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Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation
Ultrasonics Sonochemistry 16 (2009) 176–183
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Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation K.R. Morison *, C.A. Hutchinson Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand
a r t i c l e
i n f o
Article history: Received 10 June 2008 Received in revised form 1 July 2008 Accepted 1 July 2008 Available online 9 July 2008 Keywords: Weissler reaction Hydrodynamic caitation Iodine
a b s t r a c t The Weissler reaction in which iodide is oxidised to a tri-iodide complex (I 3 ) has been widely used for measurement of the intensity of ultrasonic and hydrodynamic cavitation. It was used in this work to compare ultrasonic cavitation at 24 kHz with hydrodynamic cavitation using two different devices, one a venturi and the other a sudden expansion, operated up to 8.7 bar. Hydrodynamic cavitation had a maximum efficiency of about 5 1011 moles of I 3 per joule of energy compared with the maximum of almost 8 1011 mol J1 for ultrasonic cavitation. Hydrodynamic cavitation was found to be most effective at 10 °C compared with 20 °C and 30 °C and at higher upstream pressures. However, it was found that in hydrodynamic conditions, even without cavitation, I 3 was consumed at a rapid rate leading to an equilibrium concentration. It was concluded that the Weissler reaction was not a good model reaction for the assessment of the effectiveness of hydrodynamic cavitation. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Weissler reaction
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.
* Corresponding author. E-mail address: [email protected] (K.R. Morison). 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.07.001
H2 O2 þ 2I ! I2 þ 2OH
ð1Þ
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. 1.2. Cavitation chemistry 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
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react with the iodide ion, through a series of intermediate steps [18,19], to form the tri-iodide complex I 3 . The key reactions are believed to be:
dard method to calibrate the sonochemical efficiency of an individual reaction system.
H2 O ! H þ OH
ð2Þ
OH þ I ! OH þ I
ð3Þ
I þ I ! I2
ð4Þ
2I2 ! I2 þ 2I
ð5Þ
I2 þ I ! I3
ð6Þ
1.3.1. Effect of geometry on ultrasonic cavitation 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 I 3 yield and showed the presence of acoustic emission peaks seen only in cavitating liquids. Various workers [25–27] 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.
Hydrogen peroxide is formed by OH radicals:
OH þ OH ! H2 O2
ð7Þ
In neutral or alkaline solutions, H2O2 can scavenge
H2 O2 þ
I3
þ
! 3I þ O2 þ 2H
I 3
[18,19]:
ð8Þ
Confusingly, much of the literature uses the terms I 3 and iodine interchangeably, often referring only to iodine measurements, or switching between the two in the same paper. This paper refers to I 3 for our own work, but when referencing a paper uses the term adopted by the authors. 1.3. Ultrasonic cavitation experiments 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 I 3 approached a maximum at higher initial iodide concentrations. When ammonium molybdate was added before ultrasound, the I 3 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 I 3 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 I 3 ) 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 I 3 yield due to reaction (8) [19]. Below pH 4, I 3 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 stan-
1.4. Hydrodynamic cavitation experiments The recent development of interest in hydrodynamic cavitation is reflected in the scarcity of its bibliography. Much of the work todate has used I 3 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. I 3 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 I 3 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. 1.4.1. Effect of geometry on hydrodynamic cavitation 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]:
Ca ¼
pd pv qm20 =2
ð9Þ
where pd is the fully recovered downstream pressure, pv is the liquid vapour pressure, q 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.
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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.
Fig. 2. Venturi device.
2. 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 used rather than ‘orifice’ because of the smooth angled entrance (Fig. 1). It was made from precision glass capillary tubing by glass-blowing one end then cutting and grinding both ends flat. It was then placed halfway down a 100 mm long acrylic tube which had been drilled from one end to provide a tight fit and leave a lip face for the sudden expansion to sit on, and glued in place. The venturi was made from glass tubing to the dimensions shown in Fig. 2. The total angle of the inlet reduction was 22° while the angle of the expansion was 25°. The hydrodynamic cavitation reactor used in this work (Fig. 3) included a variable-speed magnetically coupled gear pump (GDM35, Micropump Inc., WA, USA) which pumped KI solution from a stainless steel beaker (open to the atmosphere) into the hydrodynamic cavitation device at the desired pressure P1 (limited by the pump’s maximum DP of 8.7 bar) before returning it to the beaker. Exposure to air was minimised by ensuring the return pipe was below the beaker’s KI solution level. The solution temperature was controlled using a stainless steel cooling coil which was placed in the beaker (not shown) through which water was circulated from a controlled refrigerated water bath with a temperature stability of ±0.1 °C. Initially a copper coil was used but it was found that this suppressed the formation of the tri-iodide complex in both ultrasonic and hydrodynamic cavitation. A pressure/flow profile was established for each device, and each was calibrated with the cooling system to obtain the
Fig. 1. Sudden expansion device.
Fig. 3. Hydrodynamic cavitation apparatus.
