RESULTS AND DISCUSSION

3.1 Effect of ceramic membrane length and gas pressure on bubbles size

Figure 4 shows typical images of bubbles generated by ceramic membranes with different lengths at a liquid flow rate of 1.0 ml/min and a gas flow rate of 10 ml/min. It can be seen that the distribution of bubbles generated by the ceramic membrane with 10 cm length is the most uniform, and the quantity of bubbles is larger than other length’s membranes. Bubbles generated from a ceramic membrane with a length of 5 cm are also well distributed, while the small effective membrane pore area which is proportional to the length, leading to less quantity of bubbles than 10cm ceramic membrane. As the length of the ceramic membrane increased, the possibility of bubble coalescence increased during the rising process, resulting in larger size and less quantity of bubbles.16 Figure 5 indicates that the average diameter of bubbles decreases first and then increases with the increase of the length of ceramic membrane. Under conditions that the length of the ceramic membrane was 10 cm, the average diameter of the bubbles was the smallest, only 320 μm, which was smaller than other ceramic membranes. Figures 6 and 7 are performed to demonstrate that the changes of gas pressure in the range of 0.2-0.6 MPa have few effects on the flow pattern, distribution, and size of bubbles. In other words, when the gas feed at low pressure (no more than 0.6 MPa), the increase of gas pressure has no effect on the average diameter of bubbles. Results also illustrate that the CMGD does not require extra energy consumption as a green and high-performance gas distributor.