What is cavitation?
Cavitation is defined as a phenomenon of formation, growth, and collapse of microbubbles or cavities, occurring in a few milli- to microseconds at multiple locations in the reactor and thus releasing large magnitude of energy in a short span of time. Cavitation is initiated with the formation of vapor cavities (bubbles or voids) when liquid enters into the low-pressure region, and subsequently these cavities attain a maximum size under the conditions of isothermal expansion.
In the successive compression cycle, an immediate adiabatic collapse occurs, resulting in the formation of supercritical state of high local temperature (1000°K-10,000°K) and pressure (100-1000 bar), known as hot spots. The intense temperature and pressure conditions that are generated in these hot spots facilitate process intensification in many areas of industry.
The mechanical effects of cavitation are mainly responsible for the intensification of physical processes such as synthesis of nanoemulsions, nanoparticle synthesis, microbial cell wall disruption, resulting in disinfection, etc.
The chemical effects, such as the generation of highly reactive free radicals in the immediate environment, are responsible for the intensification of chemical processes such as: depolymerization, catalyst free molecular degradation, synthesis of chemicals, oxidation of organic pollutants, etc.
Cavitation can be generated in a liquid medium either through flow variation in a flowing liquid known as hydrodynamic cavitation (HDC) or by exposing the liquid medium to ultrasonic waves known as acoustic cavitation (AC).
In this technique, the pressure variations in the liquid are affected by sound waves, usually ultrasound (> 20 kHz). The acoustical energy is generated by the transmission of ultrasound waves that consist of the rarefaction and compression cycles traveling through the liquid medium. In the rarefaction (expansion) cycle, a large negative acoustic pressure is developed, which increases intermolecular distances, and cavities are formed where it exceeds the critical molecular distances.
In the compression cycle, the positive acoustic pressure pushes the cavities together and compresses them, which eventually leads to their violent collapse. The final collapse phase is adiabatic in nature and, thus, locally produces high-temperature and high pressure conditions.
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Hydrodynamic cavitation (HDC)
Cavitation is produced by affecting pressure variations in a flowing fluid by allowing the fluid to pass through a constriction in a pipe. The variation of pressure through a constriction channel such as venturi, orifice, etc., with different geometries leads to the generation of cavities. HDC can also be produced by mechanical rotation of an object within a liquid. Although there are other HDC devices such as high-speed homogenizer, high-pressure homogenizer, and high-speed rotor, which can create cavitation conditions, they have been limited in terms of their applications because of high energy cost involved in generating high pressure and speed. Orifices and venturis are the most widely used cavitating devices for generating HDC.
HDC produces mostly low-intensity cavity collapse as compared with AC, but more cavities are generated in HDC, resulting in higher volumes making it more efficient than AC. Despite many laboratory scale, and pilopt scale studies, acoustic cavitation has found few uses on an industrial scale because of many limitations, such as poor transmission of acoustic energy in large volume of liquid, higher operational cost and low energy efficiencies. Voltek designs, builds and utilizes HDC in it’s processes. The cavitation device is designed for each application because cavitation occurs when the pressure is reduced to a point lower than the vapor pressure of the liquid, and must be controlled to produce either spherical or non spherical collapse (see below).
Sudden pressure and velocity variation cause dynamic cavity oscillations, and as the cavity collapses, certain physical and chemical effects occur in its vicinity leading to the desired transformations. Also, the type of cavity collapse controls the desired transformation. There are two conditions of cavity collapse: symmetric and asymmetric. The cavity may remain spherical till the point of collapse or nonspherical because of the presence of interface at the boundary wall or other particle/bubble surface near the oscillating cavity. The formation of reactive free radicals for oxidation, thermal pyrolysis of organic molecules and other chemical transformations are favorable under spherical collapse, i.e. symmetric collapse.
By contrast, nonspherical collapse, i.e. asymmetric collapse, produce high-velocity liquid jets and intense local turbulence, which are beneficial for physical transformation applications such as intense mixing, nanomaterial production, etc. Although both symmetric collapse and asymmetric collapse receive the same energy from pressure fluctuations, energy is delivered in different forms, i.e. either in the form of intense turbulence or in the form of extreme condition of pressure and temperature.
