Ind. Eng. Chem. Res. 2004, 43, 2775-2780
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Bubble Size Distribution Formed by Depressurizing Air-Saturated Water Apostolos G. Vlyssides,* Sofia T. Mai, and Elli Maria P. Barampouti Chemical Engineering School, National Technical University of Athens, 9 Heroon Polytechniou Street, Zographou 157 00, Greece
Wastewater treatment by dissolved air flotation (DAF) relies on the adhesion of particles or flocs to air bubbles, formed when pressurized and air-saturated water is released in the DAF reactor. Bubbles sizes are of paramount importance. A differential gravimetric bubble separation method was used for the estimation of the bubble size distribution. The relation between the air bubble diameter after water depressurization and the pressure of the air-saturated water, its temperature, and the ratio of air saturation was studied. The bubble size distribution was proved to depend on the pressure of the air-saturated water. The temperature of water and its percentage of air saturation did not have a considerable effect. The experimental results and their treatment define the bubble size distribution, which is a dominant factor for highly efficient DAFsflotation. Introduction Dissolved air flotation (DAF) is the most commonly used flotation process in the field of water and wastewater treatment. Bubbles are generated by the release of pressurized, air-saturated water. The degree of saturation at a given pressure and temperature is determined by Henry’s law. When the saturated water is depressurized, air bubbles are released: the decrease of hydrostatic pressure causes a phenomenon of deaeration and, as a consequence, microbubbles appear in the water. The microbubbles thus obtained have a diameter generally between 5 and 220 µm. This size range of bubbles is very convenient for separating particles that may be found in water or wastewater.1,2 Takahashi et al.3 (1979) were among the first to make a fundamental study of bubble formation in DAF. They also observed that the bubbles were subject to coalescence after having been formed, thus provoking a gradual increase of the average bubble diameter. Recently, effective DAF models1,4-6 have been proposed: they describe the collision between a bubble and a particle in the contact zone. According to these models, it is confirmed that the fraction Xn of particles (based on number) successfully attached to air bubbles depends on the size of these bubbles. The collision between bubbles and particles due to interception is more effective when the bubble diameter is smaller: the size of the bubbles that are released is of paramount importance for the efficiency of the formation of air-particle agglomerates and subsequently for the efficiency of a DAF process. During the depressurization of water saturated with air, first bubbles smaller than 1 µm are formed. Through mechanisms not thoroughly understood,2,7,8 they join and produce a range of bubbles, with sizes between 1 µm and a few millimeters. Bubbles larger than 1 mm should be removed before their input in the contact zone because they might destroy the flocs.1,4 It is, moreover, considered that effective bubble-particle collisions for * To whom correspondence should be addressed. Fax: +302107723269. E-mail:
[email protected].
the formation of agglomerates are achieved by bubbles of 10-120 µm2. The parameters that have a strong effect on the final bubble size distribution are the design characteristics of the depressurization mechanism (e.g., Reynolds tube), the pressure of the air-saturated water,2,4,7 and the amount of air released.4 Different researchers have studied the mechanisms9-13 of bubble formation and the bubble sizes formed (especially the air bubble median diameter).1,4 A few more detailed references2,4,7 are available; however, they are insufficient for the design of a DAF. This is due to the relative inaccuracy of the microphotography2,14 method that was used for the determination of the bubble size distribution. This work uses a method of differential gravimetric bubble separation based on their critical velocities. This method, as described below, is simple and accurate. Its results can be applied for the design of a DAF system. Bubble size distributions and related parameters will be determined. Methods and Materials Principle of the Bubble Gravity Separation. The critical rise velocity of a bubble (ucr) in a liquid depends on the bubble diameter, the liquid specific gravity, the bubble specific gravity, and the liquid viscosity (as affected by its temperature). When the liquid-bubble mixture moves downward in a vertical flow tube, the direction of the bubble movement will depend on the ratio of the liquid velocity (V) and the rise velocity of the bubbles. If the bubbles have a large range of diameters, then for a given liquid velocity Vi, a certain bubble percentage for which uj > Vi will flow countercurrently to the liquid flow direction, while the rest of uj < Vi will follow the liquid’s flow direction. The bubble diameter at uj ) Vi is a critical bubble diameter db,cr,i for the given liquid velocity Vi. Bubbles with db > db,cr,i will flow upward, while bubbles with db < db,cr,i will flow downward together with the liquid. The percentage of air bubbles with db > db,cr,i can be estimated, given the total amount of air that the liquid contains and collecting the air that flows upward. Subsequently, when the
10.1021/ie0307176 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004
2776 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004
Figure 2. Relation between the critical bubble diameter and water flow rate.
Figure 1. Estimation of the bubble concentration (of size db,i) by subtracting two volumes of released air, as fixed by db,cr,i and db,cr,i+1 critical bubble diameters.
liquid velocity is changed and the air percentage that is separated is measured, the concentration of air bubbles that have a diameter between db,cr,i < db,i < db,cr,i+1 can be estimated (Figure 1). The experimental apparatus, which is applied and described below, had the distinctive ability to estimate bubble concentrations for a diameter difference of up to 5 µm. Theoretical Approach. For an air bubble-water system and for the laminar region (Reb < 1), the critical bubble diameter can be calculated using Stokes law.4,6,15,16 However, for bubbles under investigation (
