Two-zone Fluidized Bed Reactor Research Articles

Catalytic selective oxidation of isobutane in a decoupled redox-process

The catalytic selective oxidation of isobutane to methacrolein (MAC) and methacrylic acid (MAA) is known to proceed via a Mars-Van Krevelen mechanism. When the reaction is performed in a common reactor with co-feeding of the reactant and oxygen, reactive oxygen species are present on the catalyst surface whereby non-selective oxidation reactions occur. In this work, we studied the possibility to decouple the redox mechanism meaning to separate the reactant oxidation step and the catalyst re-oxidation step for the selective oxidation of isobutane (IBAN) to MAC and MAA. For this purpose, two reactor configurations were employed: a periodic reactor and a two-zone fluidized bed reactor (TZFBR). Whereas the periodic reactor allows the temporal separation of the two steps, the TZFBR enables the spatial separation.A Cs-based Keggin type heteropolycompound was employed as catalyst and characterized before and after reaction by XRD, Raman and XPS to better understand the structural and chemical changes during the periodic operation. The effect of the oxygen/IBAN concentration on the performance was studied. For the periodic reactor it was found that the conversion of isobutane and the formation of products decreased with the consumption of available oxygen species in the catalyst during a period. For the whole periodic tests, low concentration of oxygen injected into the reoxidation part of the cycles led to a high selectivity to the desired products (MAC+MAA).

Methanol to gasoline (MTG): Parametric study and validation of the process in a two-zone fluidized bed reactor (TZFBR)

Methanol to Gasoline (MTG) process transforms methanol to hydrocarbons within the boiling point range of gasoline. The result is a wide spectrum of products (olefins, paraffins, aromatics and naphthenics, among others), with the total conversion of methanol to hydrocarbons and water. Catalyst deactivation by coke is a main problem in this process. This work aims to determine the feasibility of carrying out the production of gasoline from methanol in a two-zone fluidized bed reactor (TZFBR). The hypothesis is that the formation of carbonaceous deposits (coke) on the catalyst particles can be counteracted by its combustion in the regeneration zone that this novel reactor presents, thus achieving stable and continuous operation. In this way, both processes (reaction and regeneration) would be being carried out simultaneously in the same reactor (process intensification). The comparison of results between a conventional fluidized bed reactor and a TZFBR shows that the second one actually provides a better stability over time.

Comparison of Conventional and Two-Zone Fluidized Bed Reactors for Methanol to Olefins. Effect of Reaction Conditions and the Presence of Water in the Feed

The effect of water over the performance of the Methanol-to-Olefins process was studied in a conventional fluidized bed reactor (CFBR) and a two-zone fluidized bed reactor (TZFBR). Both of them contained the same SAPO-34-based catalyst, prepared by agglomeration with bentonite and alumina. In the first section of this paper, the performance was studied in the TZFBR for a wide range of temperatures (350–600 °C, ΔT = 50 °C), and two oxygen concentrations fed to the lower zone (7.5 and 10.8% for 500, 550, and 600 °C). In the second section, a comparison between the performance over the two previously mentioned reactors is presented, followed by a comparison between feeding water or not doing so to the TZFBR. Results showed that the TZFBR increased the initial yield to olefins and the production time of them, an effect that was increased by feeding a mixture of methanol–water (1:1 molar ratio) instead of pure methanol.

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

Abstract 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.

Pure hydrogen from biogas: Intensified methane dry reforming in a two-zone fluidized bed reactor using permselective membranes

Methane dry reforming of biogas can be a sustainable source of hydrogen but the development of this technology is hindered by limitations such as endothermicity and catalyst deactivation by coke. A two zone fluidized bed reactor coupling permselective Pd/Ag membranes counteracts them and allows to intensify the process obtaining a stable pure hydrogen production. Here we report the effect of operation variables (i.e., temperature, total bed height, nature and partial pressure of regenerative agent, relative height of the regeneration and reaction zones, and use of an activation period) on the yield to hydrogen and stability of the process. Hydrogen over-yields, compared with the conventional fluidized bed reactor, in the range of +200% to +100% were obtained for the entire interval of temperatures 475–575 °C whilst maintaining stable operation by continuous catalyst regeneration. Around 70% of it was pure hydrogen coming from the permeate side of the membranes. The proposed reactor configuration greatly increases both methane conversion and selectivity to hydrogen (expressed as H2/CO ratio), not only in relation to our own conventional reactor findings but also regarding other published results.

