DELINCUENCIA JUVENIL: ¿UNA OPCION DE TRABAJO?

The main objective of the present research is to numerically and experimentally study particle attrition using supersonic nozzles in high temperature fluidized beds. The specific objective is to improve particle grinding efficiency, and finally reduce the attrition gas consumption in the fluid coking process.

1.6.1

Study of solids entrainment into attrition jets in fluidized beds

Supersonic nozzles are applied to various fluidized bed processes, such as the production of pharmaceutical powders, fluid catalytic cracking, and fluid coking. In applications such as jet milling, it is essential to entrain a maximum flow-rate of solids from the fluidized bed into the jet cavity. Studies of solid entrainment rate into gas jets have been mostly conducted with subsonic jets. The purpose of this research is to study solids entrainment into jets issuing from supersonic convergent-divergent nozzles, and particularly the influences of nozzle size, nozzle mass flow-rate, the properties of injection gas, and the bed properties. A novel accurate technique is developed to measure solids entrainment into jets. This investigation can be found in Chapter 2.

1.6.2

Penetration of high velocity horizontal gas jets into a fluidized

bed at high temperature

High velocity horizontal gas jets are applied to various industrial processes. In this work, an optimized thermal technique has been developed to measure the penetration length of horizontal gas jets. Experiments were carried out in a fluidized bed with a height of 1.23m and a rectangular cross section of 0.10m × 0.50m. The fluidized bed particles, which were either petroleum coke or sand, were heated by an in-bed electrical heater to temperatures between 300°C and 500°C. Cold gases, such as helium, nitrogen, carbon dioxide, were injected into the hot fluidized bed via a horizontal nozzle operating over a range of velocities. Based on the experimental results, a new empirical correlation was developed to predict the penetration length of jets issuing from the horizontal sonic nozzle at high temperature. The details of this study are discussed in Chapter 3.

1.6.3

Particle attrition with supersonic nozzles in a fluidized bed at

high temperature

Fluidized beds are used for a variety of processes such as food, pharmaceutical, petrochemical and energy production. The fluid coking process, a typical application of fluidized beds, uses thermal cracking reactions to upgrade heavy oils and bitumen from oil sands. Supersonic nozzles injecting steam are used in the fluid coking process to control coke particle size, which is essential to maintain a well-fluidized bed and a satisfactory reaction rate. Maintaining a high attrition rate with a lower steam flowrate would reduce energy consumption, increase reactor throughput, and reduce sour wastewater production. The objective of the present research is, therefore, to study particle attrition with supersonic convergent-divergent nozzles in a fluidized bed at high temperatures, under conditions such as encountered in the fluid coking process. According to the experimental results, the grinding efficiency is significantly affected by fluidized bed temperature, attrition gas properties, and nozzle size. The experimental data further suggests that particle fragmentation is the dominant attrition process using supersonic nozzles in a hot fluidized bed. The results of this study are presented in Chapter 4.

1.6.4

Modeling of horizontal jet penetration in fluidized beds at high

temperature

Supersonic nozzles as a type of efficient jet have been applied in various jet-induced attrition processes such as jet milling and the fluid coking processes. In jet-induced particle attrition, the penetration length of the supersonic nozzles is a critical property to investigate the attrition mechanism in the fluidized bed. A numerical model was developed to predict the penetration length of the horizontal supersonic nozzle in a high temperature fluidized beds, based on an Eulerian-Eulerian multiphase model and granular kinetic theory. The predicted results of the jet penetration length are in very good agreements with the experimental data and the predictions of Li's correlation. The simulation results have also demonstrated that the fluidization velocity and bed temperature have a little influence on jet penetration length. This research work is detailed in Chapter 5.

1.6.5

Numerical simulation of particle attrition with a convergent

divergent nozzle in fluidized beds at high temperature

The attrition process is a critical step in the fluid coking process to control the particle size distribution. Previous studies have shown that some particle and bed properties affect jet-induced particle attrition in a fluidized bed. Because of the multiphase interactions and the complex flow behavior in the bed and jet system, it is desirable to develop a numerical model for the attrition process that combines theoretical and experimental techniques. Therefore, a model for jet-induced attrition in fluidized beds at high temperature has been proposed and developed. The model is a coupled Eulerian-Eulerian multiphase model coupled with a population balance method. Moreover, particle-particle interactions are described with the kinetic theory of granular flow. The model is solved using the discrete method and the quadrature method of moments. The vital adjustable parameters of the model were determined from experimental data. It is found that the best prediction was obtained using the Ghadiri breakage kernel, the Diemer-Austin generalized daughter size distribution function, and the discrete solution method. The details of this study can be found in Chapter 6.

1.6.6

Experimental and numerical study of twin-jets nozzle in

particle attrition process

The convergent-divergent nozzle is popularly used in a number of industrial applications. For example, in the fluid coking process, a convergent-divergent nozzle is employed to control the particle size distribution in a reacting system. Studies for aerospace applications have found that twin-jet nozzles enhance flow mixing, entrainment, and jet thrust. Therefore, the aim of the present study is to investigate the fluid dynamic mechanism of twin-jet nozzles and their practical application to particle attrition in fluidized beds. The details of this study are described in Chapter 7.