Volume 17, Issue 11 -

In practical engineering applications, the mixing and separation behavior of multi-component particles is importance to the fluidized bed operation. The development of many practical processes is inseparable from the knowledge of particle mixing and separation, such as material processing of ash-soluble coal gasification, multi-phase flow in boilers, and petrochemical catalytic processes. In recent years, due to the obvious advantages of the Eulerian–Eulerian model, many researchers at home and abroad have used it to study the mixing and separation behavior of particles. The paper reviews the use of Eulerian–Eulerian model to study the mixing and separation of multi-component particles in fluidized beds. The Eulerian–Eulerian model describes the gas-phase and each of the individual particles as continuums. The mechanism of particle mixing and separation, the influence of different factors on the particle mixing and separation including differences in particle size and density, the differences in apparent air velocity, the differences in model factors are discussed. Finally, an outlook for the use of Eulerian–Eulerian model to study the mixing and separation behavior of three component particles and related research on the drag model between particles.

In the biodiesel production, acidic catalysts are ideally suitable for reacting with different oil sources at various free acid levels. On the other hand, the nanocatalysts can easily be propagated in the reaction medium and provide more accessible active sites for reaction. The aim of this work was to synthesize an acidic nanocatalyst based on boehmite nanoparticles then studying it to biodiesel production from soybean oil. Up to now, no reports were found on biodiesel production by this catalyst. After the synthesis and characterization of the catalyst, using response surface methodology (RSM), the optimized conditions for transesterification were 4.87 wt.% for catalyst dosage, 13:1 for the molar ratio of methanol to oil, 60 °C for reaction temperature, and 3 h for reaction time. At the optimal point, the production yield was 99.8 %. After six consecutive use of the catalyst, the yield dropped slightly (88 %). Consequently, the catalyst can be employed efficiently several runs in the production process.

The flow field in a transparent stirred tank with a diameter of D  = 0.42 m equipped with dual-layer improved Intermig impellers was numerically investigated using a multi-reference frame method based on the computational fluid dynamics (CFD) simulation code: Fluent. The simulation results were validated by experiments based on a stirred tank testing platform and a Particle Image Velocimetry (PIV) testing instrument. The effects on the flow field arising from the diameter of impellers, the installation height of impellers ( C 1 ), the rotational speed as well as the distance between two impellers ( C 2 ) were investigated. It was revealed that the distance between two impellers and the installation height has obvious influences on the local flow field around impellers. Appropriately increasing the two parameters can enhance the axial flow and achieve a better mass transfer effect. For the tank model rescaled for experiments, the optimized geometrical parameters are C 1  = 0.36 D and C 2  = 0.49 D that can result in full occupation of large eddies in the entire volume. The mixing process simulation indicates that the mixing efficiency is the best in the region near the impellers while it is the worst in the bottom of the tank. An appropriate increase in the diameter of the lower impellers and a reduction in the distance from the lower impellers to the tank bottom can improve the mixing efficiency in the central region near the tank bottom.

The methanation of carbon dioxide (CO 2 ) via the Sabatier reaction is an exothermic process that needs the continuous removal of the heat produced for avoiding the sintering of the catalyst. A novel milli-channel reactor with internal diameter of each channel in the millimeter size has been designed, manufactured and tested for the Sabatier reaction. Thanks to its configuration, this reactor could control effectively the heat generated by the reaction, attaining an intensification of the process. A CFD model was used to study the reaction phenomena occurring inside the reactor and the corresponding heat transfers. The kinetic parameters of the reaction have been obtained, and employing different process condition (different temperatures, pressures and gas flow rates) the results obtained have been studied. At the evaluated parameters, the CFD model fits the experimental results in the developed reactor.

Six different Ni-based fluidizable catalysts were synthesized using both incipient impregnation and co-impregnation. Ni-based catalysts were also promoted with 2.0 wt% La or alternatively with 2 wt% Ce. The preparation procedure included catalysts treated at high temperatures and under free of oxygen conditions. Catalysts were characterized using BET, XRD, AA, PSD, TPR, TPD, H 2 -chemisorption. TPR and H 2 chemisorption showed good metal dispersion with 10 nm- 40 nm metal crystallites. Glucose catalytic gasification runs were performed in a CREC Riser Simulator to evaluate the following catalysts: (a) 5 %Ni/γ-Al 2 O 3 , (b) 5 %Ni-2 %La/γ-Al 2 O 3 and (c) 5 %Ni-2 %Ce/γ-Al 2 O 3 . In all cases, the preparation steps involved acid solutions with pHs of 1 and 4. In between consecutive runs, different approaches were considered: (a) A catalyst was regenerated by air, (b) A catalyst was regenerated by air followed by hydrogen pretreatment, (c) A catalyst was reused directly without any regeneration or hydrogen pretreatment. It was observed that Ni-based catalysts, which were subjected after every run, to both, air regeneration and hydrogen pretreatment, displayed the best yields in close agreement with thermodynamic equilibrium. On the other hand, Ni-based catalysts regenerated with air only, showed the worst hydrogen yields. In between these two-hydrogen yield limits, where catalysts not contacted with air nor hydrogen, with these yields being moderately below chemical equilibrium. This shows that Ni-based fluidizable catalysts can perform on stream for extended periods, requiring limited reactivation with air and H 2 . This makes of gasification using the catalysts of the present study, a viable process alternative that could be implemented at industrial scale.

