Computational investigation of hydrodynamics, flow regimes and bubble size distribution in an airlift reactor (original) (raw)
Related papers
Processes, 2019
In this work, we employed a computational fluid dynamics (CFD)-based model with a Eulerian multiphase approach to simulate the fluidization hydrodynamics in biomass gasification processes. Air was used as the gasifying/fluidizing agent and entered the gasifier at the bottom which subsequently fluidized the solid particles inside the reactor column. The momentum exchange related to the gas-phase was simulated by considering various viscous models (i.e., laminar and turbulence models of the re-normalisation group (RNG), k-ε and k-ω). The pressure drop gradient obtained by employing each viscous model was plotted for different superficial velocities and compared with the experimental data for validation. The turbulent model of RNG k-ε was found to best represent the actual process. We also studied the effect of air distributor plates with different pore diameters (2, 3 and 5 mm) on the momentum of the fluidizing fluid. The plate with 3-mm pores showed larger turbulent viscosities above the surface. The effects of drag models (Syamlal-O'Brien, Gidaspow and energy minimum multi-scale method (EMMS) on the bed's pressure drop as well as on the volume fractions of the solid particles were investigated. The Syamlal-O'Brien model was found to forecast bed pressure drops most consistently, with the pressure drops recorded throughout the experimental process. The formation of bubbles and their motion along the gasifier height in the presence of the turbulent flow was seen to follow a different pattern from with the laminar flow.
Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD
Chemical Engineering & Technology, 2003
Computational Fluid Dynamics (CFD) is used to compare the hydrodynamics and mass transfer of an internal airlift reactor with that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulation velocities are significantly higher in the airlift configuration than in bubble columns, leading to significantly lower gas holdups. Within the riser of the airlift, the gas and liquid phases are virtually in plug flow, whereas in bubble columns the gas and liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k L a, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k L a are practically identical.
CFD simulation of hydrodynamics of gas–liquid–solid fluidised bed reactor
Chemical Engineering Science, 2009
A three dimensional transient model is developed to simulate the local hydrodynamics of a gas–liquid–solid three-phase fluidised bed reactor using the computational fluid dynamics (CFD) method. The CFD simulation predictions are compared with the experimental data of Kiared et al. [1999. Mean and turbulent particle velocity in the fully developed region of a three-phase fluidized bed. Chemical Engineering & Technology 22, 683–689] for solid phase hydrodynamics in terms of mean and turbulent velocities and with the results of Yu and Kim [1988. Bubble characteristics in the racial direction of three-phase fludised beds. A.I.Ch.E. Journal 34, 2069–2072; 2001. Bubble-wake model for radial velocity profiles of liquid and solid phases in three-phase fluidised beds. Industrial and Engineering Chemistry Research 40, 4463–4469] for the gas and liquid phase hydrodynamics in terms of phase velocities and holdup. The flow field predicted by CFD simulation shows a good agreement with the experimental data. From the validated CFD model, the computation of the solid mass balance and various energy flows in fluidised bed reactors are carried out. The influence of different interphase drag models for gas–liquid interaction on gas holdup are studied in this work.
International Journal of Thermal Sciences, 2018
In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flow inside the fluidized bed reactor are investigated systematically. To this end, 3D simulations have been carried out by incorporating the appropriate model parameters and using the well-known Euler-Euler two fluid methodology in ANSYS FLUENT. The methodology is validated against the experimental results available in the literature. Temperature, air velocity and particle sizes are varied systematically. Results show that the variation of minimum fluidization velocity with temperature depends upon the particle size. For small particles (Geldart's group B), the minimum fluidization velocity decreases with an increase in temperature and the trend is reversed when large particles (Geldart's group D) are fluidized. This behavior is explained by analyzing the modification in inter-phase momentum exchange coefficient due to temperature variation. Voidage profiles, particle velocity and bubble characteristics are also seen to be influenced by the thermal conditions of the fluidized bed reactor. Particle axial velocity tends to decrease with an increase in the operating temperature. For group B particles, temperature has negligible effect on bubble size and expanded bed height. On the other hand, bubble size and expanded bed height decrease with an increase in temperature for group D particles.
