Draught tubes

Eig. 5. Examples of air driven bioreactors (a) bubble column, (b) draught tube, and (c) external loop. 

Two types of bioreactors have been used to produce SCP from sugar sources, namely stirred/baffled type and the air-lift with draught tube type (figure 4-5), with various capacities up to 400 m. 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

Hano, T., Matsumoto, M., Kuribayashi, K., and Hatate, Y., Biological Nitrogen Removal in a Bubble Column with a Draught Tube, Chem. Eng. Sci., 41 3131 (1992)... 

The Standard-Messo turbulence crystalliser, Figure 15.18, is a draught-tube vacuum unit in which two liquor flow circuits are created by concentric pipes an outer ejector... 

Figure 15.17. Swenson draught-tube-baffled (DTB) crystalbser...

Ford und Ulbrecht [41] performed homogenization measurements with aqueous CMC and PAA solutions in a vessel with a screw stirrer arranged in a central draught tube. The pumping direction of the screw could be changed as well. Initially, the liquid with a lower viscosity rested in a layer on top of the more viscous one (volume ratio cp = 1). The data measured were first represented in the space n0, Reefr, Fig. 22, whereby peff was taken from the flow curve of the homogenized mixture at the shear rate of y 5 s-1, which was effective in the draught tube. 

Fig. 23 Homogenization characteristic of a screw stirrer in draught tube for non-Newtonian liquids with pseudoplastic viscoelastic properties. For the sign explanation see Fig. 22, from [41],... 

Fig. 23. Some mixers for viscous liquids, (a) Six-blade turbine (b) three inclined-blade paddles (c) helical screw (d) helical ribbon (e) propeller in draught tube (f) anchor. (Reprinted with permission from Chemical Engineering Science, 37, 813, C. J. Hoogendoorn and A. P. Den Hartog, 1967 Pergamon Press pic).

Prinzing and Htibner [447] investigated the effect of the aspect ratio H/D upon the n6(Re) dependence in a tank (D = 288 mm) with a propeller stirrer in the draught tube. They found that n3 did not depend upon Re in the range Re =... 

This relationship is reproduced in Fig. 3.12a with the test results from Hoogen-dom and den Hartog [220]. It shows that FIs = const applied in the laminar range for several axially conveying stirrer types (helical ribbon stirrers propeller stirrers in a draught tube), i.e. the stirrer power per unit volume, which is required for a particular homogenization time, is here proportional to the viscosity n of the liquid and independent of the tank diameter. FIj and therefore P/D P/V in this range is actually a scale-up criterion for these stirrers. 

Whereas Mersmann and Einenkel [112] correlated their experimental data on the basis of energy equations and Liepe and Joschek [334] set the power needed for particle suspension proportional to the stirrer power, Zehner [603] made use of the laws of momentum transfer. This appeared more reasonable to him, because in this case the efficiency of mechanical energy transfer did not have to be introduced. In his first fundamental investigations he utilized the jet loop, which due to the directional flow in the draught tube and in the annular space enabled easier balancing. 

Table 1 shows a number of aerobic fermentation systems which are schematically classified into (i) internal mechanical agitation reactors, (ii) external circulation reactors, and (Hi) bubble column and air-lift loop reactors. This classification is based on both agitation and aeration as it relates to oxygen supply. In this table, reactor 1 is often used at the industrial level and reactors (a)2, (b)2, (c)2, and (c)3, can be fitted with draught tubes to improve both mixing and oxygen supply efficiencies. 

Figure 18. Different types of bioreactors for plant cells, tissues and organs. (A) Shake Flask. (B) Aeration-Agitation. (C) Percolated Impeller. (D) Draught Tube Air-lift. (E) Draft Tube with Kaplan Turbine. -Air-liftloop. (Gj Rotating Drum. (7/1 Light Emitting Draught Tube. (I) Spin Filter. (J) Bubble Column. (K) Aeration. (L) Gaseous Phase.

Spouted beds (Figure 4.3e) with and without internal draught tubes are also used sometimes for blending particles with similar privities. For practical reasons, however, spouting is confined to coarse solids (essentially Group D and larger Group B particles) such as polymer chips. There are two reasons for this ... 

The use of a draught tube generally leads to a reduction in both Na and ( )js when used in conjunction with either a propeller or an angle-blade turbine (see Table 16.3). However, care should be taken with the position and size of the draught tube. For instance, if the bottom of the tube restricts the impeller outflow, the local head loss can cause a marked enhancement of (i-r)]s - Ideally, the flow area in the core, in the annulus, between the bottom of the tube and the base and between the top and the liquid surface should all be equal. Finally, the use of a specially contoured base, maintaining the flow area constant Figure 16.5) leads to an even further reduction in A(is and (er)js by eliminating ail dead spots. 

With a contoured bottom, the circulation velocity, is constant at all points. It was found that for just complete suspensions was of the order of the still fluid terminal velocity, Uj, with (Vc/ur) decreasing from 2 to 1 as the particle size increased. The lower values of (v /ur) for larger particles is probably due to their greater protuberance into the main flow. With a wellsized draught tube and a propeller, iVj is lower by a factor of about 1.7 with a contoured bottom and by atout 1.3 with a flat bottom. Since Po is hardly... 

Satisfactory withdrawal requires iso-kinetic sampling or at least a good approximation to it. Draught-tube, bafiled tanks have a relatively well-defined vertical flow, particularly around the mid-depth of the draught-tube. A withdrawal pipe then can be aligned with the flow but the velocity must be greater than Sut for satisfactory transporT and at the same time it must satisfy the stoichiometry. These two requirements cause problems in small scale systems but intermittent withdrawal at the point in the annulus of maximum velocity may provide an answer. 

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