refrigeration temperature required to maintain the desired system temperature for a range of device inlet pressures. For each experimental run 500 mL of 0.1 M KI solution was pored into the beaker, then an initial KI sample was extracted and placed in a cuvette. The pump speed was gently ramped up to fill the system and remove air bubbles from the apparatus. During startup, the pressure was maintained below 1 bar to prevent any cavitation induced reactions occurring (the onset of audible cavitation was found to occur in the venturi at about 1.8 bar). The system remained in this condition while it was brought to the desired experimental temperature. The pump speed was then quickly increased to bring the inlet pressure to the experimental value. The cooling water temperature setpoint was reduced immediately to that obtained from the above calibration so that temperature deviations in the solution were minimised. The temperature of the solution was typically maintained within ±0.5 °C and always within ±1 °C of the desired value. Hydrodynamic cavitation power (in watts) from the pump was calculated from QDP where Q is the mass flow rate (m3 s1) and DP is the pressure drop across the hydrodynamic cavitation device (Pa). Ultrasonic cavitation experiments were done using a 24 kHz ultrasonic processor (UP400S, Dr. Heilscher GmbH) with a 7 mm transducer probe immersed vertically from the top in 500 mL of 0.1 M KI. The same beaker and temperature control arrangement used for the hydrodynamic cavitation experiments was used. Experiments were performed using de-aerated and air-saturated KI solutions, at temperatures of 10, 20, and 30 °C. Initial poor reproducibility led to the finding that the absorbed power (determined calorimetrically) increased linearly with probe immersion depth. Over a range of probe depths from 4 to 50 mm the power increased linearly from 40 to 54 W. Therefore a constant immersion depth (47 mm) was used giving a power output of 52 W. This phenomena has been noted and examined [27]. In both ultrasonic cavitation and hydrodynamic cavitation experiments, 2 mL samples were extracted from the beaker at regular intervals for up to 60 min using a plastic syringe and were transferred into plastic cuvettes with path length of 1 cm. Absorbance at
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353 nm was obtained using a UV spectrophotometer (Ultrospec 2100 pro, Amersham Biosciences) and the concentration of I 3 was calculated using an absorption coefficient of 26200 M1 cm1 [32]. A set of cuvettes were tested and chosen to have a UV absorbance at 353 nm within ±0.002 of each other using deionised de-aerated water. The uncertainty of the concentration obtained was estimated to be ±8 108 mol L1. The repeatability of the set of cuvettes was tested during the runs. 3. Results 3.1. Ultrasonic cavitation The ultrasonic production of I 3 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 I 3 conversion rate than saturated solutions at the same temperature, with 20 °C yielding the 9 most I mol s1. For the constant 52 W power 3 at a rate of 3.9 10 input, the maximum corresponding cavitation efficiency was 7.6 1011 mol J1 (moles I 3 produced per joule). Results are shown in Fig. 5. For clarity error bars are shown for only one data set. These results can be compared with an I 3 production rate and cavitation efficiency of approximately 6.5 1010 mol s1 and 1.5 1011 mol J1, respectively, using a similar experimental setup [25], cavitation efficiencies of 6 1011 mol J1 [23], and 5.5 1010 mol J1 [19] using different reactor geometry and ultrasonic transducer types, and 6.8 1010 mol J1 using a high frequency ultrasonic cavitation reactor [8].
Fig. 5. Cavitation efficiency for ultrasonic production of I 3.
Table 1 Experimental conditions for hydrodynamic cavitation Cavitation device
D Pressure (bar)
Flow rate (mL s1)
Temperature (°C)
Venturi
8.7 8 7 6 4 2
38.5 37 35 33 28 21
10 32, 30, 30, 10 20,
8.7 8 7 6 4 2
35 33 31 28 23 16
20 (sat), 20, 10 30, 20, 20, 12 20, 10 30, 20, 10 20, 10 20, 10
Sudden expansion
3.2. Hydrodynamic cavitation
20, 10 30, 20, 10 20, 10 10
sat = air-saturated.
Hydrodynamic cavitation experiments were conducted at the flow conditions listed in Table 1, the maximum of 8.7 bar gauge being a limitation of the magnetically coupled pump. In all cases the pressure immediately downstream of the cavitation device (pd in Eq. (10)) was very close to atmospheric pressure. All solutions were made using de-aerated water except for one set made using air-saturated water. At the flow rates obtained the time per pass ranged 13–24 s in the venturi and 14–31 s in the sudden expansion. The audible onset of cavitation, detected by placing a stethoscope on the venturi, occurred at 1.8 bar. Backlit microscopic photography of the venturi was used for visual confirmation (Fig. 6). A cavitation scar is visible across the circumference of the venturi. At 1.3 bar a faint bubble cloud appeared on the venturi nozzle outlet indicating that the local pressure was below the vapour pressure of water. The smooth whooshing sound of flow (via the stethoscope)
Fig. 6. Visible bubble cloud formation (right side) and the cavitation scar (top to bottom on left) in the venturi. Flow is from left to right, upstream pressure 6 bar, image height is 1 mm.
Fig. 4. I 3 production from ultrasonic reaction of de-aerated 0.1 M KI solution at 20 °C.
remained unchanged. At 1.8 bar the sound changed into distinct crackling, while the cloud thickened sharply. As the pressure increased the bubble traces from the scar became more apparent and at 8 bar flow was fully developed across the nozzle to leave a thick cloud (assumed to be bubbles) downstream from the scar. The width of the scar indicates that the most intense cavitation occurs over a very small distance and time. All hydrodynamic cavitation experiments produced similar trends of concentration over time. High initial I 3 formation rates
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decreased until an equilibrium limit was reached (Fig. 7). Neither of the hydrodynamic cavitation devices was consistently more effective that the other. The sudden expansion gave the highest I 3 yield (at 8.7 bar, 20 °C) and at this pressure always gave greater yields than the venturi at the same temperature. At lower pressures the venturi was more effective. For a particular hydrodynamic cavitation device at a given temperature the higher pressures always yielded more I 3. De-aerated KI solutions yielded greater I 3 than saturated solutions as seen for the sudden expansion at 8.7. bar and 20 °C (Fig. 7). 3.2.1. Initial reaction rates An examination of initial reaction rates (taken at five minutes) reveals different behaviour for the two hydrodynamic cavitation devices. In general, higher pressures produced higher reaction rates. At 10 °C the venturi showed a clear (and linear within experimental error) increase in I 3 generation with pressure. At 20 °C a similar trend was evident, though not as distinct (Fig. 8 – for clarity only one set of error bars is shown). The sudden expansion produced a similar trend with pressure at 10 °C (Fig. 9) but yield seemed to be more strongly dependent on pressure.
Fig. 8. Initial generation rate of I 3 for the venturi at various pressures and temperatures.
3.2.2. Initial reaction cavitation efficiency Initially (i.e. at five minutes) the venturi has constant cavitation efficiency (within experimental error) across the range of pressures at 10 °C (Fig. 10). Indeed, the venturi was clearly more energy efficient at 10 °C. However, no clear trend was seen (within experimental error) at higher temperatures, though as pressure decreases there is a general decrease in efficiency. On the other hand, the efficiency of the sudden expansion at 10 °C (Fig. 11) increases significantly with pressure. Again, at higher temperatures the efficiency response to pressure is more variable. 3.2.3. Initial reaction rate and cavitation number The cavitation number (Eq. (10)) potentially combines the effects of pressure, temperature and velocity into one number and might give clearer trends than Figs. 8 and 9. As the cavitation number increased in the venturi (Fig. 12) at 10 °C the initial reaction rate decreases. This behaviour fits the definition of cavitation number as a measure of resistance to cavitation [15], with decreasing cavitation number meaning the hydrodynamic cavitation reactor state is less resistant to cavitation. Though the same trend is apparent for higher temperatures, it is less clear. Similar trends are seen in the sudden expansion (Fig. 13) with a distinct increase in reaction rate with lowering cavitation number at 10 °C and I 3 yield fluctuation at higher temperatures. It can be seen that the cavita-
Fig. 9. Initial generation rate of I 3 for the sudden expansion at various pressures and temperatures.
