Competing flow and collision effects in a monodispersed liquid–solid fluidized bed at a moderate Archimedes number

Abstract

We study the effects of fluid–particle and particle–particle interactions in a three-dimensional monodispersed reactor with unstable fluidization. Simulations were conducted using the immersed boundary method for particle Reynolds numbers of 20–70 with an Archimedes number of 23 600. Two different flow regimes were identified as a function of the particle Reynolds number. For low particle Reynolds numbers ( $20 < Re_p < 40$ ), the porosity is relatively low and the particle dynamics are dominated by interparticle collisions that produce anisotropic particle velocity fluctuations. The relative importance of hydrodynamic effects increases with increasing particle Reynolds number, leading to a minimized anisotropy in the particle velocity fluctuations at an intermediate particle Reynolds number. For high particle Reynolds numbers ( $Re_p > 40$ ), the particle dynamics are dominated by hydrodynamic effects, leading to decreasing and more anisotropic particle velocity fluctuations. A sharp increase in the anisotropy occurs when the particle Reynolds number increases from 40 to 50, corresponding to a transition from a regime in which collision and hydrodynamic effects are equally important (regime 1) to a hydrodynamic-dominated regime (regime 2). The results imply an optimum particle Reynolds number of roughly 40 for the investigated Archimedes number of 23 600 at which mixing in the reactor is expected to peak, which is consistent with reactor studies showing peak performance at a similar particle Reynolds number and with a similar Archimedes number. Results also show that maximum effective collisions are attained at intermediate particle Reynolds number. Future work is required to relate optimum particle Reynolds number to Archimedes number.

CFD-accelerated bioreactor optimization: reducing the hydrodynamic parameter space

“…53 Moreover, due to non-uniform particle sizes, the fluidized bed in the P-MBR forms segregated layers of particles with larger particles (2–4 mm) located at the bottom of the bed. Yao et al 32 found that the maximum collision frequency is attained at intermediate porosity for 2 mm particles. Low porosity is characterized by more frequent weak collisions while high porosity is dominated by flow rather than collisions.…”

“…The hydrodynamics of fluidized-bed reactors have been investigated experimentally 28,29 and with simulations, 30–34 but membrane bioreactor studies of microbial activity have largely focused on experimental testing. 35,36 These studies do not track particle dynamics at high volume fraction (low porosity), but instead focus on macroscopic behavior such as fluidization stability and expansion.…”

“…2). Recently, Yao et al 32 investigated particle dynamics by varying upflow velocity in simulations of a monodispersed/single-size fluidized bed with particle properties similar to those of pilot-scale and lab-scale reactors. 21 Within FBRs, porosity controls both the horizontal mixing and collisions between particles.…”

“…By comparing high- and low-strength FBRs, Shin et al 44 successfully modeled low-strength FBRs by assuming small b det . By examining particle dynamics in a fluidized bed, Yao et al 32 discovered that collisions dominate over hydrodynamic effects at low porosity. At intermediate porosity, both collisions and hydrodynamic effects are important while hydrodynamic effects dominate at higher porosity.…”

“…49 The effect of collisions alone can be accurately described by the collision pressure which is known to have a maximum and zeros for both single-particle ( ε ≈ 1) and close-packed reactors ( ε ≈ 0.4). 43,50 After close examination of particle dynamics in fluidized bed simulations, Yao et al 32 suggested that mass transfer is most likely maximized within the intermediate porosity regime at which point collisions and hydrodynamic factors are equally important, leading to optimal biological performance. Although not yet experimentally validated, pilot- and lab-scale reactors operated at this intermediate porosity (bed expansion of 40% to 60%) have achieved optimal treatment performance.…”