Catalytic Dehydration of Glycerol to Acrolein in a Two-Zone Fluidized Bed Reactor.

The gas-phase catalytic dehydration of glycerol to acrolein was carried out in a Two-Zone Fluidized-Bed Reactor (TZFBR) using a 20 wt. % phosphotungstic acid (H3PW12O40) catalyst supported on CARIACT-Q10 commercial silica. In the first step, a hydrodynamic study of the reactor was performed. A quality of fluidization of more than 80% was obtained. In the second step, the mechanical stability of the catalyst was studied. It was found that only the external layer of active phase is eliminated under the conditions of operation whereas the global composition of the catalyst was not significantly affected after 44 h of fluidization. Finally, in a third step, the influence of the main operating parameters on the overall catalytic performances (glycerol/oxygen molar ratio and relative volumes of the reaction and regeneration zones) was investigated, showing notably the importance of the O2/glycerol ratio, resulting in an inverse trend between conversion and selectivity. Increasing O2/glycerol ratio led to higher conversion (lower coke deposit as shown by TGA analysis), but to the detriment of the selectivity to acrolein, supposedly due to the presence of O2 in the reaction zone causing the degradation of glycerol and acrolein.

Methane Aromatization in a Fluidized Bed Reactor: Parametric Study

Methane aromatization is a promising technology for the transformation of natural gas to liquid products, but suffers the problem of catalyst deactivation by coke. A Two-zone fluidized bed reactor has been proposed as a tool to counteract the catalyst deactivation, by providing continuous catalyst regeneration in the same vessel where the main reaction is carried out. This work shows the effect of the main operating conditions (carburization temperature, reaction temperature, carburization time, nature of regenerating agent and feed flow and height of the hydrocarbon entry point) .

On the hydrodynamics of a pseudo two-dimensional two-zone gas-solid fluidized bed

The hydrodynamics of a pseudo 2-dimensional two-zone fluidized bed reactor has been studied using both experimental and numerical techniques. In the experimental section a coupled Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) technique was employed to obtain the averaged particle circulation pattern, bubble dynamics, bed surface fluctuations and the average bubble induced void fraction. To obtain more insight about the hydrodynamics of these fluidized bed types the Eulerian-Eulerian approach was adapted in the numerical section. The results show an alternative reciprocating trend in the bed surface in the left and right sides of the fluidized bed. It was shown that this reciprocating trend originates from two main reasons: the presence of the cavities (small bubbles) below the nozzle tips and the gas hydrodynamics inside the T-shape nozzle. Finally, the average particles circulation patterns, average solid concentration and the granular temperature variation were studied.

Biogas to high purity hydrogen by methane dry reforming in TZFBR+MB and exhaustion by Steam-Iron Process. Techno–economic assessment