Fig. 10. Cavitation efficiency of the venturi.
tion number alone is not sufficient to describe, or predict, the reaction rate in this work.
Fig. 7. Production of I 3 with the venturi (V) and sudden expansion (SE) at various upstream pressures and temperatures. Run with air-saturated solution is designated ‘‘sat”.
3.2.4. An I 3 consumption reaction The variable I 3 yield at temperatures above 10 °C, and the fact that all hydrodynamic cavitation runs reached an equilibrium limit indicated that a reverse, or subsequent, reaction occurred which consumes I 3 (this reaction is referred to as the consumption reaction in this paper). The runs at 7 bar and 30 °C were much less reproducible than the others (Fig. 14). Having done about 35 runs, each with at least seven sample points, giving well over 500 UV absorption tests, we were confident that experimental error was much less than the variability observed. It seemed possible that,
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Fig. 14. Evidence of a consumption reaction.
Fig. 11. Cavitation efficiency of the sudden expansion.
Fig. 12. Effect of cavitation number on reaction rate in the venturi. Fig. 15. Concentration of I 3 during circulation through the venturi at 1 bar without cavitation.
Fig. 13. Effect of cavitation number on reaction rate in the sudden expansion.
at 7 bar, 30 °C, the forward and consumption reactions competed in some unstable manner. To test this, an experiment was conducted at 7 bar and 30 °C in the venturi using a initial solution that had been irradiated with ultrasound to produce an amount of I 3 . The response in Fig. 14 clearly shows that I 3 is being consumed by another reaction. Gutiérrez et al. [18] discussed the possibility of reaction (8) occurring in non-acidic conditions during ultrasonic cavitation. The pH after a standard cavitation run for 30 min at 30 °C and 8 bar was measured to be 6.6. In a further run 100 mL of phosphate buffer with pH 5.9 was added to water to make 500 mL 0.1 M KI solution. The pH at the start of this experiment was 5.87, and at the end was 6.01. Fig. 14 shows the concentration of I 3 was no more stable at the lower pH. We were confident that the erratic concentrations were real. These runs provided strong evidence of a consumption reaction leading to an equilibrium. In the ultrasound pre-irradiated experi-
ment, I 3 was consumed during hydrodynamic cavitation until an equilibrium concentration was reached. The equilibrium concentration was similar to that obtained from a fresh KI solution with and without pH control. Interestingly the erratic concentration obtained from fresh KI never exceeded the equilibrium concentration. To test for the presence of the consumption reaction in the absence of cavitation, 500 mL of 0.1 M de-aerated KI solution was irradiated with ultrasound for 5 minutes to generate I 3 . 20 mL was removed from the irradiated KI and held separately as an irradiated baseline sample. The remaining KI solution was circulated through the venturi at 1 bar (well below audible cavitation) and at 10 °C. The cavitation number was 0.45 which was less than the critical value of 1.0 but well above the other experimental values in Figs. 12 and 13. Samples were extracted at intervals, tested for UV absorbance at 353 nm, and returned to the circulating solution. A sample of the irradiated baseline solution was also tested for UV absorbance concurrently with the circulated sample. The consumption reaction was found to occur even at low pressures, well below the onset of detectable cavitation (Fig. 15). The I 3 concentration in the baseline solution increased slightly before stabilizing. However, the I 3 concentration in the circulated solution decreased substantially before reaching an equilibrium concentration after about 80 min. The equilibrium concentration of 1.4 106 mol L1 is close to the concentration of 1.5 106 mol L1 produced after 30 min in the standard run at 8.7 bar and 10 °C (Fig. 7). 4. Discussion 4.1. Iodide oxidation pressure and temperature response Neither the venturi nor sudden expansion produced measurable I 3 below 4 bar, though cavitation was detectible at 1.8 bar. Other
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researchers have detected I 3 at lower pressures. Vichare et al. [28] detected I 3 (reported as iodine) at 0.7 bar using a multi-hole orifice plate while Suslick et al. [15] were not able to generate any I 3 below 150 bar with their jet fluidizer. Generally the response of KI solutions to increased inlet pressure was increased I 3 production in both the sudden expansion and venturi though at higher temperatures the effect was less clear. At higher inlet pressures the internal velocities are much greater and hence the driving force for bubble formation is greater. Further the time for travel to a lower pressure region is less. Both these effects are likely to encourage greater cavitation. Suslick et al. [15] reported a linear increase in I 3 concentration with increasing liquid pressure with a CCl4 saturated KI solution. Senthil Kumar et al. [2] reported a similar response to increasing pressure, but they reported a maximum yield of 5.7 106 g L1 (1.5 108 mol L1 of I 3 ) at about 2.8 bar before it decreased at a higher pressure. This is a low concentration, and is below the detection limit of 3.8 108 mol L1 for I 3 using photometric detection methods [19]. It is therefore difficult to know how to treat these results. Inlet pressure has featured as an important parameter in attempts to model hydrodynamic cavitation reactors [29,33–35]. Arrojo and Benito [31] have concluded that the physical significance of inlet pressure is undefined because of the lack of consistency in the results from this parameter for the same reaction (such as iodide oxidation). The present authors agree. This work showed that inlet pressure was a factor in I 3 yield under some conditions (and certainly there was a minimum pressure threshold before any reaction was seen). However, separating its significance from other parameters such as downstream pressure, temperature, hydrodynamic cavitation device geometry and, in the case of the iodide oxidation reaction, the influence of an I 3 consumption reaction (discussed below), is necessary before drawing conclusions. In both hydrodynamic cavitation devices, as temperature decreased I 3 yield increased, which is consistent with previous work [15,16]. 4.2. I 3 Concentration degradation The ultrasonic production of I 3 normally gave a straight line of concentration with time (Fig. 4) indicating that the concentration of I in 500 mL of 0.1 M KI solutions was well in excess and did not limit the reaction. In hydrodynamic cavitation, an equilibrium concentration was approached. Other researchers have reported this reduction in initial rate. Senthil Kumar et al. [2] attributed it to degassing and temperature increase of the KI solution as it is repeatedly circulated through the cavitation device. Vichare et al. [28] reported that their experimental conditions were not constant for the initial 5–10 min, and also propose that this is due to degassing. Neither degassing nor a temperature increase explain the reduction in the production rate of I 3 seen in this work. In this work, freshly (and thoroughly) de-aerated water was used which would remove the potential for further degassing, and also the temperature was controlled to within ±0.5 °C. Here there were strong indications that there was a significant consumption reaction, and this was confirmed by hydrodynamically degrading some ultrasonically produced I 3 (Fig. 14). In most cases, including the degradation of ultrasonically produced I 3, the reaction could be modelled as a zeroth order generation with a first order degradation (Eq. (9)) but almost certainly the mechanism is much more complex than implied by this equation.
