Elsevier

Powder Technology

Volume 343, 1 February 2019, Pages 49-57
Powder Technology

Analysis of the pressure fluctuation in the flow field of a large-scale cyclone separator

https://doi.org/10.1016/j.powtec.2018.11.007Get rights and content

Highlights

  • Inner vortex induce a dominant frequency in the vortex finder

  • Dominant frequency in the vortex finder changes with inlet gas velocity and particle concentration.

  • PSD of the outer vortex was divided into two parts.

  • The dominant frequency region was related to rotation of the outer vortex

  • There were special pressure transfer characteristics in the cyclone separator

Abstract

Flow field in a cyclone separator is not stable and show complex dynamic properties. Dynamic pressure signals were measured at different positions in a large-scale gas-solid cyclone separator such as the vortex finder, outer vortex and inner vortex, using a micro-differential pressure transmitter with different inlet gas velocities and inlet particle concentrations. Power spectral density (PSD) and fast Fourier transform (FFT) were used to analyze the characteristics of these dynamic pressure signals. The results showed that the PSD of the outer vortex was divided into two parts: part 1 in a low frequency region (0–n Hz) with high magnitude and part 2 in a high frequency region (n–50 Hz) with low magnitude. The dominant frequency region was related to outer vortex, and its range and magnitude increased with increased inlet gas velocity. There was a dominant frequency in the vortex finder of the cyclone separator, which was related to inner vortex rotation. The dominant frequency increased with increased inlet gas velocity and decreased with increased particle concentration. Using data processing methods, including an FFT filter and standard deviation, particle effects were found to be focused in lower frequency region parts, while the effects were not linearly depend on the particle concentration. Furthermore, there were special pressure transfer characteristics in the cyclone separator, which resulted in forming the pressure fluctuation. Through PSDs and the coherence analysis of the dynamic pressure in two positions in the vortex space of the cyclone separator, the outer vortex was found to induce a low frequency range pressure fluctuation with high magnitude and the inner vortex induced a new main frequency because of the periodically-rotating flow behavior. Considering the resonance phenomenon, the natural frequency of cyclone separator should be kept away from the frequency of pressure fluctuation, which concentrated below 50 Hz.

Introduction

Cyclone separators are widely used in many industrial fields, such as circulating fluidized bed and residue fluid-catalytic cracking. It is mainly used for cleaning gas streams and removing dust particles or for catalyst recovery. A swirling flow is formed in the cyclone separator after the particle-laden gas stream enters the cyclone tangentially, which centrifuges the particles toward the wall and thus enhances separation process.

Cyclone separators have been widely used for a long time (the first cyclone patent was granted to John M. Finch back in 1885). The performance of the cyclone separator have been improved largely in both structure and operation by virtue of their simple structure, with the characteristics of gas-solid flow in cyclone separators continuously studied for almost 100 years [1]. In most past studies, high separation efficiency and low pressure drop have been considered two important parameters, which have been improved by means of analyzing the flow field [2]. Most of the previous experimental work [[3], [4], [5], [6], [7], [8], [9]] and numerical simulations [[8], [9], [10], [11], [12], [13], [14]] have shown that the flow field in the cyclone is a highly turbulent three-dimensional swirling flow. Some basic characters about the three-dimensional velocity distribution and turbulence characteristics of the swirling flow were explained. However, these studies mainly focus on the distribution of the time mean velocity or the description of the steady flow field. However, although the time mean flow field in a cyclone separator has been extensively studied, the dynamic property research in this field still remained lacking. In the cyclone separator, the experimental study and numerical simulations have shown that the rotation center of the gas swirling flow is random distributed in the center region and the center line of the gas swirling flow along the axial direction of the cyclone has an oscillated profile [[11], [12], [13], [14]], which results in a fluctuation of the flow parameters, that is dynamic instability.

Since the fluctuation of the flow parameters results in a pressure fluctuation and velocity fluctuation at the same time, the dynamic property in a cyclone separator can be characterized by wall pressure measurements. As measurement of pressure is relatively easier to implement than measurements of the velocity and the particle concentration, the existing literature regarding dynamic characteristics of the gas solid two-phase flow fields in cyclone separators have been mainly based on the measurement and analysis of pressure fluctuations [15,16]. The pressure fluctuation can not only reveal the unstable characteristics of the flow field, which directly affect the separation and diffusion of particles, reduce the separation efficiency of cyclone separator, and ultimately affect the performance of the whole device, but also affect the equipment service life since the pressure fluctuation induces vibration of the equipment. Thus, the pressure fluctuation was the main measurement parameter adopted in this work.

