Tracer particles, concentration

The bed was first operated at the preselected conditions at a steady state then about 455 kg of the coarse crushed-acrylic particles, similar to that used as the bed material but of sizes larger than 6-mesh, were injected into the bed as fast as possible to serve as the tracer particles. Solids samples were then continuously collected from five different sampling locations at 30-second intervals for the first 18 minutes and at 60-second intervals thereafter. The samples were then sieved and analyzed for coarse tracer particle concentration. Typical tracer particle concentration profiles vs. time at each sampling location are presented in Figs. 38-42 for set point 3. 

Typically it took about 160 to 200 seconds to inject a pulse of about 455 kg coarse tracer particles into the bed pneumatically from the coaxial solid feed tube. It can be clearly seen from Figs. 38 to 42 that the tracer particle concentration increases from essentially zero to a final equilibrium value, depending on the location of the sampling port. The steady state was usually reached within about 5 minutes. There is considerable scatter in the data in some cases. This is to be expected because the tracer concentration to be detected is small, on the order of 4%, and absolute uniformity of mixing inside a heterogeneous fluidized bed is difficult to obtain. 

If the light-scattering objects originally present in the airflow are unsuitable for LDA measurements due to insufficient concentration or incorrect estimated flow-tracking capability, the air must be seeded with oil smoke, tobacco smoke, or titanium dioxide tracer particles or droplets. A simple smoke candle is generally suitable for seeding, even if the enclosure is large and the air path is not closed as in several cases of industrial ventilation. 

Solids mixing was also studied by Carlos(42) in the same apparatus, starting with a bed composed of transparent particles and a layer of tracer particles at the base of the bed. The concentration of particles in a control zone was then determined at various intervals of time... 

The observations described above indicate that, with good control, the concentration of the tracer particles in the out stream of the screw feeder can be determined to be a known function of time, and, furthermore, it is feasible to use such a known function as the input signal to the impinging stream equipment to be tested. In this way the experimental procedure can be greatly simplified. Of course, this scheme calls for corresponding mathematical relationship(s) for data interpretation. 

In order to protect the flow stability from turbulence caused by the input signal, the properties of the tracer used should be as close as possible to those of the process particles. In the investigation carried out by the author of this book the process particles are yellow millets while purplish-red rape seeds are used as the tracer, the properties of which are very similar those of the millets. The properties of the process and the tracer particles are listed in Table 3.1. The concentration of the tracer is represented in terms of mass fraction, and is measured by manually separating the tracer from the process particles according to the difference in color and weighing the amount of tracer. This is laborious and time-consuming work, but it can yield reliable data. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

In section 4.5, the following designations are made Qi, Qij and qij are the mass flow rates of the particles stream (kg particles/sec) and Cj is the concentration of the tracer particles (kg tracer particles/kg particles). In such systems, the tracer particles are, usually, those of the original ones. They are, however, made radioactive or are painted, in order to distinguish them from the original particles. The latter makes it possible to determine their concentration versus time in the RTD experiments [73, p.l76], thus their mean residence time tm in the system. tm =... 

The tracer diffusion coefficient however is not the transport coefficient to be used in the linear relationship (Fick s law) Jc = —D /c between the diffusion current Jc and the particle concentration c or in the diffusion equation dc/9Z = DN c. The coefficient D in these equations does depend on correlations between the motions of different molecules. We have used the notation Dtr above to distinguish the tracer diffusion coefficient from the so-called chemical diffusion coefficient D that appears in the diffusion equation. 

If a quantity of tracer particles, m kg, is introduced into a bed containing M kg powder, and is dispersed instantaneously, tracer will appear immediately in the offtake at a concentration Co = mIM, and its concentration in the offtake, C, at any time t will fall according to ... 

The oscillatory deep-channel rheometer described by Nagarajan and Wasan (227) can be used to examine the rheological behavior of liquid/liquid interfaces. The method is based on monitoring the motion of tracer particles at an interface contained in a channel formed by two concentric rings, which is subjected to a well-defined flow field. The middle liquid/liquid interface and upper gas/liquid interface are both plane horizon tal layers sandwiched between the adjacent bulk phase. The walls are stationary while the base moves. In the instrument described for dynamic studies of viscoelastic interfaces the base oscillates sinusoidally. This move ment induces shear stresses in the bottom liquid that are transmitted to the interface. The interfaces are viewed from above through a microscope attached to a rotary micrometer stage which is coaxial to the cylinders. 

Fluorescent polystyrene particles of radius a = 465 nm (Duke Scientific Co.) were used for tracking the flow. Such tracer particles have the excitation and emission wavelength of 540 and 610 nm, respectively. In all experiments, the number concentration of tracer particles was approximately about 2T0 particles per ml. 

Fig. 3 Measured zeta potentials versus solution concentration (moleA) of the sodium chloride and horic acid electrolytes for (a) the microchannel surface and (b) the tracer particles...

For the measurement of velocity fields, tracer particles are added to the flow. These are usually labeled with a fluorescent dye for a more intensive illumination in laser light (PIV). For the measurement of concentration fields, a fluorescent dye is dissolved in the hquid whereas the concentration of dye corresponds to its fluorescence intensity. 

In a similar attempt, Hatano and Ishida (1986) studied the particle gas mass transfer coefficient in a three-dimensional fluidized bed of nonadsorbing glass beads with = 0.18 mm using optical fiber probes. Tracer gas concentrations in and around rising single bubbles was measured continuously by the penetrative probes, while the bubble boundary and the zone with the prominent particle movement were detected by... 

Radial particle dispersion in CFB risers can be studied by measuring radial concentration profiles of tracer particles injected at a single point upstream of the measurement location (van Zoonen, 1962 Wei et al., 1995b). A two-dimensional dispersion model for fully developed axisymmetric flow with constant dispersion axial and radial coefficients, and D t, gives... 

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