5748
Ind. Eng. Chem. Res. 2006, 45, 5748-5754
Annular Downflow Layer in a Recirculating Fluidized Bed Munish K. Chandel and Babu J. Alappat* Department of CiVil Engineering, I.I.T Delhi, Hauz Khas, New Delhi, India 110 016
The effect of various operational and design parameters on the annular downflow layer thickness is studied experimentally in a recirculating fluidized bed. Inventory of solids, superficial gas velocity in the riser, particle size, axial height, and spacing between the riser bottom and the distribution plate are the various operational and design parameters considered in this study. A correlation between the annular layer thickness and the operating parameters is developed based on the experimental results. The correlation developed can be useful in the better understanding of the hydrodynamics of the recirculating fluidized beds. The results indicate that the annular layer thickness decreases with an increase in superficial gas velocity as well as with the axial position in the riser. The annular layer thickness increases with the increase in the spacing between riser bottom and distribution plate. The fine sand has a relatively stable annular downflow layer as compared to the coarse sand. 1. Introduction The flow in the riser of a circulating fluidized bed (CFB) under fast fluidization regime is a complex phenomenon. Research at industrial and laboratory scale shows the coreannulus flow structure in the riser of the CFBs. The particle agglomerates commonly known as “clusters” exist in the riser of a CFB. The core region of the riser has fewer clusters than the annulus, and these clusters mostly flow upward. However, the annular region of the riser is dominated by downflowing clusters, which have less velocity and large size as compared to the core region. The phenomenon of clustering is important as it affects the heat transfer rate and chemical interactions between gas and solids.1 The transfer of heat from bed to wall is very much influenced by clusters in the annular region.2 The cluster changes the gas-solid contact efficiency because of extensive backmixing of solids.3 Other aspects that will be affected by clusters are average slip velocity, mixing, flow pattern, and wall erosion.4-6 The downflowing annular region clusters form a layer known as annular downflow layer or annular film.7-10 These sheets fall in wavelike patterns along the wall of the riser, sometimes breaking and rejoining in the upflowing core region. Thickness of the annular film may be defined as the region adjacent to the riser wall in which the net flow is downward.11 The particle concentration in the annular layer is higher as compared to that in the core region. The boundary between the core and the annulus corresponds to the point where the net solid flux and the velocity of the core annulus flow are zero. Recirculating fluidized bed (RCFB) is a type of fluidized bed involving two concentric tubes. The smaller central tube carries the particles upwards with the fluidizing gas and the outer tube carries the free falling particles downwards. The flow regime in the riser of a RCFB can be the same as of a CFB. Basically, the riser of a RCFB can be operated in the same manner as that of the CFB. However, there are some differences in the overall configurations of both the reactors. The riser and the downcomer are concentric in the case of RCFBs.12,18 That is the reason some of the researchers called them internally circulating fluidized beds.13 Another major difference is in the air distribution to the riser. There exists a spacer section between the riser bottom * To whom correspondence should be addressed. Tel.: +91(11)2659 6254. Fax: +91(11)-2658 1117. E-mail:
[email protected],
[email protected].
and the perforated distribution plate so that the circulation of solids can take place in a RCFB. The riser of the RCFB contains core-annular flow structure, and downflowing annular layer can be seen on the wall of the riser depending upon the operating conditions. This flow pattern can be seen with the fine as well as the coarse particles. Although, there are a few attempts for the measurement of annular layer thickness in CFBs, there is no cited work on the annular layer in RCFBs. In the present work, annular layer thickness in the riser of a RCFB is measured. The annular downflow layer thickness, δ, is measured as the region adjacent to the riser wall where the net flow is downward. A frequency factor, f, is defined for signifying the time of occurrence of the annular layer in the riser. A mathematical correlation is developed between the annular layer thickness and the operational and design parameters on the basis of the experimental results. 2. Experimental Methodology The experimental setup was a cold model RCFB, semicircular in shape made of transparent Perspex. The riser was of 1 m height (Figure 1). Other dimensions of the reactor were as follows: inner diameter of the draft tube, 0.05 m; inner diameter of the downcomer, 0.15 m; air jet diameter, 0.032 m; free board diameter, 0.29 m; and spacings between the riser bottom and the perforated distribution plate, 0.03, 0.05, and 0.095 m. Air was used as the fluidizing gas, and sand particles of two different sizes (Geldart D and Geldart B) were used as the fluidizing media. The small-size particles are described as fine particles and the larger size are described as coarse particles in this paper. The properties of the fluidizing media are given in Table 1. Fluidizing air was supplied from an air compressor. Gas flow rate was measured by a gas rotameter and was regulated by a gate valve. Fluctuations in the air flow were regulated using a pressure regulator before the rotameter. U-tube manometers were used to measure the pressure difference through the riser and downcomer and between the riser bottom and downcomer bottom. The solid circulation rate was calculated by eq 1, in which the particle velocity in the downcomer Upd was calculated using tracer particles (measuring the time required by the particles to travel a known distance). Tracer particles were sand particles of the same size as that of the fluidized bed media colored with red dye.
Ws ) Upd(1 - ed)FsAd
10.1021/ie050456v CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006
(1)
Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006 5749
Figure 1. Schematic of experimental facility (recirculating fluidized bed). Table 1. Characteristics of Fluidizing Media fluidizing media Indian standard sand
coarse (Geldart D) fine (Geldart B)
sieve size passing (mm)
retained (mm)
density (kg/m3)
bulk density (kg/m3)
voidage
Umf (m/s)
1.700 0.600
1.400 0.425
2622.91 2606.03
1528.4 1503.4
0.412 0.42
1.00 0.21
A high-speed video camera was used for the annular layer thickness measurement. The technique is based on image processing and is fundamentally similar to that used by Shen et al.14 The logic behind the use of this technique is that the annular layers are having low velocity clusters and are moving downward, whereas the clusters as well as suspension in the core are moving upward with a very high velocity as compared to the annular layer. Because of the difference in the velocity of core and annular clusters, there is a difference in the threshold values in an image, which can be measured. Three axial positions, h ) 0.15, 0.50, and 0.75 m along the riser, were selected to get the complete picture of the annular layer in the riser. The video camera was focused in such a manner that it covered 0.10 m of the axial riser length. A scale was attached and marking was done on the concentric downcomer of RCFB for identification of the exact location by the video camera. On each axial measurement position, the video was recorded for 1 min for each operating condition. The video recorded was transferred to the computer through a video card interface. The frames were grabbed from the video using frame-grabber software. The annular layer thickness was measured in the grabbed frame by image analysis technique. The software used for the image analysis was “Image J”. This software could measure area and length by pixel counting. Calibration was done
by setting the known diameter of the riser equals to the number of pixels covered by it. Annular layer thickness in the grabbed frame was measured at three points for each case. For example, at h ) 0.15 m, annular layer thickness was measured on the points 0.10, 0.15, and 0.20 m of the riser. The total area covered by the annular layer in each frame was also measured. For the measurement of annular layer thickness, the numbers of video frames were optimized. Initially, 90 frames from 1 min video were selected for some of the experiments. It has been found that the deviation between the average value of annular layer thickness of 90, 60, and 30 frames was
