Article pubs.acs.org/IECR
Experimental Investigation on Interaction of Side Gas Injection with Gas Fluidized Bed Using γ‑Ray Transmission Technique Sandip Bhowmick,† Vijay Kumar Sharma,‡ Jitendra S. Samantray,‡ Harish J. Pant,*,‡ Kalsanka Trivikram Shenoy,† Ashutosh Dash,‡ and Saswati B. Roy§ †
Chemical Engineering Division, ‡Isotope Production and Applications Division, and §Chemical Engineering Group, Bhabha Atomic Research Centre, Mumbai 400085, India ABSTRACT: The present paper describes measurements of time-averaged voidage distribution in a pilot-scale fluidized bed using γ-ray transmission technique. The voidage profiles in absence of side gas injection showed that the tracks of bubble flow change from one side of the wall to the center of the bed with increasing superficial gas velocity. The side gas injection can drastically alter the shape of the bed voidage distribution. At incipient fluidization the jet bubbles and distributor bubbles were found to be moving without interacting with each other. At 1.5umf the jet bubbles and the distributor bubbles move from the wall to the central region of the bed with the increasing bed height above the nozzle plane. At 2umf the jet bubbles and distributor bubbles follow the same track from the nozzle plane. The void fraction measurement below the nozzle showed that there was no back mixing of gas injected through the nozzle. different from that of a three-dimensional system.28 They studied the extent of the void formed by the horizontal jet in a rectangular fluidized bed of Englelhard HFZ-33 FCC cracking catalyst (d̅p = 59 μm; ρp = 1450 kg/m3) with an X-ray system. The catalyst particles were Geldart Group A type.29 Franka and Heindel investigated the gas holdup in a 102 mm diameter fluidized bed with side air injection using X-ray computed tomography.30 The beds were composed of glass beads, ground walnut shells, and ground corncobs (d̅p = 500−600 μm, Geldart Group B29). The side air was introduced through an 11 mm inside diameter (i.d.) tube located just above the distributor plate. Deza et al. studied the effect of side port air flow on particle mixing in a biomass fluidized bed having similar dimensions as previously referenced.31 Drake and Heindel applied the 3D X-ray computed tomography to determine local time averaged gas holdup in fluidized beds with side air injection considering scale-up of such devices.32 Min et al. compared the computational fluid dynamics simulation to the experimental data of a laboratory scale fluidized bed with and without side gas injection.33 In the present work the voidage distributions in a pilot-scale fluidized bed (250 mm i.d.) were measured in the presence and absence of side injection of gas. Glass beads (Geldart Group B) were used as bed material and air as fluidizing agent. A commercial nozzle was used for the gas injection considering its vast applicability in fluidized-bed processes. Among various noninvasive void measurement techniques,34−36 γ-ray transmission technique was adopted for the current study. Till date, the voidage distributions in a gas fluidized bed have been studied widely but very limited investigations have been published on the effect of side injection of gas on the bed
1. INTRODUCTION In a large number of industrial processes the liquid feed is introduced into the fluidized bed using pneumatic atomizing nozzles. These processes include combustion of liquid fuels,1−5 thermal denitration of uranyl nitrate,6,7 metal nitrate wastes,8 and ammonium nitrate,9 granulation,10 and coating.11 The disintegration of a liquid stream into multiple droplets is needed to achieve a large increase in its surface area and thereby corresponding increase in rate of evaporation.12 Generally, endothermic and exothermic reactions are carried out in such liquid-sprayed gas fluidized beds. Therefore, the transfer of heat between particles and gas as well as between bed and surface (wall or immersed surface) is important. In that case the knowledge of the bed voidage and its distribution is