid model simulations

A geometrical technique based on shape construction was employed to reconstruct the simulated domain of 3D bubbles in a gas-solid fluidized bed, from two-fluid model (TFM) simulations. The Delaunay triangulation of the cloud of points that represent volume fraction iso-surfaces in transient TFM simulations was filtered by means of the so-called a-shapes, allowing a topologically accurate description of 3D bubbles within a fluidized bed. Consequently, individual 3D bubble properties such as size and velocity were measured. Simulated bubble characteristics were further compared to those measured on pseudo-2D bed facilities by image techniques in order to illustrate the effect of the bed geometry on the bubbling behavior under mimicked operational conditions