dC I3 ¼ r 0 ðP; TÞ kðP; TÞC I3 dt
ð10Þ
Here the forward rate, r0, and the consumption rate constant; k, depended on the temperature and upstream pressure. The value of r0 corresponded to the initial rates given in Figs. 8 and 9 and k was found to range from 0.00075 to 0.0026 s–1. From the experiments done it was not possible to determine the rate of the forward reaction with any certainty. It was quite possible that the consumption reaction reduced the apparent forward reaction in some cases to zero (as at 7 bar 30 °C, Fig. 14). Given that this uncertainty made comparison of cavitation efficiency very difficult, further examination of the reaction mechanism or rates was beyond the scope of the work reported here. The presence of an I 3 consumption reaction (reaction (8)) has been discussed in relation to ultrasonic cavitation [18,19] where it has been shown to become significant in neutral or alkaline solutions. There is some evidence that a consumption reaction can become prominent in ultrasonic cavitation at high power/high frequency [8]. Hart and Henglein [20] stated that their results with ammonium molybdate (catalysing reaction (1)) confirmed that H2O2 was involved in the reaction. A complementary hypothesis is that H2O2 is involved in the consumption reaction and that catalysed reaction (1) (or reactions (2)–(6)) consumes the H2O2 and prevents the consumption reaction (8) from occurring. The continuation of the consumption reaction during circulation at low pressure (Fig. 15) shows that it does not require cavitation and suggests that H2O2 already in solution is used. Given that the higher temperature experiments gave lower equilibrium concentrations it seems likely that the consumption reaction was dependent on the bulk solution temperature, while the forward reaction was dependent on the intensity of cavitation. Given the apparent effect of the copper coil mentioned in the experimental section the possibility exists that the consumption reaction is a surface catalysed reaction. We offer no evidence to determine the cause of the difference between ultrasonic and hydrodynamic cavitation. Clearly the forward reaction dominates over the consumption reaction in ultrasonic cavitation. Similarly there is no evidence to determine why the different hydrodynamic devices responded differently to different pressures and temperatures. Almost certainly all three types of cavitation have different forms of bubble formation and collapse which might influence any of the possible reactions. When reviewed in the context of these results, other previous work is not inconsistent with the current findings. The general result of lower production of I 3 at higher temperatures is consistent with the proposal that the consumption reaction rate increases at higher solution temperatures. This work has not determined the mechanisms of the reactions but it has shown that the Weissler reaction cannot be used as a simple indicator for the effectiveness of process intensification by hydrodynamic cavitation.
5. 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 I 3 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 I 3 occurs during hydrodynamic flow, with and without cavitation. The uncertainty added by the consumption reaction causes the Weissler reaction to be unsuitable for the comparison of ultrasonic and hydrodynamic cavitation, and not even suitable for comparison of different hydrodynamic cavitation systems.
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Acknowledgement We wish to thank Mr. Rob McGregor, Chemistry Department, University of Canterbury, for the glass-blowing of the venturi and sudden expansion. References [1] P.R. Gogate, Chem. Eng. Proc. 47 (2008) 515–527. [2] P. Senthil Kumar, M. Sivar Kumar, A.B. Pandit, Chem. Eng. Sci. 55 (2000) 1633– 1639. [3] M.H. Entezari, C. Pétrier, P. Devidal, Ultrason. Sonochem. 10 (2003) 103–108. [4] P. Ning, H. Bart, Y. Jiang, C. Tien, Sep. Purif. Technol. 41 (2005) 133–139. [5] P. Raviyan, Z. Zhang, H. Feng, J. Food Eng. 70 (2005) 189–196. [6] P. Piyasena, E. Mohareb, R.C. McKellar, Int. J. Food Microbiol. 87 (2003) 207– 216. [7] I. Hua, J.E. Thompson, Wat. Res. 34 (2000) 3888–3893. [8] J.D. Seymour, H.C. Wallace, R.B. Gupta, Ultrason. Sonochem. 4 (1997) 289–293. [9] V.S. Moholkar, A.B. Pandit, Chem. Eng. Sci. 56 (2001) 6295–6302. [10] J. Ji, J. Wang, Y. Li, Y. Yu, Z. Xu, Ultrasonics 44 (2006) e411–e414. [11] Z.A. Zhou, Z. Xu, J.A. Finch, H. Hu, S.R. Rao, Int. J. Miner. Process. 51 (1997) 139– 149. [12] B. Balasundaram, S.T.L. Harrison, Biotechnol. Progr. 22 (2006) 907–913. [13] S.S. Save, J.B. Joshi, A.B. Pandit, Trans. I Chem. E. 75 (Part C) (1997) 41–49. [14] M. Sivakumar, A.B. Pandit, Ultrason. Sonochem. 9 (2002) 123–131.
