Development and application of a novel swirl cyclone scrubber: (2) Theoretical

doi:10.1016/j.jhazmat.2008.08.023

Journal of Hazardous Materials

Volume 164, Issue 1, 15 May 2009, Pages 315-321

https://doi.org/10.1016/j.jhazmat.2008.08.023 Get rights and content

Abstract

The swirl cyclone scrubber analyzed in this paper is a novel aerosol filtering device in which a uniflow cyclone and a scrubber are combined. Systematic experiments showed that the swirl scrubber is a promising device that has minimal installation, operational, and maintenance costs. In this article, theoretical analyses are developed for the swirl cyclone scrubber. The dependency of particle collection efficiency on the design and operating parameters observed by experiments is explained using a theoretical parameterization to provide guidelines for optimal design and operation of the device. Discussion on possible variations of the swirl cyclone scrubber is also presented based on the theoretical parameterization developed.

Introduction

Cyclones are very useful pre-cleaning devices that can effectively remove airborne particles larger than 10 μm in their aerodynamic diameter. Cyclones are widely used because of their low costs of installation, operation and maintenance. Their particle collection efficiency, however, rapidly decreases with decreasing particle size [1], [2]. High collection efficiency for small particles can only be obtained with significant pressure drop and operation cost increase. Contamination of inner wall by collection of adhesive particulates such as tars is another disadvantage of cyclones.

Wet scrubbers have been widely used for removing gaseous and particulate air pollutants with significantly reduced risks of fire, explosion and erosion [3], [4], [5]. Conventional wet scrubbers often suffer from clogging and fouling problems by salt formation at the tip, the inside and outside of the nozzles, the tubes and the walls of scrubbers. Another drawback of conventional wet scrubbers is high pressure drop resulting from the improvement of collection efficiency [6].

Cyclone scrubbers have been developed to enhance the particle removal efficiency of conventional cyclones by adding the wet scrubbing effect. Although cyclone scrubbers have shown better particle removal performance than conventional cyclones [7], [8], they still share the same problems with conventional cyclones such as contamination by adhesive particles.

We have developed a novel aerosol filtering device, the swirl cyclone scrubber (swirl scrubber hereafter) that mainly consists of a cyclone and a swirl scrubber with a rod impact plate and swirl plates (Fig. 1), to overcome the many drawbacks of conventional cyclones and wet scrubbers. The lower zone of the device is a uniflow tangential-inlet cyclone. Aerosol particles entering the device are collected on the cyclone wall by centrifugal force. The upper zone was designed to be a wet scrubber in which water fed through a nozzle impinges on the impaction plate, generating water spray. Particles are removed by collisions with drops from the spray. Water drops deposited on the scrubber wall form a water film flowing down along the wall, which provides more adhesive deposition surface for particles collected by centrifugal force in the cyclone zone. The film also prevents contamination of the scrubber wall by sticky or corrosive particulates. Placed between the lower and upper zones is a swirl plate that not only sustains the swirling air flow, but also maintains a thick water layer on it. This, which is the most important difference from other cyclone scrubbers, provides another chance for aerosol particles to be filtered when particle-laden air passes through the water layer, a process called pool scrubbing. For more detailed information on the design and operation of the swirl scrubber, one can refer to the sister paper of this article [9].

Systematic experiments showed that the swirl scrubber is a promising air filtering device with minimal installation, operational, and maintenance costs [9]. It is expected to be particularly useful when one needs to control adhesive and/or corrosive particulates. One possible drawback of the swirl scrubber is relatively large water consumption and the need of waste-water treatment. Recycling of water, however, is expected to be able to reduce this problem significantly. In this article, theoretical analyses are developed for the swirl scrubber. The dependency of particle collection efficiency on the design and operating parameters observed by experiments are explained theoretically to provide guidelines for optimal design and operation of the device.

Section snippets

Experiment

The experiment for development and evaluation of the swirl scrubber is briefly summarized here although the detailed information is reported in the sister paper [9].

The swirl scrubber (Fig. 1, Fig. 2) consists mainly of a cyclone, swirl plates, a scrubber, feeding and circulation devices of scrubbing medium, and demister. A gas stream containing particulates enters the cyclone with tangential direction through the vane attached on the bottom part of the swirl scrubber. After the gas stream

Particle collection theory

The swirl scrubber analyzed in this article can be regarded as a serial combination of a uniflow cyclone, a pool scrubber, and a wet scrubber (Fig. 1). When particle collection by those three devices takes place independently in series, the combined collection efficiency can be expressed as $η = 1 - (1 - η_{c}) (1 - η_{p}) (1 - η_{w}) .$

Impact of operating conditions on collection efficiency

Among the parameters needed to calculate the collection efficiency of the swirl scrubber shown in the previous section, the scrubber dimensions, air stream flow rate, pool scrubbing layer thickness, water drop diameter, and water drop volume fraction are controlled by design and operational conditions. Smaller device size is preferable for higher performance of the cyclone, but limits the flow rate of the air stream that the device can treat. On the other hand, a larger device has higher wet

Analyses of experimental results

Fig. 4 compares the collection efficiencies obtained for a swirl plate angle of 15°, a nozzle diameter of 7.5 mm, a water flow rate of 17.5 lpm, and a nozzle pressure of 0.7 kgf/cm² with the theoretical parameterization developed in Section 3. ‘Cyclone (experiment)’ represents the experimentally obtained collection efficiencies without applying scrubbing medium. ‘Total (experiment)’ represents the experimentally obtained collection efficiencies with applying scrubbing medium. ‘Scrubber

Discussion on possible variations

In our experiments, rod impaction plates were designed as shown in Fig. 6, installed, and tested to further reduce water drop size while all other conditions were left unchanged. Fig. 7 shows the results together with the best-fit theoretical parameterization curves. In this figure, the legend “without rod” represents Case II shown in Table 1, while “with rod” refers to the new test case with addition of the rod impaction plate. The best fit was obtained with D_d = 0.5 mm and α = 1.5E−3 for the new

Conclusions

Theoretical analyses have been carried out for the swirl scrubber. The particle collection efficiencies obtained from experiments under various design and operating parameters were explained theoretically to provide guidelines for optimal design and operation of the device. Under the assumption that cyclone filtering, pool scrubbing, and wet scrubbing take place independently in series, a theoretical parameterization has been developed. The suggested parameterization adequately explained the

Acknowledgements

This study was funded by ECO-TSC through the Industrial Corporation of the University of Ulsan, Ulsan, Korea. The authors wish to acknowledge the excellent assistance provided by Ju-Hyuck Lee and Jung-Gi Kim who generously helped us conduct the performance tests of the NSCS system.

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View full text

Development and application of a novel swirl cyclone scrubber: (2) Theoretical

Abstract

Introduction

Section snippets

Experiment

Particle collection theory

Impact of operating conditions on collection efficiency

Analyses of experimental results

Discussion on possible variations

Conclusions

Acknowledgements

Powder Technol.

J. Hazard. Mater.

J. Hazard. Mater.

J. Hazard. Mater.

Sep. Purif. Technol.

Sep. Purif. Technol.

J. Aerosol Sci.

J. Hazard. Mater.

J. Aerosol Sci.

Particulate control

Air Pollution: Its Origin and Control

The cyclone scrubber—a high efficiency wet separator

Chem. Eng. Technol.

Effect of cyclone dimensions on gas flow pattern and collection efficiency

Aerosol Sci. Technol.

Gravity and centrifugal separation

Ind. Hyg. Q.

Collection efficiency of cyclone separators

AIChE J.