Herbert Weinstein

Flow characterization in high-velocity fluidized beds using pressure fluctuations

Abstract Fast fluidization flow characteristics were studied using instantaneous pressure signals in a fast fluidization unit with 0.152 m i.d. and 8.4 m height using a Zeolite FCC catalyst. The data confirmed that a fast fluidized bed can usually be divided into two main regions, a dense region at the bottom and a dilute region at the top. Within both regions the mean void fractions are essentially constant along the height. The void fraction in the dense region appears to be independent of the solid circulation rate and varies only slightly with the superficial gas velocity. Upward moving instabilities were detected within the dense region of the bed with a fairly constant wave speed over a wide range of flow parameters. These have similarity to phenomena in slugging fluidized beds. The results suggest that the solids in the dense region of a high-velocity fluidized bed are present in three different forms—relatively stagnant in a dense annulus region, very dilute in a core region, and in the form of waves rolling upward between the core and dense annulus.

An experimental comparison of gas backmixing in fluidized beds across the regime spectrum

Abstract An investigation of gas backmixing in fluidized beds spanning the entire range from the bubbling regime to the dilute transport regime was conducted using continuous injection of a helium tracer. The focus was to compare backmixing levels in the different regimes. While time averaging at a point averages over the phase heterogeneities in the bubbling and slugging regimes, it does not average these out in the high-velocity regimes of turbulent and fast fluidization where the heterogeneities are distributed in space rather than time. It is also shown that, in the bubbling and slugging regimes, the concentration of backmixed tracer is almost independent of the gas velocity while, in the turbulent and fast regimes, the concentration is strongly dependent on the gas velocity.

DESIGN PARAMETERS DETERMINING SOLID HOLD-UP IN FAST FLUIDIZED BED SYSTEM

ABSTRACT The solid holdup in a fast fluidized bed has already been shown to be a function of system solid inventory as well as gas velocity and rate. In this paper experimental results are reported which show that the solid hold-up is also strongly influenced in a complex fashion by system design. Entrance and exit configurations as well as system air supply characteristics all interact in determining the solid hold-up.

Measurement of normal stress and hindrance factor in a collapsing fluidized bed

Abstract An experimental apparatus and approach were developed to measure the instantaneous changes in pressure gradient and solid volume fraction with the time along a collapsing fluidized bed. Measurements were made for a single fluid cracking catalyst. The pressure transducer and X-ray data were in excellent agreement. These data along with a straightforward one-dimensional description of both gas and solid phases were used to obtain the solid stress modulus and the drag coefficient for the catalyst powder in the solid fraction range between minimum fluidization and loose packing.

Numerical analysis of confined turbulent flow

Abstract This work outlines a second order accurate, coupled, conservative new numerical scheme for solving a two dimensional incompressible turbulent flow filed. Mean vorticity, ω, and mean stream function, ψ, are used as the mean flow dependent variables. The turbulent kinetic energy k and the turbulent energy decay rate, ϵ, are used to define the turbulent state. In the present computational scheme two systems of equations and variables are considered: the mean flow system, ψ-ω, and the turbulent state system, k-ϵ . Every system is solved implicity in a coupled double loop manner, and all the flow equations are solved iteratively in the global sense. Since the turbulence boundary conditions have a non-regular variation near a solid wall, they are coupled to the equations implicitly in both systems. In this way the numerical instabilities due to the irregular form of the equations near the solid walls are suppressed. The rate of convergence of the new numerical scheme of the coupled systems ψ-ω and k-ϵ is twice that realized when solving these equations separately. The necessary conditions for convergence of the numerical equations are investigated as well as the rate of convergence features. The detailed stability conditions are derived. As an example, the axisymmetric mixing of two confined jets with an internal heat source is considered with this numerical scheme.

A model for the collapse of a fluidized bed

A one-dimensional hydrodynamic model for the collapse of a fluidized bed following the instantaneous shut-off of fluidizing air was developed in terms of the sedimentation of a dense system of monodisperse particles. This model is based on the continuity and momentum equations written for the fluid and solid phases. The solution of the problem was formulated in terms of two distinct regions separated by a discontinuity. All hydrodynamic properties in these regions together with the speed of propagation of the discontinuity were calculated analytically. The predicted pressure distribution compares favorably with experimental data from the literature.