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[15] K.S. Suslick, M.M. Mdleleni, J.T. Ries, J. Am. Chem. Soc. 119 (1997) 9303–9304. [16] A.B. Pandit, P. Senthil Kumar, M. Siva Kumar, Chem. Eng. Prog. 95 (5) (1999) 43–50. [17] A. Weissler, H.W. Cooper, S. Snyder, J. Am. Chem. Soc. 72 (1950) 1769–1775. [18] M. Gutiérrez, A. Henglein, F. Ibañez, J. Phys. Chem. 95 (1991) 6044–6047. [19] Y. Iida, T. Yasui, M. Sivakumar, Microchem. J. 80 (2005) 159–164. [20] E.J. Hart, A. Henglein, J. Phys. Chem. 89 (1985) 4342–4347. [21] D.V.P. Naidu, R. Rajan, R. Kumar, K.S. Gandhi, V.H. Arakeri, S. Chandrasekaran, Chem. Eng. Sci 49 (1994) 877–888. [22] D.M. Kirpalani, K.J. McQuinn, Ultrason. Sonochem. 13 (2006) 1–5. [23] S. Koda, T. Kimura, T. Konda, H. Mitome, Ultrason. Sonochem. 10 (2003) 149– 156. [24] P.R. Gogate, P.A. Tatake, P.M. Kanthale, A.B. Pandit, AIChE J. 48 (2002) 1542– 1560. [25] Z.L. Wu, J. Lifka, B. Ondruschka, Chem. Eng. Technol. 29 (2006) 610–615. [26] I.Z. Shirgaonkar, A.B. Pandit, Ultrason. Sonochem. 4 (1997) 245–253. [27] J. Klíma, A. Frias-Ferrer, J. González-Garcia, J. Ludvík, V. Sáez, J. Iniesta, Ultrason. Sonochem. 14 (2007) 19–28. [28] N.P. Vichare, P.R. Gogate, A.B. Pandit, Chem. Eng. Technol. 23 (2000) 683–690. [29] P. Senthil Kumar, A.B. Pandit, Chem. Eng. Technol. 22 (1999) 1017–1027. [30] S. Arrojo, C. Nerín, Y. Benito, Ultrason. Sonochem. 14 (2007) 343–349. [31] S. Arrojo, Y. Benito, Ultrason. Sonochem. 15 (2008) 203–211. [32] A.D. Awtrey, R.E. Connick, J. Am. Chem. Soc. 73 (1951) 1842–1843. [33] P.R. Gogate, A.B. Pandit, AIChE 46 (2000) 1641–1649. [34] P.R. Gogate, A.B. Pandit, Ultrason. Sonochem. 12 (2005) 21–27. [35] J.S. Krishnan, P. Dwivedi, V.S. Moholkar, Ind. Eng. Chem. Res. 45 (2006) 1493– 1504.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Limitations of the Weissler reaction as a model reaction for measuring the efficiency of hydrodynamic cavitation K.R. Morison *, C.A. Hutchinson Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand
a r t i c l e
i n f o
Article history: Received 10 June 2008 Received in revised form 1 July 2008 Accepted 1 July 2008 Available online 9 July 2008 Keywords: Weissler reaction Hydrodynamic caitation Iodine
a b s t r a c t The Weissler reaction in which iodide is oxidised to a tri-iodide complex (I 3 ) has been widely used for measurement of the intensity of ultrasonic and hydrodynamic cavitation. It was used in this work to compare ultrasonic cavitation at 24 kHz with hydrodynamic cavitation using two different devices, one a venturi and the other a sudden expansion, operated up to 8.7 bar. Hydrodynamic cavitation had a maximum efficiency of about 5 1011 moles of I 3 per joule of energy compared with the maximum of almost 8 1011 mol J1 for ultrasonic cavitation. Hydrodynamic cavitation was found to be most effective at 10 °C compared with 20 °C and 30 °C and at higher upstream pressures. However, it was found that in hydrodynamic conditions, even without cavitation, I 3 was consumed at a rapid rate leading to an equilibrium concentration. It was concluded that the Weissler reaction was not a good model reaction for the assessment of the effectiveness of hydrodynamic cavitation. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Weissler reaction
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.
* Corresponding author. E-mail address: [email protected] (K.R. Morison). 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.07.001
H2 O2 þ 2I ! I2 þ 2OH
ð1Þ
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. 1.2. Cavitation chemistry 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
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177
react with the iodide ion, through a series of intermediate steps [18,19], to form the tri-iodide complex I 3 . The key reactions are believed to be:
dard method to calibrate the sonochemical efficiency of an individual reaction system.
H2 O ! H þ OH
ð2Þ
OH þ I ! OH þ I
ð3Þ
I þ I ! I2
ð4Þ
2I2 ! I2 þ 2I
ð5Þ
I2 þ I ! I3
ð6Þ
1.3.1. Effect of geometry on ultrasonic cavitation 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 I 3 yield and showed the presence of acoustic emission peaks seen only in cavitating liquids. Various workers [25–27] 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.
Hydrogen peroxide is formed by OH radicals:
OH þ OH ! H2 O2
ð7Þ
In neutral or alkaline solutions, H2O2 can scavenge
H2 O2 þ
I3
þ
! 3I þ O2 þ 2H
I 3
[18,19]:
ð8Þ
Confusingly, much of the literature uses the terms I 3 and iodine interchangeably, often referring only to iodine measurements, or switching between the two in the same paper. This paper refers to I 3 for our own work, but when referencing a paper uses the term adopted by the authors. 1.3. Ultrasonic cavitation experiments 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 I 3 approached a maximum at higher initial iodide concentrations. When ammonium molybdate was added before ultrasound, the I 3 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 I 3 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 I 3 ) 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 I 3 yield due to reaction (8) [19]. Below pH 4, I 3 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 stan-
1.4. Hydrodynamic cavitation experiments The recent development of interest in hydrodynamic cavitation is reflected in the scarcity of its bibliography. Much of the work todate has used I 3 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. I 3 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 I 3 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. 1.4.1. Effect of geometry on hydrodynamic cavitation 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]:
Ca ¼
pd pv qm20 =2
ð9Þ
where pd is the fully recovered downstream pressure, pv is the liquid vapour pressure, q 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.
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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.
Fig. 2. Venturi device.
