Acoustic irradiation

Kuusisto, J., Mikkola, J.-P., Salmi, T., and Murzin, D. (2006) Suppression of catalyst deactivation by means of acoustic irradiation - application on fine and specialty chemicals. Chem. Eng.J., 120, 91-98. 

Toukoniitty B, Mikkola J-P, Murzin DYu, Salmi T (2005) Utilization of electromagnetic and acoustic irradiation in enhancing heterogeneous catalytic reactions. App Cat A Gen 279 1-22... 

A number of terms in this area will be unfamiliar to most chemists. Cavitation is the formation of gas bubbles in a liquid and occurs when the pressure within the liquid drops significantly below the vapor pressure of the liquid. Cavitation can occur from a variety of causes turbulent flow, laser heating, electrical discharge, boiling, radiolysis, or acoustic irradiation. We will be concerned... 

Figure 2. Pressure drop evolution for the middle part of the core (ri/ a, in Fig. 1) during acoustic irradiation. The acoustic power is 25% of the maximum power (2 kW).

During the evaluation of our calculations we noticed, that the dissipation of the acoustic waves has an important influence on the temperature of the liquid. The dissipation caused an increase of the liquid temperature, which in its turn caused a decrease of the liquid viscosity and, as a result, the pressure gradient over the core decreased at constant liquid flow rate. This phenomenon is completely responsible for the pressure drop effect. It is a measurable effect and has to be taken into account when studying acoustic irradiation of porous materials. From the evaluation of the calculations we could also conclude, that the momentum transfer of the acoustic waves to the liquid, i.e. acoustic streaming, has a negligible effect on pressures and temperatures of the liquid, although the effect is measurable. 

It should be noted that acoustic irradiation is a mechanical energy (no quantum), which is transformed to thermal energy. Contrary to photochemical processes, this energy is not absorbed by molecules. Due to the extensive range of cavitation frequencies, many reactions are not well reproducible. Therefore, each publication related to the use of US generally contains a detailed description of equipment (dimensions, frequency used, intensity of US, etc.) [709]. Sonochemical reactions are usually marked )))), in accordance with internationally accepted usage [708], For successful application of US, the influence of various factors can be summarized as follows [710] ... 

Acoustic irradiation appears to be able not only to boost chemical reactions but also to intensify mass transfer processes in multiphase systems. A twofold increase of k a using ultrasound has been observed [150], but depends strongly on the reaction conditions. Other authors have reported instead higher intensification factors. The enhancement is probably related to a reduction of the boundary layer thickness due to the microscale turbulence and reduction of the viscosity in the boundary layer. 

Catalytic Hydrogenation of Citral The Effect of Acoustic Irradiation... 

To study whether acoustic irradiation might have a beneficial effect on the hydrogenation velocity, or on the obtained selectivity to the desired products, a series of experiments with on-line unltrasonic treatment of the reaction mixture was performed. 

Hydrogenation experiments with simultaneous acoustic irradiation were carried out by using iso-propanol (2-propanol) as solvent in an automatic laboratory-scale autoclave (Parr 4560) with an effective liquid volume of 250 ml. The operating conditions were as follows 50 bar hydrogen pressure and 70°C (343 K) as the reaction temperature. The catalyst-to-citral ratio was 1 25 (wt wt) in the beginning of the reaction. A commercial molybdenum promoted Raney nickel catalyst with a mean particle size of approx. 22 pm and the specific surface area in the range of 80 m /g was used in the experiments. The reactor contents were analyzed oiF-line with gas chromatography (GC). 

Figure 3 A schematic illustration of the equipment for simultaneous hydrogenation and acoustic irradiation. The enlargement illustrates principally the nature of the acoustic field it is at its strongest close to the tip of the vibrating horn.

When the catalyst was recycled for use in the second batch, the deactivation was found to be rather severe. This, however, was expected - mainly due to the very small amount of catalyst applied into the hydrogenation batch. Nevertheless, the results indicated that ultrasound enhanced the reaction rate. The maximal yields were 0.60/0.08 at 220 min, compared with those obtained by conventional hydrogenation technology, 0.42/0.06 at 345 min. It is probable that the yield of OL would eventually revert in the absence of acoustic irradiation, as well. 

Figure 4 Hydrogenation of citral in the presence and absence of acoustic irradiation. The by-product emerging towards the end of the batch (batch with acoustic irradiation) is isopu-legol.

The acoustic field is not homogeneous over the entire reactor volume. This fact, indeed, is one of the complications related to the acoustic irradiation technology the results tend to be equipment dependent, since the field strength is never homogenous over the complete volume space. Moreover, the design and placement of the hom varies from equipment to equipment. 

The experiments illustrated the beneficial effects of acoustic irradiation in the hydrogenation of citral to citronellal and citronellol in a polar solvent. This is in line with previous studies conducted with different chemical systems, such as hydrogenation of xylose to xylitol (5, 6). The future challenge lies in detailed understanding of the microscopic phenomena taking place in the system exposed to the acoustic irradiation although numerous investigations have been conducted in the use of ultrasound in chemistry, the exact mechanisms still remain a secret to the scientific community. 

Use of electromagnetic and acoustic irradiation to enhance heterogeneous catalytic reactions has been recently reviewed [83]. Alternative use of ultrasound and microwave irradiation for hydrocarbon oxidation and catalyst preparation, to improve selectivity, was also recently reported [84]. 

Ultrasound irradiation has been used in the process of catalyst preparation. Acoustic irradiation increases the dispersion of the active metal on the support,depassivates the metal, and reduces the particle size to nanometer scale.In the case of palladium supported on active carbon prepared under ultrasound with extremely high surface area, not only was a greater metal dispersion achieved, but a larger penetration of metal inside the pores of the support and an easier elimination of chloride ion were observed as well. ... 

Cholesteric liquid crystals can also be used to display sound fields, especially ultrasonic fields [13, 14]. The ultrasonic source is immersed in water and directed at a black foil on the water surface. The upper side of the foil is coated with a cholesteric liquid crystal film. The acoustic irradiation of the indicator foil generates colored images of the ultrasonic source and also various interference patterns. The reason for colors appearing can be attributed to conversion to heat of the ultrasonic energy absorbed by the foil. 

Figure 5 The NMR spectra of particulate trisodium phosphate dodecahydrate suspended in a bromoibnn/chloroform mixture (A) without acoustic irradiation and (B) and (C) irradiated with 2 M Hz ultrasound at 30 and 50 W cm- respectively. Reproduced with permission from AL Weekes, Ph.D. thesis, Aston University, 1998.

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