(29b) Gas / Solids Mixing IN Rotating Fluidized Beds IN A Static Fluidization Chamber

The gas and solids mixing patterns in a rotating fluidized bed in a static geometry are experimentally investigated. To quantify the solids mixing, a colored particles step response technique and mixtures of different types of particles are used. The gas phase mixing pattern is visualized by means of smoke. The influence of the fluidization gas flow rate and the solids loading is studied.

1. INTRODUCTION

The concept of a rotating fluidized bed in a static fluidization chamber with a static or rotating chimney was recently presented and experimentally proven [de Broqueville, 2004; de Broqueville and De Wilde, 2007; De Wilde and de Broqueville, 2007, 2008, 2008b]. The particle bed rotational motion is introduced by injecting the fluidization gas tangentially in the fluidization chamber, via multiple slots in the fluidization chamber outer wall. The rotating chimney can be used to reduce the rate of solids losses via the chimney at a given solids loading, but its direct impact, that is, on the particle bed rotational speed, is limited to the vicinity of the chimney.

Fig. 1: Schematic representation of a rotating fluidized bed in a static fluidization chamber with a static or rotating chimney. Application of smoke visualization technique.

Whereas previous work mainly focused on proof of concept aspects [De Wilde and de Broqueville, 2007, 2008b] and stability considerations [De Wilde and de Broqueville, 2008, 2008b], the present work addresses the gas and solids mixing patterns and how these are affected by the fluidization gas flow rate and the solids loading in the fluidization chamber. An improved understanding and the quantification of the gas and solids mixing patterns must facilitate the development of simple engineering models for reactor design and scale-up.

2. PARTICLE BED MIXING AND SEGREGATION BEHAVIOR

Details on the geometry of the fluidization chamber and chimney can be found in De Wilde and de Broqueville [2007, 2008, 2008b]. The fluidization chamber is 24 cm in diameter and 11.5 cm long and contains 24, 2.3 mm wide gas inlet slots. The chimney is 16 cm in diameter and consists of 32 blades. The chimney can be rotated fast around its axis of symmetry by means of a motor. Chimney rotational speeds between 0 and 2800 rpm (rotations per minute) can be set. The fluidization gas flow rate is controlled with a mass flow controller and can be varied between 650 and 900 Nm³/h. The solids are fed from a container using a screw feed controller and a rotating sealing valve.

First, the particle bed mixing was studied experimentally using colored particles of the same type and applying a step response technique. The latter is schematically represented in Fig. 2. The rotating fluidized bed is operated in a continuous solids feeding mode with the solids outlet closed. This implies that the rate of solids losses via the chimney equals the solids feeding rate. Initially, white particles are fed to the fluidization chamber and the flow pattern is fully developed. At time t₀, the solids feeding is switched to colored particles. The particles are fed via a tube through one of the fluidization chamber end plates. Radially, the solids feeding tube is positioned in the middle of the particle bed. Hence, both longitudinal and radial mixing and segregation can be studied. For comparison reasons, the response to the step change in the colored particle concentration (from 0 to 100 wt%) is measured, both inside the particle bed and in the chimney outlet. To quantify the behavior in the particle bed, a fast digital camera is used which faces the fluidization chamber plexi-glass end plate opposite the solids inlet. The concentration of colored particles in the chimney outlet is measured by sampling (Fig. 3).

Next, the particle bed mixing and segregation behavior with particles of different density and / or size was investigated.

Fig. 2: Colored particles step response technique.

                                                       (a)                                                                     (b)

Fig. 3: Response of the concentration of colored, but otherwise identical particles to a step change in the solids inlet colored particle concentration at time t₀ = 0 s from 0 to 100 wt%. Particles: 2-5 mm polymer pellets. (a) In the chimney outlet for fluidization gas flow rates G of 650 and 800 Nm³/h; (b) At the end plate of the fluidization chamber opposite the solids inlet for a fluidization gas flow rate G of 800 Nm³/h. Theoretical ideal CSTR behavior of the particle bed shown in (a) for comparison.

3. FLUIDIZATION GAS MIXING BEHAVIOR

To visualize the gas phase flow path and mixing behavior in the rotating fluidized bed, smoke has been used as a tracer (Fig. 1). The smoke is injected through a section of one of the gas inlet slots of the reactor, which was compartimented from the gas inlet slots (Fig. 1) and the gas distribution chamber and which was positioned close to one of the end plates of the fluidization chamber to ensure visibility. The smoke injection compartment surface area has been chosen such that (i) with the smoke flow rate generated, the smoke is sufficiently concentrated to be visible; (ii) smoke injection velocities comparable with the gas injection velocities in the gas inlet slots could be obtained. To avoid gas bypassing smoke rich zones and a maldistribution of the gas resulting in poor smoke injection and an incorrect visualization of the gas flow pattern, the smoke requires a separate flow rate control system, supplied by a separate compressor or gas bottles. Equal smoke and main gas injection velocities have to be ensured. The smoke flow path is captured by means of different rapid cameras (Fig. 4). Because smoke detection within the particle bed is not possible at this stage, observations focused on the particle bed freeboard where the smoke leaves the particle bed.

A comparison with Computational Fluid Dynamics (CFD) simulations is made for an improved understanding of the gas and solids flow patterns.

Fig. 4: Gas flow path and mixing visualization by means of smoke. (a) Before smoke injection; (b) During smoke injection; (c) Flow paths of gases injected via successive gas inlets.

REFERENCES

de Broqueville, Axel : Catalytic polymerization process in a vertical rotating fluidized bed :  Belgian Patent 2004/0186, Internat. Classif. : B01J C08F B01F; publication number : 1015976A3.

de Broqueville, A., De Wilde, J., Central rotating chimney apparatus for obtaining a rotating fluidized bed in a cylindrical fixed or rotating fluidization chamber and processes using this apparatus, April 23, 2007: PCT/EP2007/053941 (F-995-WO), European and United States of America patent application.

De Wilde, J., de Broqueville, A., Rotating Fluidized Beds in a Static Geometry: Experimental Proof of Concept, AIChE J., 53(4), pp. 793-810, 2007.

De Wilde, J., de Broqueville, A., Experimental Investigation of a Rotating Fluidized Bed in a Static Geometry, Powder Technol., 183:(3), pp. 426-435, 2008.

De Wilde, J., de Broqueville, A., Experimental Study of Fluidization of 1G-Geldart D-type Particles in a Rotating Fluidized Bed with a Rotating Chimney, AIChE J., 54:(8), pp. 2029-2044, 2008b.