Processes, cavitation-controlled

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Moser, W. R., Giang, T., Nguyen, S., and Kozyuk, O. V., A new route to cavitational chemistry and chemical processing by controlled flow cavitation, in Process Intensification for the Chemical Industry, 3rd International Conference (A. Green, Ed.), pp. 38, 173, BHR Group, London (1999). 

Cessna (9) carried out similar experiments as those reported here on several commercial impact thermoplastics over a range of strain rates. His work suggested that the classes of impact plastics studied exhibit a similar transition from volume-conserving to cavitation-controlled deformation processes as deformation rates are increased or temperature decreased. The present work supports those findings, as do the predictions of Bucknall and Drinkwater. Unlike Cessna, in no case did we find evidence of closure of cavities by shear yielding after cavitation. 

If the size of the bubble reactor is small compared with the mean free path of molecules in the circulation flow, the diffusion supply of reagents cannot be a limiting factor in the kinetics of synthesis. All these factors together are the reasons for the activation of chemical processes, and control particle size in the range of a few nanometers, i.e. one order of magnitude smaller than that without cavitation. 

WATER-SOFTENER SELECTION AND ANALYSIS 19.20 COMPLETE DEIONIZATION OF WATER 19.22 COOLING-POND SIZE FOR A KNOWN HEAT LOAD 19.24 PROCESS TEMPERATURE-CONTROL ANALYSIS 19.26 CONTROL-VALVE SELECTION FOR PROCESS CONTROL 19.27 CONTROL-VALVE CHARACTERISTICS AND RANGEABILITY 19.29 CAVITATION, SUBCRITICAL, AND CRITICAL-FLOW CONSIDERATIONS IN CONTROLLER SELECTION 19.30 INDIRECT DRYING OF SOLIDS 19.34 VACUUM DRYING OF SOLIDS 19.36 ESTIMATING THERMODYNAMIC AND TRANSPORT PROPERTIES OF WATER 19.37... 

Control of sonochemical reactions is subject to the same limitation that any thermal process has the Boltzmann energy distribution means that the energy per individual molecule wiU vary widely. One does have easy control, however, over the energetics of cavitation through the parameters of acoustic intensity, temperature, ambient gas, and solvent choice. The thermal conductivity of the ambient gas (eg, a variable He/Ar atmosphere) and the overaU solvent vapor pressure provide easy methods for the experimental control of the peak temperatures generated during the cavitational coUapse. 

High pressure homogenizers are especially suitable for the emulsification processes in the food, pharmaceutical and bioprocess industries. A general disadvantage of these type of reactors is that there is no precise control over the cavitationally active volume and the magnitude of the pressure pulses that will be generated at the end of the cavitation events (cavitational intensity), unless the valve seat designs are substantially modified. 

A primary limitation of sonochemistry remains its energy inefficiency. This may be dramatically improved, however, if a more efficient means of coupling the sound field with preformed cavities can be found. The question of selectivity in and control of sonochemical reactions, as with any thermal process, remains a legitimate concern. There are, however, clearly defined means of controlling the conditions generated during cavitational collapse, which permit the variation of product distributions in a rational fashion. 

In thermal solar power plants, high-temperature oil flows need to be controlled in hydrogen processes, very-low-temperature LH2 flows have to be controlled. For these reasons, the emphasis in this treatment will be on control valve designs that are suited for these applications and on phenomena (such as noise and cavitation) that are common in these applications. 

As was already outlined, the experimental conditions for sonochemistry must be carefully considered when a process is designed, and these conditions must be carefully controlled during operation. Here is a brief account of the main parameters influencing cavitation chemistry. 

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