4 Experimental work on bubbling and circulating fluidized bed reactors
4.2 Cold model of circulating fluidized bed reactor
This was because the main interest of comparison was minimum fluidization conditions and not the fixed bed conditions. The results confirms good agreement between experimental and computational pressure drops at the ambient condition.
The pressure data of olivine particles at high temperature conditions show that the pressure drop at high temperature conditions is similar to glass particles with lower particle size at ambient condition. More details of the experimental and computational procedures and results can be found in Paper A.
Figure: 4.2: Experimental vs computational pressure drop at ambient and high temperature conditions
The CFD model is also used to investigate the applicability of Glickman’s full, simplified and viscous limit sets of scaling parameters. The results of the investigation are presented in Paper B and Paper C. Experiments were performed in the cold model of the bubbling fluidized bed reactor to validate the CPFD models as well. The paper containing the validation results is not included in this work [61]. The validated CFD and CPFD models are further used in the computational study of bubbling fluidized gasification reactor.
4.2 Cold model of circulating fluidized bed reactor
The experimental set up of the cold model circulating fluidized bed is located in University of Natural and Life sciences (BOKU) in Vienna, Austria. The set up consists of a circulating fluidized bed of height of 1.6 m and diameter 0.05 m as shown in Figure 4.3. The cold model includes a riser, cyclone separator, down comer and siphon.
34 CHAPTER 4. EXPERMENTAL WORK
The cold flow model is made of plexiglas which makes it easy to visualize the fluidization inside the riser, cyclone and downcomer. Pressure tapping points are connected to 15 points throughout the reactor as shown in Figure 4.3(a). The pressure tapping points are connected to the pressure acquisition system with pressure sensors which are connected to the computer program to record pressure readings.
The cold model is wrapped with copper wire to avoid electrostatic effects that make the particles stick to the wall. The bed and the siphon is fluidized by compressed air.
Figure 4.3: (a) CFB cold model with airflow regulation and pressure measurement arrangements (b) pressure tapping points
The location of the pressure tapping points are shown in Figure 4.3 (b) and their corresponding heights are shown in Table 4.1.
4.2 COLD MODEL OF CIRCULATING FLUIDIZED BED REACTOR 35
The pressure measurements are performed via hoses connecting all the tapping points to a pressure gauge. The device can take readings of 24 pressure tapping points to measure absolute or differential pressure. The pressure sensors are grouped according to their capacity of pressure measurement range. 14 sensors measure in the range of 0-100 mbar, 6 in the range of 0-250 mbar and 4 of them in the range of 0-500 mbar. The tapping points with possible higher pressure are connected to the high-pressure range sensors.
The pressure measurements are recorded as a function of various airflow rates. For each of the air flow rate, the pressure data are registered for 2-3 minutes and averaged.
Table 4.1: Height of the pressure tapping points
Labelling Position Height
[mm]
P1 Siphon top 665
P2 Siphon top 665
P3 Down comer 1010
P4 Exit Filter 1685
P5 Intersection Precipator 1595
P6 Reactor 1535
P7 Reactor 1330
P8 Reactor 1170
P9 Reactor 1005
P10 Reactor 850
P11 Reactor 610
P12 Reactor 525
P13 Reactor 365
P14 Reactor 205
P15 Reactor 40
P16 Siphon bottom 425
P17 Siphon bottom 205
Pressure reduction valves regulate the ambient airflow and the flow is measured by rotameters shown in Figure 4.4. The characteristics of the rotameters used in the experiments are presented in Table 4.2.
36 CHAPTER 4. EXPERMENTAL WORK
Figure 4.4: Rotameters for primary and secondary fluidization Table 4.2: Flow range of rotameters
The bed is fluidized with constant rate of airflow and the steady state circulation of bed materials is achieved. The particle height in the down comer is measured.
Then the fluidization of the siphon is suddenly interrupted. The particle level at the downcomer increases over a given interval of time. The particle height is measured again. The difference of initial and final height gives the height of the accumulated particles. Knowing the cross sectional area, the amount of solid circulation during the given time interval is determined.