A techno-economic study has been carried out with the aim of analyzing the performance (product distribution and energy yields) and estimating the production costs of high purity hydrogen obtained from biogas. For such purpose and taking advantage of empirical data developed in our laboratory, it has been proposed a system consisting of a two-zone fluidized bed reactor aided by a system of permselective (Pd/Ag) metallic membranes inserted in the fluidized bed (TZFBR+MB), and a battery of several fixed bed reactors operating cycles of reduction and oxidation (Steam-Iron Process -SIP-). The feed has always been an equimolar mixture of CH4 and CO2 simulating a sweetened biogas. The first reactor (TZFBR+MB) can produce a stream of pure hydrogen (i.e. PEMFC quality) as permeated flow through the MB, and a retentate stream rich in all species resulting from the methane dry reforming reaction (MDR) and the water gas shift equilibrium (WGS). The singularity of this kind of complex reactors is that regeneration of the catalyst is performed in the same reactor and simultaneously to the MDR reaction because of the two-zone. Due to the reductive behavior of the retentate stream, it can be fed to a bed of solid where up to two different oxygen carriers (iron oxide with additives and cobalt ferrite) can be reduced to their metallic state. Once the solid has been completely reduced, it can be reoxidized with steam releasing a high purity hydrogen stream. Both reactors (i.e. TZFBR+MB and SIP) have been coupled in different degrees. A performance (hydrogen and energy yields) as well as costs analysis (fixed assets and operating costs) have been performed with the aid of Aspen HYSYS v9.0, used for dimensioning the equipment needed to process up to 1350 kg/h of biogas. On this way, the integrated process enhances the efficiencies of every single process allowing pure hydrogen yields up to 68% at 575 °C in the TZFBR+MB and an overall energy efficiency greater than 45%. Production costs have been found to be in the range from 4 to 15 €/kg, still high but not so far away from the target of DOE fixed in 2 $/kg by 2020.

Using two-zone fluidized bed reactor in propane aromatization over Zn/HZSM-5 catalyst

Propane conversion to aromatics was carried out in a two-zone fluidized bed reactor (TZFBR) over Zn/HZSM-5 catalyst. At the steady state condition the coke formation in the TZFBR was counteracted with continuous catalyst regeneration by oxygen. In the lower part of the reactor, the oxidation and combustion of deposited coke on the catalyst surface were achieved with a necessary amount of oxygen. The aromatization reaction takes place in the upper part. In order to ensure the stability of the TZFBR performance, the propane aromatization was carried out in the conventional fluidized and fixed bed reactors in the absence of oxidant. Because of the catalyst regeneration, the high propane conversion and selectivity to aromatics were obtained in the TZFBR. The effects of the main operating variables such as: O2 mole percentage, height of the propane feed entry point, bed temperature and relative velocity were studied and the optimum experimental conditions were considered.

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.

Dry reforming of biogas in fluidized bed: Process intensification

Biogas is a renewable resource obtained mainly from the anaerobic fermentation of agro-industrial and anthropogenic residues. The production of hydrogen by dry reforming of methane represents a potential application for this renewable energy carrier. This could play a positive contribution towards meeting the challenge of providing a global supply of energetically sustainable and environmentally friendly energy. This work combines a catalytic reaction, a separation and the catalyst regeneration in a single reactor. To this end, a two zone fluidized bed reactor (TZFBR) with hydrogen selective membranes has been employed (TZFBR + MB). The operating conditions for the process of dry reforming of biogas have been optimized experimentally, both in TZFBR and TZFBR + MB. Several catalysts were prepared (Ni/Al2O3, NiCe/Al2O3, NiCo/Al2O3), characterized and tested in reactions in both TZFBR and in TZFBR + MB. Finally, the influence of using oxygen or carbon dioxide as regenerating gases in the process has been studied. Experimental results show the feasibility of using CO2 for in situ catalyst regeneration, avoiding the potential problems associated with the use of O2.

Honeywell UOP introduces range of new hydrotreating catalysts, expanding offerings to support production of cleaner-burning gasoline and diesel

Methanol to Gasoline (MTG) process transforms methanol to hydrocarbons within the boiling point range of gasoline. The result is a wide spectrum of products (olefins, paraffins, aromatics and naphthenics, among others), with the total conversion of methanol to hydrocarbons and water. Catalyst deactivation by coke is a main problem in this process. This work aims to determine the feasibility of carrying out the production of gasoline from methanol in a two-zone fluidized bed reactor (TZFBR). The hypothesis is that the formation of carbonaceous deposits (coke) on the catalyst particles can be counteracted by its combustion in the regeneration zone that this novel reactor presents, thus achieving stable and continuous operation. In this way, both processes (reaction and regeneration) would be being carried out simultaneously in the same reactor (process intensification). The comparison of results between a conventional fluidized bed reactor and a TZFBR shows that the second one actually provides a better stability over time.

