Statistical and frequency analysis of pressure fluctuation in an annular spouted bed of coarse particles

doi:10.1016/j.powtec.2017.05.007

Powder Technology

Volume 317, 15 July 2017, Pages 216-223

https://doi.org/10.1016/j.powtec.2017.05.007 Get rights and content

Highlights

•
Peaks in PSD of pressure fluctuation in annular spouted bed were first examined.
•
Peak frequencies and behavior of bubbles were observed in good agreement.
•
Peaks in PSD are corresponded to bubble generation, eruption and bed oscillation.

Abstract

The pressure fluctuation characteristics of an annular spouted bed filled with coarse particles were studied by statistical and frequency analysis. The effects of spouting gas velocity, static bed height, and measuring point height on pressure fluctuation standard deviation and major frequency were systematically examined. The peak frequencies and the behavior of bubbles in the gas-solid spouted bed were observed. Experimental results show the unique features of the novel spouted bed filled with coarse particles: the pressure fluctuation standard deviation increases with the increasing of spouting gas velocity and static bed height; however, it decreases with the increasing of measuring point height. For U_g < U_ms, two distinct peaks with one around 0.5 Hz and the other around 5.5 Hz are shown; while for U_g > U_ms, three peaks with the highest one at around 4 Hz can be observed in the spectra, and the major frequency decreases apparently with the increasing of spouting gas velocity, and also decreases with the increasing of static bed height or measuring point height. The peaks in the power spectral density (PSD) of pressure fluctuation are in good agreement with the observed behavior of bubbles in the annular spouted bed. The results would be helpful for the identification of flow pattern in the annular spouted bed of coarse particles.

Graphical abstract

Introduction

The conventional spouted bed (CSB) is a gas-particle contactor in which the gas is introduced through a single nozzle at the center of a conical or flat base [1]. As an alternative to fluidized beds for handling coarse particles, spouted bed technique has found application in many industrial processes such as drying of granular materials, blending of polymer chips, coating of tablets, and granulation of fertilizers and other materials [2], [3], [4], [5], [6], [7]. However, along with the in-depth study of CSBs, some crucial shortcomings of the CSBs - such as low annulus aeration, slow solids turnover, and bad mixing effect - were gradually recognized by many researchers [8], [9], [10], [11], [12], and many improved designs for spouted bed have been proposed and studied [13], [14], [15], [16], [17], [18], [19], [20]. These improvements are concerned with changes in the vessel geometry, spouting operation and mechanism, air supply, etc. Compared with the fluidized bed, these improved spouted beds have high efficiency in gas-solid contact, large capacity for handling coarse particle, stable operation in a wide range of gas flow rates, small bed pressure drop, little fluidizing air flow rates, and so on [21], [22], [23], [24], [25], [26], [27], [28], [29].

Based on the conventional cylindrical/square spouted bed, a novel annular spouted bed with multi-air-nozzles has been proposed by our research group [30], [31]. Compared with the CSBs, this annular spouted bed has many advantages including excellent mixing effect, high heat transfer rate of air-solids, high particle load of unit air, and being matched to the recuperative heat exchanger. In previous research, some hydrodynamic characteristics of this annular spouted bed were obtained [32]. However, other spouting characteristics, such as bubble action, gas diffusion, bed pressure fluctuation behavior and even scale-up criterion, need to be further investigated.

Pressure fluctuation signals of gas-particle contactors contain much dynamic information. These signals, which consist of stochastic and deterministic components, are obviously periodic, and can reflect the information of particle property, geometric property and bubble action. Recently, with the development of signal processing technology, much research focusing on the flow behavior in fluidized or spouted beds have utilized time series analysis on the pressure fluctuation signals [33], [34], [35]. Piskova et al. [36] systematically studied the pressure fluctuations of a spouted bed under atmospheric conditions. Different fluidization regimes in spouted beds have been characterized by the analysis of pressure fluctuations signals. Mollick and Sathiyamoorthy [37] used standard deviation and power spectral densities of time-series pressure data to assess the stability of a spouted bed. Mostoufi et al. [38] examined the hydrodynamics of a conical spouted bed by analyzing pressure fluctuation signals in the frequency domain. It was found that, the peaks in PSD of pressure fluctuations correspond to the movement of bulk of solids (at low frequency, < 5 Hz), particle transport in the spout (at medium frequency, between 5 and 15 Hz), and clustering and motion of clusters throughout the bed (at high frequency, between 15 and 130 Hz) respectively. Xu et al. [39] employed statistical and frequency analyses of the differential pressure fluctuation signals to recognize and characterize the flow regimes in spouted bed filled with 1–2 mm particles. However, very few literature showed the application of time series analysis on a spouted bed of coarse particles (5–6 mm in mean diameter) [40]. It is believed that, due to the differences in size and voidage, the flow characteristics of the coarse particles should be different from that of those fine particles. The studies on the spouted bed of coarse particles by time series analysis of pressure signals are required.

