Process Intensification and Green Processing Using Cavitational Reactors - Process Intensification for Green Chemistry - page 208

The following important design recommendations can be made based on a detailed
analysis of the existing literature [20,26,27]:

1. An orifice flow configuration is more suitable for applications requiring intense
cavitational conditions, whereas for milder processes (typically requiring collapse
pressure pulses between 15 and 20 bar) and for transformations based on physical
effects, a venturi configuration is more suitable and more energy efficient.
2. In the case of a venturi flow, the most economical technique for increasing cavitation
intensity is to reduce the length of the venturi, but for higher volumetric flow rates
there might be a limitation caused by the possibility of flow instability and super-
cavitation. A similar argument can be made for the enhancement in the cavitation
intensity caused by reducing the venturi throat to pipe diameter ratio.
3. In the case of an orifice flow configuration, the most convenient way of controlling
the cavitation intensity is to control the orifice to pipe diameter ratio and to control the
cross-sectional flow area by manipulating the number and diameter of the hole on the
orifice plate, although indiscriminate growth of bubbles downstream of the orifice can
lead to splashing and vaporization (super-cavitation) of the flow.
4. Increasing pipe size downstream of the orifice (which offers a faster pressure
recovery) is another option for intensifying cavitation effects, but using pipes of a
larger size will require higher volumetric flow rates for the same cavitation number,
resulting in an increase in processing costs.
5. In the case of orifice plates, it will be a good idea to have a higher number of holes on
the plates for similar free-flow areas, so that the number of cavitational events and
hence the cavitationally active volume is increased.
6. Higher inlet pressures into the system result in higher cavitational intensity, but
optimum values should be used so as to avoid occurrence of super-cavitation, where a
single large slug is formed with much lower cavitational effects.
7. The recommendations for liquid physicochemical properties are exactly similar to
those discussed for sonochemical reactors.

7.6 Intensification of Cavitational Activity

At times, the net rates of chemical processing achieved using cavitational reactors may not
be sufficient to prompt industrial-scale operation. This is especially important given the
possibility of an even more uneven distribution of cavitational activity in large-scale
reactors. It is thus important to look into supplementary strategies with the aim of
intensifying the cavitational intensity. Two types of operating strategy can be recom-
mended, depending on the type of application:
1. The use of process-intensifying parameters such as the presence of dissolved
gases and/or continuous sparging of gases such as air, ozone and argon (to a
limited extent), the presence of salts such as NaCl, NaNO 2 and NaNO 3 and the
presence of solid particles such as TiO 2 , CuO and MnO 2 , which can also act as
catalysts in some cases.
2. The use of a combination of cavitation and other processes with similar mechanisms
such as advanced oxidation processes, including photocatalytic oxidation and Fenton