CFD Modeling of Bubble Rise Velocity in Fluidized Bed Reactor
In this research the ability of computational fluid dynamics to predict bubble diameter and bubble rise velocity with different particle diameters in a gas-solid fluidized bed reactor has been investigated. A multi fluid Eulerian model incorporating with the KTGF has been applied to simulate the unsteady-state behavior of this reactor. Results have been compared with some correlations. Pressure drop distributions predicated by CFD at different particle diameter were in a good agreement with correlations. The CFD results reveal that smaller bubbles are produced at the bottom of the bed. It has been found that the bubble diameter and bubble rise velocity are sensitive to particle diameter.
This paper presents hydrodynamic simulation and experimental studies of gas-solid flow in an internally circulating bubbling fluidized bed (ICBFB) using concentric cylinders. An Eulerian-Eulerian Model (EEM) incorporating the kinetic theory of granular flow (KTGF) was applied in order to simulate the gas-solid flow behavior. The gas and solid dynamic was simulated using a computational fluid dynamics (CFD) software package, Fluent 6.3. Three-dimensional (3D) geometry was used to represent the key parts of a pilot scale of this reactor. Simulations were conducted to investigate the effect of the changes of six operating parameters: the fluidization flow rate of the draft tube (Q dt), the aeration flow rate of the annulus (Q an), the initial bed static height (H bs), the draft tube height (H dt), the draft tube diameter (D dt), and the orifice diameter (D or) on the solid flow characteristics in terms of the solid circulation rate (G s). The results were then validated with the experimental results. All the investigated operating parameters have strong effect on G s .
CFD simulation of gas-solid bubbling fluidized bed: A new method for adjusting drag law
The Canadian Journal of Chemical Engineering, 2009
This research includes both experimental and CFD investigation in the hydrodynamic behavior of a new type of column tray introduced as "Concap" tray. The proposed column tray is used in contactor columns especially in stripping columns. The hydrodynamics of this Concap tray is investigated in a 1.2m in diameter column. Using air-water system, the experiments were performed for different liquid and vapor loads at constant ambient temperature and pressure. The clear liquid height and total pressure drop were measured. The results were compared with the experimental data of a valve tray in the same column simulator rig. The total pressure drop and clear liquid height of Concap tray were similar to a valve tray, having equal 14% open holes area. Liquid velocity distribution on the tray has been predicted by CFD and is reported.
Powder Technology, 2015
Computational fluid dynamics (CFD) Eulerian two-fluid model Solid stack volume (SSV) Hydrodynamic characteristics in a wind-box and bubbling fluidized-bed (BFB) gasifier zone of a dual fluidizedbed (DFB) were investigated by a two-phase three-dimensional computational fluid dynamics (CFD) model. The gas and solid phases were treated by an Eulerian-Eulerian two-fluid model, coupled with the realizable kepsilon turbulence model and the kinetic theory of granular flow (KTGF) describing the random motion of solid particles. Pressure drops obtained from the cold-rig CFD simulation were validated with experimental data which were measured in a pilot-scale BFB using air as a fluidization agent and sand as heat carrier particles at an operating temperature of 800°C. Hydrodynamics of the fluidized-bed with a uniform gas distributor (Ideal case) and a nozzle-type gas distributor (Real case) were evaluated in terms of the pressure drop, solid volume fraction (SVF), uniformity index (UI), and solid stack volume (SSV) for three inlet air flow rates (low, medium and high). Similar behaviors were shown for both the two cases in pressure drop along the gasifier height. However, significant differences were observed in SVF, UI, and SSV. A threshold changing the slope of SSV to the air flow rate was found at a fluidization index (u/u mf) of 2.9.
Processes
Even bubble column reactors (BCR) and airlift reactors (ALR) have been developed in terms of various related aspects towards mass transfer enhancement, the effective analysis of gas diffuser types on mass transfer and gas–liquid hydrodynamic characteristics is still limited. Therefore, the present study aims to analyze the relative effect of different types of air diffusers on bubble hydrodynamics and mass transfer performance to understand their behaviors and define the best type. The experiments were conducted by varying different diffuser types, reactor types (BCR and ALR), and superficial gas velocity (Vg) (0.12 to 1.00 cm/s). Five air diffusers including commercial fine sand (F-sand) and coarse sand (C-sand) diffusers, and acrylic perforated diffusers with orifice sizes of 0.3 mm (H-0.3), 0.6 mm (H-0.6), and 1.2 mm (H-1.2), were used in this study. For every condition, it was analyzed in terms of bubble hydrodynamics and oxygen mass transfer coefficient (KLa). Lastly, the selec...