Path and wake of cylinders falling in a liquid at rest or in a bubble swarm towards the hydrodynamical modeling of ebullated bed reactors

• Main Author: Toupoint, Clément
• Other Authors: Institut National Polytechnique de Toulouse - INPT (FRANCE)
• Format: Others
• Language: en
• Published: 2018
• Subjects: Dynamique des Fluides; Lits bouillonnants; Ecoulements multiphasiques; Lâchers tourbillonaires; Sillages
• Online Access: http://oatao.univ-toulouse.fr/24120/1/Clement_Toupoint.pdf

The origin of this PhD thesis lies in the study of Ebullated Bed Reactors (EBRs). These chemical reactors are very active research topics in chemical processes, notably thanks to their usage in heavy oil processing. Many complex phenomena take place within EBRs, and make their design and optimization difficult. In fluid mechanics, a lot of physical mechanisms present in EBRs are active fields of study (three-phase flow, fluid-body interaction...). Hence, in the present work, a study of the mechanisms participating in the hydrodynamics of an EBR with cylindrical catalysts is performed. In a first part, the impact of the catalyst anisotropy on its fall is investigated. In order to gain insight on the effect of the body anisotropy on its fall dynamics, we investigate experimentally the free fall of a solid cylinder in a fluid at rest. The sensitivity to two dimensionless parameters, the Archimedes number (Ar) and the aspect ratio of the cylinder (L/d) is examined. Experiments are conducted with two orthogonal cameras, and advanced image processing techniques are developed in order to measure the position and orientation of the cylinder in 3D. Within the range of parameters studied (200 < Ar < 1100, 2 < L/d < 20), the cylinders adopt different types of falling motion. Two main types of paths are observed, the first one is a rectilinear fall of the cylinder that keeps its axis horizontal, and the second one is a fluttering oscillatory motion. Other more complex types of motion are observed and discussed. The fluttering motion of the cylinder is analyzed in details. On top of the study of the body motion, the cylinder wake is also visualized and characterized. A large number of particles are present at the same time inside an EBRs (about 40% of the mass). Interactions between multiple objects have a strong impact on the motion of each individual particle, but are very complex. In a first approximation, we take into account the presence of numerous particles by introducing a confined medium. We study experimentally the fall of a single cylinder in a confined vertical thin-gap cell, where the cylinders are free to move in only two directions. The cylinder elongation ratio (3<L/d<40) and density ratio ( c / f = 1.16, 2.70, 4.50) are the two parameters of interest. The Archimedes number of the cylinder lies within the same range as in the unconfined medium, and the two main modes of motion of the cylinder are a rectilinear motion, and a fluttering one. However, for the same parameters (Ar,L/d), the motion of the cylinder in the confined cell is strongly different in form to that in the unconfined medium. We also studied the interaction between a freely falling cylinder and a rising swarm of bubbles. This investigation was performed experimentally, in the confined cell used in the second part. Cylinders of various density ratio ( c / f = 1.16, 2.70, 4.50) and elongation ratio (3<L/d<20) are released in a bubble swarm of gas volume fraction between 2% and 5%. The cylinder motion is greatly modified by the bubble swarm. Several mechanisms of interaction between the cylinder and the bubbles are identified (direct contact, interactions with fluid perturbations...), and their effect is characterized. We perform a statistical analysis of the cylinder motion in the swarm, and compare it to results in the confined fluid at rest. The cylinder density ratio and elongation ratio both play an important role in its motion in the bubble swarm. Conditional statistics allow us to further investigate the effect of the contact between the cylinder and a bubble, and of the cylinder orientation in the swarm. Finally, the dispersion of the cylinder motion in the swarm is characterized. A major effect of the bubble swarm is to increase, through bubble-cylinder contacts, the probability of the cylinder to be in nearly vertical orientations. This drastically changes the kinematics of the cylinder as compared to its motion in the fluid at rest