Maximum Semi-fluidization Velocity of Gas-liquid-solid Semi-fluidized Beds

 

Pooja V Shrivastava^1, A. B. Soni¹, H. Kumar²

¹Chemical Engg. Department, National Institute of Technology, Raipur -492001, India.

²Chemical Engg. Department, Raipur Institute of Technology, Raipur -492001, India.

 

ABSTRACT:

Semi-fluidized beds are found to have wide applications in process industries. In the present study hydrodynamic viz., maximum semi-fluidization velocity of gas-liquid-solid semi-fluidized bed has been studied. The necessity of generalized correlation for the prediction of maximum semi-fluidization is emphasized. Experiments have been conducted in a 100 mm ID, 1.8m height vertical Plexiglas column using air, water and solid (of different densities) in order to develop a good understanding of each flow regime in gas-liquid-solid semi-fluidization. It is found that maximum semi-fluidization velocity is strong function of superficial gas velocity, particle size, particle density and bed expansion ratio. Values of the maximum semi-fluidization velocity, obtained from experimental investigations have been compared with calculated values.

 

 

 

INTRODUCTION:

Semi-fluidization is a novel fluid-solid contacting technique¹. The increasing popularity of semi-fluidized beds is because of its unique operation in overcoming some inherent disadvantages of both fluidized and fixed beds.   Gas-liquid-solid semi-fluidization is defined as an operation in which a bed of solid particles is suspended in upward flowing media gas and/or liquid due to the net gravitational force on the particles and the motion of the particles is restricted by a top restraint. Various authors^2,3,6,8,9 have enumerated advantages of the semi-fluidized beds relating to studies on hydrodynamics, reaction kinetics , mass transfer and other unit operations.  

 

A semi-fluidized bed is characterized by a fluidized bed and a fixed bed in series within a single contacting vessel. The internal structure of a semi-fluidized bed can easily be altered to create an optimal operating configuration. This unique feature of a semi-fluidized bed allows it to be utilized for a wide range of physical, chemical and biochemical applications.

 

For successful design and operation of such reactors the knowledge of pressure drop, minimum semi-fluidization velocity, top packed bed formation etc are required. The study of semi-fluidized bed has been broadly classified as prediction of minimum and maximum semi-fluidization velocities, prediction of top packed bed height, and prediction of pressure drop across the semi-fluidized bed.

 

In the present study experiments have been conducted for prediction of maximum semi-fluidization velocity in which co-current flow of air and water takes place in a bed of solid particles of various sizes and densities.

 

Maximum semi-fluidization velocity:

Maximum semi-fluidization velocity (Umsf) of the fluid may be defined as the fluid velocity corresponding to which the entire solid particles of fluidized bed are transferred upward to form a complete packed bed just below the top restraint. Theoretically this velocity corresponds to the free fall velocity (Ut) of the particles. But several other researchers have suggested the use of experimental determination of the Umsf.  Apparently the reason is that, whereas the terminal velocity predicted from the above laws is valid for a single particle only, the actual value of the Umsf should include effect of presence of other particles⁴.

In actual experiments very often it is not possible to transfer the entire mass of material to the top restraint. However, there are few methods available for the prediction of the Umsf obtained from experimental data.

 

There are:

(i)      By extrapolation of the plot between ratio of height of the packed bed formed to the initial bed height vs. superficial velocity of curve to hpa/hs =1.0.

(ii)    By extrapolation of the plot between porosity (єf) vs. superficial velocity of curve to hpa/hs =1.0.

(iii)    By calculation of terminal free fall velocity.

 

Experimental setup

A schematic representation of the experimental setup is shown in Figure 1. The vertical Perspex fluidizer column is of 100 mm ID with a maximum height of 1.8m. The column consists of three sections, viz., the gas-liquid distributor, test section, and gas-liquid disengagement section. The gas-liquid distributor is located at the bottom of the test section and is designed in such a manner that uniform distribution of the liquid and gas can be maintained in the column. The distributor section is a conical frustum of 14 cm in height, with diameter of 5.1 cm and 8 cm at the two ends and having liquid inlets. A perforated plate of 23 cm ID 1 mm thick, 11.5 cm diameter, of about 300 numbers of 2, 2.5 and 3mm perforations is placed at the top of this section. There is an air sparger consisting of 48 numbers of 1mm perforations in triangular pitch. In this section the gas and liquid streams get mixed and passed through the perforated grid. Bed pressure drop has been measured using U-tube mercury manometers.

 

Materials of different densities, water and compressed air (oil free) were used as solid, liquid and gas phases respectively. The ranges of variables for experimental studies are shown in Table-1. The flow of air and water is concurrent and upward. Accurately weighed amount of materials was charged into the column and adjusted for some initial static bed height. The liquid flow rate was varied for a constant gas flow rate using the control valves and bypass adjustment. The bed pressure drop was measured from manometer reading.

 

RESULTS AND DISCUSSIONS:

In the present study the maximum semi-fluidization velocity has been calculated by first method stated in previous section. For gas-liquid-solid systems, variation of hpa/hs with superficial liquid velocity at constant gas velocity, particle size, particle density, expansion ratio and initial bed height have been shown in figures 2, 3, 4, 5 and 6 respectively. Using the values of experimental maximum liquid semi-fluidization velocity a correlation from regression analysis has been developed.

 

Umsf=0.009 Ug ^-0.19 R ^0.51 hs ^0.08 dp ^0.56 ρs ^0.62

 

It has been observed that Umsf increases with bed expansion ratio, particle size and particle density of the solids, but insignificant effect with static bed height.

