Influence of cross-sectional ratio of down comer to riser on the efficiency of liquid circulation in loop air lift bubble column

Loop type bubble columns have good performance of liquid circulation and mass transfer by airlift effect, where the liquid circulation time is an important measurable characteristic parameter. This parameter is affected by the column construction, the aspect ratio of the column, the cross-sectional area ratio of down comer to riser (R), and the superficial gas velocity in the riser (UGR). In this work, the mean gas holdup and the liquid circulation time (TC) have been measured in four types of loop airlift type bubble column: concentric tube internal loop airlift type, rectangular internal loop airlift type, external loop airlift type, external loop airlift with separator. Air and tap water were used as gas and liquid phase, respectively. The results have demonstrated that the mean gas holdup in riser increases in proportion to UGR, and that it in downcomer changes according to the geometric parameters of each bubble column. TC has been found to conform to an empirical equation which depends on UGR and the length of draft tube or division plate in the region of 0.33 < R < 1.


Introduction
Various styles of bubble column are used as gas-liquid two phase or gas-liquid-solid three phase reactor in the plants for energy supply and environment conservation.The loop type bubble column is particularly important because of good performance of liquid circulation and mass transfer by airlift effect [1,2].In this type bubble column, the liquid circulation time and the gas holdup are the important and characteristic parameters [3,4], including the column construction, the aspect ratio of the column, the cross-sectional area ratio of down comer to riser (R) and the superficial gas velocity in the riser (UGR) [5].For a well-understandings of the behavior of the loop type bubble column, we have to collect the data on various styles of the loop type bubble column.
We have already presented the behavior of the mean gas holdups (εGR, εGD) in riser and downcomer and TC measured in rectangular internal loop airlift type bubble column in a previous congress [6].In this paper, we report these behaviors observed in four types of loop airlift bubble column: concentric tube internal loop airlift type, rectangular internal loop airlift type, external loop airlift type, external loop airlift type with separator.

Experimental
Four types of airlift bubble column used in this work are illustrated in Figure 1.The details of rectangular internal-loop type were presented previously [6].The dimensions of bubble column are tabulated in Table 1.Air and tap water at 303K were used as the gas and liquid phase, respectively.A perforated plate with many holes was used as a gas disperser.The non-aerated liquid level (HD) was kept at 0.12m from the upper end of the draft tube or the division plate.The mean gas holdups (εGR, εGD) in riser and downcomer sections were calculated by measuring the liquid height with a static pressure manometer.A tracer technique was used to measure the liquid circulation time (TC) with a conductance meter and 20wt% sodium chloride aqueous solution.

Bubble Distribution Pattern
Shapes of bubble generated through the perforated plate were predominantly spherical.The bubbles rose with a constant velocity without colliding or coalescing one another inside the riser in low UGR region.When UGR was increased, some bubbles entrained into the main liquid flow from the riser to the downcomer.The amount of entrained bubbles decreased when the clearance between the lower end of inner tube or division plate and the bottom of column was small.

Mean Gas Holdup
In the case of rectangular internal-loop bubble column (RIL) for example, the mean gas holdup in riser εGR increased in proportion to UGR in the higher range of UGR as demonstrated in Figure 2. The same behaviour was observed in the case of external-loop reactor (EL and ELS).The solid line in Figure 2 shows an Akita-Yoshida equation [7] obtained empirically in the loop bubble column with a concentric-tube using a single hole gas sparger.εGR was usually larger than εGD in the observed region of UGR.A relation of εGD to εGR depends generally on R and HC which is the clearance between the bottom of the column and the lower end of the division plate.In the case of HC = 0.24 m, εGD increased in proportion to εGR from a threshold value of εGR as Figure 3, which also suggests that the decrease of R tends to intensify the dependency of εGD to εGR.The decrease of crosssectional area of down downcomer was empirically found to encourage the formation of liquid down flow containing bubbles.The bubble entrainments have been also observed in the external-loop type airlift bubble column, resulting in another threshold value of εGR.On the other hand, in the case of HC = 0.015 m, εGD kept to be approximately zero with increasing εGR.

Fig. 2 .
Fig. 2. Changes of εGR with UGR and R in case of HC = 0.24m.

Table 1 .
Dimensions of loop airlift bubble column employed.