INTRODUCTION:
Fluidizing behavior in columns and vessels:
Several research works are available the field of three phase bubble column and three phase gas-liquid-solid fluidization. Krishna et al. 1993 reported that hydrodynamics behavior of bubble column and fluid beds can be described using an analogous relation.The importance of diameter to height ratio of column has been discussed by many researchers as this ratio affects significantly the hydrodynamic behavior. Similarly,hydrodynamics of the stationary liquid fluidization is also affected by the L/d ratio (length verses diameter)of the fluidizing column (Abraham et al. 1992). It has already been established that hydrodynamics of the columns having higher values of L/d ratio (Vidal et al. 2016), is different from the columns having lower value of length to diameter ratio (less than or equal to 1). In fact, the columns having lower values of L/d ratio are called horizontal vessel.
The difference in the hydrodynamic behavior is mainly due to the fact that for stationary liquid fluidization in the column gas drags the solids and then buoyancy of liquid gives the further addition on the drag of the solids.Hydrodynamic behavior of fluidization of solids in horizontal vessel having stationary liquid is quite different and mainly depends upon the gas velocity. For higher gas velocity gas creates single and central passage which creates compressive action on the periphery of the passage as the solids tend to move towards the wall of the vessel (Vidal and Espci 2012). In case of fluidization with lower gas velocity gas creates a number of passages of small diameter and inclined in nature. Further, the hydrodynamic behavior is also affected by the height of liquid present in the column or vessel.Mariyama, et al. (1981) reported regarding circulatory motion of liquid within the column under specific conditions
Flow regimes in slurry column and fluidizing column:
The hydrodynamic behavior is greatly affected by the flow regimes and pattern of solid movements. This mapping of solid movement has been given in figure 1 and figure 2 which is common for column and horizontal vessel. In case of three phase column there are two different classifications. First is gas-solid-liquid fluidizing column and second is gas-solid-liquid slurry column. However the mapping of the flow regime is almost similar for both the case.The first flow regimes is mainly homogenous type of flow regime which is also known as bubbly flow regime, heterogeneous type of regime which is also known as churn turbulent type regime and last one is slug flow regime. Apart from these three flow regimes there is one more flow regime which is called foaming regime which is found is only in some specific type gas-solid-liquid systems. The flow regime map presented in Figure 1 is given by Shah et al., 1982 which differentiates the different flow regimes on the basis of the column diameter and gas flow rate. The intermediate regions between different flow regimes have been shaded and known as transition flow regime.However, regarding the boundary of the transition regime, different investigators have given different findings (Joshi et al., 1981). The shape and solid movement pattern is further studied by the Bouaifiet al.( 2001) which is presented in Figure 2.
Figure 1: Flow regime map for stationary liquid batch bubble column containing a low viscosity liquid phase (FromAbrahamet al., 1982)
The flow regime map for fluidizing column is very much identical to the regime of the bubble column. The two main flow regimes which are present during fluidization are dispersed flow regime which is identical to the homogeneous flow regime of the bubble column and coalesced or turbulent flow regimes which is identical to the heterogeneous flow regime of the bubble column.
Figure 2: Schematic of possible flow regimes in bubble columns (From Bouaifi et al., 2001)
Zhang (1996) further differentiated these two regimes on the basis of bubble pattern into seven different flow regimes which are applicable during gas-solid-liquid fluidization. It is presented in Figure 3.These seven flow regimes are dispersed bubble flow, discrete bubble flow, coalesced bubble flow, slug flow, churn flow, bridging flow and annular flow regimes.
Figure 3: Flow regime map for three-phase fluidization (From Zhang et al., 1997)
Although the flowregimes are identical for these two processes (bubble column and fluidization) butmany investigators given their opinion for differentiating the process on the basis of other parameters. Letzelet al. 1997 differentiated the bubble column and fluidizing column on the basis of pressure fluctuation. Before the work of Letzel et al. (1987), Fan et al. (1987) differentiated these two processes on the basis of terminal velocity and suggested for terminal velocity range of 0.03 m/s to 0.07 m/s the regimes of the bubble column and gas-liquid-solid fluidizing column are having similar nature.Kumar (2012) related the particle shape with the terminal velocity and suggested that shape of the solid also influences the hydrodynamics of the process.
MATERIALS AND METHOD:
Shaikh and Dahhan (2007) described regarding the different methods for identifying the flow regimes. These methods are identification using hydrodynamic parameters, temporal signature, advanced measurement technique and visual observation type methods. In the present research work, visual observation type method has been used for identifying the flow regimes.
Figure 4:Experimental set-up
For identifying the flow regime during stationary liquid fluidization, experiments have been carried out. The schematic diagram of the experimental set-up is shown in Figure 4. The experiment set-up has been standardized by applying gauge reproducibility and repeatability theory (Kumar et al. 2015). It consists of vertical column made of acrylic having internal diameter 90 mm. The vertical column is attached with air compressor. For restricting the motion of liquid in the air line a non-return valve has been used. Flow rate of air can be adjusted by the flow control valve. Proper provision has been made for feeding into and removal of the material from the column. The pressure drops have been measured by using pressure gauge.
Identification of flow regimes:
The flow regimes of the stationary liquid fluidization have been identified by conducting the experiments on air-water-coal system. The ranges of experiments are presented in the Table 1.
Table 1: Range of parameters for identifying the regime
|
Sr |
Parameter |
Range |
|
01 |
Solid bed height |
25, 30 and 35 cm |
|
02 |
Height of stationary water |
10, 15, 20, 25, 30, 35, 40, 45, 50, 55 and 60 cm |
|
03 |
Flow range |
0 – 60 lpm |
|
04 |
Average solid size |
0.002 m |
Experiments have been performed by filling the water in the column up to a particular height then solids have been charged into the column up to the desired height. The bubble flow pattern has been closely monitored for identifying the flow regime.
Figure 5: Flow regime map of the present research work
Blue curve discriminates homogenous and heterogeneous flow regime and green curve differentiates between slug flow regime and churn flow regime. Region under blue curve is for heterogeneous slug and churn flow behavior for that particular flow rate and that liquid height. Similarly region below green curve indicates the turbulent churn flow region. The present investigation has been conducted in this region only. The space between red and blue curve is meant for transition zone. It is found that above 6.5 lpm flowrate and at least liquid height upto 60 cm, bubble behavior clearly exhibits churn flow regime. Further, it is clear from the figure that for increasing stationary liquid height flow transition velocity increases.
CONCLUSION:
Hydrodynamic studies of the fluidized bed and slurry column have been discussed and different flow regimes have been identified. Then an attempt has been made for describing the flow regimes during stationary liquid fluidization. It has been found that two flow regimes i.e. homogenous and heterogeneous flow regimes, which are common in gas-liquid-solid bubble column and fluidizing column, are also present during fluidization of solids in stationary liquid.
REFERENCES:
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Bouaifi, M., Hebrard, G., Bastoul, D., andRoustan, M. (2001). A comparative study of gas hold-up, bubble size, interfacial area and mass transfer coefficients in stirred gas–liquid reactors and bubble columns. Chemical engineering and processing: Process intensification, 40(2), 97-111.
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Received on 25.03.2018 Accepted on 21.04.2018 © EnggResearch.net All Right Reserved Int. J. Tech. 2018; 8(1): 11-15 DOI:10.5958/2231-3915.2018.00003.2 |
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