Effect of superficial liquid velocity

Experiments in annular flow were performed by Hetsroni et al. (2003b) to study the flow regimes and heat transfer in air-water flow in 8° inclined tubes of inner diameter 49.2 mm and 25 mm. 

Droplets appeared on the surface of the pipe (Fig. 5.33b) after increasing the water flow rate up to I/ls = 0.007 m/s. Spedding et al. (1998) referred to this regime as film plus droplet pattern. When the water flow rate increased and superficial liquid velocity was Gls = 0.03 m/s (Fig. 5.33c) droplets began to roll back into the liquid film. Kokal and Stanislav (1989) identified such a regime as annular plus roll wave flow pattern. The experimental facility used in the present study allowed us to achieve values of superficial gas velocities up to 20 m/s in the 49.2 mm pipe. 

Therefore, to study the flow regimes at higher superficial gas velocities the pipe diameter was decreased. 

The local heat transfer coefficients on the surface of the pipe may not be uniform, though the surface is heated by uniform heat flux. This irregularity is due to the distribution of the air and liquid phase in the pipe. The temperature distribution along the pipe perimeter shows a maximum at the top and a minimum at the bottom of the pipe. In Fig. 5.36a-c, the heat transfer coefficients are plotted versus angle 0. These results were compared to simultaneous visual observations of the flow pat- 

Experiments in annular and slug flow were carried out also by Ghajar et al. (2004). The test section was a 25.4 mm stainless steel pipe with a length-to-diameter ratio of 100. The authors showed that heat transfer coefficient increases with increase in liquid superficial velocity not only in annular, but also in slug flow regimes. 

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Fig. 5.42 Effect of superficial liquid velocity on heat transfer in parallel triangular micro-channels of Jh = 130 pm...

Figure 4.6 Effect of superficial liquid velocity an average liquid holdup...

Kato et al. [43] used the pressure profile method and the electroconductivity probe in a bidimensional column. The liquid hold-ups obtained by the two methods agreed well except near the top of the fluidized bed. Because this study involved experimentation with air, aqueous solutions of carboxymethyl cellulose at different concentrations and glass particles of different sizes (0.42 mm, 0.66 mm, 1.2 mm, 2.2 mm) the effects of superficial liquid velocity, liquid viscosity and particle size on the liquid hold-up were considered. With this data a correlation similar to the one derived by Richardson and Zaki for liquid-solid fluidized beds was proposed [43]. 

Most of heat transfer correlations are based on data obtained in flow boiling from relatively large diameter conduits and the predictions from these correlations show considerable variability. Effects of superficial liquid and gas velocity on heat transfer in gas-liquid flow and its connection to flow characteristics were studied by Hetsroni et al. (1998a,b, 2003b), Zimmerman et al. (2006), Kim et al. (1999), and Ghajaret al. (2004). However these investigation were carried out for tubes of D = 25—42 mm. These data, as well as results presented by Bao et al. (2000) in tubes of L> = 1.95 mm and results obtained by Hetsroni et al. (2001), Mosyak and Hetsroni (1999) are discussed in the next sections to clarify how gas and liquid velocities affect heat transfer. Effects of the channel size and inclination are considered. 

There is a lack of information on the effect of superficial liquid and gas velocities on heat transfer in micro-channels. We studied this problem in the test section that contained 21 parallel triangular micro-channels of rfh = 130 pm. 

The effect of superficial gas velocity on gas-liquid interfacial areas for different operating pressures is shown in Fig. 1. For very low liquid or gas flow rates, a is almost independent of a change in pressure. However, for higher liquid flow rates, an increase in pressure induces a significant increase in a for gas superficial velocities above a critical gas velocity uGc = 2 to 3 cm/s. Gas and liquid superficial velocities over which pressure effects are no longer negligible are coincident with the ones for which gas hold-up becomes pressure-sensitive [6], Thus a relation between the change in interfacial areas and hold-ups due to pressure is expected. 

