The thesis work entitled Investigation of Gas – Solid Circular Fluidized Beds in Two Scale Using Experimental and Numerical Techniques by Premkumar K, Student, Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India, for the award of Ph.D. in philosophy was completed under my supervision and this work was not submitted elsewhere for a degree.
ABSTRACT
Introduction
- Introduction
- Circulating fluidized bed
- Solid-catalyzed gas-phase reactions Fluid catalytic cracking
- Hydrodynamics of circulating fluidized bed
- Scale up of circulating fluidizing bed
- Motivation
- Objectives
- Structure of the thesis
This is mainly due to a lack of understanding of scale effects in the circulating eddy layer. Eds.), Circulating Fluidized Bed Technology In: Proceedings of the Fifth International Conference on Circulating Fluidized Beds.
Radioactive Particle Tracking (RPT) and Benchmarking for High Velocity Conditions
Introduction
To obtain the directional speed, one of the beam frequencies is shifted, thus creating the motion of the edge in the predetermined direction. In the case of solid phase tracking, the size, shape and density of the tracer particles must be the same as the solids present in the system.
Radioactive particle tracking technique
- Methodology
- Calibration
- Reconstruction algorithm
- Optimal design of RPT experiments
D is the mass attenuation coefficient of the detector crystal material is the penetration depth of photons in the detector crystal. Penetration depth of the photon into the crystal depends on the point of entry and exit. The path of the tracer particle is reconstructed using counts recorded by the scintillation detectors.
Equation 2.1 is then used to generate an estimated count for each position of the tracer particle relative to the detector.
Implementation of RPT technique for high velocity system
- Experimental setup
- Resolution and sensitivity
- Conclusions
However, the effect of data acquisition frequency on the ability of RPT measurement to locate the stationary position of the tracer particle has not been investigated. The results show that the standard deviation of the error in displacement increases with a decrease in data acquisition frequency. Furthermore, it is observed that the error in particle position reconstruction increases with the increase in tracer particle velocity for the same frequency.
However, it decreases with increasing data acquisition frequency for the same tracer particle velocity.
Summary
Experimental investigation of local solids Fluid dynamics in various industrial circulating fluidized beds with optical probes (Doctoral dissertation). A comparative study of hydrodynamics in a bubble column using computer-automated radioactive particle tracking (CARPT)/computed tomography (CT) and particle image velocimetry (PIV). Investigation of fluid hydrodynamics in bubble columns using computer-automated radioactive particle tracking (CARPT) (D.Sc. Dissertation).
Investigating the hydrodynamics of binary fluidizing beds via radioactive particle tracking and dual-source densitometry.
Introduction
However, for multiphase systems where two different phases must interact with each other, quantification based on global mixing does not provide the necessary information to fully characterize the flow. For complete characterization of the flow beyond time-averaged velocity and distribution of phases, local phase fraction and velocity distribution are needed. It is still difficult to map the entire flow field in gas-solid circulating fluidized beds to capture all the interactions that occur at different length and time scales.
Furthermore, it is still difficult to map the flow field at different scales with the same accuracy to match the phenomena occurring at the different scales.
Literature on velocity studies in CFB
In the flow of the core annulus structure, solids flow upwards into the core and downwards near the wall. This type of CFB is commonly referred to as a high-density circulating fluidized bed (Issangya et al., 1997). Very steep profiles are usually the hallmarks of dense suspension upflow, where there is also a high density of solids in the system (Wang et al., 2014).
For example, both parabolic and power profiles are reported for dilute flow conditions (Wei et al., 1998; Pantzali et al., 2013).
Experimental setup
A reciprocating compressor is used to supply drive air, which is supplied through a secondary intake. The experiments were carried out with glass beads with a mean particle size of 500 µm and a density of 2500 kg/m3.
Solid flux measurement
- Velocity and volume fraction measurement methods
So if the velocity vs and the volume fraction of solids are known, solid flux can be obtained. Therefore, solid flux can be measured by measuring solid volume fraction and solid velocity individually. Solid flux can be changed either by changing the solids inventory in the system or by changing the superficial gas velocity.
Thus, for continuous solid flux monitoring during RPT measurements, the circulation number method is used.
Radioactive particle tracking (RPT) measurements
- Visual observation of riser
- Lagrangian track of particle position
It should be noted that the tracer remains only a fraction of the circulation time in the study zone. Studies of fixed flux described in section 3.5 are used to select operating conditions. Except at low speed conditions (7 m/s and 7.8 m/s), changes in speeds are indistinguishable with operating conditions.
A typical tracer particle trajectory for single circulation is shown in Figure 3.9 in the x-z, y-z, r-z and r-θ planes.
