Experimental Investigations on Solid-Liquid Mass Transfer in GLS Fluidized Beds at low Reynolds Numbers

 

Vaishali Pendse1, Dr. Bidyut Mazumdar*1, Dr. H. Kumar2

1Dept. of Chemical Engg, National Institute of Technology, Raipur (CG) 492009

2Dept. of Chemical Engg, Raipur Institute of Technology, Raipur (CG) 492101

*Corresponding Author E-mail: bmazumdar.che@nitrr.ac.in

 

ABSTRACT:

Solid -Liquid mass transfer coefficient (K) was measured in Gas-Liquid-Solid Fluidized Beds by using the system of dissolution of benzoic acid in water. Benzoic acid pellets of various diameter- height combinations were prepared by pelletizing machine and effect of various parameters (superficial gas velocity, superficial liquid velocity, Reynolds number, particle Reynolds number based on gas and liquid velocity) on mass transfer of benzoic acid pellets in water has been studied. It was observed gas velocity plays an important role on mass transfer coefficient value. The effect of Reynolds number based on superficial gas velocity on rate of mass transfer was investigated with varying liquid velocity, bed height and pellet size. Increase in mass transfer coefficient was observed with increase in gas velocity, liquid velocity, bed height at fixed pellet size. As the pellet size decreases in one run, increase in mass transfer with increasing rate of remaining parameter was observed. Comparative study on effect of Reynolds number and particle Reynolds number along with liquid velocity, gas velocity on mass transfer has been shown that mass transfer coefficient increases with increase in Reynolds no., liquid velocity, gas velocity, bed height and with decrease particle Reynolds number.

 

KEYWORDS: Gas-Liquid-Solid fluidization, Solid- liquid mass transfer, Reynolds number based on gas velocity, bed height, mass transfer coefficient.

 

 


1. INTRODUCTION:

Solid -liquid- gas three-phase fluidized beds have been used for various industrial processes including physical, chemical, petrochemical, electrochemical, and biochemical operations. The performance of the three-phase fluidized-bed reactor in these processes often depends on the rate of mass and heat transfer between solid particles and a liquid. Particle-Liquid mass transfer in fluidized beds is a very important transport phenomenon in many chemical engineering operations such as adsorption, desorption, drying, ion exchange and evaporation. The measurements with liquids are mainly concerned with fixed bed, liquid solid fluidized beds but relatively few data are available for three phase fluidized beds particularly with large particles(1,3,4). The Reynolds numbers(7) used are

 

 

 

Reg   -   Reynold number (gas)

Reg’  -   Particle Reynold number (gas)

Rel    -   Reynold number (liquid)

Rel’   -   Particle Reynold number (liquid)

 

The present work extends the solid-liquid mass transfer data for three phase fluidized bed of large particle size ie benzoic acid pellets. It covers a Reynolds number (2709 and 3510), particle Reynolds number range of (90-595) based on liquid velocity and Reynolds number (69.8, 93.86, 125.55, 156.64), particle Reynolds number range (3.13-6.26) based on gas velocity. The experiments have been performed with obtaining mass-transfer data for the dissolution of the compressed pellets of benzoic acid in water. On the basis of the present as well as available published data on identical systems, an attempt has also been made to study the effect of Reynold number, gas velocity, liquid velocity, bed height and pellet size to judge the suitability of the particle Reynolds numbers in correlating the mass-transfer data for fluidized beds.

 

 

 

2. MATERIALS AND METHOD:

Preparation of the benzoic acid pellets: 500gm of benzoic acid was taken in a first batch to make pellets of required size. Powdered Benzoic acid was introduced from the hopper. To convert powdered benzoic acid into pellets compressive strength was applied in machine shown in Fig 1.