Veera, U.P., Kataria, K.L., and Joshi, J.B. (2004), Effect of superficial gas velocity on gas hold-up profiles in foaming liquids in bubble column reactors, Chemical Engineering Journal, 99(1) 53-58. 

The effect of superficial gas velocity (SGV) on the performance at an inlet SO2 concentrations of 0.3 is shown in Figure 3 using AC as the flushing solvent. The average superficial liquid velocity (SLV) was 0.953 mm/s and the experiments were performed at a bed temperature of 18°C. Oxygen concentration was 5 vol% and water was 2.9 vol%. As expected from contact-time considerations, recovery of SO2 from the gas and its conversion to acid decreases with increasing SGV. The removal curve extrapolates smoothly to approximately 100% as the gas velocity goes to zero. Since conversion is measured on the... 

The ratio k pp/k is defined by Satterfield [67] as the contacting effectiveness, nc deviates from unity when one of the above assumptions becomes unrealistic. Circumstances for which this occurs are rather difficult to predict. Satterfield has estimated from data taken on commercial hydrotreating units as a function of superficial liquid velocity Vq. tIc was found to depend on the... 

Fig. S9. Influence of superficial liquid velocity on the ratio of the effective surface area to tfae total sur ce area of the P27-60 packing at different values of the liquid phase viscosity.

In processing the quantitative effect of the liquid flow-rate, the superficial gas velocity, and the total reactor pressure on the dynamic liquid hold-up, good results have been obtained by using ... 

Figure 5.2-34. Effect of the superficial gas velocity at different superficial liquid velocities on the dimensionless interfacial area for the CO2-N2-DEA-H2O-40% ETG system (P = 25 bar, porous AI2O3 cylinder) (after Wammes et al. [32]).

Within the gas (0-10 cm/s superficial velocity) and liquid (0.6-2.5 cm/s superficial velocity) flow ranges investigated, a good liquid distribution was observed at all conditions, as manifested by uniformity factor in excess of 70%. The liquid saturation increases with increasing superficial liquid velocity as well as down the column height. Within the conditions studied, the effect of gas velocity was, in general, found to be minimal. 

Fig. 9 illustrates the effect of superficial gas and liquid velocities on the cross-sectionally averaged liquid saturation at the middle of the column (2.5D axial position). It is obvious that the effect of gas velocity on the liquid saturation is not significant within the range of flows studied. This could be due to the fact that solid and liquid holdups are very small, leaving enough space for the gas to flow upwards without significant interactions with the liquid phase flowing downward. 

Fig. 8. Effect of superficial gas and liquid velocities on the liquid saturation Fig. 9 Effects of gas and liquid superficial velocities on the cross-radial profile at axial position of 2.5D. sectionally averaged liquid saturation.

The liquid saturation increases with increasing superficial liquid velocity. Moreover, the liquid saturation increases as the liquid phase moves downward. The liquid distribution was found to be fairly uniform in general as expressed by the uniformity factor which was relatively large, between 70 to 95%. Liquid distribution was better at the bottom of the bed, compared to the upper section. The effect of gas velocity, was in general, found to be very small at the conditions used in this study. 

Here, the parameter F = Uo]dJ2De( — t) considers the effect of intraparticle diffusion, Pe = V dJlEzi. takes into account the effect of axial dispersion, S = 3(1 — e)Kt/U0L considers the effect of total external mass-transfer resistance, and A0 = /j (l — )k dp/2UoL considers the effect of surface reaction on the conversion. In these reactions L/0l, s the superficial liquid velocity, dp is the particle... 

Figure 5 Effects of superficial (a) gas and (b) liquid velocities on volumetric gas-liquid mass transfer coefHcients, in monolith reactors with different channel sizes. (From Ref. 18.)...

