Hydrodynamique de l'écoulement diphasique solide-gaz, dans un réacteur à lit fluidisé circulant interne

In petroleum industry, as in any other chemical industry, secondary reactions are very expensive. They are the main cause for selectivity decline. The simplest and efficient way to avoid or to minimize their impact is by reducing the contact time. In other words, the solid residence time in the reaction zone should be easy to control.

To meet this objective numerous technologies were derived from the conventional fluidized bed, as spouted beds and circulating fluidized beds CFB (internal and external). Internal circulating fluidized beds (ICFB) for instance, were designed and developed for the short contact time applications (order of magnitude of milliseconds). In ICFB units, the residence time is an important variable, thus to ensure short solid residence time, the use of high gas flow rate and short and small riser diameter is necessary. However, in these conditions the hydrodynamics of the ICFB is far from what is already established and well known in other units like CFBs and spouted beds.

ICFB could be the suitable technology for several applications if the hydrodynamic is well understood. The compact and complex geometry of this kind of unit make the access to the internal zone a difficult task. The position of the riser in the hearth of the ICFB and the small riser diameters used in this kind of units makes impossible the use of the conventional measurement tools (intrusive probes). The reader should keep in mind that the few works done about the hydrodynamic aspects of the ICFB were based either on computational fluid dynamic CFD or on visualization.

In this work certain hydrodynamic aspects were investigated in the ICFB riser, using radioactive particle tracking (RPT) and fiber optic needle probe techniques. The key parameters in the one-dimensional model were calculated in the same conditions as the usual operating conditions of an ICFB riser.

The first aspect studied here concerns the Particle-Wall Friction Factor (PWFF). The variation of PWFF along the riser was investigated in an ICFB riser 1m in length and 0.052 m in diameter.

The second aspect of this study was the drag coefficient. In this work a new correlation for the purpose of evaluating the effective drag coefficient of an ascending gas-solid flow was developed.

The third aspect presented in the following paragraph is about the effects of riser exit geometry on pressure drop and solid behaviour inside the ICFB riser. Different riser exit geometries have been investigated in an ICFB at several operating conditions. (Abstract shortened by UMI.)