 

Fluidized bed column: A schematic diagram of the experimental set-up is shown in Fig 2. The test section is the main component of the fluidized bed where fluidization takes place. It is a vertical cylindrical acrylic column of 90mm internal diameter and total 1350mm height consisting 3 pieces of Perspex columns of lengths, 300, 450 and 600mm respectively assembled by flanges and nut bolt arrangement with rubber gasket in-between. The gas liquid distributor is located at the bottom of the test section. The gas-liquid disengagement section at the top of the fluidizer is a cylindrical section of 300 mm diameter and 250 mm height, assembled to the test section having 600 mm length which allows gas to escape and liquid to be circulated through the outlet of 50 mm internal diameter at the bottom of this section.

 

The experiment was carried out for various superficial gas velocities corresponding to different constant superficial liquid velocities, bed height and pellet size (Ref Table I). Fresh water was allowed to pass through the bed and the flow rate was adjusted by control valve. Gas was used as mass transfer aid. Liquid flow rate, manometer readings and bed height were noted (6). After every 15 min time elapsed, a sample was collected from the top of the fluidized bed and analyzed for solid concentration by volumetric titration method for fixed liquid velocity and increased gas velocity. The same procedure was repeated for different sized particles with different liquid and gas velocity combinations. Mass transfer coefficient, Reynolds number, particles Reynolds numbers based on gas and liquid, bed porosity were calculated(8) for different superficial liquid and gas velocities.

 

 

Fig 1. Pelletizing machine

 

Fig 2. Three phase fluidized bed set up

 

Table I: Range of operating parameter

S. No

Parameters

Ranges

1

Vl

0.031,0.040 m/s

2

Vg

0.0112, 0.0157, 0.021, 0.026 m/s

3

H

1,2,3 cm

4

PS (d*h)

0.2*0.4, 0.4*0.6,0.6*0.8 cm*cm

5

Reg

69.8,93.86,125.55,156.64

6

Reg

3.13 - 6.26

7

Rel

2709,3510

8

Rel

90 - 595

9

vl

10, 15 lpm

10

vg

4.5, 6, 8, 10 lpm

 

3. RESULTS AND DISCUSSION:

The effect of Reynold numbers based on gas velocity on mass transfer coefficient:

 This study was carried out to check the influence of superficial gas velocity (Vg) parameter in terms of Reynolds number on mass transfer coefficient (K). The variation of K with Re is for three pellet sizes of benzoic acid, each of which for three bed height shown in Fig (3a, 4a, 5a) at 0.030 m s -1 liquid velocity and Fig (3b, 4b, 5b) for 0.040 m s -1liquid velocity. It can be seen that value of Re varies with varying gas velocity and value of K is more at 156.64 and less is at 69.8 for all bed heights. This Reynolds number range comes under lower range and in this range gas enters through the bed as gas bubbles which appeared to be uniformly distributed over the hole cross section of the bed. As gas velocity increases up to 0.026 ms-1 number of bubbles increases and results increase in mass transfer rate(2).

 

 

Fig 3 (a) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl =0 .03m/s, PS = (0.2*0.4) cm

 

Fig 3(b) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl = 0.04m/s, PS=(0.2*0.4)cm

 

Fig 4 (a) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl =. 03m/s, PS= (0.4*0.6) cm

 

 

Fig 4(b) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl = 0.04m/s, PS= (0.4*0.6) cm

 

 

Fig 5 (a) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl = 0.03m/s, PS=(0.6*0.8)cm

 

Fig 5 (b) Reynolds number(gas) vs mass transfer coefficient

for different bed height at Vl = 0.04m/s, PS=(0.6*0.8)cm

 

The effect of superficial liquid velocity on mass transfer coefficient:

Experimental runs have been taken for two liquid velocities ie 0.03 ms-1 and 0.4 ms-1. Mass transfer coefficient value at 0.03 ms-1 and 11 lpm ref Fig (3a, 4a, 5a) is less than the 0.4 ms-1 and 15 lpm ref Fig (3b, 4b, 5b) for specific pellet sized and bed height. Figures show effect of mass transfer ie it increases with liquid velocity(5). At minimum fluidization velocity 0.03 ms-1 bed has less void space for uniform distribution of bubbles than at higher liquid velocity for fixed size and bed height of solid however no measurements were taken at liquid velocity higher than 0.04 ms-1.

