for the cell sensitive biological culture and harvesting as well as the reasonable bubble size generated work well in the mixing process in the bioreactors. Therefore, it is important and essential to choose a suitable generation method depending on the treatment application. As mentioned previously, microbubble size is the essential key in controlling the performance in all the application. the bubble size generated from different microbubble generation method should be studied in order to choose a suitable bubble generation method for the desired bubble required to improve the efficiency of the application. 2.4 Bubble Size Measurement Techniques 2.4.1 Introduction This section mainly illustrates the ‘pros and cons’ of the two bubble size measurement methods. The two methods are discussed in this chapter which are optical (photography method) and photonic method (Spraytec). This chapter illustrates the working principle, using procedure, specifications and the analysis methods of all the bubble size measurement methods. 2.4.2 Optical Methods –Photography Technique Figure 2.21: Schematic diagram of the optical method experimental setup 2.4.2.1 Specification and Working Principle The apparatus for the analysis system includes a camera (FastCam SA3 Model120K) and a 40 Watt halogen light with light source diffuser sheet (Caddilac). The camera resolution used was 1024 X 1024 pixels at 2000 fps and provided with a macro lens (Nikon Nikkor Lens, 1.8 D). The calibration scale factor measured 25.0
2.4.4 Summarized Method Usage Photography Method Spraytec Method level required Low Low User friendly Medium Medium Calibration Manual Automatic Area of interest Large and adjustable with different camera lens. Fix laser beam size (10mm) Work space required Small Huge Working Bench Not provided Came with the equipment and very stable but heavy Data processing program Third party software such as “labview” and “Image J” Spraytech software with the function of the data acquisition, average etc. Post Processing Required an image processing software and programme code. Process manually with processing software and programme. Raw image or data can be review. Can be easily done by using the equipment software, easy to be operated. Rapid results and process automatically by the software. No raw data available to review. Sampling chamber Yes, transparent Yes, transparent Bubble Can be solved with the image processing software. Not suitable for very high density bubbles measurement. 2.2: Comparison table for the photography and Spraytec method Summary of the photography method and Spraytec method are listed in the Table 2.2. The bubble sizing measurement method mainly depends on the uses, availability, expertise and the application, different methods can be used to obtain the bubble size distribution and upon significant inference, accurate bubble size distributions can be
Figure 3.9 shows the experimental setup for determining the microbubble rise velocity and liquid flow velocity in the ALB. The aeration method was used as the microbubble generator system. The microbubble is generated by supplying a steady air flow directly to the porous aerator in the fluid. The bubble rise velocity and the liquid flow velocity of the five bubble sizes in the ALB were measured. The ALB is filled with approximately 3 Litres of de-ionized water and the distance between the water level and the ceramic aerator surface is approximately 195 mm. At a given 0.1 l/min steady flow rate, 1020 µm, 910 µm, 755 µm, 690 µm and 440 µm bubbles were generated by the aerator with pore sizes of 150 µm, 50 µm, 20 µm, 16 µm and 2 µm respectively. In order to maintain a relatively constant condition, the inlet flow rate was controlled by using rotormeter and a pressure regulator was used to maintain a constant pressure for the system. The experimental condition is presented in Table 3.3. The operating pressure was caused by different as different pore size ceramic aerator. Therefore, a correction factor given by the manufacturer was applied to obtain require flow rate. Table 3.3: Experimental 3 Operating Conditions Airlift Bioreactor Geometry Inside diameter : 140mm Height : 260mm Diameter of draft tube : 84mm Height of draft tube : 130mm Ceramic Aerator Geometry Inside diameter : 55mm Area : 2375.83mm2 Pore diameter : 2-150 µm Operating Conditions Gas Flow Rate : 0.1 l/min Operating Pressure : 0.05-1bar Water Temperature : 20±1°C Volume of water : 3 l Medium Height : 195mm Flow condition : Steady flow
In a conclusion, the experimental data shows that microbubble improves the airlift reactor efficiency. In addition, bubble size is an important parameter in the design of an ALB rather than only the physical parameters of the ALB. Chapter 6: Effects of the Draft Tube on Liquid Flow Velocity in ALB 6.1 Introduction The design of vessel significantly influences the performance of airlift reactor, in particularly, the tube and draft tube diameter ratio, the distance between the upper edge of the draft tube to the dispersion surface (CT) and the distance between the lower edge of the draft tube to the aerator surface (CB). Over last decade, there have been lot of studies on the column diameters to draft tube diameter ratio. However, the effect of the top and bottom clearance on the performance of the ALB and the effect of the position of the draft tube on the flow behaviour in the ALB have not been studied to such an extent. Weiland (1984) and Chisti, et al. (1989) reported that that there is a certain range of top clearance affecting the ALB performance and the effects of the bottom clearance was neglected. Sandiani (2012) and Al-azzi & Al-kuffe (2010) neglected the bubble size which affects the flow velocity in the ALB. Sandiani (2012) and Al-azzi & Al-kuffe (2010) varied the clearance distance by changing the water height. Therefore, after completing the previous chapter, illustrating the importance of bubble size on the performance of the ALB, it is necessary to investigate the effect of the draft tube geometry on the ALB performance, with maintaining a consistent bubble size. Therefore, this chapter presents the effects of CT (the distance between the top of