Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation
Introduction
Of the various modes of generating cavitation, acoustic and hydrodynamic cavitation have retained the interest of researchers due to their ease of use and ability to generate conditions suitable for different physical and chemical transformations [1]. The use of ultrasound to generate chemical changes in liquids is well established [2]. Ultrasonic cavitation has been proposed as a method for the intensification of reaction in many applications. Mass transfer applications include mixing, emulsification, extraction, impregnation, filtration, precipitation, surface cleaning, solid disruption and degassing [1]. Biological disruption has been applied in water and wastewater treatment [3], [4], food processing [5], [6], disinfection [7], and sterilization [8].
Difficulties in scaling up ultrasonic reactors, and possible energy efficiency gains, have led to the study of hydrodynamic cavitation as a new generation gas–liquid reactor with potential for application in process industries. Hydrodynamic cavitation has been shown to be more efficient for large scale operation than ultrasonic cavitation in chemical processes such as fatty acid hydrolysis [9]. Furthermore, it has been deployed in other areas, such as the preparation of bio-diesel from soybean oil [10], fine particle flotation for separation [11], biological disruption [12], [13], wastewater treatment [14], the preparation of nanostructured catalytic materials [15], chemical synthesis [1], and as an alternative to conventional agitation [16].
The oxidation of iodide to iodine has been used as a test reaction for the effect of ultrasonic and hydrodynamic cavitation since the late-1920s. Weissler et al. [17] discovered that, in the presence of ultrasonic cavitation, the yield of iodine was linearly proportional to time of ultrasonic irradiation but relatively independent of the initial potassium iodide concentration. They proposed that the iodine produced indicated the extent of another more fundamental reaction which produces oxidising agents, and suspected this was ultrasonically produced H2O2 which oxidised KI (reaction (1)). This reaction is generally known as the Weissler reaction.
Subsequent work [18] has shown that this mechanism was more complex, but the essential proposition, that iodide oxidation is an indicator of the presence of another oxidising agent was correct. This discovery led to this so-called Weissler reaction being widely used for the study of the extent and efficiency of ultrasonic cavitation, and later of hydrodynamic cavitation. It has been used extensively to compare different ultrasonic reactor configurations and to construct a theoretical base for the subject.
It is now known that the high temperatures and pressures generated in a bubble cavity ‘hot spot’ during its collapse results in complex chemistry which causes the homolytic dissociation of H2O to form OH radicals (designated here as OH). These radicals react with the iodide ion, through a series of intermediate steps [18], [19], to form the tri-iodide complex . The key reactions are believed to be:
Hydrogen peroxide is formed by OH radicals:
In neutral or alkaline solutions, H2O2 can scavenge [18], [19]:
Confusingly, much of the literature uses the terms and iodine interchangeably, often referring only to iodine measurements, or switching between the two in the same paper. This paper refers to for our own work, but when referencing a paper uses the term adopted by the authors.
In early experiments it was found that iodine was produced only when iodide solutions contained dissolved air while other experiments obtained reaction rates five times faster when carbon tetrachloride was present [17]. Hart and Henglein [20] showed that H2O2 is the product of sonolysis. When KI was subjected to ultrasound the production of approached a maximum at higher initial iodide concentrations. When ammonium molybdate was added before ultrasound, the yield was higher and independent of iodide concentration (reaching the H2O2 yield when pure water is irradiated). The ammonium molybdate catalysed reaction (1).
Gutiérrez et al. [18] looked at formation and the effect of initial I− concentration on the amount of H2O2 produced. In a buffered solution of KI (pH 5.9) the total yield of oxidised products (H2O2 and ) was independent of iodide concentration, and equal to the H2O2 yield in pure water. The amount of H2O2 decreases with increasing iodide concentration. They also mentioned previous studies where iodine was formed only when iodide concentrations exceeded that of H2O2.
For a pH above 8, air-saturated KI solutions produced a lower yield due to reaction (8) [19]. Below pH 4, yields increased markedly due to iodide oxidation by oxygen gas. Without pH control, their 0.1 M solution had a pH of 6.5 after 200 s.
The iodine liberation was found to be directly proportional to the formation of hydroxyl radicals [21]. The radicals are generated under certain conditions favourable to the pyrolysis of water; above a threshold pressure inside the collapsing cavities. Beyond this pressure iodine liberation increases linearly as the pressure pulse from the ultrasonic horn increases.
Kirpalani and McQuinn [22] examined the effect of temperature control. With temperature control, the yield increased asymptotically to a maximum yield after 20 min. Without, the iodine yield was higher but the reaction kinetics more unstable. They proposed that cooling of the KI solution might be advantageous to optimizing the cavitation intensity in high frequency reactors.
Koda et al. [23] compared 11 different ultrasound devices and three different chemical reactions; the Fricke reaction, KI oxidation, and decomposition of porphyrin derivatives. For ease of use they proposed the use of KI oxidation of 0.1 M KI solution as a standard method to calibrate the sonochemical efficiency of an individual reaction system.
The effect of geometry and other critical parameters on cavitational yields is not well understood, though the Weissler reaction has been used to conclude that the ultrasonic reactor configuration was a significant factor in overall reactor efficiency [8]. Gogate et al. [24] found that variation between sample vessels could contribute significantly to the yield and showed the presence of acoustic emission peaks seen only in cavitating liquids. Various workers [25], [26], [27] have demonstrated the importance of the geometrical nature of the ultrasound reactor with parameters such as probe position and reactor dimensions influencing the efficiency of ultrasound cavitation.