very useful for improving the heat-transfer rate as well as the process efficiency. The gas, utilized in the nozzle for liquid atomization, is ultimately introduced into the fluidized bed. This excess gas injection may significantly change the voidage distribution. Sometimes this excess gas flow increases the bed voidage and becomes the main cause of bypassing of liquid vapor without making contact with the solid particles. Hence, the efficacy of such processes can be improved by understanding the dynamics of the gas and solid motion resulting from the interaction between atomization gas and fluidized bed. The bed voidage and its distribution are commonly measured to predict the quality of fluidization.13−17 According to Schouten and Bleek, the fluidization quality is described as the condition of the fluidized bed that leads to an optimal mixing of solids and excellent contact between gas and particles throughout the bed.18 There are a number of selected papers dealing with injection of gas into fluidized beds.19−25 These researchers were mainly concerned with the estimation of penetration length of the jet. Xuereb et al. described the behavior of horizontal and inclined gas jets in a twodimensional fluidized bed.26,27 According to Chen and Weinstein, the behavior of a two-dimensional system is quite © 2015 American Chemical Society
Received: Revised: Accepted: Published: 11653
July 25, 2015 September 22, 2015 October 29, 2015 October 29, 2015 DOI: 10.1021/acs.iecr.5b02741 Ind. Eng. Chem. Res. 2015, 54, 11653−11660
Article
Industrial & Engineering Chemistry Research voidage distribution. The main objective of this work was to investigate the interaction of side gas injection with fluidized bed. Experimental details and observations are presented in this paper.
( ) = ε −ε 1−ε ln( ) ln
Ia Is
2. γ-RAY TRANSMISSION TECHNIQUE γ-Rays are an electromagnetic radiation with energies above 10 keV. This type of radiations is weakly attenuated by the material and it can permeate very deep into the matter. Therefore, γ-ray transmission technique can successfully be used to measure the densities of materials inside the chemical process systems. Noteworthy applications of this technique are found not only in the industrial37 but also in the agricultural38 fields. A suitably selected γ-ray source and a detector can be placed outside the measuring system, providing a noninvasive measuring method that does not interfere with the internal flow. A radioisotope Cesium-137 (137Cs) of 10 mCi activity and a NaI (Tl) scintillation detector were used in the present study. The 137Cs decays to the Barium-137m (137mBa) emitting beta (β) radiation with a half-life of 30.2 years. The 137mBa de-excites by emitting a 661.65 keV γ photon with a half-life of 2.55 min. The activity of the source during the experiments can be considered as constant because of the long half-life of 137Cs. Consider a narrow beam of monoenergetic photons with an incident intensity I0 (photons/s) passes through the absorber material m with thickness d. The intensity of transmitted beam I is then given by the following Beer−Lambert law. I = I0e−μ(E ,m)d
⎛I ⎞ ln⎜ a ⎟ = −μair (l − 2t ) − μacrylic (2t ) ⎝ I0 ⎠
(3)
When the column was filled with the solid particles with static void fraction εs, detected number of photons Is can similarly be written as ⎛I ⎞ ln⎜ s ⎟ = −[εsμair + (1 − εs)μsolid ]d i − μacrylic (2t ) ⎝ I0 ⎠ − μair (l − d i − 2t )
(4)
In the fluidization state with void fraction of εf, the intensity of transmitted γ-rays If can be given as ⎛I ⎞ ln⎜ f ⎟ = −[εf μair + (1 − εf )μsolid ]d i − μacrylic (2t ) ⎝ I0 ⎠ − μair (l − d i − 2t )
s
s
(6)