Experimental and simulated solids mixing and bubbling behavior in a scaled two-section two-zone fluidized bed reactor

Digital Image Analysis techniques, a phenomenological Counter-Current Back-Mixing model (CCBM) and Two-Fluid Model (TFM) simulations were employed to evaluate the effect of scale on the Two-Section Two-Zone Fluidized Bed Reactors (TS-TZFBR) fluid dynamics, i.e. bubble characteristics, axial mixing of solids and defluidization phenomena. The reactor scaling did not affect the quality of the TFM bubble size predictions. A bubble size correlation previously proposed by the authors for TS-TZFBR units was able to predict the experimental axial bubble size evolution at the different reactor scales and gas velocities (ugas/umf=1.5–3.0) with a relative error under 17%. The TFM simulated axial solid mass fluxes were same order as these obtained by Particle Image Velocimetry for every reactor size. However, the classical CCBM model was unable to predict the effect of scale on the solids axial mixing in a TS-TZFBR. The inclination angle of the defluidized bed regions found within the TS-TZFBR tapered zone, β, increased by (ugas/umf)0.25 when duplicating the reactor size. Nevertheless, it did not exceed the prescribed upper limit of β=80° for any of the conditions tested.

Oxidative dehydrogenation of n-butane to butadiene with Mo-V-MgO catalysts in a two-zone fluidized bed reactor

The oxidative dehydrogenation of n-butane has been studied in a two-zone fluidized bed reactor using two Mo-V-MgO catalysts. Both catalysts have been prepared by incipient wetness impregnation and were calcinated at different temperatures. The operating conditions temperature, flow velocity, n-butane inlet height and oxygen/n-butane molar ratio were varied to maximize 1,3-butadiene yield. At suitable conditions, the two-zone fluidized bed reactor can be operated at steady state performing chemical conversion and catalyst regeneration in a single vessel. The regeneration zone at the bottom of the fluidized bed was used to burn coke depositions as well as to fill up lattice oxygen of the catalyst. After regeneration the catalyst particles can reach the reaction zone due to particle mingling inside the fluidized bed. Different behaviors for both catalysts, despite equal metal content, were observed. The catalyst calcinated at lower temperature tends more to coke formation. Raman spectroscopy, XRD, and BET were used to identify the reason for this. With Raman spectroscopy, small graphite particles were identified on the low temperature calcined catalyst surface. These graphite particles seem to be precursors for coke deposition. The Mo-V-MgO catalyst calcinated at 720°C leads to an improved selectivity to 1,3-butadiene being 51% and a 1,3-butadiene yield of 32.7% at steady state.

Hydrodynamic study of a Two-Section Two-Zone Fluidized Bed Reactor with an immersed tube bank via PIV/DIA

The hydrodynamic behaviour of a pseudo-2D Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR) with an immersed tube bundle in its lower zone has been studied using non-invasive Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA). Coupled porosity distribution maps from DIA post-processing and solids velocity vector fields from PIV analysis allowed the reconstruction of the transient and time-averaged solid fluxes along the bed. Six different tube bank configurations at several different superficial gas velocities have been tested to evaluate the hydrodynamic behaviour within the lower zone of the TS-TZFBR. The solids axial mixing along the vertical bed position has been quantified for this novel reactor configuration. Besides, the effect of the internals on the gas-solid mass transfer has been estimated by means of bubble size. A five-staggered tube bundle configuration was able to diminish the average bubble size by 30% within the lower bed zone at usual TS-TZFBR gas flow rates, ugas/umf=3.0, without increasing the solids circulation time dramatically.

