Page 176 - Industrial Wastewater Treatment, Recycling and Reuse

150 Industrial Wastewater Treatment, Recycling, and Reuse

3.2.2 Hydrodynamic Cavitation
One of the alternative techniques for the generation of cavitation is the use
of hydraulic devices where cavitation is generated by the passage of the liq-
uid through a constriction such as a valve, orifice plate, or venturi (Gogate
and Pandit, 2005; Moholkar et al., 1999).
Hydrodynamic cavitation (HC) can simply be generated by the passage
of the liquid through a constriction such as an orifice plate. When the liquid
passes through the reduced cross-sectional flow area of the constriction, the
kinetic energy/velocity of the liquid increases at the expense of the pressure.
If the throttling is sufficient to cause the pressure around the point of vena
contracta to fall below the threshold pressure for cavitation (usually vapor
pressure of the medium at the operating temperature), cavities are generated
locally. Subsequently, as the liquid jet expands downstream of the constric-
tion, the pressure recovers, and this results in the collapse of the cavities.
During the passage of the liquid through the constriction, boundary layer
separation occurs and a substantial amount of energy is lost in the form of
a permanent pressure drop. Very high intensity turbulence occurs on the
downstream side of the constriction; its intensity depends on the magnitude
of the pressure drop, which, in turn, depends on the geometry of the con-
striction and the flow conditions of the liquid. The intensity of turbulence
has a profound effect on the cavitation intensity (Moholkar and Pandit,
1997). Thus, by controlling the geometric and operating conditions of
the reactor, one can produce the required intensity of the cavitation so as
to bring about the desired chemical and/or physical change with maximum
efficiency. Also, the collapse temperatures and pressures generated during
the cavitation phenomena are a strong function of the operating and geo-
metric parameters (Gogate and Pandit, 2000b).
Figure 3.2 shows the hydrodynamic generation of cavities schematically.
The pressure-velocity relationship of the flowing fluid as explained by Ber-
noulli’s equation can be exploited to achieve this effect. When flowing liq-
uid passes through a mechanical constriction (either orifice or venturi;
Figure 3.2a), its velocity increases, accompanied by an increase in the kinetic
energy and a corresponding decrease in the local pressure (Figure 3.2b). If
the throttling is sufficient to reduce the absolute local pressure below the
vapor pressure (at the operating temperature), spontaneous vaporization
of the medium in the form of microbubbles (nucleation) occurs. With con-
tinued lowering of the pressure, the cavity continues to grow by further
vaporization or desorption of gases (usually some gases are dissolved