Chum-turbulent

Figure 5.3-8. Details of the chum-turbulent flow regime of BSCR according to Inga [1], (db)0-gas bubble size at atmospheric conditions in the absence of solid particles, (d, )ps- bubble size at the operating pressure and catalyst concentration. Column height = 2.8 m, internal diameter = 0.316 m. P < 8 bars. Organic media, catalyst diameter < 100 pm.

Krishna R, Urseanu MI, van Baten JM, Ellenberger J. Influence of scale on the hydrodynamics of bubble columns operating in the chum-turbulent regime experiments vs. Eulerian simulations. Chem Eng Sci 1999 54 4903 -911. 

Axial mixing in the liquid, induced by the upflow of the gas bubbles, can be substantial in commercial-scale bubble columns, especially in the chum turbulent regime. Due to typically small particle size, the axial dispersion of the solid catalyst in slurry bubble columns is expected to follow closely that of the liquid exceptions are high-density particles. The liquid axial mixing can be represented by an axial dispersion coefficient, which typically has the form... 

Of interest is a recent theoretical relation for eg in gas-liquid bubble columns based on liquid circulation and claimed to be valid both in the homogeneous bubble flow regime and in the chum-turbulent regime, also for non-Newtonian fluids. For power law fluids with... 

Recommendations for p range from 1 to 1.5 and for q from 0 to 2. Rice et al. [62] concluded that this was due to the different regimes and recommended q values of 2, 1, 1/3 and 0 for chain bubbling, bubbling flow, chum-turbulent flow and slug flow, respectively. 

Figure 5 gives a few examples from Kara et al. [70] together with two correlations. Recent experiments with a large-scale slurry bubbling column (dc = 0.57 m, H = 7.62 m) operating in the chum-turbulent regime, revealed that the liquid phase could be considered as ideally mixed [71],... 

For the industrially most important chum-turbulent regime much less information is available. 

Only some scattered information on the influence of pressure and gas density on k a is available. Recently, Dewes et al. [51] reported results from a 0.115m diameter column operated in all but the chum-turbulent regime, both at 1 bar and 8 bar, with 2 vol% of solids. The influence of solids was negligible (see Fig. 8), but, as expected from Wilkinson s findings on the influence of pCl on r< [44], A Ltf increased substantially with pressure. Dewes et al. [51] were able to show that the pressure affected a only, with A l- remaining independent of both pressure and the presence of solids. 

The significance of the Jadhav and Pangarkar equation stems not only from the large column diameters for which it has been tested but also from the relatively large superficial velocities, which are well into the industrially important chum-turbulent regime. Using their equation (eq 55, see Table 6) and substituting the maximum possible value of t/L, i.e.,... 

Chum-Tuiimlent Vessel Model The chum-turbulent vessel model is also based on uniform vapor generation throughout the liquid but with considerable vapor-liquid disengagement. The liquid phase is continuous with coalesced vapor regions of increased size relative to the bubble vessel model. 

NonboUing Height Model This model applies the chum-turbulent assumptions to only a top portion of the fluid in the protected equipment. Below this portion, boiling does not occur and there is no liquid swell. The location of this nonboiling height is estimated from a balance ofthe hydrostatic effects and the recirculation effects. 

Bubble columns Loop reactors Stirred tanks Hydrocyclones Reasonable Reasonable Reasonable Reasonable Modeling of chum-turbulent flow regime Modeling of chum-turbulent flow regime Improved geometrical representation of impeller and baffles Improved geometrical representation of system... 

The Chum turbulent vessel model (11) assumes uniform vapor generation throughout the liquid with considerable vapor-liquid disengagement in the vessel. The degree of vapor-liquid disengagement is represented by the following relationship ... 

Methods are given for the bubbly and chum-turbulent flow regimes within the vessel. The criterion for safe use to take account of disengagement is, GAh vJ Vv,(l-a,) - However, when disengagement is not accounted for, then = 1.0, and the above equation is not required. 

CFD approach. This model has too many unknown parameters when applied to multiphase flow. The kind of model used by Dudukovic group (72) computed the Reynolds stresses in agreement with measurements done in L.S. Fan s laboratory. In the Matonis et al (75) paper we show our capability to compute turbulence in a slurry bubble column in the chum-turbulent regime in agreement with our measurements. 

Three-phase fluidized beds and slurry reactors (see Figs. 30g-l) in which the solid catalyst is suspended in the liquid usually operate under conditions of homogeneous bubbly flow or chum turbulent flow (see regime map in Fig. 33). The presence of solids alters the bubble hydrodynamics to a significant extent. In recent years there has been considerable research effort on the study of the hydrodynamics of such systems (see, e.g., Fan, 1989). However, the scale-up aspects of such reactors are still a mater of some uncertainty, especially for systems with high solids concentration and operations at increased pressures it is for this reason that the Shell Middle Distillate Synthesis process adopts the multi-tubular trickle bed reactor concept (cf. Fig. 30e). The even distribution of liquid to thousands of tubes packed with catalyst, however poses problems of a different engineering nature. 

The optical impression of a bubble flow system is characterized by a great number of finely dispersed small bubbles, while in the case of the chum turbulent flow a growing bubble size is observed on its way to the liquid surface. 

Calculation of the characteristic dimensionless velocity at the liquid surface for chum turbulent flow Wg a according to Eq. (10.13) gives... 

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