Types of Bubble Columns

Two main groups of bubble columns can be distinguished, these are simple bubble columns (Fig 8.1) and bubble columns with enforced circulation via a loop (Fig 8.6). Depending on the way in which the loop is realized, the latter group may be subdivided into bubble columns with internal or external loop. Compared to the types of bubble columns with reduced back-mixing 

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Several types of bubble column reactor are used in practice, some of which are shown in Fig. 11.1. As can be seen from this figure, several different modes of... 

Three phase fluidized bed reactor/slurry reactor FIGURE I l.l Types of bubble column reactors (from Lee and Tsui, 1999). 

Fig. 8.6. Special types of bubble column reactors. Figures depicted from Onken [109] (Reproduced by permission from Verein Deutscher Ingenieure (VDI)- Gesellschaft Verfahrenstechnik und Chemieingenieurwesen 2007) and Deckwer [28] (Copyright John Wiley Sons Limited. Reproduced with permission). Similar figures can also be found in Shah and Sharma [133] and Shah et al [132].

In the simplest type of bubble column, gas is dispersed at the bottom and bubbles are present throughout the reactor, as shown in Figure 7.10a. In a loop or air-lift reactor (Fig. 7.10b), gas is introduced beneath a central draft tube, and rising bubbles carry liquid upward. After most of the bubbles disengage, liquid flows downward in the annulus. The direction of flows could be reversed by feeding gas to the annulus. The columns may have internal coils for heat transfer and baffles to decrease axial mixing. 

Bubble columns with vibrating internals and columns with low-amplitude pulsations are currently under investigation in an attempt to improve the column performance. Vibrating internals consist of helical springs leading to 140% gas hold, which is much higher than the ones without internals. Sauter mean bubble diameter is estimated around 0.3 cm for the new type of bubble columns. 

FIGURE 7.4 Various types of bubble columns for gas-liquid reactions. (Data from Tram-bouze, R, van Landeghem, H., and Wauquier, J.P., Chemical Reactors—Design/Engineering, Editions Technip, Paris, 1988.)... 

Figure 7.6 Types of bubble column reactors, (a) Bubble column (b) gaslift with inlemal loop (c) gas-lift wilfi external loop.

The gas-liquid flow characteristics of stirred vessels depend both on the level of agitation and the rate of gas flow and can vary from the case of bubble column type operation to that of a full circulating tank, as shown in Fig. 5.123. The mixing characteristics and gas distribution obtained, obviously exert a considerable influence on the rate of mass transfer obtained (Harnby et al., 1985). 

Fig. 2.4p shows three types of post-column reactor. In the open tubular reactor, after the solutes have been separated on the column, reagent is pumped into the column effluent via a suitable mixing tee. The reactor, which may be a coil of stainless steel or ptfe tube, provides the desired holdup time for the reaction. Finally, the combined streams are passed through the detector. This type of reactor is commonly used in cases where the derivatisation reaction is fairly fast. For slower reactions, segmented stream tubular reactors can be used. With this type, gas bubbles are introduced into the stream at fixed time intervals. The object of this is to reduce axial diffusion of solute zones, and thus to reduce extra-column dispersion. For intermediate reactions, packed bed reactors have been used, in which the reactor may be a column packed with small glass beads. 

A new alternative approach for Stage I screening in liquid phase is the use of bubble column-type reactors. These parallel bubble columns can operate in batch and fed-batch mode regarding the reaction mixture, while a continuous stream of gas is used as reactant (H2, 02, or others) as well as for the intense agitation of the reaction mixture (Figure 11.39). 

It is seldom realized that many rules of thumb utilized for scale-up of different types of equipment are represented by quantities, which fulfill only a partial similarity. As examples, only the volume-related mixing power P/ V widely used for scaling-up mixing vessels and the superficial velocity V which is normally used for scale-up of bubble columns, should be mentioned here. 

Stirred tank reactors (STR) are the most frequently used reactors in lab-scale ozonation, partially due to the ease in modeling completely mixed phases, but they are very seldom used in full-scale applications. There are various modifications with regard to the types of gas diffusers or the construction of the stirrers possible. Normally lab-scale reactors are equipped with coarse diffusers, such as a ring pipe with holes of 0,1-1.0 m3 diameter. The k/ a-values are in the range of 0.02 to 2.0 s (see Table 2-4 ), which are considerably higher than those of bubble columns. From the viewpoint of mass transfer, the main advantage of STRs is that the stirrer speed can be varied, and thus also the ozone mass transfer coefficient, independently of the gas flow rate. 

We can construct a distillation column that has separate steps or plates (i.e., a bubble-cap type distillation column). Each plate would correspond to an evaporation-condensation step. The phase diagram for such a system is illustrated in Figure 2.3b. This type of a column is a useful example for describing the concept of plates. 

Partition of bubble column Figure 2.10(a) (Concentric doughnut-type regions). 

FIG. 19-31 Some examples of bubble column reactor types, (a) Conventional bubble column with no internals. (6) Tray bubble column, (c) Packed bubble column with the packing being either an inert or a catalyst. [From Mills, Ramachandran, and Chaudhari, Multiphase Reaction Engineeringfor Fine Chemicals and Pharmaceuticals, Reviews in Chemical Engineering, 8(1-2), 1992, Figs. 2, 3, and 4.]... 

In the present research, we study two fundamental properties of bubble columns liquid hold-up and mixing. Both of these properties depend on the flow rates of the gas and liquid phases. These two properties may be considered response variables in the sense that their values depend on the way bubbles are formed. We present results for two types of bubble generating devices (or, for short, spargers) i.e. perforated rigid plates and perforated rubber sheets. An advantage of the rubber-sheet sparger is the self-cleaning feature. This is... 

As mentioned earlier many two-phase gas-liquid flows are highly dynamic and this property requires the use of dynamic simulation methods. A well-known example of such a type of flow is encountered in bubble columns where recirculating flow structures are present which are not stationary but which continuously change their size and location in the column. Due to their frequent application in chemical reaction engineering and their relatively simple geometry, CFD analyses of bubble columns have received significant attention and are discussed in more detail here. 

Many types of fractionating columns have been designed. However, most of these have proved relatively inefficient, and the principal types presently in use are the Vigreux, bubble-cap, packed, spinning-band, and concentric-tube. Each of these will be considered in turn. 

The major advantage of the bubble column is the simple design of this type of reactor. Mixing of the liquid is achieved by the upflowing gas bubbles, and, as a consequence, mass transfer rates are limited. Another disadvantage of bubble columns is that they have limited temperature and pressure ranges, while they are also unsuitable for processing viscous fluids. 

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