3. EXPERIMENTAL SETUP A schematic sketch of the experimental setup is shown in Figure 1. The bed consists of an acrylic cylinder with an inner diameter of 250 mm and a height of 1650 mm. The thickness of the acrylic column is 25 mm. A freeboard section having inner diameter of 500 mm is provided to prevent the entrainment of particles. Fluidizing air was introduced into the plenum chamber using a 40 NB size drilled pipe. The pipe has 12 rows of holes spaced 30° apart aligned along the longitudinal length of the pipe. The longitudinal distance between the holes is 15 mm, and there are 14 No. holes of 2 mm diameter per row. This sparger pipe provides a uniform distribution of the fluidizing air throughout the plenum. The sparger pipe also serves gradually expanded air into the plenum chamber and thus prevents jetting phenomena. A perforated plate-type distributor was placed into the bottom of the column to achieve equal flow over the entire cross section. The 6 mm thick distributor plate consists of 937 No. aeration holes of 0.6 mm diameter on 8 mm triangular pitch. The total open area of the distributor plate is 0.54%. According to Zuiderweg, the thumb rule which should be followed to achieve equal flow over the cross section of the bed is that the pressure drop across the distributor should be 0.2−0.4 times of the bed pressure drop.39 The distributor was designed such that it satisfies the pressure drop criteria even at minimum fluidization velocity and ensures uniform air distribution and stable operation at all the experimental conditions. A nozzle (Spraying System (India) Pvt. Ltd.; Type 1/4 JCO; Fluid Cap 2850; Air Cap LP6065060) was located horizontally 640 mm (h/Dt = 2.56) above the distributor. It is an external mix type nozzle. The nozzle head consists of 6 No. peripheral holes of 1.3 mm diameter for injection of the air and a central hole for injection of the liquid. The side air was introduced only through the air side of the nozzle. The liquid line of the nozzle was plugged as no fluid was introduced through the liquid side. The usage of such nozzle can be found in various references.6,7 The nozzle was mounted with the face flush with the wall of the bed (y/R = −1). The volumetric air flow rate (SLPM) was measured using transmitting type metal tube rotameters (Krohne Marshall India, Model H250). The calibration pressure and temperature of rotameters were 2 bar (g) and 30 °C, respectively. Glass beads were used as bed material. The particle density of glass beads was 2500 kg/m3 and the bulk density was 1575 kg/m3, which corresponds to a void fraction of 0.37. The particle size distribution was measured using an electromagnetic sieve shaker unit (EMS-8). The size analysis result is presented in Table 1. The volume surface mean diameter of the glass beads was 385 μm. Purge method of pressure measurement was adopted to measure the pressure drop across the fluidized bed and the distributor. The pressure drops were measured by ABB
In the above equation μ(E, m) is the linear attenuation coefficient of the absorber material m for photon energy E. As energy E is a constant (661.65 keV), it can be omitted. In γ-ray transmission technique setup the source and detector were located in the same horizontal plane and the linear distance between them was l. An acrylic column of internal diameter di and thickness t was placed between the source and the detector. Now the detected number of photons Ia in the detector is given by the following equation. (2)
f
Using the above equation, the local void fraction of a fluidized bed can be estimated. Equation 6 is free from any dimensional parameter. Though the central location of the column (y/R = 0) was chosen for derivation of this equation, it can be used in any axial location (−1 ≤ y/R ≤ 1) to determine the local void fraction. The εs can be evaluated from the static bed height, particle density, and quantity of particle.
(1)
Ia = I0e−μair (l − 2t ) − μacrylic (2t )
If Is
(5)
From eqs 3, 4, and 5, we get the following equation: 11654
DOI: 10.1021/acs.iecr.5b02741 Ind. Eng. Chem. Res. 2015, 54, 11653−11660
Article
Industrial & Engineering Chemistry Research
Table 1. Particle Size Distribution of the Glass Bead Used mesh range (μm)
weight fraction (xi)
500−425 425−350 350−300 300−250 250−212 Σxi
0.5037 0.2019 0.1783 0.0969 0.0193 1.00
Figure 2. Determination of minimum fluidization velocity without side air injection.
