Fluidization of Particles and Agglomerates in Wet Gas-Fluidized Beds

Gas-fluidized beds of dry particles exhibit complex behavior with instabilities giving rise to inhomogeneities that span a wide range of time and length scales. Adding liquid to the system such that particles support a liquid film coating introduces further complexity, allowing for the fluidization of both particles and agglomerates.  Constitutive models for the effective fluid-particle drag force in such systems must account for inhomogeneous structures¹, such as agglomerates.  Thus, our primary goal is to develop an effective drag force model for use in large-scale simulations of wet gas-fluidized beds.   We perform Euler-Lagrange simulations of a fluidized bed using a 3D periodic domain 2 cm³ in size with as many as 250,000 particles.  The domain-average slip velocity serves as a quantitative measure of the inhomogeneous structure within the system.  We find that the system behavior may be characterized by a modified Bond number: Bo Φ^α , where α is a constant and Φ is the liquid loading level.  Bo = 3γ/(2r²ρg), where γ is the surface tension, ρ is the liquid density, and r is the particle radius.  As shown in Figure 1, the collapse of the domain-average slip velocity with respect to modified Bond number is achieved regardless of domain size or whether liquid spreading is permitted.  This observation suggests that the approximation of constant liquid bridges is valid^2,3; moreover, the robust collapse suggests that the drag in a wide variety of systems may be predicted with a simple model.  

[1] Igci, Y., Andrews, A. T., Sundaresan, S., & O'Brien, T. (2008). Filtered Two-Fluid Models for Fluidized Gas-Particle Suspensions. AIChE Journal, 54(6), 1431–1448.

[2] Mikami, T., Kamiya, H., & Horio, M. (1998). Numerical simulation of cohesive powder behavior in a fluidized bed. Chemical Engineering Science, 53(10), 1927–1940.

[3] Shi, D., & McCarthy, J. J. (2008). Numerical simulation of liquid transfer between particles. Powder Technology, 184(1), 64–75.

 

                  Figure 1: Domain-average slip velocity vs. modified Bond number.  Domain-average slip velocity is scaled against that of a comparable dry system.  Bo is Bond number, φ is solid volume fraction, and Φ is liquid loading level.