**4.6.2 Results and discussion**

358 Mass Transfer - Advanced Aspects

The effluent flow rate displays a decreasing trend with increasing effective height. This can be explained by the fact that the higher the effective height in the atmosphere is, the larger the hydraulic head loss and the weaker the airlift effect are. DO saturation rate shows a propensity of going up of an increase in effective height. It can be easily understood that increased effective height leads to the prolonged contact time between the effluent water and air. Furthermore, in the case where the amount of the effluent water through the airlift pipes is reduced, a thinner liquid film can be formed, which can enhance the oxygen

As a result, the oxygen transfer amount per unit air aeration volume approaches its maximum value under the experimental conditions of 1 and 5 cm in effective height, and 4

Using highly oxygen-transfer-efficient airlift part with a pipe diameter of 4 cm and effective height of 1 cm, as shown in Fig. 21, we examine the effect of airlift part configuration by arranging 1, 2 and 3 pipes inside. To keep aeration flux constant, air flow rate normalized to each piece of airlift pipe is set at 6 L/min. The detailed experimental conditions are

> Air flow rate 12 L/min

> > area (cm2)

1 12.56 6

3 37.48 18

Air flow rate 18 L/min

Air flow rate per unit airlift pipe number (L/min)

Area of airlift part 37.48 cm2

Area of airlift part 25.12 cm2

**4.6 Effect of the distribution of airlift pipes on oxygen supply efficiency** 

Pipe diameter 4cm

Area of airlift part

Air flow rate 6 L/min

Side view of apparatus Top view of apparatus

(cm) No. of airlift pipe Cross-sectional

4 1 2 25.12 12

Table 5. Experimental conditions relating to the configuration of the airlift part

dissolution efficiency.

cm in pipe diameter.

tabulated in Table 5.

Pipe diameter (cm)

height 1cm

Fig. 21. The configuration of airlift part

Effective height

**4.6.1 Experimental conditions and methods** 

12.56 cm2 Effective

Figs. 22, 23, 24 and 25 respectively illustrate the effect of No. of airlift pipe on the effluent water flow rate per unit airlift pipe number, DO saturation rate in the effluent water, oxygen mass flow rate per unit airlift pipe number and oxygen transfer amount per unit crosssectional area (cm2) of airlift part.

No. of airlift pipe

Fig. 22. Effect of airlift pipe No. on the effluent water flow rate per unit airlift pipe number

No. of airlift pipe

Fig. 23. Effect of airlift pipe No. on the DO saturation rate

As shown in Fig. 23, the lower the airlift pipe number is, the higher the DO saturation rate is. As revealed in Fig. 22, the effluent water flow rate per unit airlift pipe number does not change notably. Thus it appears that the aeration is evenly distributed among the airlift pipes. Namely, the air flow rate is the same through every airlift pipe. As revealed in Figs. 24 and 25, respectively, the oxygen transfer amount per unit airlift pipe number and oxygen transfer amount per unit airlift part cross-sectional area both exhibit a decreasing trend with increasing pieces of airlift pipes. If the air flow rate of every airlift pipe was same

Improvement of Oxygen Transfer Efficiency in

in Table 6.

Pipe diameter (cm)

**4.7.2 Results and discussion** 

to 1 cm2 of cross-sectional area is 1.1 L/min.

0.0

Fig. 26. Effect of air flow rate on the liquid/gas ratio

0.5

1.0

1.5

Liquid/gas ratio

2.0

2.5

**4.7.1 Experimental conditions and methods** 

**4.7 Effect of air flow rate on oxygen supply efficiency** 

Effective height (cm)

Table 6. Experimental conditions focusing on the variation in air flow rate

DO saturation rate and oxygen transfer amount normalized to 1 L of air (E0).

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 361

At a cross-sectional area of 12.56 cm2, the impact of air flow rate on oxygen supply efficiency is studied experimentally. The experimental conditions include the air flow rate ranging from 6 to 18 L/min with an increment of 2 L/min. The pipe diameter of 4 cm and effective height of 1 cm are chosen for the LFFA. The experimental conditions in detail are presented

> Cross-sectional area (cm2)

4 1 12.56 6, 8, 10, 12, 14, 16, 18

Figs. 26, 27 and 28 respectively present the effects of air flow rate on the liquid/gas ratio,

As indicated in Fig. 26, at all air flow rates except for that of 6 L/min, any liquid/gas ratio does not change too much. It is thus deduced that the energy loss is very low in the airlift part. The findings of DO saturation rate are shown in Fig. 27. Below 12 L/min, it does not make a difference. In contrast, beyond 14 L/min, DO saturation rate exhibits an attenuating tendency. Thus, the excess air flow rate brings about the energy waste. As revealed in Fig. 28, oxygen transfer amount normalized to 1 L of aeration air is gradually increasing in the 6- 12 L/min range. However, below 14 L/min, it tends to decrease gradually. Owing to nearly unchanged liquid/gas ratio shown in Fig. 26, the dominant factor affecting oxygen transfer rate is DO concentration. The reason lies in that with air flow rate increasing, it will lead to increasing effluent flow rate, rendering too much effluent water flowing through the airlift part in relation to gas bubble number, thereby inhibiting the liquid-film formation. In summary, under the present experimental conditions, the optimal air flow rate normalized

Air flow rate (L/min)

18 16 14 12 10 8 6

Air flow rate (L/min)

(6 L/min), the same result is supposed to be obtained. On the contrary, the experimental data from Figs. 24 and 25 both tend to decay. This is due to the fact that for 1 piece of airlift pipe, the effluent can completely overflow through the periphery of the pipe. However, for 2 or 3 pieces of airlift pipes, a fraction of effluent overflowing from a certain airlift pipe can flow back into another airlift pipe, or 2 streams of effluents overflowing from 2 airlift pipes can hinder with each other, and thus flow back into the respective airlift pipes again, thereby causing the reduced effluent flow rate. Otherwise, with increasing airlift pipe number, the coverage area of the capture part is also increased. However, since only one diffuser is serving, it is impossible for aeration to uniformly distribute towards every airlift pipe. As a consequence, when designing the system, every airlift pipe should be separately installed. In the meanwhile, the design of the capture part should take into consideration that the gas bubbles can be well captured, and subsequently uniformly distributed to every airlift pipe.

Fig. 24. Effect of airlift pipe No. on the oxygen mass flow rate per unit airlift pipe number

Fig. 25. Effect of airlift pipe No. on the oxygen transfer amount per unit airlift part crosssectional area (cm2)

### **4.7 Effect of air flow rate on oxygen supply efficiency 4.7.1 Experimental conditions and methods**

At a cross-sectional area of 12.56 cm2, the impact of air flow rate on oxygen supply efficiency is studied experimentally. The experimental conditions include the air flow rate ranging from 6 to 18 L/min with an increment of 2 L/min. The pipe diameter of 4 cm and effective height of 1 cm are chosen for the LFFA. The experimental conditions in detail are presented in Table 6.


Table 6. Experimental conditions focusing on the variation in air flow rate
