BACKGROUND AND FORMULATION OF WORK
1.3 Different Aspects of Microbubble Flotation for Fine Separation
1.3.1 Engineering Aspects
1.3.1.2 Design of Unit
The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters (Shah, 1979). During the past decade, extensive research efforts have been made in order to improve our knowledge in these areas (Shah et al., 1982). Advancement on flotation technology has come about through the application of new theories (i.e. adsorption mechanism, electrochemical control) novel flotation reagents, various alternative flotation technique and so on.
In the last decay new horizons have been opened for this process in chemical technology, particularly in its use as a selective separation technique. The design of a flotation column where separation would be carried out on a commercial scale would depend on multiple aspects of chemical engineering (Mathis, 1995). The design and efficient operation of ionic flotation column also require knowledge of what is happening inside the column: how much time is allowed for the particles to mix with the microbubble? Are there any dead spaces, where mixture gets trapped, or any bypassing, with either air or pulp leaving the column too soon? All these are cases of malfunctioning results non-ideal flow and lead to poor performance of the unit (Matis and Mavros, 1991).
1.3.1.3 Microbubble Generation
Microbubbles can be generated by number of ways. Researchers had made several efforts to develop a generator which can produce microbubbles with a minimal power requirement. A brief description of different microbubble generation methods are as follows:
1.3.1.3.1 Methods based on the Mechanics of Flowing Fluid
The mechanics of flowing liquid is used to create the microbubbles. Some of the important methods for microbubble generation under this category have been described in the following section:
(i) Spherical body in a flowing tube: Microbubbles can be generated by introducing pressurized water in a pipe having spherical body in the core region. The spherical body increases the velocity of liquid in the pipe, due to which pressure reduces and creates automatic suction of gas in liquid, results in generation of microbubbles of approximately 50 µm (Sadatomi, 2003).
(ii) Rotary liquid flow type: In this method, pressurized water is pumped to create a rotary type of flow of liquid which sequentially causes a reduction in pressure in its central axial part. Gas is automatically induced to the reduced pressure area. The air-liquid mixture then develops microbubbles of approximately 63 µm, due to high smashed and shears (Koichi and Yasuyuki, 2007). Ohnari (2006) developed a cylindrical microbubble generator by this principle. The liquid flow rate can be up to 12 litres per hour and the rotary speed of the gas-liquid mixture can have 300-600 revolution per sec. The 63.
(iii) Static mixture type: The smashing of flow in a static mixture with a swirl pattern can produce fine bubbles. The gas-liquid flow is channelized into a swirl pattern by guided vanes. The smashing of flow is done by two mushroom shaped projections. This type of microbubble generator generates microbubbles having size ranges from 5 to 50 μm at maximum rate of 1500 l/min (Uematsu, 2006).
(iv) Venturi type: In this method, gas and liquid are passed simultaneously through the venturi tube to generate microbubble. It is composed of an inlet section, a tubular part,
a throat and a tapered out flow section. Pressurised fluid is introduced in the tabular part through the inlet section. The throat of venturi-system accelerates the two phase fluid, due to which the pressure in this part decreases. Because of decrease in pressure cavitation occurs in the system which generates microbubbles (Fujiwara, 2006).
Yoshida et al. (2008) used this method to generate microbubble having diameter of approximately 70 µm.
(v) Ejector type: Ejector uses the venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction gas. After passing through the throat of the injector, the gas-liquid mixer expands and the velocity is reduced. The shearing of gas- liquid mixture in turbulent flow creates the microbubbles of diameter less than 58 µm (Nakatake et al., 2007;Chu et al., 2008).
(vi) Multi fluid mixture device: Sadatomi and kawahara (2008) invented a new device which can generate microbubbles as well as mists. They used orifice and porous pipe instead of spherical body and small drilled holes. When pressurized water enters in the generator, the velocity of water increases due to the presence of orifice, from the principle of energy conservation, the pressure at a little downstream of the orifice becomes negative, this creates automatic suction air in water from porous pipe, which results in generation of microbubble.
(vii) Pressurized dissolution type: In this method air is allowed to dissolve in water by applying a pressure of about 3-4 atm and water is flushed through a nozzle inside the water pool. Because of reduction in pressure the supersaturated air is released into expelled water in the form of microbubbles of approximately 52 µm (Tsuge, 2010).
1.3.1.3.2 Methods without Accompanying Liquid Flow
These methods of microbubble generation has only gas phase in motion, and the blowing gas used to generate microbubbles. Some of the methods for microbubble generation with gas phase in motion are rotary gas flow type, porous mullet ceramics technique, porous membrane type, etc.
(Kukizaki et al., 2004; Okada et al., 2010; Onoe et al., 2002). Some of low power generation methods are also used to generate microbubble such as are flow focusing technique, microchannel technique, ultrasonic systems, micro-bubbler technique, etc. (Gañán-Calvo and Gordillo, 2001;
Lee et al., 2005; Makuta et al., 2006; Yasuno et al., 2004). Microbubbles can also be generated by some other techniques such as by applying electric field, from porous plates, by spinning disk and by electrohydrodynamic atomization (Burns et al., 1997; Farook et al., 2007; Fujikawa et al., 2003;
Weber and Agblevor, 2005). More details of all the method are published by us (Parmar and Majumder, 2013).