The effect of gas extraction through vertical membranes on the bubble hydrodynamics in a fluidized bed reactor

Recently membrane-assisted fluidized bed reactors (MAFBRs) have emerged as a cutting-edge technology for intensification of a number of industrially important chemical processes. Such reactors combine the benefits of excellent separation properties of membranes and the excellent heat and mass transport characteristics of fluidized beds. Hydrogen production from methane reforming is one of the main applications of MAFBRs where H2 perm-selective membranes are employed to extract ultra-pure hydrogen and increase product yield. Understanding the effect of inserted membranes on the hydrodynamic behaviour of the fluidized bed reactor is of high importance for the design and optimization of MAFBRs. In particular, bubble properties (size, number and velocity) strongly influence the performance of fluidized bed reactors as they play a major role in heat and mass transfer phenomena. This work presents the experimental results of the effect of gas extraction via vertical membranes on the bubble properties using a Digital Image Analysis (DIA) technique. A pseudo 2D experimental setup ( Fig. 1) with a multi-chamber porous plate mounted at the bottom of the back plate is used to simulate vertical membranes. This setup allows for gas extraction in specific locations from the back of the column. Thus the effects of vertical membranes (gas extraction rates and locations) on the bubble properties for different particle sizes and fluidization velocities are studied in great detail. Results showed that variation of gas extraction rates ( Fig. 2a) and locations ( Fig. 2b) significantly influences bubble properties. Please click Additional Files below to see the full abstract

On the novel chemical switching reforming (csr) reactor for hydrogen production with integrated co2 capture

Membrane reactors has recently emerged as one of the most promising technologies for pure hydrogen production as these reactors integrate the catalytic reactions, mostly reforming and water-gas shift reactions for hydrogen generation, and separation through membranes in a single unit. This combination of process units brings a high degree of process intensification with additional benefits in terms of increased process efficiencies. Recently, a novel MA-fluidized bed reactor concept has been proposed for pure hydrogen production with integrated CO2 capture from steam methane reforming. The so-called Chemical Switching Reforming reactor utilizes an oxygen carrier which acts as catalyst and heat carrier to the endothermic reforming reaction and is periodically exposed to fuel/steam and air streams; when air is fed to the reactor, the oxygen carrier is heated by the exothermic solids oxidation reaction, this heat is then utilized in the fuel stage where endothermic reduction and catalytic reactions regenerate the oxygen carrier and produce syngas. A hydrogen perm-selective membrane (¨thin-Pd-membrane) is used to directly recover pure hydrogen produced during steam-methane reforming while simultaneously shifting steam reforming and water-gas shift reactions equilibria towards complete conversion at lower temperatures. The CSR concept brings large benefits in design simplification, scale up and ease of operation at elevated pressure. This paper presents preliminary experimental tests on a lab scale CSR reactor ( Fig. 3) without membrane operated under auto-thermal reforming (ATR) conditions. Experimental tests investigating the influence of reactor temperature, steam to carbon ratio and feed flowrate on the reactor performance will be presented. Please click Additional Files below to see the full abstract

The effect of sorbent regeneration enthalpy on the performance of the novel Swing Adsorption Reactor Cluster (SARC) for post-combustion CO2 capture

The recently proposed SARC concept combines a temperature swing and a vacuum swing to efficiently regenerate a CO2 capture sorbent. A heat pump is used to facilitate the temperature swing by transferring heat from the exothermic carbonation to the endothermic regeneration reaction. This study uses combined reactor and power plant simulations to investigate an interesting trade-off presented by the SARC concept: sorbents with higher regeneration enthalpies will require more heat transfer between carbonation and regeneration, but also allow for a smaller temperature swing, thus enabling the heat pump to transfer this heat more efficiently. Studies showed that the optimal process efficiency is achieved at a sorbent regeneration enthalpy in the range of 100-150 kJ/mol, thus avoiding the need for novel sorbents with very low regeneration enthalpies. Simulation results also highlighted another interesting feature of the SARC concept: the generally undesired adsorption of water vapour by the sorbent does not reduce the overall SARC efficiency because the release of water vapour under regeneration enhances the pressure swing. Finally, a central composite design was conducted to fully understand the SARC behaviour when using sorbents with different regeneration enthalpies. These insights are important for optimal sorbent selection for the SARC concept in future studies

Experimental demonstration of control strategies for a gas switching combustion reactor for power production with integrated CO2 capture

This paper reports on operational control strategies for a Gas Switching Combustion (GSC) reactor concept for power generation with integrated CO2 capture. This concept uses a single reactor where air and gaseous fuel feeds are alternated into a bed of oxygen carrier. No external solids circulation is required in this concept and thereby many technical and scale-up challenges that are encountered in the conventional Chemical Looping Combustion processes based on interconnected fluidized bed systems are circumvented. A first demonstration of this concept has shown that the reactor temperature undergoes large variations across the redox cycle due to the highly exothermic reaction of air with the oxygen carrier. This paper presents and demonstrates the experimental feasibility of a number of heat management techniques implemented in order to minimize the temperature variation and thereby improve the electric efficiency of the GSC concept when integrated into a combined cycle power plant. Three heat management strategies have been devised based on theoretical calculations; two of them involve air feed dilution using nitrogen while the third uses a secondary air injection point to force part of the air feed during the oxidation stage to slip through the bed without reacting with the oxygen carrier. The three investigated heat management strategies succeeded to greatly reduce the temperature rise across the redox cycle although to different extents. However, the one with the secondary concentrated air injection shows greater economic potential for implementation in the GSC reactor, since it does not require a depleted air recirculation that is required for the two other strategies