In this research, coarse particles are selected as testing materials. A multi-channel differential pressure signal acquisition system is used to record the pressure fluctuations in an annular spouted bed under the different operating condition (spouting gas velocity, static bed height, and measuring point height). The statistical analysis and frequency analysis of time series pressure fluctuation signals are also presented in this paper.

Section snippets

Time series analysis of pressure fluctuation signals

Time series signals can be analyzed in three different ways: statistical analysis, frequency analysis, and chaotic analysis. In this paper, the former two methods were employed.

Experiments

The experiments were carried out in the annular spouted bed with multiple air nozzles already described in a previous work [30]. The experimental set-up consists of an air supply system, a feeding system, an annular spouted bed, and a signal acquisition system (shown in Fig. 1). The annular spouted bed includes two homocentric upright cylinders, eight nozzles and an air chamber, the detailed description of its functional architecture was given in our previous work [30], [31].

The spouting air is

Pressure drop

The determination of pressure drops was carried out as follows: Firstly, obtain the overall pressure drop between a measuring point at a given height and the air chamber below the nozzle; then, subtract the nozzle pressure drop from the overall pressure drop. The nozzle pressure drop at various spouting gas velocities was measured with the empty bed.

Fig. 4(a) shows the total bed pressure drop in the freeboard region at different spouting gas velocities. With the increasing of spouting gas

Conclusions

In this paper, the coarse particles, 6 mm in mean diameter, are adopted as the bed materials. A multi-channel differential pressure signal acquisition system was used to record the pressure fluctuations in the annular spouted bed under different operating condition (spouting gas velocity, static bed height and measuring point height). The pressure fluctuation characteristics were studied by statistical and frequency analysis. It was found that the time traces of differential pressure fluctuation

Nomenclature

height, mm

H_b

height of measuring point above the nozzle outlet, mm

H_g

distance of measuring point away from the inlet of nozzle, mm

H_s

static bed height, mm

sampling frequency, Hz

length, mm

U_s

spouting gas velocity, m·s^− 1

U_ms

minimum spouting gas velocity, m·s^− 1

width, mm

ρ_b

bulk density, kg·m^− 3

ρ_s

particle density, kg·m^− 3

voidage, [1 − (ρ_b/ρ_s)]

Acknowledgements

We gratefully acknowledge financial support by the National Natural Science Foundation of China (51476100).

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Statistical and frequency analysis of pressure fluctuation in an annular spouted bed of coarse particles

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Time series analysis of pressure fluctuation signals

Experiments

Pressure drop

Conclusions

Nomenclature

Acknowledgements

Chem. Eng. J.

Powder Technol.

Chem. Eng. Process.

Chem. Eng. Process.

Chem. Eng. Process.

Chem. Eng. Sci.

Powder Technol.

Chem. Eng. J.

Chem. Eng. J.

Chem. Eng. Sci.

Powder Technol.

Chem. Eng. Sci.

Powder Technol.

Powder Technol.

Powder Technol.

Powder Technol.

Exp. Thermal Fluid Sci.

Chem. Eng. Sci.

A technique for contracting gases with coarse solid particles

AICHE J.

Mixing behaviour of solids in multiple spouted beds

Can. J. Chem. Eng.

Combustion of rice husk in a multiple-spouted fluidized bed

Energy Sources

Stability of slot-rectangualr spouted beds with multiple slots

Can. J. Chem. Eng.