flow rate of 400 SLPM (Qmf). During determination of umf, 50 kg glass beads were poured into the column and the static bed height was 645 mm. Franka et al. studied the effects of side air injection on minimum fluidization velocity.40 According to their observations, the side air injection has a larger impact on reduction of the pressure required to fluidize the bed. As a significant pressure is required to fluidize the glass bead, the variation of umf for the bed of glass beads with side air injection is insignificant. Two movable pieces of mild steel (MS) equal angles (size: 50 mm × 50 mm × 5 mm) were installed over fixed angles. The contact surfaces were smoothened. The friction between movable and fixed angles was minimized by applying grease on contact surfaces. The horizontal movement of movable angles parallel to the y-axis was adjusted by screw thread. Two lead collimators with outer diameter of 50 mm were placed on the movable angles. The collimators were positioned perpendicular to the column. The collimators had a narrow opening of 3 mm diameter. One collimator contains encapsulated γ-ray source (137Cs) of 10 mCi strength. The opposite collimator carries NaI (Tl) scintillation detector. The photon counting system consists of a photomultiplier tube, preamplifier, amplifier, and multichannel analyzer. The counter was connected with a data acquisition system with software interface to record the counts per specified time. The openings of both collimators should be aligned in straight line and the alignment was checked by a laser beam passing through the holes of both collimators. The face-to-face linear distance between the lead collimators was 318 mm. A pointer and scale was used to get the exact axial location (along y-axis) of the beam. Both sides of fixed MS angles were connected with pipes by welding joints. These pipes were inserted into fixed pipes of bigger diameter. So the fixed MS angles can traverse vertically
Figure 1. Schematic sketch of the experimental setup. (a) Top view, (b) side view. N1, fluidizing air inlet; N2, nozzle for gas injection; N3, off-gas outlet; P1 and P2, pressure taps for bed pressure drop measurement; P3 and P4, pressure taps for distributor pressure drop measurement. All dimensions are in mm.
600T series differential pressure transmitters. The minimum fluidization velocity (umf) without side air injection was determined by plotting average bed pressure drop against superficial air velocity as presented in Figure 2. The umf was in the order of 0.156 m/s, which corresponds to a volumetric air 11655
DOI: 10.1021/acs.iecr.5b02741 Ind. Eng. Chem. Res. 2015, 54, 11653−11660
Article
Industrial & Engineering Chemistry Research (along z-axis) and their vertical position can be fixed by locking pins. At a certain height (z), the source and detector traverse linearly parallel to the y-axis as shown in Figure 1a. Hence, the γ-beams travel through the bed parallel to the x-axis. This measurement technique provides the line-averaged void fraction over the length of the beam that crosses the inside area of the column. Thus, at a specific horizontal plane the void fractions values are obtained for different y/ R. Initially, the number of photons transmitted in 20 s through the empty column was detected. Five photon counts each of 20 s span were measured and the average of these readings was considered as Ia. After that the column was filled with 77 kg glass beads. The static bed height was 995 mm. The number of photons transmitted in 20 s through the static bed was measured and the average of five readings was taken as Is. After the air flow was started, similar measurements were done to determine If. The measurements of Ia, Is, and If were carried out at 19 different y/R locations at the interval of 12.5 mm for each vertical location.
4. RESULTS AND DISCUSSION In the present study, the superficial gas velocity was varied from 0.795 to 2.5umf. The ratio of the static bed height to the bed diameter was 3.98. The measurement of the time-averaged void fraction was carried out at three different heights, 400, 640, and 790 mm (h/Dt = 1.6, 2.56, and 3.16), respectively. The volumetric air flow rate through the nozzle was varied from 75 to 200 SLPM (0.1875−0.5Qmf). During gas injection the radial void fraction was measured at the nozzle plane (h/Dt = 2.56) and two different vertical locations (h/Dt = 1.6 and 3.16). Five repeat readings each for 20 s (counts/20 s) were taken at each measurement point and the average of these readings was considered for voidage estimation. The relative standard deviation ((standard deviation/mean) × 100) of five repeat readings was determined for all the measurement points (h/Dt, y/R) and the values were found to be
