Doctor of Philosophy
Chemical and Biochemical Engineering
de Lasa, Hugo I.
Particle cluster dynamics in downflow reactors are of great importance for the implementation of large scale, environmentally friendly catalytic processes. Studies should address particle cluster velocities, solids holdups, and individual cluster sizes to establish reliable models for the unit scale up.
In this PhD dissertation, the individual characteristics of particle clusters, such as cluster size, velocity, and particle volume fraction, were measured in the feeding, intermediate, and fully developed flow sections of a cold-flow model unit using CREC-GS-Optiprobes. The downer unit employed in this research had a 0.051 m ID and a 2 m high acrylic column. The feeding section included a cyclone and a ring gas injector with eight nozzles angled at 45º. A fluid catalytic cracking (FCC) catalyst with a mean diameter of 84.4 μm and a density of 1722 kg/m3 was used. The operating conditions for the experiments were superficial gas velocities of 1.0-1.6 m/s and solids mass fluxes of 30-50 kg/m2s. The results obtained showed close to normal particle cluster size distributions near the feeding region, and skewed distributions with a higher frequency of short clusters in the fully developed flow section. Additionally, significant changes were noticed when clusters evolved from the feeding section to the fully developed flow section: the average cluster size changed from 7-9 particles to 3-4 particles, and 0.5-0.9 m/s cluster slip velocities in the downer entrance increased to 1.1-1.4 m/s in the stabilized region.
Regarding the obtained findings, it was observed that the cluster slip velocity is a function of the measured axial cluster length. On the basis of the data obtained, it was also established a quasi-spherical shape for the clusters in the entry downer section and a strand shaped cluster for clusters in the stabilized downer region.
Furthermore, by using computational fluid dynamics simulations (Multiphase Particle-in-Cell (MP-PIC) Method) and accounting for the experimentally determined cluster size distribution, a Hybrid Experimental-Numerical Cluster Model was postulated and successfully validated.
Finally, and to establish the relevance of the fluid dynamic model, a fluidized catalytic cracking (FCC) pilot-scale downer unit, was simulated using the developed Hybrid MP-PIC Model and kinetics obtained in a CREC Riser Simulator. Radial and axial temperature distributions show the adequacy of the gas-solid feeder employed. This was the case given the very effective gas-solid mixing leading to quick gas-solid radial thermal stabilization. On this basis, it was proven that flow stabilization can be achieved in a 1-2 m downer unit length, and this for typical FCC operated with 5-7 C/O (catalyst/oil) ratios.
Summary for Lay Audience
Chemical Engineering is the discipline that combines chemistry and physics to study the transformation of raw materials into more useful and valuable products. Within this field, reaction engineering is the branch responsible for the design of the vessel where the chemical reaction occurs.
Catalytic processes are responsible for the production of fuels and chemicals, among many other important products that contribute to the world’s economic progress. Out of these catalytic processes, one of the most important is fluid catalytic cracking (FCC), which is a process that is at the heart of an oil refinery, taking the product of upstream units and producing gasoline and other precursors for the petrochemical industry.
Even though the FCC process has seen many improvements over the last 40 years, the need to meet stricter environmental regulations poses the challenge to develop better technology to perform this chemical process. The downflow fluidized bed reactor has been proposed as an improvement in the FCC process that will allow for the better utilization of the catalytic resources. Understanding the particle clusters behavior in the entrance region of the downflow reactor is crucial for the successful implementation of this unit.
With this objective in mind, the present PhD dissertation studied the gas-solid fluid dynamic characteristics of the downflow reactor, such as particle cluster velocity, size, and concentration, with a special focus on the entrance zone, using the advanced CREC-GS-Optiprobes fiber optic devices. Additionally, a numerical model was developed based on the experimental results and it was employed in the simulation of a pilot-scale fluid catalytic cracking downflow reactor. As a result, this project aims to contribute to the understanding of particle clusters in downflow fluidized bed reactors and their application in fluid catalytic cracking.
Medina Pedraza, Cesar Alejandro, "Cluster Acceleration and Stabilization in Downflow Catalytic Reactors: Experimental and CPFD Simulation Studies" (2021). Electronic Thesis and Dissertation Repository. 7611.