According to the hot spot theory, each cavity acts as a microreactor where high-pressure and high-temperature conditions are created and release large amount of energy in a very short duration. The theory describes the following three reaction zones in the cavitating system:
(a) In the core of the cavities, the temperature and the pressure reach the highest peak (10,000° K and 1000 atm, respectively) during the collapse of the cavities. Inside this region, the entrapped molecules dissociate into smaller intermediates and also produce reactive free radicals upon dissociation. These radicals attack the targeted molecules, initiating further reaction. For example, water molecules dissociate into ˙OH and ˙H radicals.
(b) In the interface region or very near to the cavity interface, high shear microjet and turbulence is created because of cavity oscillation and its subsequent collapse. The temperature may go up to 2000° K in the proximity of the cavity interface. This high turbulence enhances the mass transport of the generated radicals, and therefore the scope of reaction of these radicals with the targeted organic molecules is higher in the cavity-liquid interface than that in the bulk liquid region. Also, the organic molecules that are present near the cavity-liquid interface get thermally decomposed because of high temperature in this region.
(c) In the bulk liquid region, the temperature is not so high and remains near the atmospheric temperature. In this region, the generated radicals diffuse into the bulk liquid and react with the targeted molecules. The quantification of the chemical effects occurring within the cavitating device in terms of generation of ˙OH radicals and reaction yields is important. The quantification is conducted by evaluating the reaction yields based on the oxidation of substrate molecule by ˙OH radicals and the oxidation products produced, which is equivalent to the quantum of ˙OH radicals being used.
HDC is usually generated by providing a suitable constriction in a liquid flow. The throttling can be created using various devices such as venturi, orifice, high-speed rotor, and homogenizer. The operating pressure and flow conditions may vary with these devices, but the phenomena of cavity generation is the same where sufficient throttling transforms the liquid into vaporous cavities. Orifice and venturi-based HDC devices are found to be the most efficient in creating an intense cavitation condition. However, larger, continuous flow rates are more efficiently treated by a dynamic cavitator with two rotors rotating counter clockwise.
Voltek designs and builds the venturi type static cavitator nozzles and a proprietary shear induced cavitator
Our venturi reactor skids are designed for pilot scale work and low flow rates. It has been successfully used in many wastwater treatment applications. The reactor skid is configured as shown in the diagram below. The liquid is pumped in a closed loop with a by pass valve to control the pressure at the venturi. pH is adjusted based on the target organic molecules. Flow, temperature, initial pressure, cavitation pressure and final pressure are adjusted based on simulations and designed into the system to achieve the maximum effect.
For higher flow rates, Voltek makes the dynamic cavitators based on the principle of shear induced cavitation. The reactor consists of two facing rotors with special radial grooves where each one is spinning in the opposite direction The rotors each have special geometry, which causes periodically repeating pressure drops. The rotating frequency of the rotors is adjusted by VFDs, creating velocities comparable to the venturi type set up. When the teeth of the two rotors are aligned, the gap between them resembles the Venturi nozzle geometry. The Venturi shape geometry of the teeth causes a low pressure zone – if the pressure is low enough, the cavitation forms. Cavitation is present in three different regions. In the gap between the rotor and the housing, where attached cavitation forms on the leading edge of the teeth. When the two grooves are aligned, cavitation forms in the gap between the rotors. Finally cavitation clouds also form in the Venturi gap between the aligned teeth. Very rapidly changing pressure field points to an aggressive cavitation process.
Effect of other operating parameters of HDC
The intensity of cavitation that occurs inside the cavitating device also depends on the operating inlet pressure and cavitation number, which are interdependent of each other. At lower cavitation number or higher inlet pressure, more cavities are formed. However, a decrease in cavitation number beyond an optimum value leads to the condition of choked cavitation, which has no practical utility. The optimum operating pressure and the cavitation number depend on the area of application for which HDC is applied. The cavitation inception is affected by the presence of dissolved gases and suspended particles in the solution