## 4 Effect of Essential Factors on CE
### Intensity and Pressure  
The generation of a number of active cavitation bubbles can be significantly altered depending on the extent of applied ultrasound intensity. The ultrasound intensity can be calculated from the following Equation (1).  
\[\mathrm{I}=\mathrm{P}/\mathrm{S} \tag{1}\]  
where \"I\" is the intensity (W/cm\({}^{2}\)), \"P\" is the ultrasound power and \"S\" is the surface area of the transducer.  
The correlation between sonochemical reactions and the applied power can be illustrated as follows: (1) The rate and the generation of number of active bubbles increase with an increase in intensity, (2) an increase in the size of individual bubbles with an increase in intensity, resulting into higher collapse temperatures because of the conversion of higher available potential energy into heat, (3) an increase in the mixing capability of solution results with an increase in the intensity because of the turbulence produced from cavitational effects.  
By increasing the power, the energy of cavitation enhanced following a more violent implosion. The optimum power dissipation for effective extraction is dependent on the configuration of the reactor and the product employed for extraction. Cavitation events could be affected or reduced marginally beyond the optimum power intensity, because of the excessive production of heat during sonication, which leads to a less violent bubble collapse [78].  
Generally, the extraction yield increases linearly with an increase in power density, but in some instances, it may be an exception depending on the properties of target molecules. [PERSON] et al. [20] observed a linear enhancement in the yield of mandarin peel extract (MPE) with an increase in power level from 30.34 to 59.36 W. [PERSON] et al. [28] indicated a decline in the extraction of polysaccharides from _Polygomum multiforum_ with a rise in power beyond 140 W. Therefore, the intensity can affect both the total amount of extracts to be recovered, as well as the proportion of final products. [PERSON] et al. [29] reported the existence of an optimum intensity for the extraction of pectin from passion fruit. Hence, the optimum intensity needs to be decided following the properties of individual compounds. Higher amplitude sonication can opt for high viscous solvents, but the optimization of intensity is necessary for the type of matrix to avoid the degradation of extracted compounds.  
In the case of hydrodynamic cavitation (HC), the variation in the inlet pressure or negative pressure can alter the cavitational phenomena. In the case of venturi type HC, deciding the value of cavitation number (Cv), can be beneficial while selecting the optimum inlet pressure for extraction. The cavitation number can be estimated based on the following Equation (2).  
\[\left(\mathrm{C_{v}}\right)=\frac{P_{2}-P_{v}}{\frac{1}{2}\rho v_{0}^{2}} \tag{2}\]  
where \(P_{2}\) is the fully recovered downstream pressure, \(P_{v}\) is the saturated vapor pressure of the liquid, \(\rho\) is the density of liquid and \(v_{0}\) is the average velocity of liquid at the constriction of the orifice. With an increase in inlet pressure and a decrease in cavitation number below 1, the intensity of cavity collapse increases with the generation of a larger quantum of active radicals. However, beyond the optimum inlet pressure, indicating supercavitation, where indiscriminate and rapid growth of bubbles takes place at the downstream of orifice constriction which leads to choked cavitation [79].