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Process Applications

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Online mixing:

Mixing operation is the heart of most of the industrial processes and is even energy intensive. Hydrodynamic cavitation can promote online uniform mixing and, even, chemical reactions without agitators. Hydrodynamic cavitation is a very effective tool based on the growth and implosive collapse of bubbles in liquid. Under the proper conditions, these bubbles undergo a violent collapse, generating very high pressures and temperatures. The growth and violent collapse of the bubbles themselves provides a desirably rigorous agitation of the liquid. The collapsing cavities generate spherically diverging shock-waves (symmetric collapse) or surface normal liquid jets (asymmetric collapse) of magnitude high enough to get uniform mixture or emulsion.

Particle size reduction:

Hydrodynamic cavitation can break down solids particles and increase their specific surface area. As a result higher rate of reaction is observed. In particular for the manufacturing of superfine-size slurries, cavitation has many advantages, when compared with common size reduction equipment, such as: colloid mills (e.g. ball mills, bead mills), disc mills, jet mills, rotor-stator mixers (ultra turrax) or high-pressure homogenizers. Cavitation allows for the processing of concentrated and viscous slurries, therefore reducing the volume to be processed.

Emulsification/ nanoparticles/ nanosuspensions:

Cavitation can be used for emulsification, nanoparticle synthesis or to produce nanosuspensions. In all of these applications a cavity needs to produce a stress greater than strength of a drop/ solid particle so that they reduce in size. Its application includes production of rubber nanosuspensions using hydrodynamic and acoustic cavitation, in hydrodynamic cavitation, nano-particles are generated through the creation and release of gas bubbles inside the sol–gel solution. Nano-suspensions have emerged as a promising strategy for an efficient delivery of hydrophobic drugs because of their versatile features such as very small particle size. The unique features of nano-suspensions have enabled their use in various dosage forms, including specialized delivery systems such as mucoadhesive hydrogels.

Chemical synthesis:

In case of homogenous system, a cavity that is formed is likely to enclose vapour of the liquid medium or dissolved volatile reagents or gases. During the collapse, the vapour or gas will be subjected to extreme conditions of high temperatures and pressures, causing molecules to fragment and generate highly reactive radicals. These radicals may then react either within the collapsing bubble or after their migration into the bulk liquid.

In liquid-solid systems intense turbulence is created due to asymmetric collapse of cavitation bubbles which forms a liquid jet targeted at the solid surface. This effect can activate the solid catalyst, increase the interfacial mass transfer and dislodge the material occupying the inactive sites, causes fragmentation of powders and particles.

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface will cause disruption of interface and mixing, resulting in the formation of very fine emulsions. When very fine emulsions are formed, the surface area available for the reaction between the two phases is significantly increased; this further increases the rate of reaction. This is very beneficial particularly in the case of phase-transfer catalyzed reactions.

The different ways in which cavitation can be used in the chemical processing applications are:

    •  Reaction time reduction
    •  Increase in reaction yield
    •  Reduction in the induction period
    •  Use of less forcing conditions (temperatures and pressure) as compared to the conventional routes
    •  Possible switching of the reaction pathways resulting in increased selectivity  Increasing the effectiveness of the catalyst used in the reaction
    •  Initiation of the chemical reaction by way of generation of the highly reactive free radicals

Surface cleaning:

Ultrasound has long being used for cleaning purpose at domestic level as well as at industrial scale. For this application the intensity of cavitation should be just enough to remove the dirt or impurities from the surface and should not be high enough to damage the surface itself. Apart from cleaning of solid surface, cavitation can also remove unwanted matter (poison) from the catalyst surface. This makes more active sites available for reactions to take place.

Microbial Cell disruption:

One of the important applications of hydrodynamic cavitation is in cell disruption. The process of microbial cell disruption occurs in several applications of ultrasound such as water disinfection, waste water treatment, enzyme recovery, etc. The cavitation effects can be classified as physical and chemical. The physical effects include the generation of shock waves, the water-hammer effect and radial bubble motion. The chemical effects of cavitation during the cell disruption are due to the generation of free radicals. Their extent depends on the intensity of cavitation which can be varied by manipulation of the operating parameters such as the cavitation number, initial concentration of the cell suspension, number of passes through the cavitation zone, temperature and viscosity. A cell is likely to get disrupted due to high velocity jet or shock wave produced by collapsing cavity. When one is interested in obtaining periplasmic enzyme, low intensity cavitation which is just sufficient to break the outer cell wall should be used. High intensity cavitation is required to reduce viable microbial count like in water disinfection applications or to recover the cytoplasmic enzyme.

Water and Effluent Treatment:

Hydrodynamic cavitation is applied for water disinfection for potable as well as for industrial use. When cavities collapses near a microbe it generates intense stress on the cell and disrupts them. This can be used for potable water disinfection, avoiding bio–fouling (by killing the microbes and/or destroying the nutrients required for their growth), etc. Hydrodynamic cavitation is an economically attractive alternative compared to the techniques such as ozonation and heat sterilization for reducing the bacterial counts.

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