Positron emission particle tracking in fluidized beds with secondary gas injection
Graphical abstract
Introduction
Fluidized beds with secondary gas injection enjoy great popularity in miscellaneous fields of process engineering. Their characteristic properties, such as intense mixing of solids, excellent heat and mass transfer conditions as well as easy solids handling, make them attractive for application in mixing and drying processes. Moreover they are implemented in power plant technology, but also as chemical reactors for heterogeneous catalysis or chemical vapor deposition [1], [2].
The fundamental setup of a fluidized bed with secondary gas injection is composed of two subsystems. The first one consists of the actual fluidized bed, in which particles are fluidized at moderate superficial fluid velocities by feeding the primary process fluid through a distributor plate at the bottom of the plant. Apart from bubble formation, which is commonly observed in gas–solid fluidized beds, solids are distributed homogeneously in the first subsystem and particle motion therein is based on random. Hence the first zone is in many cases considered an ideally mixed system, which provides gradient-free conditions with respect to heat and material distribution. The second subsystem is induced by the injection of a secondary fluid, which is fed through the orifice of a nozzle. Depending on the velocity of the injected secondary fluid, a region with reduced solids concentration is formed at the top of the nozzle orifice. According to Merry [3] this region is termed the jet region. In contrast to the statistical particle motion observed in the suspension phase of the fluidized bed, the jet region is characterized by an oriented flow field.
Several studies have been undertaken in the past to describe the interaction of the jet region with particles of the suspended phase and to derive correlations and design criteria for the description of fluidized beds with secondary fluid injection [3], [4], [5], [6]. A large number of the conducted studies aimed at the investigation of the physical dimensions of the jet region, namely its penetration depth into the fluidized bed and the jet opening angle, in dependence of the prevailing operating conditions [7], [8], [9]. For that purpose invasive measurement techniques were often applied. As alternative, two-dimensional replicas of the systems were constructed, which facilitate visual observation of the bed and the jet region. However, a drawback of previously applied procedures consists in their invasive nature. By changing the geometry of the bed or penetrating the bed with probes, the flow field is influenced, which has a detrimental effect on the significance of the obtained measurement data.
More recent studies focused on the effect of secondary gas injection on the bubble size and behavior in gas–solid fluidized beds. Using a fractal injector system, Kleijn van Willingen et al. found that the injection of secondary gas leads to a reduction of the average bubble size within the suspension phase of the bed, which leads to an improved gas–solid contact [10]. With regard to the implementation as a chemical reactor system the residence time behavior of the injected fluid was investigated and allowed to draw conclusions on the dispersion of the fluid within the bed [11], [12].
The objective of the present article consists in the non-invasive investigation of a three-dimensional gas–solid fluidized bed with secondary gas injection through a single, centrally arranged nozzle. Apart from the determination of the jet characteristics an investigation of the behavior of single particles is aimed at for the first time. For that purpose a particle is randomly selected from the fluidized bulk material and labeled radioactively. By means of positron emission particle tracking (PEPT) the motion of the particle is analyzed with high temporal and spatial resolution. A method for PEPT-data evaluation is presented, by means of which the local solids hold-up in the bed can be inferred. The derived solids concentrations are juxtaposed to those measured invasively, using capacitance probes. In addition to that, the residence time behavior of the labeled particle in the jet region and in the suspended phase is analyzed. With regard to the flow characteristics of single particles in jetted fluidized beds the flux of particles across the boundaries of the jet region is determined and dispersion coefficients are derived.
Section snippets
Experimental setup
In the following sections the applied materials are introduced and the experimental setup of the fluidized bed with secondary gas injection is presented. Besides that, information on the operating conditions adjusted in the course of the measurements is given. Subsequently the applied measurement techniques are introduced and, in this regard, the measuring procedure is explained.
PEPT-data analysis
PEPT-measurements yield a data set containing the spatial coordinates of the tracer particle position as a function of time [17]. In the following sections the procedures are explained, according to which several process-relevant properties of the investigated system can be derived from the PEPT-raw data set.
Results and discussion
The data obtained from particle tracking and evaluated according to the methods described in the previous sections are presented in the following. In order to validate the significance of the PEPT-measurement and the subsequent evaluation of raw data, PEPT-derived solids concentrations are confronted with those obtained by invasive measurements using capacitance probes.
For better comparability of the presented data set to measurements in similar systems with different scales, all length-related
Conclusions
The focal objective of the presented work consisted in the non-invasive investigation of the motion of single particles in a fluidized bed with secondary gas injection by means of positron emission particle tracking (PEPT). For this purpose a particle was selected randomly from the bulk material and labeled radioactively for use in a gas–solid fluidized bed with secondary gas injection through a vertically arranged nozzle. Subsequent tracking of the particle motion resulted in a data set
Acknowledgments
The authors would like to thank the collaborators in the team of Prof. David Parker at the School of Physics and Astronomy at the University of Birmingham for their great support in conducting the particle tracking experiments. Moreover, the authors would like to express their gratitude to Prof. Jonathan Seville for providing valuable incentives and his readiness for fruitful discussions. The financial support by the German Research Foundation DFG (EXC315/2) in the framework of the Cluster of
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T. Hensler and M. Tupy contributed equally.