Geometric and operational optimization of 20-kHz probe-type sonoreactor for enhancing sonochemical activity

https://doi.org/10.1016/j.ultsonch.2020.105065Get rights and content

Highlights

  • Effects of geometric and operational factors were investigated in a 20-kHz sonicator system.

  • Optimal conditions for the highest sonochemical activity were suggested.

  • Formation of circular-shaped active zone is required for the highest sonochemical activity.

Abstract

The use of a 20-kHz probe-type sonicator irradiating downward in a 500 mL vessel was optimized for the enhancement of the sonochemical activity in terms of the geometric and operational factors. These factors included the probe immersion depth (the vertical position of the probe), input power, height of the liquid from the bottom, horizontal position of the probe, and thickness of bottom plate The sonochemical oxidation reactions were investigated both quantitatively and qualitatively using calorimetry, KI dosimetry, and luminol (Sonochemiluminescence, SCL) techniques. The sonochemical activity was very positively affected by the vertical boundaries. The highest sonochemical activity was obtained when the probe was placed close to the bottom of the vessel (immersion depth of 60 mm), with a high input power (input power of 75%), and optimal liquid height condition (liquid height of 70 mm). The SCL image analysis showed that the cavitational activity zone gradually expanded around the probe body and changed into a circular shape as the experimental conditions were optimized, and consequently the sonochemical activity increased. The formation of a large bright circular-shaped activity zone could be attributed to the strong reflections of the ultrasound firstly, at the vessel bottom and secondly, at the liquid surface. On the other hand, the cavitational activity zone and the sonochemical activity were negatively affected by the horizontal boundaries when the probe was placed close to the side wall of the vessel. In addition, it was found that the sonochemical activity was also significantly affected by the thickness of the support plate owing to the reflection and transmission of the ultrasound at the boundary between the liquid and the solid media.

Introduction

A probe-type sonicator is one of the most commonly used laboratory devices for the development of acoustic cavitation [1], [2], [3], [4], [5]. The probe-type sonicator works by irradiating ultrasound at the end of a tip with a small area. As a result, a highly concentrated cavitational activity zone is formed just under the tip. A strong spray is also generated from the tip through a concentrated ultrasound irradiation, which can induce a violent macro-scale agitation of a liquid in a reactor [6], [7]. The previous studies in this area have successfully proved the remarkable performance of ultrasound in diverse processes including oxidation/degradation [6], [8], [9], emulsification [10], [11], extraction [12], exfoliation/dispersion [13], [14], and precipitation [7] that prefer using sonochemical and sonophysical effects to conventional technologies.

To enhance and optimize the cavitational activity, ultrasonic factors, namely frequency, input power, and continuous/pulse mode, as well as solution factors, namely temperature, pH, dissolved ion, and dissolved gas have been mainly studied in the field of sonochemistry [15], [16], [17]. Recently there has been a growing interest in geometric factors for the optimal design of industrial-scale sonoreactors. Some previous researchers reported that small variations in a sonoreactor geometry could change the cavitational activity significantly owing to the deformation of cavitational activity zone, specifically in bath-type sonoreactors [15], [17], [18], [19], [20].

The effect of geometry on cavitational activity has also been investigated in a probe system. The acoustic pressure distribution was numerically simulated under various geometric conditions that included the probe position and, vessel shape, and an optimal condition was suggested considering the calorimetric energy generation, nanomaterial dispersion, and aluminum foil erosion [4], [13]. The degree of emulsification, flow rate, sound intensity, chemical reaction/degradation, and precipitation were analyzed sonophysically and sonochemically under different probe immersion depth conditions [5], [6], [7], [10], [11], [14]. In addition, Ye’s group reported different trends for chemical degradation and extraction processes under the same probe immersion using the same probe system [8], [12].

Although some simulations have been reported on sound pressure distribution [4], [13], the optimization of a cavitational activity zone by altering the probe position and operational conditions that are directly related to the sonochemical activity, has been barely investigated in the previous studies. We briefly reported the significant enhancement of a cavitational activity zone and its sonochemical activity in our recent research using the probe system [5].

In this study, the cavitational activity under various applicable experimental conditions using a 20 kHz-probe system was investigated as one of the basic steps for understanding the various sonochemical and sonophysical applications of the 20 kHz-probe system. The experimental conditions included the probe immersion depth, input power, liquid height from the bottom, horizontal position of the probe, and thickness of the bottom plate. The calorimetry and KI dosimetry were used to quantitatively compare the cavitational activity, as the aforementioned conditions were varied. The luminol method was used to visualize the cavitational activity zone.

Section snippets

Chemicals

Potassium iodide (KI) and sodium hydroxide (NaOH) were acquired from Junsei Chemical Co., Ltd. (Tokyo, JPN). Luminol (3-aminophthalhydrazide) was acquired from Sigma–Aldrich Co. (St. Louis, USA). All chemicals were used as received.

Experimental setup

A 20-kHz horn-type sonicator (VCX–750, Sonics & Materials Inc., USA), equipped with a threaded-end type probe and a replaceable tip of 13 mm diameter, made of a titanium alloy was used in this study. The probe was submerged in a 500 mL circular glass vessel (a

Effect of probe immersion depth

To understand the sonochemical oxidation in the 20-kHz sonicator system, the effect of the probe immersion depth in the vessel, which is one of most commonly applicable experimental conditions, was investigated using KI dosimetry and the SCL method. Fig. 2 shows the electrical power (Pelec), calorimetric power (Pcal), and concentration of the triiodide ion (I3), which is the final product of the sonochemical oxidation in the KI dosimetry, at each probe immersion depth from 10 to 60 mm. It

Conclusions

The effect of geometric and operational factors, including the probe immersion depth, input power, liquid height from the bottom, horizontal position of the probe, and thickness of bottom plate on sonochemical reactions was investigated in a 20-kHz probe-type sonicator system. Some guidelines for the optimal use of the probe-type sonicator irradiating downward in a 500 mL vessel for sonochemical oxidation reactions were suggested using the results of this study. Firstly, the highest

CRediT authorship contribution statement

Younggyu Son: Conceptualization, Validation, Methodology, Writing - original draft, Writing - review & editing, Visualization, Supervision. Yunsung No: Methodology, Investigation. Jeonggwan Kim: Methodology, Writing - review & editing.

Acknowledgments

This work was supported by the National Research Foundation of Korea [Grant No. NRF-2018R1D1A1B07048124].

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