Table of Contents
The nature of the gas distribution system as well as the hydrodynamics of rising swarms of bubbles can create various profiles. Three radial gas holdup profiles have been identified: parabolic, saddle-shaped and W-shaped. Radial gas holdup profiles have been measured in a large-scale laboratory flotation column using the same multi-cell conductivity probe. Results on three bubble generation systems: vertical filter cloth spargers, and horizontal Cominco-type and filter cloth spargers are presented.
Experimental
The column is illustrated in Figure 1. It was made in three sections of PVC and was 50cm in diameter and 400cm in total height.
Vertical Cloth Sparger System
The diameter of the sparger was 10cm and the height was 12cm. The supporting frame was made of PVC and was cylindrical with a large number of perforated orifices (approximately 1.0cm in diameter) uniformly distributed over the surface of the cylindrical frame (there were no orifices in the two ends).
Horizontal Cominco Sparger System
It was made of a stainless steel tube of length 30cm and outside and inside diameter 1.9cm and 1.2cm, respectively. There were two rows of five identical orifices which were evenly distributed. The diameter of the orifice was 0.1cm. .
Horizontal Cloth Sparger System
Three such spargers were mounted in the column bottom section in the same way as for the Cominco spargers (Figure 5(b)). Air was introduced, as in the vertical filter cloth sparger system, through individual air flowmeters.
Multi-Cell Conductivity Probe
The approach used here to measure gas holdup profiles was that described by Xu et al. (1992).
In most of the systems studied to date, Maxwell’s model for conductivity of a two-phase dispersion with a non-conducting dispersed phase (air bubbles in this case) applies. Re-arranging, gas holdup εg is given by,
εg = 1 – γ/1 + 0.5γ……………………………………………………….(1)
where γ=kLG/kL and kLG, kL are the conductivity of the gas/liquid mixture and liquid alone, respectively. With the appropriate cell geometry, γ can be substituted by the conductance ratio.
Data Acquisition System
The conductivity signals from the multi-cell probe were collected automatically by a data acquisition system. The system consisted of a micro computer, a conductivity meter, a 24-channel relay board, an I/O interface to drive the relay board and an A/D converter.
Results
The profiles were symmetrical and relatively uniform. In this case, the probe was directly above spargers No.2 and No.6 (defining the angle α=0°). The actual variation in gas holdup was about 3% over the diameter of the column. The profiles suggest two maxima at r/R = ±0.4. These peaks are not directly above the spargers (the spargers were positioned in a circle with r/R=0.68).
Horizontal Cominco Sparger System
The profiles using Cominco spargers were obtained for the following conditions: Jg= 1.0 (three spargers in use) or 0.67cm/s (two spargers in use); water addition rate to the spargers JWs=0.053 or 0.035cm/s, Hp=70 or 150cm and Cf = 10ppm D250.
Horizontal Cloth Sparger System
The profiles for the horizontal cloth sparger system were obtained under the following conditions: Jg = 1.0, 1.5cm/s, Hp=70 or 150cm and Cf=10ppm D250. Figure 14 presents the profiles when three spargers were in use: (a) at α=0° (perpendicular to sparger), and (b) α=90° (parallel to spargers).
Discussion
Gas holdup variations were of the order of ±2- 3 % (absolute) which is about the order of experimental uncertainty: the least variability occurred with the Cominco-type spargers.
Again from the practical viewpoint, the vertical sparger arrangement does not seem suitable. It proved impossible to eradicate asymmetry from the profile at certain angles of rotation. (Non-symmetry was also evident in the horizontal case along the sparger length but this was understandable). This may mean that a large number of small spargers is not the direction to take.