Nevertheless, as a first approximation for design, use of Eq. (Ill) is recommended with the coefficient doubled for a gas-liquid reaction in pulse or spray flow over inert packing. For the bubble flow regime, with G > 0.01 kg/m sec, it is conservative to assume Al = 0.15 sec for any gas-liquid reaction. For the dependence of effective interfacial area, despite the lack of a general correlation, it is judicious to consider that this area will vary with the 0.5 power of superficial gas velocity regardless of packing size and type, column diameter, and liquid superficial velocity. 

The superficial liquid velocity varies over the range of 0-2.18 cm/sec for the data available this seems to have very little effect except at very low... 

The main contribution from the work of Luo [95, 96] was a closure model for binary breakage of fluid particles in fully developed turbulence flows based on isotropic turbulence - and probability theories. The author(s) also claimed that this model contains no adjustable parameters, a better phrase may be no additional adjustable parameters as both the isotropic turbulence - and the probability theories involved contain adjustable parameters and distribution functions. Hagesaether et al [49, 50, 51, 52] continued the population balance model development of Luo within the framework of an idealized plug flow model, whereas Bertola et al [13] combined the extended population balance module with a 2D algebraic slip mixture model for the flow pattern. Bertola et al [13] studied the effect of the bubble size distribution on the flow fields in bubble columns. An extended k-e model was used describing turbulence of the mixture flow. Two sets of simulations were performed, i.e., both with and without the population balance involved. Four different superficial gas velocities, i.e., 2,4,6 and 8 (cm/s) were used, and the superficial liquid velocity was set to 1 (cm/s) in all the cases. The population balance contained six prescribed bubble classes with diameters set to = 0.0038 (m), d = 0.0048 (m), di = 0.0060 (m), di = 0.0076 (m), di = 0.0095 (m) and di = 0.0120 (m). 

F = uoLdp/2Dsff — 6p) considers the effect of intraparticle diffusion uql is the superficial liquid velocity and is the particle void fraction. 

The wall shear term in Eq. (4.10) increases significantly with increasing superficial liquid (I/l) and gas (11 ) velocities and can amount to 20% of the total gas holdup (Hills, 1976 Merchuk and Stein, 1981). This is because the wall shear stress increases significantly with [/l and f/g (Liu, 1997 Magaud etal., 2001 Wallis, 1969). When the liquid phase is highly viscous, the wall shear term can be significant even at superficial liquid velocities on the order of 2-lOcm/s (Al-Masry, 2001). Hence, it is necessary to include the wall shear effect in the total gas holdup value for most cocurrent or viscous flow bioreactors. 

PBRs and TBRs, respectively, share some basic flow characteristics, but major differences exist, which lead to differing reactor performance and application. At very low superficial gas velocity, the superficial liquid velocity has a significant impact on the relative gas velocity for both PBR and TBR operations. Furthermore, liquid holdup is solely a function of the superficial liquid velocity (Alix and Raynal, 2008). Under PBR operation, increasing the superficial liquid velocity causes an increase in the relative gas velocity. This effect, in turn, increases the pressure drop of wetted to dry packing. TBR operation, on the other hand, leads to the opposite effect. 

For mixtures of liquids, the situation is more complex (103). Although the superficial liquid velocity u does not have a large effect, it can easily be taken into account. For example, in the case of cocurrent flow, a is corrected to the true holdup a by the relation Uq/u = Uq/terminal velocity is taken into account. An alternate, and general correlation, that has been tested in industrial equipment for the oxidation of toluene, cyclohexane, and alipharic acids under pressure, has been proposed by Van Dierendonck et al. (84)... 

The procedure, then for the use of the model to simulate an industrial packed column is outlined in Figure 21. First, we assume that we have specified the nature of packing material, the height of the column, the superficial liquid velocity and the gas to liquid ratio from these follow the values of ki, kQ the effective interfacial area and the liquid hold-up (that is. Stage 1 of Figure 1). It is assumed that these latter quantities... 

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