2. 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 used rather than ‘orifice’ because of the smooth angled entrance (Fig. 1). It was made from precision glass capillary tubing by glass-blowing one end then cutting and grinding both ends flat. It was then placed halfway down a 100 mm long acrylic tube which had been drilled from one end to provide a tight fit and leave a lip face for the sudden expansion to sit on, and glued in place. The venturi was made from glass tubing to the dimensions shown in Fig. 2. The total angle of the inlet reduction was 22° while the angle of the expansion was 25°. The hydrodynamic cavitation reactor used in this work (Fig. 3) included a variable-speed magnetically coupled gear pump (GDM35, Micropump Inc., WA, USA) which pumped KI solution from a stainless steel beaker (open to the atmosphere) into the hydrodynamic cavitation device at the desired pressure P1 (limited by the pump’s maximum DP of 8.7 bar) before returning it to the beaker. Exposure to air was minimised by ensuring the return pipe was below the beaker’s KI solution level. The solution temperature was controlled using a stainless steel cooling coil which was placed in the beaker (not shown) through which water was circulated from a controlled refrigerated water bath with a temperature stability of ±0.1 °C. Initially a copper coil was used but it was found that this suppressed the formation of the tri-iodide complex in both ultrasonic and hydrodynamic cavitation. A pressure/flow profile was established for each device, and each was calibrated with the cooling system to obtain the
Fig. 1. Sudden expansion device.
Fig. 3. Hydrodynamic cavitation apparatus.
refrigeration temperature required to maintain the desired system temperature for a range of device inlet pressures. For each experimental run 500 mL of 0.1 M KI solution was pored into the beaker, then an initial KI sample was extracted and placed in a cuvette. The pump speed was gently ramped up to fill the system and remove air bubbles from the apparatus. During startup, the pressure was maintained below 1 bar to prevent any cavitation induced reactions occurring (the onset of audible cavitation was found to occur in the venturi at about 1.8 bar). The system remained in this condition while it was brought to the desired experimental temperature. The pump speed was then quickly increased to bring the inlet pressure to the experimental value. The cooling water temperature setpoint was reduced immediately to that obtained from the above calibration so that temperature deviations in the solution were minimised. The temperature of the solution was typically maintained within ±0.5 °C and always within ±1 °C of the desired value. Hydrodynamic cavitation power (in watts) from the pump was calculated from QDP where Q is the mass flow rate (m3 s1) and DP is the pressure drop across the hydrodynamic cavitation device (Pa). Ultrasonic cavitation experiments were done using a 24 kHz ultrasonic processor (UP400S, Dr. Heilscher GmbH) with a 7 mm transducer probe immersed vertically from the top in 500 mL of 0.1 M KI. The same beaker and temperature control arrangement used for the hydrodynamic cavitation experiments was used. Experiments were performed using de-aerated and air-saturated KI solutions, at temperatures of 10, 20, and 30 °C. Initial poor reproducibility led to the finding that the absorbed power (determined calorimetrically) increased linearly with probe immersion depth. Over a range of probe depths from 4 to 50 mm the power increased linearly from 40 to 54 W. Therefore a constant immersion depth (47 mm) was used giving a power output of 52 W. This phenomena has been noted and examined [27]. In both ultrasonic cavitation and hydrodynamic cavitation experiments, 2 mL samples were extracted from the beaker at regular intervals for up to 60 min using a plastic syringe and were transferred into plastic cuvettes with path length of 1 cm. Absorbance at
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353 nm was obtained using a UV spectrophotometer (Ultrospec 2100 pro, Amersham Biosciences) and the concentration of I 3 was calculated using an absorption coefficient of 26200 M1 cm1 [32]. A set of cuvettes were tested and chosen to have a UV absorbance at 353 nm within ±0.002 of each other using deionised de-aerated water. The uncertainty of the concentration obtained was estimated to be ±8 108 mol L1. The repeatability of the set of cuvettes was tested during the runs. 3. Results 3.1. Ultrasonic cavitation The ultrasonic production of I 3 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 I 3 conversion rate than saturated solutions at the same temperature, with 20 °C yielding the 9 most I mol s1. For the constant 52 W power 3 at a rate of 3.9 10 input, the maximum corresponding cavitation efficiency was 7.6 1011 mol J1 (moles I 3 produced per joule). Results are shown in Fig. 5. For clarity error bars are shown for only one data set. These results can be compared with an I 3 production rate and cavitation efficiency of approximately 6.5 1010 mol s1 and 1.5 1011 mol J1, respectively, using a similar experimental setup [25], cavitation efficiencies of 6 1011 mol J1 [23], and 5.5 1010 mol J1 [19] using different reactor geometry and ultrasonic transducer types, and 6.8 1010 mol J1 using a high frequency ultrasonic cavitation reactor [8].
Fig. 5. Cavitation efficiency for ultrasonic production of I 3.
Table 1 Experimental conditions for hydrodynamic cavitation Cavitation device
D Pressure (bar)
Flow rate (mL s1)
Temperature (°C)
Venturi
8.7 8 7 6 4 2
38.5 37 35 33 28 21
10 32, 30, 30, 10 20,
8.7 8 7 6 4 2
35 33 31 28 23 16
20 (sat), 20, 10 30, 20, 20, 12 20, 10 30, 20, 10 20, 10 20, 10
Sudden expansion
3.2. Hydrodynamic cavitation
20, 10 30, 20, 10 20, 10 10
sat = air-saturated.
Hydrodynamic cavitation experiments were conducted at the flow conditions listed in Table 1, the maximum of 8.7 bar gauge being a limitation of the magnetically coupled pump. In all cases the pressure immediately downstream of the cavitation device (pd in Eq. (10)) was very close to atmospheric pressure. All solutions were made using de-aerated water except for one set made using air-saturated water. At the flow rates obtained the time per pass ranged 13–24 s in the venturi and 14–31 s in the sudden expansion. The audible onset of cavitation, detected by placing a stethoscope on the venturi, occurred at 1.8 bar. Backlit microscopic photography of the venturi was used for visual confirmation (Fig. 6). A cavitation scar is visible across the circumference of the venturi. At 1.3 bar a faint bubble cloud appeared on the venturi nozzle outlet indicating that the local pressure was below the vapour pressure of water. The smooth whooshing sound of flow (via the stethoscope)
Fig. 6. Visible bubble cloud formation (right side) and the cavitation scar (top to bottom on left) in the venturi. Flow is from left to right, upstream pressure 6 bar, image height is 1 mm.