The recent development of interest in hydrodynamic cavitation is reflected in the scarcity of its bibliography. Much of the work to-date has used yield as an indicator of cavitation intensity, adapting ultrasonic cavitation methodology to hydrodynamic cavitation. Suslick et al. [15] and Pandit et al. [16] reported that the effect of some experimental parameters was the same for both hydrodynamic and ultrasonic cavitation. Yields increased linearly with pressure over a minimum threshold, and decreased with increased solution temperature. yield increased 20 fold in the presence of CCl4.
Vichare et al. [28] and Senthil Kumar et al. [2] reported similar results, and also found that orifice plate geometry considerably affected iodine liberation. Senthil Kumar and Pandit [29] developed a hydrodynamic cavitation model that compared favourably with experiments using KI decomposition, including showing the possible existence of some optimal operating conditions. Senthil Kumar et al. [2] found an optimal operating pressure to maximize the hydrodynamic cavitation effect, and had iodine formation rates three times those achieved from ultrasonic cavitation. A slowing of the iodine rate was explained by degassing of the KI solution over time, resulting in fewer cavitation events.
Alternatives to yield as an indicator of OH radical generation have been considered. Arrojo et al. [30], and Arrojo and Benito [31] believe the knowledge acquired in ultrasonic cavitation has not been adapted to hydrodynamic cavitation, which yields inconsistent results, and has no clear theoretical base. They argued that because iodide oxidation does not differentiate from other oxidation mechanisms (e.g., H2O2 and O2), attempts to characterize hydrodynamic cavitation bubble behaviour using iodide are insufficient. They conclude salicylic acid is especially suitable for monitoring hydrodynamic cavitation’s characteristic timescales and the generation of OH radicals. Not only is it oxidised exclusively by those radicals, but its reaction products can be analysed with more sensitivity than those of the Weissler or Fricke reactions.
Attempts to characterize the hydrodynamic cavitation in terms of the geometry of reactors has led to the definition of cavitation number (Ca) to describe the resistance of flow to cavitation [14]:where pd is the fully recovered downstream pressure, pv is the liquid vapour pressure, ρ is the fluid density, and vo is the orifice velocity. Cavitation is not generally possible unless the cavitation number is less than 1.0 and it is expected to be more intense at lower values of the number.
However, the cavitation number has some limitations and was found unsatisfactory in work on multiple hole orifice plates, so it was redefined to include other reactor parameters [14]. Also, a venturi and orifice with the same cavitation number could have quite different cavitation conditions, especially because the flow path to the point of defined down stream pressure is different. The usefulness of the cavitation number to describe hydrodynamic cavitation needs to be experimentally confirmed.
The aim of this work was to compare two different hydrodynamic cavitation devices with each other and with ultrasonic cavitation so as to draw conclusions about the most effective form of hydrodynamic cavitation. This would test the usefulness of the cavitation number. A subsequent aim that arose during the work was to demonstrate that the Weissler reaction is not suitable as a measure of cavitation intensity in hydrodynamic devices.
Section snippets
Experimental procedure
All experiments were conducted using deionised reverse osmosis water. This was de-aerated by boiling at 65 °C for 30 min under vacuum or saturated with air by bubbling air through the water at room temperature for at least 1 hour using a fish tank pump. Solutions of 0.1 M potassium iodide (AnalaR, 99.5% purity, BDH) were prepared using either type of water.
Hydrodynamic cavitation experiments were performed using two devices, a sudden expansion and a venturi. The term ‘sudden expansion’ has been
Ultrasonic cavitation
The ultrasonic production of always gave a straight line of concentration with time (Fig. 4) indicating that the iodide ions were well in excess and were not a limiting reactant.
In general, de-aerated KI had a higher conversion rate than saturated solutions at the same temperature, with 20 °C yielding the most at a rate of 3.9 × 10−9 mol s−1. For the constant 52 W power input, the maximum corresponding cavitation efficiency was 7.6 × 10−11 mol J−1 (moles produced per joule). Results are
Iodide oxidation pressure and temperature response
Neither the venturi nor sudden expansion produced measurable below 4 bar, though cavitation was detectible at 1.8 bar. Other researchers have detected at lower pressures. Vichare et al. [28] detected (reported as iodine) at 0.7 bar using a multi-hole orifice plate while Suslick et al. [15] were not able to generate any below 150 bar with their jet fluidizer.
Generally the response of KI solutions to increased inlet pressure was increased production in both the sudden expansion
Conclusion
Neither the venturi nor sudden expansion cavitation devices were found to consistently out perform the other. At 10 °C it was found that ultrasonic cavitation gave better yields from the Weissler reaction, and better yield efficiencies, than both of the hydrodynamic cavitation devices used here. The presence of the consumption reaction precludes a conclusion that hydrodynamic cavitation is not more efficient than ultrasonic cavitation.
There is little doubt that a reaction that consumes
Acknowledgement
We wish to thank Mr. Rob McGregor, Chemistry Department, University of Canterbury, for the glass-blowing of the venturi and sudden expansion.
References (35)
Chem. Eng. Proc.
(2008)- et al.
Chem. Eng. Sci.
(2000) - et al.
Ultrason. Sonochem.
(2003) - et al.
Sep. Purif. Technol.
(2005) - et al.
J. Food Eng.
(2005) - et al.
Int. J. Food Microbiol.
(2003) - et al.
Wat. Res.
(2000) - et al.
Ultrason. Sonochem.
(1997) - et al.
Chem. Eng. Sci.
(2001) - et al.
Ultrasonics
(2006)