CFD model prediction of the Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR) hydrodynamics

Abstract Computational simulations and experiments have been carried out to study the bed hydrodynamics in a Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR). An Eulerian–Eulerian two-fluid approach has been used to simulate the reactor bubbling behavior, while cold fluidization measurements have been made in pseudo-2D TS-TZFBR facilities. The characteristic bubble shrinkage within the transition bed section as well as the eventual formation of defluidized bed regions have been well predicted by the CFD model. The effect of TS-TZFBR geometry, i.e. the transition section angle ( α ), superficial gas velocity in the bed and immersed gas distributor location, on the experimental hydrodynamic behavior has been compared to that in computational simulations, resulting in a reasonably good agreement. Bubble related properties have been analyzed for both simulations and experimental measurements. The experimental results validate the standard Eulerian–Eulerian model for simulating the TS-TZFBR bubble hydrodynamics in a wide range of operational conditions: u gas – u mf = [5–20] cm/s, α = [0°–85°].

A non-parametric bubble size correlation for a Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR)

The Two-Zone Fluidized Bed Reactor (TZFBR) provides a high level of process integration, allowing heterogeneous catalytic reaction and simultaneous catalyst regeneration in two different zones of a single fluidized bed. This work considers a new reactor design that incorporates a different cross-sectional area in each zone to allow low flow rates in the regeneration zone. The reactor geometry, e.g. the transition angle α between the two zones in this Two-Section Two-Zone Fluidized Bed Reactor (TS-TZFBR), has a strong effect on the bubbling behavior. The present study focuses on obtaining a bubble size (db) correlation able to predict the experimental hydrodynamic behavior inside this novel TS-TZFBR. Experimental studies have been conducted on acrylic glass pseudo-2D TS-TZFBRs using several particle types and reactor geometries (α). Experimental results validate classical db correlations within the straight region of the reactor. A novel non-parametric bubble size correlation has been developed to predict the bubbling behavior within the tapered bed section, where an additional gas feed is also applied. This non-parametric model agrees with experimental results along the whole TS-TZFBR bed height for a wide range of operational conditions: ugas/umf=1.5–3.5, ugas−umf=5–25cm/s, α=0°–85°, umf=1.2–20.5cm/s.

Particle Mixing in a Two-Section Two-Zone Fluidized Bed Reactor. Experimental Technique and Counter-Current Back-Mixing Model Validation

Effective particle circulation between the two zones is a prerequisite for ensuring the simultaneous reaction and catalyst regeneration inside a two-zone fluidized bed reactor (TZFBR). An appropriate degree of particle mixing provides a steady state catalytic operation, whereas poor solid circulation between the bed zones leads to unhampered catalyst deactivation. To achieve effective control of the fluid dynamic regime within the two bed regions, a new design has been proposed with a different cross sectional area in each zone. The transition angle (α) between these zones represents the most characteristic feature of the so-called two-section TZFBR (TS-TZFBR). In the present study, the influence of operational variables (α, gas velocities, gas distributor location) on the particle circulation has been determined. Phosphorescent particles have been used as optical tracers to measure the solid axial mixing between the zones in a cold pseudo-2D TS-TZFBR facility. Additionally, a modified counter-current back-mixing model (CCBM) without adjustable parameters has been developed to explain and predict the solid mixing rate for different TS-TZFBR geometries. The experimental mixing results show a reasonably good agreement with the suggested mixing model predictions in a wide range of operational conditions: ugas–umf = [5–20] cm3/(cm2 s), α = [0°–85°].

Stable hydrogen production by methane steam reforming in a two zone fluidized bed reactor: Experimental assessment

The Two Zone Fluidized Bed Reactor concept is proposed for hydrogen production via the steam reforming of methane (SRM) including integrated catalyst regeneration. In order to study the effect of the contact mode, the oxidative SRM has been carried out over a Ni/Al2O3 catalyst using a fixed bed reactor (fBR), a conventional fluidized-bed reactor (FBR) and the proposed two-zone fluidized bed reactor (TZFBR). The technical feasibility of these reactors has been studied experimentally, investigating their performance (CH4 conversion, CO and H2 selectivity, and H2 global yield) and stability under different operating conditions.Coke generation in the process has been verified by several techniques. A stable performance was obtained in the TZFBR, where coke formation was counteracted with continuous catalyst regeneration. The viability of the TZFBR for carrying out this process with a valuable global yield to hydrogen is demonstrated.