Fig. 4. I 3 production from ultrasonic reaction of de-aerated 0.1 M KI solution at 20 °C.
remained unchanged. At 1.8 bar the sound changed into distinct crackling, while the cloud thickened sharply. As the pressure increased the bubble traces from the scar became more apparent and at 8 bar flow was fully developed across the nozzle to leave a thick cloud (assumed to be bubbles) downstream from the scar. The width of the scar indicates that the most intense cavitation occurs over a very small distance and time. All hydrodynamic cavitation experiments produced similar trends of concentration over time. High initial I 3 formation rates
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decreased until an equilibrium limit was reached (Fig. 7). Neither of the hydrodynamic cavitation devices was consistently more effective that the other. The sudden expansion gave the highest I 3 yield (at 8.7 bar, 20 °C) and at this pressure always gave greater yields than the venturi at the same temperature. At lower pressures the venturi was more effective. For a particular hydrodynamic cavitation device at a given temperature the higher pressures always yielded more I 3. De-aerated KI solutions yielded greater I 3 than saturated solutions as seen for the sudden expansion at 8.7. bar and 20 °C (Fig. 7). 3.2.1. Initial reaction rates An examination of initial reaction rates (taken at five minutes) reveals different behaviour for the two hydrodynamic cavitation devices. In general, higher pressures produced higher reaction rates. At 10 °C the venturi showed a clear (and linear within experimental error) increase in I 3 generation with pressure. At 20 °C a similar trend was evident, though not as distinct (Fig. 8 – for clarity only one set of error bars is shown). The sudden expansion produced a similar trend with pressure at 10 °C (Fig. 9) but yield seemed to be more strongly dependent on pressure.
Fig. 8. Initial generation rate of I 3 for the venturi at various pressures and temperatures.
3.2.2. Initial reaction cavitation efficiency Initially (i.e. at five minutes) the venturi has constant cavitation efficiency (within experimental error) across the range of pressures at 10 °C (Fig. 10). Indeed, the venturi was clearly more energy efficient at 10 °C. However, no clear trend was seen (within experimental error) at higher temperatures, though as pressure decreases there is a general decrease in efficiency. On the other hand, the efficiency of the sudden expansion at 10 °C (Fig. 11) increases significantly with pressure. Again, at higher temperatures the efficiency response to pressure is more variable. 3.2.3. Initial reaction rate and cavitation number The cavitation number (Eq. (10)) potentially combines the effects of pressure, temperature and velocity into one number and might give clearer trends than Figs. 8 and 9. As the cavitation number increased in the venturi (Fig. 12) at 10 °C the initial reaction rate decreases. This behaviour fits the definition of cavitation number as a measure of resistance to cavitation [15], with decreasing cavitation number meaning the hydrodynamic cavitation reactor state is less resistant to cavitation. Though the same trend is apparent for higher temperatures, it is less clear. Similar trends are seen in the sudden expansion (Fig. 13) with a distinct increase in reaction rate with lowering cavitation number at 10 °C and I 3 yield fluctuation at higher temperatures. It can be seen that the cavita-
Fig. 9. Initial generation rate of I 3 for the sudden expansion at various pressures and temperatures.
Fig. 10. Cavitation efficiency of the venturi.
tion number alone is not sufficient to describe, or predict, the reaction rate in this work.
Fig. 7. Production of I 3 with the venturi (V) and sudden expansion (SE) at various upstream pressures and temperatures. Run with air-saturated solution is designated ‘‘sat”.
3.2.4. An I 3 consumption reaction The variable I 3 yield at temperatures above 10 °C, and the fact that all hydrodynamic cavitation runs reached an equilibrium limit indicated that a reverse, or subsequent, reaction occurred which consumes I 3 (this reaction is referred to as the consumption reaction in this paper). The runs at 7 bar and 30 °C were much less reproducible than the others (Fig. 14). Having done about 35 runs, each with at least seven sample points, giving well over 500 UV absorption tests, we were confident that experimental error was much less than the variability observed. It seemed possible that,
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Fig. 14. Evidence of a consumption reaction.
Fig. 11. Cavitation efficiency of the sudden expansion.
Fig. 12. Effect of cavitation number on reaction rate in the venturi. Fig. 15. Concentration of I 3 during circulation through the venturi at 1 bar without cavitation.
Fig. 13. Effect of cavitation number on reaction rate in the sudden expansion.
at 7 bar, 30 °C, the forward and consumption reactions competed in some unstable manner. To test this, an experiment was conducted at 7 bar and 30 °C in the venturi using a initial solution that had been irradiated with ultrasound to produce an amount of I 3 . The response in Fig. 14 clearly shows that I 3 is being consumed by another reaction. Gutiérrez et al. [18] discussed the possibility of reaction (8) occurring in non-acidic conditions during ultrasonic cavitation. The pH after a standard cavitation run for 30 min at 30 °C and 8 bar was measured to be 6.6. In a further run 100 mL of phosphate buffer with pH 5.9 was added to water to make 500 mL 0.1 M KI solution. The pH at the start of this experiment was 5.87, and at the end was 6.01. Fig. 14 shows the concentration of I 3 was no more stable at the lower pH. We were confident that the erratic concentrations were real. These runs provided strong evidence of a consumption reaction leading to an equilibrium. In the ultrasound pre-irradiated experi-
ment, I 3 was consumed during hydrodynamic cavitation until an equilibrium concentration was reached. The equilibrium concentration was similar to that obtained from a fresh KI solution with and without pH control. Interestingly the erratic concentration obtained from fresh KI never exceeded the equilibrium concentration. To test for the presence of the consumption reaction in the absence of cavitation, 500 mL of 0.1 M de-aerated KI solution was irradiated with ultrasound for 5 minutes to generate I 3 . 20 mL was removed from the irradiated KI and held separately as an irradiated baseline sample. The remaining KI solution was circulated through the venturi at 1 bar (well below audible cavitation) and at 10 °C. The cavitation number was 0.45 which was less than the critical value of 1.0 but well above the other experimental values in Figs. 12 and 13. Samples were extracted at intervals, tested for UV absorbance at 353 nm, and returned to the circulating solution. A sample of the irradiated baseline solution was also tested for UV absorbance concurrently with the circulated sample. The consumption reaction was found to occur even at low pressures, well below the onset of detectable cavitation (Fig. 15). The I 3 concentration in the baseline solution increased slightly before stabilizing. However, the I 3 concentration in the circulated solution decreased substantially before reaching an equilibrium concentration after about 80 min. The equilibrium concentration of 1.4 106 mol L1 is close to the concentration of 1.5 106 mol L1 produced after 30 min in the standard run at 8.7 bar and 10 °C (Fig. 7). 4. Discussion 4.1. Iodide oxidation pressure and temperature response Neither the venturi nor sudden expansion produced measurable I 3 below 4 bar, though cavitation was detectible at 1.8 bar. Other