 

Fig. 1 Schematic representation of the experimental setup.

 

Fig. 2. Variation of hpa/hs with superficial velocity of liquid for different gas velocities of Lime stone  particle at hs= 0.17m, R=2 and dp= 4.05mm

 

Fig. 3. Variation of hpa/hs with superficial velocity of liquid for different diameter of  Lime stone  particle at hs= 0.17m, R=2 and Ug = 0.076m/s

 

Fig. 4. Variation of hpa/hs with superficial velocity of liquid for  4.05mm diameter  particle of different densities at hs= 0.17m, Ug= 0.076m/s

 

Fig. 5. Variation of hpa/hs with superficial velocity of liquid for  4.05mm lime stone particle of different expansion ratios at hs= 0.17m, Ug= 0.076m/s

  

Fig. 6. Variation of hpa/hs with superficial velocity of liquid for  4.05mm lime stone particle of different static bed height at hs= 0.17m, Ug= 0.076m/s

 

 Fig. 7. Comparison between Experimental and calculated values of Umsf

 

The curve of variation of hpa/ hs with superficial liquid velocity has been extrapolated to hpa/hs =1, to get the value of maximum liquid semi-fluidization velocities for different gas velocities, particle sizes, particle densities, expansion ratios and initial bed heights. Figure 2 shows Umsf decreases with increase in gas velocity with constant superficial liquid velocity. Figure 5 shows the variation of hpa/hs with superficial liquid velocity at different expansion ratios. Figure shows the steep increase in hpa/hs values for lower bed expansion ratios compared to higher bed expansion ratios indicating higher packed bed formation in beds with lower expansion ratios. It is seen from the plot that maximum liquid semi-fluidization velocity increases with bed expansion ratio.  It is seen from the figure 3 and 6 that maximum liquid semi-fluidization velocity increases with particle size and initial static bed height.

Table 1- Range of Operating Parameters 

Particle name	Particle size (mm)	Particle density (kg/m3)	Static bed height (cm)	Expansion Ratio	Fluidizing media
					Liquid	Gas
Sand	1.67	1550	15	2.0	Water	Air
Stone chips	2.18	2010	17	2.5	Density-  999 kg/m3	Density- 1.16 kg/m3
Lime stone	2.58	2552	21	3.0
Dolomite	3.07	2900	25	3.5	Viscosity-0.099Pa	Viscosity- 0.0019Pa
Iron ore	4.05	3455	29

 

 

The correlation is found to agree well with experimentally determined maximum liquid semi-fluidization velocities and is shown in Figure-7. It is clear from the plot that the correlation perfectly fits other experimental values obtained by varying the parameters like Ug, R, hs, ρs and dp.

 

CONCLUSION:

The hydrodynamic study of the three-phase semi-fluidized beds reveals that the maximum liquid semi-fluidization velocity (Umsf) is a strong function of gas superficial velocity, particle size, bed expansion ratio but not of the initial static bed height. The maximum semi-fluidization velocity is decreases with gas superficial velocity and increases with particle size and bed expansion ratio. Fig. 7 shows that, the developed correlation agrees well with experimental results. The study will help in successful design and operation of a three-phase semi-fluidized bed reactor.

 

REFERENCE:

1.       Chern, S. H.,  Fan L. S. and Muroyama K., Hydrodynamics of Cocurrent gas-Liquid Solid Semi-fluidization with Liquid as the Continuous Phase, AIChE Journal. 1984; 30(2); 288-294.

2.       Fan, L.T. and Wen, C.Y., Mechanics of Semi-fluidization of single size particles in solid-liquid systems, AIChE Journal. 1961; 7; 606-610.

3.       Ho, T.C., Yau S.J. and Hopper, J.R., Hydrodynamics of Semi-fluidization in Gas-Liquid systems, Powder Technology. 1987; 50; 25-34.

4.       Jena, H. M.,  Roy, G. K. and Meikap, B. C.,  Prediction of gas holdup in three phase fluidized bed from bed pressure drop measurement, Chemical Engineering Research and Design. 2008; 86; 1301-1308.

5.       Kunii D. and Levenspiel O., Fluidization Engineering. 2nd ed. Butterworth-Heinemann, MA, USA, 1991.

6.       Murthy J. S. N. and Roy G. K., Semi-fluidization: a Review, Indian Chemical Engineer 1986; 29 (2); 9-22.

7.       Shrivastava P. V., Soni A. B., Kumar H. K., Prediction of Minimum semi-fluidization Velocity of Gas-liquid-solid Semi-fluidized beds, International Journal of Chemical Sciences. 2012; 10(4).

8.       Roy, G. K. and Sharma, K. J. R., Dynamics of Liquid-Solid Semi-fluidization: Prediction of Semi-fluidization Velocity and Packed Bed Formation, Indian Journal of Technology. 1973; 2; 237.

9.       Sen Gupta, P. and Roy, G. K., Prediction of the Packed Bed Height in Gas-Solid Semi-fluidization, Ind. Eng. Chem. Process Des. Develop. 1974; 13(63); 219-221.

10.     Singh, R. K.,  Maharathy, A. K. and Mahapatra, A. K., Prediction of Pressure Drop in Three-phase Semi-fluidized Bed for Non-spherical Particles, Chemical Engineering World. 2005; 40 (9); 86-88.

11.     Soni A. B., Kumar H. and  Shrivastava Pooja V., Studies on Momentum transfer aspects of semi-fluidized bed in annular section, IUP Journal of Chemical Engineering, 2010; 2(4); 37-42.

 

 

 

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