The experimental results of pressure drops and solid circulation rates are used to validate a CPFD model. The experimental and computational solid circulation rates as a function of air flow are presented in Figure 4.5.
Air feed Range of volume
flow [Nm3/h]
Primary fluidization 5.5 - 55 Secondary fluidization 2.9 – 29 Siphon fluidization 0.2 - 2
4.2 COLD MODEL OF CIRCULATING FLUIDIZED BED REACTOR 37
Figure 4.5: Experimental and computational solid circulation rate
The solid circulation rate is highest at gas flow rate of 20 Nm3/h. The computational and experimental results agree well to each other. The deviation between the results are 2% to 10%. The CPFD model is then used to investigate various fluid dynamic properties of the bed.
The primary airflow is introduced while maintaining a constant bottom air feed rate of 15 Nm3/h. The primary air feed rate is 5 Nm3/h. The primary air feed position is varied from the height of 200 mm to 1200 mm from the bottom of the riser with an interval of 200 mm. For every primary air feed position, the total air feed rate in the simulation is constant and 20 Nm3/h which is the sum of bottom and primary air flow. The total air feed of 20 Nm3/h is used because the highest circulation rate is achieved at this flow rate as presented in Figure 4.5. Solid circulation rate as a function of the primary air feed position is shown in Figure 4.6. The solid circulation rate is decreasing with increase in the height of primary air feed position.
38 CHAPTER 4. EXPERMENTAL WORK
Figure 4.6: Solid circulation rate vs Primary air feed position
The highest solid circulation rate is achieved when the ratio of primary air feed position to the total height of the reactor is 0.125.
Chapter 5
Mathematical Model
There are generally two directions in Computational Fluid Dynamics (CFD) modeling of two-phase gas-particle flow. One of them uses Eulerian continuum governing equations for both gas and particle phases [62]. The second one uses Lagrangian description for the particle phase and an Eulerian continuum description for the gas phase [16]. Sections 5.1 and 5.2 give a short description of each of the modeling approaches.
5.1 Euler-Euler method
The Euler-Euler method treats the continuous fluid and dispersed solid as interpenetrating continua. The fluid and solid are treated as primary and secondary phases respectively. The two phases interact with each other by momentum exchange. The model solves a set of conservation equations (e.g. continuity and momentum) for the primary and secondary phases. The secondary phase is differentiated by the solid particle diameter. Each group of particles with a unique diameter is regarded as a separate phase. A single pressure is shared by all the phases. A short description of the method is given in this chapter [63]. The continuity equation for the secondary phase is given by Equation 5.1.
𝜕𝜕
𝜕𝜕𝑝𝑝(𝛼𝛼𝑠𝑠𝜌𝜌𝑠𝑠) +∇ ∙(𝛼𝛼𝑠𝑠𝜌𝜌𝑠𝑠𝑢𝑢�⃗𝒔𝒔) =𝑚𝑚̇𝑔𝑔𝑠𝑠 (5.1) Where 𝑚𝑚̇𝑔𝑔𝑠𝑠is the mass transfer rate, for example due to chemical reaction or evaporation. The granular phase momentum equation is expressed by:
𝜕𝜕
𝜕𝜕𝑝𝑝(𝛼𝛼𝑠𝑠𝜌𝜌𝑠𝑠𝑢𝑢�⃗𝑠𝑠) +∇ ∙(𝛼𝛼𝑠𝑠𝜌𝜌𝑠𝑠𝑢𝑢�⃗𝑠𝑠𝑢𝑢�⃗𝑠𝑠)
=−𝛼𝛼𝑠𝑠∇𝑃𝑃𝑔𝑔+∇𝜏𝜏𝑠𝑠+��𝑅𝑅�⃗𝑔𝑔𝑠𝑠+𝑚𝑚̇𝑔𝑔𝑠𝑠𝑢𝑢�⃗𝑔𝑔𝑠𝑠�+𝐹𝐹⃗𝑠𝑠 𝑛𝑛
𝑠𝑠=1
(5.2)
39