Hydrogen production with integrated CO2 capture in a novel gas switching reforming reactor: proof-of-concept

This paper reports an experimental investigation on a novel reactor concept for steam-methane reforming with integrated CO2 capture: the gas switching reforming (GSR). This concept uses a cluster of fluidized bed reactors which are dynamically operated between an oxidation stage (feeding air) and a reduction/reforming stage (feeding a fuel). Both oxygen carrier reduction and methane reforming take place during the reduction stage. This novel reactor configuration offers a simpler design compared with interconnected reactors and facilitates operation under pressurized conditions for improved process efficiency. The performance of the bubbling fluidized bed reforming reactor (GSR) is evaluated and compared with thermodynamic equilibrium. Results showed that thermodynamic equilibrium is achieved under steam-methane reforming conditions. First, a two-stage GSR configuration was tested, where CH4 and steam were fed during the entire reduction stage after the oxygen carrier was fully oxidized during the oxidation stage. In this configuration a large amount of CH4 slippage was observed during the reduction stage. Therefore, a three-stage GSR configuration was proposed to maximize fuel conversion, where the reduction stage is completed with another fuel gas with better reactivity with the oxygen carrier, e.g. PSA-off gases, after a separate reforming stage with CH4 and steam feeds. A high GSR performance was achieved when H2 was used in the reduction stage. A sensitivity analysis of the GSR process performance on the oxygen carrier utilization and target working temperature was carried out and discussed

The effect of frictional pressure, geometry and wall friction on the modelling of a pseudo-2D bubbling fluidized bed reactor

The two fluid model (TFM) closed by the kinetic theory of granular flows (KTGF) has been developed to a high level of maturity over the past three decades. However, significant uncertainties still remain about the influence of various closure models on the predictions of the hydrodynamics and especially the reactive performance of fluidised bed reactors. The three factors investigated in this study – frictional pressure, geometry (2D/3D) and friction at the walls – all have significant influences on model predictions of the behaviour of a pseudo-2D bubbling fluidised bed reactor. This study aims to quantify the influence of these important factors on simulation output both in terms of hydrodynamics and reactive performance. Simulations designed to evaluate the effects of these factors were carried out over a wide range of fluidisation velocities, bed loadings and particle sizes to reveal significant impacts on the results. Differences in simulation results varied significantly with changes in the three operating variables investigated (fluidisation velocity, bed loading and particle size) and were analysed in detail. Finally, 3D simulations with wall friction and frictional pressure included showed qualitatively very similar hydrodynamic behaviour to that observed in the experiments. Quantitatively, measurements of the bed expansion ratio compared well for different fluidisation velocities and the particle sizes, but some unexplained differences were still observed in response to changes in the bed loading

The effect of gas permeation through vertical membranes on chemical switching reforming (CSR) reactor performance

A novel membrane assisted fluidized bed reactor concept has been proposed for ultra-pure hydrogen production with integrated CO2 capture from steam methane reforming. The so-called Chemical Switching Reactor (CSR) concept combines the use of an oxygen carrier for supplying heat and catalysing the steam methane reforming reaction and hydrogen perm-selective membrane (thin Pd-membrane) for hydrogen recovery. However, extraction of gas through the membranes influences the hydrodynamics of the fluidized bed by altering the bubble behaviour and the extent of gas back mixing. Bubble properties (size, number and velocity) strongly influence the performance of fluidized bed reactors as they play a major role in heat and mass transfer phenomena. This work experimentally investigates the effects of gas extraction via vertical membranes on the bubble properties using Digital Image Analysis (DIA) technique and numerically using the Two Fluid Model approach (TFM) closed by the kinetic theory of granular flow. The simulation studies were extended to investigate real reactive conditions. A pseudo 2D experimental setup with a multi-chamber porous plate mounted at the bottom of the back plate was used to simulate vertical membranes. This setup allowed for gas extraction in specific locations from the back of the column, thus facilitating studies on the effect of gas extraction rates and locations on the bubble properties. Results show that variation of gas extraction flow rates slightly influences the bubble behaviour, whereas variation of gas extraction locations (varying the area) significantly influences bubble properties. Cold flow simulations showed a reasonable comparison to experimental measurements and reactive simulations revealed very similar hydrodynamic responses to changes in gas extraction rate (membrane permeability) and location. Shifting gas extraction towards the centre of the bed proved to be beneficial in reducing gas back-mixing. Specifically, reducing the number of vertical membranes from 7 to 5 by removing the outer two membranes showed a slight increase in hydrogen extraction performance