Elsevier

Powder Technology

Volume 360, 15 January 2020, Pages 112-119
Powder Technology

Mass flow rate measurement of bulk solids based on microwave tomography and microwave Doppler methods

https://doi.org/10.1016/j.powtec.2019.09.087Get rights and content

Highlights

  • Developed a novel microwave method for mass flow rate measurement of bulk solids.

  • Applied microwave tomography imaging to visualize and obtain solid concentration.

  • Applied microwave Doppler and novel data analysis for average velocity measurement.

  • Established a model for mass flow rate measurement of bulk solids in a flow rig.

  • Verified the accuracy of the novel method through laboratory experiments.

Abstract

Accurate mass flow rate measurement of multiphase flows is a major challenge in the process industry. In this paper, a microwave tomography sensor for the concentration measurement and a microwave Doppler sensor for the velocity measurement are presented for the determination of mass flow rate of gas-solid flows in a pipeline. The method of approach including data acquisition, image reconstruction and short-time FFT is described. With the technique, the concentration of bulk solids in the flow is measured and visualized using the microwave tomography sensor. Based on the velocity and concentration of bulk solids, a time varying mass flow rate can be calculated. The experimental study shows that the mass flow rates measured with the microwave sensors agree well with their actual values. The test results are also presented.

Introduction

Pneumatic transportation of bulk solids has been widely used in many applications, such as energy, food, medicine and chemistry, due to its advantages of nonintrusive, convenience, high-speed and automation [1]. As the most important evaluated parameter of the transportation, the mass flow rate of bulk solid flows plays an important role in measurement and control systems, which is determined by accurate flow velocity and concentration measurement of solids.

Tomography is imaging by sectioning, through the use of a penetrating wave or field. In broad terms, it is a technology to get target features, such as concentration and distribution by a non-contact or non-invasive means. Currently electrical capacitance tomography (ECT) [[2], [3]] and electric resistance tomography (ERT) [[4], [5]] are the common methods for industrial tomographic imaging. However, ERT is not suitable for the imaging of pneumatic transportation. Besides, both ERT and ECT have low image resolution. On the other hand, microwave tomography (MWT) which has a wide range of frequency can produce images of higher resolution than ECT and ERT. MWT can also be used to measure materials with high permittivity and conductivity, making it attractive for a range of industrial imaging applications, including industrial process monitoring and concentration measurements [[6], [7], [8], [9], [10], [11]], the comparisons of ERT, ECT and MWT are shown in Table 1.

To meet the needs of industrial production, a large number of velocity meters such as spatial filtering method, cross correlation method, laser Doppler method [12] have been introduced over the past decades [13,14]. Among them, the spatial filtering method is very complex. The cross correlation method has a poor spatial sensitivity with restricted signal processing and laser Doppler equipment is expensive. Therefore, it is urgent to develop a new method for the velocity measurement and analysis of bulk solid flows. In recent years, with the rapid development of microwave technology, remarkable progresses with diversified microwave sensors have been made for industrial applications [15,16]. Besides, Doppler method offers non-intrusive point velocity measurement with fine temporal resolution and high accuracy. With microwave Doppler sensors, consistent real-time velocity measurements can be achieved. The comparison between laser Doppler and MWD is shown in Table 2. Based on analysis and comparison above, a microwave Doppler (MWD) sensor is adopted to measure the flow velocity of solids in this investigation as a part of the mass flow rate measurement system [17].

The paper is organized into 5 sections. Following the introduction in Section 1, the test facility, MWT sensor and MWD sensor used in the study are described in Section 2. In Section 3, tomography measurements of bulk solids are presented, including the analysis of solid concentration and different flow regimes from the images obtained. Besides, average velocity measurement of bulk solids is described. In Section 4, the measured results of mass flow rate of bulk solids using MWT sensor and MWD sensor are presented and the errors are analyzed. Conclusions are given in Section 5.

Section snippets

The method of approach

The mass flow rate is the mass of a substance which passes per unit of time. The mass flow rate M for the bulk solid flows can be estimated using the “velocity-concentration” method, i.e.,M=AυβSρwhere A is a system's constant, υ is the velocity of bulk solid flows, β is the solid concentration value, S is the cross sectional area of pipe, and ρ is the density of bulk solids.

Thus, the measurement of mass flow rate is converted to the measurement of velocity and concentration of bulk solid flows.

Measurement and results

In this section, the applicability of the proposed MWT sensor for imaging of bulk solids is investigated by conducting measurements and reconstructing the relative permittivity distribution. Images of solid particle flow with different types of flow regimes can be reconstructed quantitatively. Besides, MWD sensor is used to measure the velocity of bulk flows.

Mass flow rate measurement and error analysis

To show that microwave sensors can be applied to bulk industrial transportation, a MWD sensor is used to measure the average flow velocity of bulk solids and a MWT sensor is used to measure the concentration of bulk solids in the flow with the measurement facility shown in Fig. 1.

The wind speed is set at 20 m/s. Fig. 9 shows the average velocity υ(t), concentration values β(t) and the product of υ(t), β(t), S and ρ against time over a period of 40 s. At first, there is no solid flowing in pipe,

Conclusions

A new mass flow rate measurement and analysis method for bulk solids based on a MWT sensor and MWD sensor has been presented. The experiments carried out show that MWT can be used to measure concentration of bulk solids and particle distribution can be observed visually over the cross section. Besides, the average velocities of bulk solids can be measured with the MWD principle and short-time FFT. At last, the mass flow rate and the total mass of bulk solids can be calculated using the proposed

Acknowledgments

The authors would like to thank the National Natural Science Foundation of China (No. 61320106004) and CAS Interdisciplinary Innovation Team for their supports. The authors thank all the staff of the RF & Microwave Technology Research Center, WUT for their support throughout the experiment and for their comments.

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