A swirling flow is created in the separation space of a cyclone separator by the tangential injection of gas and forms an outer vortex that works as a quasi-free vortex in the outer region and an inner vortex that works as a quasi-forced vortex in the inner region. The flow patterns of these vortices make a large contribution to the unsteady flow field. There are some studies on the dynamic property of these vortices. Solero [4] performed a flow field measurement by LDV, which proved the existence of the fluid dynamic instabilities. Derksen [12,13] simulated the gas flow field in a cyclone separator by LES and observed the core of the main vortex moving about the geometrical axis of the cyclone in a quasi-periodic manner. As a result, the instantaneous velocity signal in a cyclone manifests itself as a quasi-periodically fluctuating signal and there is a peak in its velocity spectral analysis [10]. Carmona [17] has found that there exist two peaks in the power spectrum density (PSD) of the tangential velocity by means of a series of simulations, indicating that there are two kinds of fluctuation behaviors in the flow fields of the cyclone separator with a 180° spiral-case. The two fluctuation patterns were considered here to originate from the inner vortex flow and its periodic swing around its axis center, which was called a processing vortex core (PVC). However, their studies have lacked substantive experimental work. Gao [16] measured and analyzed wall pressures in a cyclone separator with a 300 mm inner diameter to investigate the gas solid two-phase flow fields in cyclone separators. He found that there exist two regions with different dominant frequencies,the outer vortex region and the inner vortex region. The pressure fluctuation frequency in the inner vortex is higher than that in the outer region. Gu [15] has studied the pressure fluctuation in different radial and axial positions in a cyclone separator, based on analytical methods of the probability density function and PSD. There were two dominant frequencies of the pressure fluctuation in the gas flow (~71 and 179 Hz). The former was related to the quasi-forced vortex and the latter related to the swing of the quasi-forced vortex core. Furthermore, when particles were fed, the particle strand formed on the wall and spiraled, which result in reduced the dispersion of pressure fluctuations, mitigating to some extent the core's off-axis phenomenon, and adding a dominant low frequency (0.31–0.86 Hz), which increased with the increased inlet solids loading.

However, few literature were reported on the experimental work on pressure fluctuation frequencies in the whole space of the cyclone separator, especially for an industrial-scale cyclone separator. Therefore, it was necessary and meaningful to study the dynamic characteristics of the flow field in a cyclone separator. On the other hand, it is reported that the separation performance and the flow field characteristics will vary due to the variation of the size of the cyclone separator [[18], [19], [20], [21]], thus the study of the pressure fluctuation in the large size cyclone separator is also very meaningful and innovative, compared with other studies on the small size equipment.

In this study, the pressure fluctuation signals along each axial position of an industrial-scale cyclone separator were measured using a multichannel dynamic pressure transducer. The pressure fluctuation characteristics were also analyzed in terms of the average pressure, the standard deviation, the power spectral density, and the FFT filter methods, which helped extend the understanding of the dynamic and unsteady flow behavior properties in cyclone separators.

Section snippets

Experimental system

A schematic diagram of the experimental cold-flow large-scale cyclone separator system is shown in Fig. 1. The cyclone separator's barrel diameter with a tangential inlet was 1236 mm and its total length was 12,033 mm. The internal diameter of the vortex finder was 406 mm and the cyclone inlet tube had a rectangular cross-section 719 mm in height and 320 mm in width. The internal diameter and the length of the dipleg were 153 and 2800 mm, respectively. Its main dimensions are listed in Table 2,

Static pressure distribution in the cyclone separator

The gas fluctuations in the cyclone separator were continuously recorded by eight micro-differential pressure transmitters. The distribution of the average pressure along the separator's axial direction under different inlet gas velocities showed that (Fig. 2), along the gas-flow direction, the negative pressure was small in the inlet pipe and in the cylinder (z = 10,973 and 10,216 mm, respectively) significantly decreased from the cone to dipleg (z = 7372 and 4760 mm, respectively), and then

Conclusions

In this paper, pressure fluctuations induced by a gas-solid two-phase flow were experimentally investigated in an large-scale cyclone separator. The main conclusions are summarized below.

  • 1.

    PSD of the pressure signal revealed a dominant frequency in the vortex finder, which was related to the inner vortex. The dominant frequency also increased with increased inlet gas velocity and decreased with increased particle concentration.

  • 2.

    PSD of the outer vortex was divided into two parts, a low frequency

Acknowledgements

The authors acknowledge the supports by the National Natural Science Foundation of China (Grant No. 21776305, 21566038) and the Science Foundation of China University of Petroleum-Beijing At Karamay (No. RCYJ2016B-02-002).

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