Membrane-assisted Fluidized Bed Reactor Research Articles

Erosion behaviors of membrane tubes in a fluidized bed reactor with hydrogen separation

Based on a hybrid Eulerian-Lagrangian approach, erosion characteristic of membrane tubes in a membrane-assisted fluidized bed reactor is numerically evaluated. The erosion rate of Pd-based membrane is integrated. The influence of hydrogen separation on gas-solid hydrodynamics and erosion characteristics is discussed. The relationship between concentration polarization and membrane tube erosion is examined. The results demonstrate that the maximum wear of Pd-based membrane occurs at the normal angle. The hydrogen separation weakens membrane tube erosion in a fluidized bed. The arrangement of membrane tube locations plays a very important role in tube erosion. The non-uniform hydrogen concentration strengthens the maximum membrane tube wear, especially for the bottom of the fluidized bed reactor.

Numerical studies of sorption-enhanced glycerol steam reforming in a fluidized bed membrane reactor at low temperature

In order to improve the hydrogen production efficiency by glycerol steam reforming, a membrane-assisted fluidized bed reactor with carbon dioxide sorption is developed to enhance the reforming process. Low-temperature operation in a membrane reactor is necessary considering the thermal stability of membrane. In this work, the sorption-enhanced glycerol steam reforming process in a fluidized bed membrane reactor under the condition of low temperature is numerically investigated, where the hydrotalcite is employed as CO2 sorbents. The impact of operating pressure on the reforming performance is further evaluated. The results demonstrate that the integration of membrane hydrogen separation and CO2 sorption can effectively enhance the low-temperature glycerol reforming performance. The fuel conversion above 95% can be achieved under an elevated pressure.

Efficient Hydrogen Production from Coke Oven Gas by Sorption-Enhanced Steam Reforming in a Membrane-Assisted Fluidized Bed Reactor

High-purity hydrogen was produced by catalytic steam reforming of coke oven gas (COG) in a fluidized bed membrane reactor (FBMR), assisted by calcined dolomite and a tubular palladium membrane. Thi...

CFD analysis on sorption-enhanced glycerol reforming in a membrane-assisted fluidized bed reactor

In this work, a three-dimensional simulation of the sorption-enhanced glycerol reforming (SEGR) process for hydrogen production in a membrane-assisted fluidized bed reactor is carried out by means of the multi-fluid model coupling with chemical kinetics model and membrane separation model. The mutual interaction mechanism of the two enhancing methods including membrane hydrogen separation and carbon dioxide sorption is revealed. The results indicate that the carbon dioxide sorption can hinder the concentration polarization resistance and improve the hydrogen permeation rate while the scope of densified zones is enlarged. Meanwhile, the increase of hydrogen separation degree is also beneficial to the carbon dioxide sorption performance. In addition, the utilization of bi-functional sorbent-catalyst particles can further promote the sorption-enhanced reforming process compared to the conventional two-pellet design.

On concentration polarization in fluidized bed membrane reactors

Palladium-based membrane-assisted fluidized bed reactors have been proposed for the production of ultra-pure hydrogen at small scales. Due to the improved heat and mass transfer characteristics inside such reactors, it is commonly believed that they can outperform packed bed membrane reactor configurations. It has been widely shown that the performance of packed bed membrane reactors can suffer from serious mass transfer limitations from the bulk of the catalyst bed to the surface of the membranes (concentration polarization) when using modern highly permeable membranes. The extent of concentration polarization in fluidized bed membrane reactors has not yet been researched in detail. In this work, we have quantified the concentration polarization effect inside fluidized bed membrane reactors with immersed vertical membranes with high hydrogen fluxes. A Two-Fluid Model (TFM) was used to quantify the extent of concentration polarization and to visualize the concentration profiles near the membrane. The concentration profiles were simplified to a mass transfer boundary layer (typically 1cm in thickness), which was implemented in a 1D fluidized bed membrane reactor model to account for the concentration polarization effects. Predictions by the TFM and the extended 1D model showed very good agreement with experimental hydrogen flux data. The experiments and models show that concentration polarization can reduce the hydrogen flux by a factor of 3 even at low H2 concentrations in the feed (10%), which confirms that concentration polarization can also significantly affect the performance of fluidized bed membrane reactors when integrating highly permeable membranes, but to a somewhat lesser extent than packed bed membrane reactors. The extraction of hydrogen also affects the gas velocity and solids hold-up profiles in the fluidized bed.