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researchers have detected I 3 at lower pressures. Vichare et al. [28] detected I 3 (reported as iodine) at 0.7 bar using a multi-hole orifice plate while Suslick et al. [15] were not able to generate any I 3 below 150 bar with their jet fluidizer. Generally the response of KI solutions to increased inlet pressure was increased I 3 production in both the sudden expansion and venturi though at higher temperatures the effect was less clear. At higher inlet pressures the internal velocities are much greater and hence the driving force for bubble formation is greater. Further the time for travel to a lower pressure region is less. Both these effects are likely to encourage greater cavitation. Suslick et al. [15] reported a linear increase in I 3 concentration with increasing liquid pressure with a CCl4 saturated KI solution. Senthil Kumar et al. [2] reported a similar response to increasing pressure, but they reported a maximum yield of 5.7 106 g L1 (1.5 108 mol L1 of I 3 ) at about 2.8 bar before it decreased at a higher pressure. This is a low concentration, and is below the detection limit of 3.8 108 mol L1 for I 3 using photometric detection methods [19]. It is therefore difficult to know how to treat these results. Inlet pressure has featured as an important parameter in attempts to model hydrodynamic cavitation reactors [29,33–35]. Arrojo and Benito [31] have concluded that the physical significance of inlet pressure is undefined because of the lack of consistency in the results from this parameter for the same reaction (such as iodide oxidation). The present authors agree. This work showed that inlet pressure was a factor in I 3 yield under some conditions (and certainly there was a minimum pressure threshold before any reaction was seen). However, separating its significance from other parameters such as downstream pressure, temperature, hydrodynamic cavitation device geometry and, in the case of the iodide oxidation reaction, the influence of an I 3 consumption reaction (discussed below), is necessary before drawing conclusions. In both hydrodynamic cavitation devices, as temperature decreased I 3 yield increased, which is consistent with previous work [15,16]. 4.2. I 3 Concentration degradation The ultrasonic production of I 3 normally gave a straight line of concentration with time (Fig. 4) indicating that the concentration of I in 500 mL of 0.1 M KI solutions was well in excess and did not limit the reaction. In hydrodynamic cavitation, an equilibrium concentration was approached. Other researchers have reported this reduction in initial rate. Senthil Kumar et al. [2] attributed it to degassing and temperature increase of the KI solution as it is repeatedly circulated through the cavitation device. Vichare et al. [28] reported that their experimental conditions were not constant for the initial 5–10 min, and also propose that this is due to degassing. Neither degassing nor a temperature increase explain the reduction in the production rate of I 3 seen in this work. In this work, freshly (and thoroughly) de-aerated water was used which would remove the potential for further degassing, and also the temperature was controlled to within ±0.5 °C. Here there were strong indications that there was a significant consumption reaction, and this was confirmed by hydrodynamically degrading some ultrasonically produced I 3 (Fig. 14). In most cases, including the degradation of ultrasonically produced I 3, the reaction could be modelled as a zeroth order generation with a first order degradation (Eq. (9)) but almost certainly the mechanism is much more complex than implied by this equation.
dC I3 ¼ r 0 ðP; TÞ kðP; TÞC I3 dt
ð10Þ
Here the forward rate, r0, and the consumption rate constant; k, depended on the temperature and upstream pressure. The value of r0 corresponded to the initial rates given in Figs. 8 and 9 and k was found to range from 0.00075 to 0.0026 s–1. From the experiments done it was not possible to determine the rate of the forward reaction with any certainty. It was quite possible that the consumption reaction reduced the apparent forward reaction in some cases to zero (as at 7 bar 30 °C, Fig. 14). Given that this uncertainty made comparison of cavitation efficiency very difficult, further examination of the reaction mechanism or rates was beyond the scope of the work reported here. The presence of an I 3 consumption reaction (reaction (8)) has been discussed in relation to ultrasonic cavitation [18,19] where it has been shown to become significant in neutral or alkaline solutions. There is some evidence that a consumption reaction can become prominent in ultrasonic cavitation at high power/high frequency [8]. Hart and Henglein [20] stated that their results with ammonium molybdate (catalysing reaction (1)) confirmed that H2O2 was involved in the reaction. A complementary hypothesis is that H2O2 is involved in the consumption reaction and that catalysed reaction (1) (or reactions (2)–(6)) consumes the H2O2 and prevents the consumption reaction (8) from occurring. The continuation of the consumption reaction during circulation at low pressure (Fig. 15) shows that it does not require cavitation and suggests that H2O2 already in solution is used. Given that the higher temperature experiments gave lower equilibrium concentrations it seems likely that the consumption reaction was dependent on the bulk solution temperature, while the forward reaction was dependent on the intensity of cavitation. Given the apparent effect of the copper coil mentioned in the experimental section the possibility exists that the consumption reaction is a surface catalysed reaction. We offer no evidence to determine the cause of the difference between ultrasonic and hydrodynamic cavitation. Clearly the forward reaction dominates over the consumption reaction in ultrasonic cavitation. Similarly there is no evidence to determine why the different hydrodynamic devices responded differently to different pressures and temperatures. Almost certainly all three types of cavitation have different forms of bubble formation and collapse which might influence any of the possible reactions. When reviewed in the context of these results, other previous work is not inconsistent with the current findings. The general result of lower production of I 3 at higher temperatures is consistent with the proposal that the consumption reaction rate increases at higher solution temperatures. This work has not determined the mechanisms of the reactions but it has shown that the Weissler reaction cannot be used as a simple indicator for the effectiveness of process intensification by hydrodynamic cavitation.
5. 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 I 3 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 I 3 occurs during hydrodynamic flow, with and without cavitation. The uncertainty added by the consumption reaction causes the Weissler reaction to be unsuitable for the comparison of ultrasonic and hydrodynamic cavitation, and not even suitable for comparison of different hydrodynamic cavitation systems.
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