Strömungsmechanische Charakterisierung des Ringwirbelschichtreakktors im Hinblick auf Scale-up (English shop)

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Since fluid–solid systems play a significant role in a large number of processes in the process industry, the search for ever more efficient and cost-effective reactors represents an ongoing scientific and economic challenge. The reactor concept of the annular fluidized bed reactor emerged from the requirement to combine a long solids residence time with good mass and heat transfer.

In order to minimize the risks involved in the design and operation of an industrial annular fluidized bed reactor, both an experimental fluid-dynamic characterization of the annular fluidized bed reactor that is as detailed and systematic as possible and its modelling were indispensable.

Within the framework of a cooperation with Outotec GmbH, methodical investigations were carried out for this purpose on a laboratory-scale and a pilot-scale plant with a cross-section-related scale-up factor of 15 under ambient conditions. After delimiting the stable operating ranges of the plants, the focus of the investigations was on the characterization of the solids entrainment at the outlet of the central nozzle as a function of numerous operational and geometric parameters. The experimental results thus obtained were then used to derive a two-part model for the quantitative and qualitative calculation of the solids entrainment.

For the investigation of the vertical gas–solid flow, both globally and locally measuring techniques were employed in this work. By means of highly time-resolved pressure measurements in the region of the central nozzle, stable operating ranges of the plants could be delimited between the onset of solids weeping through the nozzle and the slugging fluidized bed. Additional axial pressure profiles along the entire plant enabled the characterization of the global development of the flow. Local information on the solids concentration, the axial solids velocity and the cross-sectional loading, in contrast, could be obtained by means of an intrusive capacitive measuring system. In evaluating the data, particular attention was paid to eliminating system- and measurement-related sources of error, which was made possible by implementing numerous selection criteria taking new concepts into account. Among the significant findings are the distinction between strand and overall solids concentration, between number-averaged and time-averaged cross-sectional loading, and the disclosure of the influence of the wall on the measurement signal of the capacitive probes.

The occurrence of solids weeping through the nozzle is of particular importance for the process-appropriate design of the inlet flow in the central nozzle. Stable operating behaviour in the plant is achieved only if the solids clusters thrown in by the bubbles are broken up again by the turbulence of the flow and entrained into the mixing chamber, or if backflow regions within the central nozzle are suppressed. For this purpose, on the one hand, the central nozzle must have an appropriate L/D ratio or be able to generate sufficient turbulence. On the other hand, a gas velocity higher than the single-particle terminal velocity of the “clusters” must be set.

All findings obtained from local and global measurements point to a solids distribution in the plant that is pronounced both radially and axially. While the annular fluidized bed exhibits a solids concentration uniform over the cross-section, similar to that at minimum fluidization of the bubbling fluidized bed, in the first centimetres of the mixing chamber there is a highly concentrated annular region, which is characterized by a significant concentration gradient towards the centre of the plant and which increases with rising solids inventory in the mixing chamber. This highly concentrated solids region surrounds a central jet directly at the outlet of the central nozzle, which is characterized by high solids velocities. In the upper part of the mixing chamber, a small pressure gradient occurs which is characteristic of the external solids recirculation and which depends decisively on the gas velocity in the central nozzle and the height of the mixing chamber. In this region, the fully developed flow profile of a circulating fluidized bed with a “core–annulus flow” is present.

The detailed consideration of the solids entrainment in the jet region above the central nozzle shows the outstanding role of the bubbles in the annulus, which, upon bursting at the edge of the central nozzle, catapult solids into the jet region. By controlling the fluidization in the annulus or its height, the bubble size and bubble velocity can additionally be adjusted and thus the intensity of the solids entrainment into the central jet can be specifically controlled: the annulus serves as a “solids feeder”. The removal of the entrained solids, in contrast, is effected by setting the gas velocity in the central nozzle.

The dominance of both mechanisms on the flow structure of an annular fluidized bed reactor can be exploited in order to predict, with the aid of a simple model, the entrainment in reactors of any size. In this two-stage model, on the one hand the amount of solids that can be thrown in by the convective transport of the bubbles above the central nozzle is estimated, and on the other hand the particle trajectory of the entrained solids is calculated as a function of the gas velocities and geometries set. It can be shown that the larger bubbles present in large plants transport “clusters” rather than single particles into the jet region and thus ensure good entrainment even with a wide central nozzle. As a scale-up quantity, the cluster diameter corresponds to 0.6 % of the bubble diameter. Furthermore, the ratio of the bubble diameter to the diameter of the central nozzle d B / D ZD has emerged as the central quantity for controlling the solids entrainment. The ratio of the annular fluidized bed diameter to the central nozzle diameter D RWS / D ZD, in contrast, only comes into play in combination with the height of the mixing chamber in determining the external solids recirculation.

Within the framework of this thesis, measurement data and a simple model for describing the flow structures in annular fluidized bed reactors have thus been presented. The findings presented here, in conjunction with the scale-up criteria derived, form an essential basis for improving the design and operation of annular fluidized bed reactors.