Pulsing flow regime

Fig. 5.2.1 Flow regimes in a trickle-bed reactor (after Sie and Krishna [2]). Typical conditions for research and industrial reactor operation are indicated. The black line indicates the boundary between the pulsed flow regime and the spray, trickle and bubble flow regimes.

The experiments for the specified study have been made in the pulsing flow regime. This is stated in the original article, and it is evident by calculating the liquid and gas mass superficial velocities, which are... 

Model application in the pulsing-flow regime The mass transfer coefficient in the liquid-solid film is evaluated by means of the Dhai wadkar and Sylvester correlation (eq.3.433), and is found to be 0.45 s. Then, the several parameters of the model eq. (5.379) are shown in Table 5.18. 

Pressure drop Using the Ergun equation, the liquid and die gas pressure drop are 0.726 and 2.12 kPa/m, respectively. Then, by using the correlation of Larkins et al., die two-phase pressure drop is equal to 10.8 kPa/m, ten times lower than in die pulsing-flow regime and low enough to assure diat the gas density is almost constant. Thus, die condition (e) is satisfied. 

The model conversion is 14.7%, which is close to the value calculated for the pulsing-flow regime. The oxygen conversion is very low, as assumed, about 0.06 %. 

In industry, TBR operate mainly in the trickling and the pulsing flow regimes. 

The transition between the trickle-flow regime and the pulse-flow regime is plotted in the Figure 5.2—4 as a function of the superficial gas velocity, uc, the liquid velocity, uL, and the total reactor pressure, P, for the water-nitrogen system [17]. This figure shows clearly that the transition depends strongly on the pressure in the reactor. 

Two sizes of glass beads (1.4 and 2 mm diameter) were used, both in the trickle-and pulsing-flow regime, as well as in the transition zone (see Table 5.2-6) at total pressures between 2 and 81 bar. 

The hydrodynamics control the mass transfer rate from gas to liquid and the same from liquid to the solid, often catalytic, particles. In concurrently operated columns not only the gas-continuous flow regime is used for operation as with countercurrent flow, but also the pulsing flow regime and the dispersed bubble flow regime (2). Many chemical reactors perform at the border be-... 

Figure 2. Relative liquid holdup versus S/u, in the pulsing flow regime. System is cur-water. Key 9,2.5 X 2.5 and X> 4 X 4 Raschig rings.

Mass transfer from gas to liquid in the pulsing flow regime... 

The distributions of gas and liquid in the pulsed-flow regime have been studied by Weekman and Myers104 and Beimesch and Kessler.6 The former investigators found that, in the pulsed-flow regime, at high liquid flow rate, the pulses tend to coalesce as they move down the column. Thus, the frequency of the pulses would be slightly lower at the bottom than at the top. These pulses... 

Beimesch and Kessler6 measured the liquid-gas distribution in the pulsed-flow regime and found that the flow distribution was significantly affected by position in the bed, nature of the packing, and gas flow rate, but it was relatively insensitive to the liquid flow rate. A new model for the phase distribution was proposed, which included liquid fingers at the center of the pulse and centered around... 

In the pulsed-flow regime, the data were correlated by the penetration model. According to this model... 

Recommendations Under trickle-flow conditions, the use of Eq. (6-68) and, in the pulsed-flow regime, the use of Eq. (6-70) are recommended. More experimental as well as theoretical work is needed, particularly with hydrocarbon systems. 

Recommendations For large-size packings, use of Eq. (7-2) is recommended for the calculation of pressure drop in the bubble- and pulsed-flow regimes. For small-size packings, wherever possible, PERC data22 or the data of Sato et al.27 for the pressure drop could be used. More experimental as well as theoretical studies on the pressure drop for hydrocarbon systems are needed. 

The experimental data were largely obtained in bubble-flow and pulsed-flow regimes. A typical radial variation in the liquid holdup obtained under pulsed-flow regime is shown in Fig. 7-11. Runs nos. 1 and 2 in this figure are duplicate runs. Although the manner in which the column was packed may have had some effect on the holdup profile, it is clear from this figure that the liquid holdup profile was relatively flat in the center of the tube and was very sharp near the wall. It should be noted that the liquid holdup in this study was defined in terms of fraction of open reactor volume occupied by the liquid. 

Recommendations The gas holdup in the bubble-flow regime can be estimated using either Cq. (7-13) or F.q. (7-14). For the estimation of liquid holdup in the bubble-flow regime, use of Eq. (7-9) is recommended. In the pulsed-flow regime, the data of PERC and Eq. (7-15) would be useful. More experimental work with the hydrocarbon systems is needed. 

Recommendations For a cylindrical packed bubble-column, the use of Eq. (7-23) for the calculation of axial dispersion coefficient in the liquid phase is recommended. The axial dispersion in the gas phase of large columns needs to be investigated. Future study on this subject should concentrate on the pulsed-flow regime and the hydrocarbon systems. 

Recommendations Under bubble-flow conditions, the use of Eq. (7-36) for large particles and the correlation shown in Fig. 7-31 for small particles is recommended. Further experimental work in the pulsed-flow regime and with hydrocarbon systems is needed. 

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