Modeling of autothermal reforming of methane in a fluidized bed reactor with perovskite membranes

The performance of perovskite membrane-assisted fluidized bed reactors for ultra-high purity hydrogen production through autothermal reforming of methane has been investigated using numerical simulations. Two different three-phase phenomenological models have been developed differing in their description of the mass transfer between the membranes and the emulsion, cloud and bubble phases, where the parameters in the oxygen permeation flux expression were determined from new experimental data. The calculation of the required oxygen-to-carbon ratio is based on overall enthalpy balance. The reactor performance without and with O2 permeable perovskite membranes have been investigated and compared. The two models of perovskite membrane-assisted fluidized bed reactors produce basically the same results, indicating that the external mass transfer from the membranes to the bulk of the fluidized bed is not rate limiting, but clearly show that the fluidized bed membrane reactor can largely outperform ordinary fluidized bed reactors for the autothermal reforming of methane. It has been demonstrated that with perovskite membrane-assisted fluidized bed reactors autothermal operation with a high CH4 conversion and H2 yield can be achieved with a relatively small catalyst inventory.

On the hydrodynamics of membrane assisted fluidized bed reactors using X-ray analysis

The application of membrane assisted fluidized bed reactors for distributed energy production has generated considerable research interest during the past few years. It is widely accepted that, due to better heat and mass transfer characteristics inside fluidized bed reactors, the reactor efficiency can outperform other reactor configurations such as packed bed units. Although many experimental studies have been performed to demonstrate and monitor the long term performance of membrane assisted fluidized bed reactors, the hydrodynamics of membrane-assisted fluidized bed reactors has thus far only been studied in pseudo-2D geometries. In this work the solids concentration inside a real 3D fluidized bed reactor geometry was measured using a fast X-ray analysis technique. Experiments were conducted in absence and presence of two different membrane modules with different configurations and number of membranes (porous Al2O3 tubes) for two types of particles, viz. 400–600μm polystyrene (Geldart B type) and 80–200μm Al2O3 (Geldart A/B type). Results from the experiments with Geldart B type particles revealed that the membrane modules (both the membranes and the spacers) can significantly reduce bubble growth along the fluidized bed resulting in a smaller average bubble diameter, expected to improve the bubble-to-emulsion mass transfer, whereas for the experiments with fine Geldart A/B particles, and at a very high extraction values (40% of the inlet flow), a densified layer with high solids concentration was formed near the membrane, which may impose an additional mass transfer resistance for gas components to reach the surface of the membranes (concentration polarization). The results from this study help designing and optimizing the positioning of the membranes and membrane spacers for optimal performance of fluidized bed membrane reactors.

Detecting densified zone formation in membrane-assisted fluidized bed reactors through pressure measurements

This work reports the results of an experimental investigation on densified zone formation in a membrane fluidized bed reactor using combined pressure fluctuation and PIV (Particle Image Velocimetry) measurements. A pseudo 2D experimental setup was used, where porous plates on the back plate of column mimicked gas extraction through flat vertically inserted membranes. The maximum in the standard deviation of pressure fluctuations, commonly employed to indicate the transition to turbulent fluidization, shifted to lower fluidization velocities with an increase in the fraction of fluidizing gas being extracted. Flow visualization showed that this result is connected to the onset of stable densified zone formation, which occurred at progressively lower fluidization velocities as the gas extraction fraction was increased. It has also been found that the extent of densified zones quantified using instantaneous particle velocity maps collected by PIV increases with increasing gas extraction rates. This effect became larger for smaller particles. Results have therefore shown that the peak in pressure fluctuations in fluidized beds with gas extraction through flat vertical membranes indicates the onset of densified zones formation rather than turbulent fluidization. Such densified zones can have substantial detrimental effects (such as induced mass transfer limitations, gas bypass etc.) on the reactor performance and can be identified via pressure measurements, as illustrated in this work.

Discrete particle simulations of micro membrane-assisted fluidized beds with H2 extraction

Recent research has shown the potential of membrane-assisted fluidized bed (MAFB) reactors for various applications, and for ultra-pure hydrogen production in particular. For micro MAFBs with gas extraction, the formation of densified zones close to the membrane walls has been recognized to potentially increase the mass transfer resistance from the bed to the membranes. In order to investigate this phenomenon in detail, an classic Euler-Lagrange type model, Discrete Particle Model (DPM) extended with gas-phase component conservation equations has been used to simulate the case with H2 extraction via perm-selective membranes mounted in the confining walls. The simulated membrane walls were set up such that they closely mimic high-flux, thin film, supported palladium-based membranes. It has been found that the formation of densified zones in micro MAFBs has a very small influence on the H2 mass transfer from the bed to the membranes. The advantages of micro MAFBs have been highlighted through a fundamental comparison with a micro packed bed membrane reactor.