Particle image velocimetry (PIV) was used to measure the flow field in the mixing chamber of a mixer settler. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate liquid-liquid two-phase flow, turbulent flow, and impeller rotation in the mixer settler, respectively. The effects of impeller type, impeller clearance and baffle configuration on the flow field were analyzed. Results showed that rigid-flexible impeller can spread the impeller energy more uniformly in the mixing chamber and increase the velocity of the continuous phase through the multi-body motion of flexible sheet compared with rigid impeller. The flow numbers of RF-RT, RF-PBT and RF-CBT systems were increased by 46 %, 54 % and 43 %, respectively, compared with RT, PBT and CBT systems. Meanwhile, the suction capacity of impeller was enhanced with a decreased impeller clearance, and the backflow was eliminated by the installation of baffles.

The synthesis of monoglycerides by the transesterification of triglycerides with glycerol was studied using zinc glycerolate as a heterogeneous catalyst. The effect of the operating variables on the triglyceride conversion and monoglyceride yield was evaluated. The maximum values of triglyceride conversion and monoglyceride yield reached at 2-hour reaction time were 83 and 49 %, respectively. These values were obtained at 240 °C with 3 % catalyst loading and glycerol/oil molar ratio of 6. When the molar ratio of the reactants was increased, the triglyceride conversion and monoglyceride yield achieved an optimum value. This behavior was related with a competitive adsorption of the reagents. It was possible to reuse the catalyst without significant changes in activity.

Supercritical water gasification (SCWG) is a novel and clean technology for lignite translating into hydrogen-rich gas. Previous experimental researches show that the use of external recycle system of liquid residual can improve the energy efficiency, but there is not a theoretical model to figure out the component of which exergy lost most and to provide guidance for further optimization of the existing system. In this paper, the thermodynamic model of liquid residual external recycle system was established, based on which energy and exergy balance of the system was evaluated and the exergy efficiency of the main equipment was calculated. Moreover, the influence of recycle flow ratio (0–37.5 %), gasification temperature (550 °C–650 °C), gasification pressure (23–25 MPa) and slurry concentration (2.73–4.15 %) on the exergy and energy efficiency were analyzed. The results showed that the exergy destruction rate of reactor was the highest, which reached 5.52 kW. Both energy and exergy efficiency increased as recycle flow ratio, gasification temperature and pressure increased. The energy and exergy efficiency of the system reached 70.26 % and 56.86 % respectively at the condition of recycle flow ratio of 30 %, gasification temperature of 650 °C, pressure of 25 MPa and slurry concentration of about 2.93 %.

A computational fluid dynamics (CFD) simulation was performed to study the hydrodynamics characteristics in a Rushton turbine stirred tank in laminar regime. The effects of operating condition, working medium and geometrical parameter on the flow field and power number characteristics were investigated. It is found that the two-loop flow pattern is formed in laminar regime when the impeller is not very close to tank bottom, while its shape and size vary with Reynolds number and impeller diameter. For a given geometrical configuration, the flow pattern, power number and dimensionless velocity profile are mainly depended on Reynolds number, and do not change with working medium and scale-up for a constant Reynolds number. When impeller off-bottom clearance is too low and Reynolds number is relatively high, the fluid flow would transit from two-loop flow pattern to sing-loop flow pattern as that occurs in turbulent regime. Power number falls for larger impeller in laminar regime. Surprisingly, in laminar regime, power number in the baffled tank with small impeller is almost identical to that in the unbaffled tank.

Supercritical fluids with excellent decomposition and mass transfer capabilities can degrade the resin matrix of carbon fiber reinforced polymer (CFRPs) to recycle high-performance carbon fibers. The degradation rate of CFRPs was influenced by the velocity and temperature distributions of supercritical fluids in the reactor with mechanical stirring. The flow field model in the reactor was established during the degradation of CFRPs by supercritical ethanol. The velocity and temperature distributions in both the axial and radial directions were simulated when the stirrer was installed in the reactor at different heights. The simulation indicated that the flow distribution was uneven in the reactor and the position with optimum flow distribution for placing CFRPs was 45 %–70 % of the installation distance between underside of the impeller and the base of the reactor. The experiment shows mechanical stirring can significantly promote CFRPs degradation. When the installing height of the stirrer is 110 mm, the degradation rate of the epoxy resin is 10 % higher than that without stirring. The degradation rate of epoxy resin was also affected by the placement position of CFRPs in the reactor, and could be improved by approximately 14 % higher than that without mechanical stirring when the CFRPs were placed in the position with optimum flow distribution.