Top viewr –z plane
Lagrangian velocity of the solids
The instantaneous Lagrangian velocities of the tracer particles are obtained by time differentiation of two consecutive positions of the tracer particle. The stationary condition is said to be satisfied if the moments of the distribution (mean, variance, etc.) do not change in the time series. In RPT experiments, with an increase in the data series (number of occurrences), the moments of the distribution should not change to have a stationarity.
The average velocity (ensemble) is calculated by adding the instantaneous velocities in the cell and dividing by the number of times they occur in the cell.
PDF of instantaneous velocity
However, in the current study, we have clearly observed the presence of the clusters through photographic studies. This can be confirmed through higher velocity distribution observed near the wall of the riser. Results show that radial velocity at all locations is negligible compared to the axial velocity of the solids.
However, a relatively larger radial velocity distribution is observed near the center of the column compared to the wall.
Velocity vector plots
Therefore, it can be stated that the presence of negative velocity alone cannot confirm the presence of the clusters. Although such structures can be observed at all locations inside the riser, the probability of occurrence is higher near the wall. Similar profiles are observed for all operating conditions to ensure that axisymmetric flow is maintained.
Furthermore, asymmetric flows are observed under all operating conditions, so the azimuth average will not influence the comparison.
Number of occurrences
Most of the literature shows that the wall peaks (Godfroy et al., 1999; Bhusarapu, 2005), but in this work a steep increase in incidence from the center to the wall is observed. So far, qualitative analysis of the system is presented to find the overall flow pattern. Average velocities and other turbulent quantities at different axial positions can shed light on the behavior of the riser in detail for the mentioned operating conditions.
Ensemble averaged velocity
Furthermore, it is noted that slip velocity is approximately 1.3 times the terminal settling velocity of the solids, assuming that gas velocity is the same as the superficial gas velocity (plug flow). Such a slip velocity is usually observed either in dilute flow conditions or in dense suspension upflow conditions.
Solid velocity fluctuations
In other words, the fluctuations due to the mean free path are still dominant and decreasing the mean free path is not critical enough to reduce the fluctuations. However, in the radial and tangential directions (Reynolds shear stresses) the values are almost zero. Insignificant values of rz may be due to particle properties, since solids have high momentum, change in direction of motion is less likely to occur.
From the present and previous studies, it can be concluded that particle properties play a major role in the radial movement of solids.
Turbulent kinetic energy
Kinetic theory of granular flow (KTGF) expresses the fluctuations in terms of granular temperature, similar to the thermodynamic temperature shown in the kinetic theory of gases. TKE values are in the order of axial RMS fluctuations and essentially follow the same trend. Trend and values of TKE are of the same order as in Tartan and Gidaspow (2004) and Pantzali et al. 2013), even though the superficial gas velocities and mass loading are higher in the present work.
Initially, the effect of superficial gas velocity on the behavior of solids flow in risers is discussed.
Effect of superficial gas velocity
Axial RMS velocities are low in the center and high near the wall for all conditions. As the inlet gas velocity increases, the axial RMS velocity decreases up to 8.8 m/s and then remains almost constant. However, no significant difference is observed in the radial RMS velocity value for different.
The turbulent kinetic energy of solids decreases with increasing superficial gas velocity and follows the same profile as the RMS axial velocity.
Effect of solid flux
These results show that initial increase in solids flux uniformly increases the solids fraction across the cross section. Increases in the gas velocity and solids flux simultaneously significantly increase the average axial solids velocity. However, increase in the average axial solids velocity is not linear with the gas velocity nor with the solids flux.
It is observed that the axial RMS velocity of solids increases with the increase in the solids flux.
Solid mixing studies
- Residence time distribution
- Trajectory length distribution (TLD)
- Local solids mixing
With an increase in the superficial gas velocity at the same fixed flux, the average residence time, variance and dispersion number decrease. However, in the case of 7 and 7.8 m/s operating conditions, an increase in the dispersion figure is observed. Average length traveled and variance decrease with increasing superficial gas velocity at the same fixed flux.
While it is increasing or almost remains the same for increase in the fixed flux at the same superficial gas velocity.
Summary
Gas - Solid Turbulent Flow in a Circulating Fluidized Bed Riser: Experimental and Numerical Study of Monodisperse Particle Systems.Ind. Measurement of radial and axial solids flux variations in the rise of circulating fluidized bed in Circulate. Detailed Measurements of Particle Velocity and Solids Flux in a High-Density Circulating Fluidized Bed Riser.
Flow characteristics in the inlet and outlet regions of a high-flux circulating fluidized bed riser.