Simulation study on the effect of gas permeation on the hydrodynamic characteristics of membrane-assisted micro fluidized beds

Recent research has shown the potential of membrane-assisted fluidized bed reactors for various applications, and for ultra-pure hydrogen production in particular. Due to the excellent mass transfer characteristics of fluidized beds, concentration polarization (i.e. mass transfer limitation) can be overcome and the production capacity of membrane-assisted fluidized bed reactors could be further improved by maximizing the installed membrane area per unit volume, leading to the concept of a micro-structured membrane-assisted fluidized bed reactor. In this study, numerical simulations have been systematically carried out with a discrete particle model to investigate in detail the effects of gas addition and extraction through the confining porous membrane walls on the hydrodynamic characteristics of a single membrane-assisted micro fluidized bed compartment. In particular, the effect of the permeation ratio (amount of gas permeated through the membrane relative to the amount fed) and the installed membrane area on the hydrodynamics was investigated. Gas addition or extraction via the porous membrane walls confining the emulsion phase was simulated via inward or outward directed fluxes of the gas phase, which was found to have a very pronounced influence on the bed hydrodynamics. The effects of gas permeation on the solids circulation pattern, solids holdup distribution and porosity probability density function in membrane-assisted micro fluidized beds have been discussed in great detail. It has been found that gas permeation can have an adverse effect on the bed expansion caused by gas by-passing either through the bed center for the case of gas extraction or close to the membrane walls for the case of gas addition. In addition, the formation of densified zones (increased solids holdup) close to the membrane wall that was observed in case of gas extraction may increase the bed-to-membrane mass transfer resistance. These effects may strongly decrease the gas–solid contacting and the gas residence time, which may deteriorate the reactor performance. On the other hand, it is shown that these problems caused by gas permeation may be avoided by properly tuning the gas velocity through the membrane via membrane area and other design parameters and operating conditions.

Influence of Reactor and Particle Scale on the Hydrodynamics of Microstructured Fluidized Bed Membrane Reactors

Microstructured membrane-assisted fluidized bed reactors have been proposed for hydrogen production as an efficient solution to strongly reduce bed-to-membrane mass transfer limitations (concentration polarization) adversely affecting other types of membrane reactors while maximizing the volumetric production capacity. In a previous work it has been demonstrated that a microstructured membrane-assisted fluidized bed can be successfully operated with state-of-the-art membranes in the turbulent fluidization regime with significantly reduced formation of densified zones. The effect of the particle size and the reactor size on the hydrodynamics of these novel reactors is not well understood. In this paper, the fluidization behavior of beds with different reactor sizes employing different particle sizes has been investigated using particle image velocimetry coupled with an advanced digital image analysis. It has been found that a bed with small width can be operated in the turbulent regime already at relatively low fluidization velocities. Also for reactors with a small bed width the extraction of gas via membranes results in the formation of densified zones close to the membrane walls, while the addition of gas results in an inversion of the solids circulation pattern. However, compared with bigger reactors, the influence of gas permeation via the membranes on the solids circulation patterns as well as bubble size and its distribution is much less pronounced in reactors with a smaller bed width. Also the effect of particle size on the hydrodynamics of the bed when extracting/adding gas through the membrane walls is discussed in this paper. The findings of this work give guidelines for tuning the fluidization regime, reactor size, and particle diameter, for a proper reactor design of microstructured membrane-assisted fluidized bed reactors that can offer significant advantages, viz. maximum volumetric production capacity while minimizing expensive reactant bypassing at the outlet and maximizing the product recovery.

Compartment model for steam reforming of methane in a membrane-assisted bubbling fluidized-bed reactor

Abstract A compartment model was developed to describe the flow pattern of gas within the dense zone of a membrane-assisted fluidized-bed reactor (MAFBR), in the bubbling mode of operation for steam reforming of methane both with (adiabatic) and without (isothermal) entering oxygen. Considering such a flow pattern and using the experimental data reported elsewhere [Roy S, Pruden BB, Adris AM, Grace JR, Lim CJ. Fluidized-bed steam methane reforming with oxygen input. Chem Eng Sci 1999; 54:2095–2102.], the parameters of the developed model (i.e., number of compartments for the bubble and emulsion phases) were determined and fair agreements were obtained between model predictions and experimental data. The developed model was utilized to describe the behavior of an industrial scale adiabatic and isothermal MAFBR. Moreover, the influences of various operating and design parameters such as steam-to-methane ratio (SMR), oxygen-to-methane ratio (OMR), operating temperature and pressure, and the number of hydrogen membrane tubes on the performance capability of the MAFBR were investigated. Furthermore, the performance capability of the MAFBR was optimized subject to the various operating and design constraints, including 1 ≤ SMR ≤ 4 and 500 ≤ T ≤ 1250 K, in the bubbling regime.

Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: Experimental demonstration and model validation

Hydrogen is emerging as a future alternative for mobile and stationary energy carriers in addition to its use in chemical and petrochemical applications. A novel multifunctional reactor concept has been developed for the production of ultrapure hydrogen ( < 10 ppm CO ) from light hydrocarbons such as methane for online use in downstream polymer electrolyte membrane fuel cells. A high degree of process intensification can be achieved by integrating perm-selective hydrogen membranes for selective hydrogen removal to shift the methane steam reforming and water–gas-shift equilibriums in the favourable direction and perm-selective oxygen membranes for selective oxygen addition to supply the required reaction energy via partial oxidation of part of the methane feed and enable pure CO 2 capture without costly post-treatment. This can be achieved in a proposed novel multifunctional bi-membrane bi-section fluidised bed reactor [Patil, C.S., van Sint Annaland, M., Kuipers, J.A.M., 2005. Design of a novel autothermal membrane assisted fluidized bed reactor for the production of ultrapure hydrogen from methane. Industrial and Engineering Chemistry Research 44, 9502–9512]. In this paper, an experimental proof of principle for the steam reforming/water–gas-shift section of the proposed novel fluidised bed membrane reactor is presented. A fluidised bed membrane reactor for steam reforming of methane/water–gas-shift on a commercial noble metal-based catalyst has been designed and constructed using 10 H 2 perm-selective Pd membranes for a fuel cell power output in the range of 50–100 W. It has been experimentally demonstrated that by the insertion of the membranes in the fluidised bed, the thermodynamic equilibrium constraints can indeed be overcome, i.e., increased CH 4 conversion, decreased CO selectivity and higher product yield (H 2 produced/CH 4 reacted). Experiments at different superficial gas velocities and also at different temperatures and pressures (carried out in the regime without kinetic limitations) revealed enhanced reactor performance at higher temperatures ( 650 ∘ C ) and pressures (3–4 bar). With a phenomenological two-phase reactor model for the fluidised bed membrane reactor, incorporating a separately developed lumped flux expression for the H 2 permeation rate through the used Pd-based membranes, the measured data from the fluidised bed membrane reactor could be well described, provided that axial gas back-mixing in the membrane-assisted fluidised bed reactor is negligible. This indicates that the membrane reactor behaviour approached that of an ideal isothermal plug flow reactor with maximum H 2 permeation.

Development of a Membrane-Assisted Fluidized Bed Reactor. 2. Experimental Demonstration and Modeling for the Partial Oxidation of Methanol

A small laboratory-scale membrane-assisted fluidized bed reactor (MAFBR) was constructed in order to experimentally demonstrate the reactor concept for the partial oxidation of methanol to formaldehyde. Methanol conversion and product selectivities were measured at various overall fluidization velocities, reactor temperatures, methanol and oxygen overall feed concentrations, ratios of gas fed via membranes relative to gas fed via the bottom distributor, and aspect ratios of the fluidized bed. High methanol conversions and high selectivities to formaldehyde were achieved with safe reactor operation (isothermal reactor conditions) at very high methanol inlet concentrations, much higher than currently employed in industrial processes. It was experimentally demonstrated that with distributive feeding of oxygen in a MAFBR the overall formaldehyde yield and throughput could be increased without a pronounced and undesirable conversion of formaldehyde to carbon monoxide. Furthermore, a one-dimensional two-phase phenomenological reactor model has been developed with which the experimentally observed conversion and selectivity as a function of the operating conditions could be well described.

Development of a Membrane-Assisted Fluidized Bed Reactor. 1. Gas Phase Back-Mixing and Bubble-to-Emulsion Phase Mass Transfer Using Tracer Injection and Ultrasound Experiments

A small laboratory-scale membrane-assisted fluidized bed reactor (MAFBR) was constructed in order to experimentally demonstrate the benefits of this reactor concept, especially the enhanced bubble-to-emulsion phase mass transfer and the reduced overall axial gas phase back-mixing, due to the presence of the membranes and permeation of gas through the membranes. With steady-state tracer gas injection experiments, it was demonstrated that the experimental reactor exhibited approximately plug flow behavior for all the operating conditions investigated in this work as a result of the elimination of macroscale circulation patterns due to the presence of the membranes and, even more importantly, the permeation of gas through the membranes. With an ultrasound technique, the gas residence time distribution (RTD) of the MAFBR was measured over a wide range of fluidization velocities for two different bed heights. Interpretation of the RTD measurements with a phenomenological two-phase reactor model extending the bubble assemblage model proposed by Kato and Wen (Kato, K.; Wen, C. Chem. Eng. Sci. 1969, 24, 1351) showed that the average bubble diameter is significantly decreased for higher ratios of gas permeated through the membranes relative to the total gas flow rate.