Distributed secondary gas injection via a fractal injector: A nature-inspired approach to improving conversion in fluidized bed reactors

Distributed secondary gas injection via a fractal injector: A nature-inspired approach to improving conversion in fluidized bed reactors

Christensen, D.O.

Coppens, M.O. (promotor)

Applied Sciences

2008-01-30

The conversion in bubbling fluidized bed reactors is suppressed because the interphase mass transfer and gas-solid contact in bubbling fluidized bed reactors are often poor. Most of the gas is present in the form of bubbles, which have low surface-to-volume ratios and are nearly devoid of catalyst particles. The chaotic behaviour of the bubbles is difficult to predict and can change with reactor size, making scale-up very difficult. The work in this thesis presents a novel approach to overcoming these difficulties in bubbling fluidized beds. Nature uses branching, fractal structures, which greatly facilitate mass transfer in natural systems, such as trees and lungs. These structures scale easily, which is a very important feature as the organism grows. This approach can also be applied to fluidized beds. A fractal injector was developed for both quasi 2-D and 3-D beds to distribute a portion of the total gas flow throughout the fluidized bed. To determine the effect of this distributed secondary gas injection on the properties of a gas-solid fluidized bed, the study is split into four topics: the effect on the hydrodynamics of the fluidized bed, the mechanisms leading to the observed changes in the hydrodynamics, the residence time and macroscopic mixing of the gas, and the influence on the performance of the reactor. The results indicate that secondary gas injection via a fractal injector effectively reduces the bubble diameter by up to 30% (~70% reduction in the volume) and increases the gas-solid contact. It is shown that these effects lead to a higher conversion and selectivity in a bubbling fluidized bed reactor. Mechanisms for these effects are proposed.

fluidized bed
bubble size
fractal injector
secondary injection
residence time distribution
collapse test
ozone decomposition
conversion

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978-90-9022769-6

Institutional Repository

doctoral thesis

(c) 2008 D.O. Christensen