 

Effect of particle Reynolds number based on liquid velocity on mass transfer coefficient:

For the chosen particle range it is observed from Fig (6,7,8) that value of K is practically dependent on particle size which is in agreement with the literature(7) and so on the particle Reynolds number. It is evident for given bed height, liquid velocity and gas velocity value of K increases with decrease in particle Reynolds number. As the small size particles move freely in comparison with bigger one and contact between particle and liquid is relatively good which leads to higher value of K.


 

Fig 6 Particle Reynolds number vs K for PS= 0.2*0.4

 

Fig 7 Particle Reynolds number vs K for PS= 0.4*0.6

 

Fig 8 Particle Reynolds number vs K for PS= 0.6*0.8

 


4. CONCLUSION:

Experiments were carried out using Benzoic acid pellets as solid phase for Reynolds number (2709 and 3510), particle Reynolds number range of (90-595) based on liquid velocity and Reynolds number (69.8, 93.86, 125.55, 156.64), particle Reynolds number range (3.13-6.26) based on gas velocity for mass transfer studies in three-phase fluidization. It was observed from the investigation that the mass transfer coefficient was significantly influenced by liquid velocity, Reynolds no based-on liquid and gas velocity etc and it increases with liquid velocity so with Reynolds number and with decrease in particle Reynolds numbers based on gas velocity.

 

5. NOMENCLATURE:

Vl - superficial liquid velocity (m/s)

Vg - superficial gas velocity (m/s)

d - diameter of benzoic acid pellets (cm)

h - height of benzoic acid pellets (cm)

H - Bed height (cm)

K - solid liquid mass transfer coefficient (cm/s)

PS - Pellet size

Reg - Reynold number (gas)

Reg’ - Particle Reynold number (gas)

Rel - Reynold number (liquid)

Rel’ - Particle Reynold number (liquid)

vl - Volumetric flowrate of liquid (lpm)

vg - Volumetric flowrate of gas (lpm)

 

6. REFERENCES:

1.      A. Prakash, C. L. Briens and M. A. Bergougnou “Mass Transfer between Solid Particles and Liquid in Three Phase Fluidized Bed “ In The Canadian Journal Of Chemical Engineering April I987; Volume 65.

2.      A. Sivalingam, N. M. Hariharan, T. Kannadasan, M. Thirumarimurugan, and S. Gopalakrishnan. Mass Transfer Studies in Three-phase Fluidized Bed Using Response Surface Method. Chem. Biochem. Eng. 2011; Q. 25 (2): 171–179.

3.      Arter D. C And L.S. Fan. AIChE Annual Meeting 1984 ; Nov 26-30.

4.      D. C. Arter and L. S. Fan. Chemical Engineering Sciences.1986; 41: 107-115.

5.      Dinesh V. Kalaga , Anu Dhar, Sameer V. Dalvi , Jyeshtharaj B. Joshi. Particle-liquid mass transfer in solid–liquid fluidized beds. Chemical Engineering Journal. 2014 ; 245: 323–341.

6.      Jiasen Song, Caroline L. Hyndman, Rajesh K. Jakher, Kelly Hamilton, Apostolos Kantzas Fundamentals of hydrodynamics and mass transfer in a three-phase Fluidized bed system. Chemical Engineering Science 1999; 54: 4967- 4973 .

7.      Siddh N. Upadhyay and Gopal Tripathi. Liquid-Phase Mass Transfer in Fixed and Fluidized Beds of Large Particles. Journal of Chemical and Engineering Data 1975 ;Vol. 20: No. 1.

8.      T koloini, M Soptic And M Iumer. Mass Transfer In Liquid-Fluidized Beds At Low Reynolds Numbers. Chemical Engineering science 1977; Vol 32 : 637-641.

 

 

 

 

 

Received on 20.02.2019           Modified on 16.03.2019

Accepted on 23.04.2019         © RJPT All right reserved

Research J. Pharm. and Tech. 2019; 12(6): 2753-2757.

DOI: 10.5958/0974-360X.2019.00461.X