Liquid phase holdup

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

Some of this theoretical thinking may be utilized in reactor analysis and design. Illustrations of gas-liquid reactors are shown in Fig. 19-26. Unfortunately, some of the parameter values required to undertake a rigorous analysis often are not available. As discussed in Sec. 7, the intrinsic rate constant kc for a liquid-phase reaction without the complications of diffusional resistances may be estimated from properly designed laboratory experiments. Gas- and liquid-phase holdups may be estimated from correlations or measured. The interfacial area per unit reactor volume a may be estimated from correlations or measurements that utilize techniques of transmission or reflection of light, though these are limited to small diameters. The combined volumetric mass-transfer coefficient kLa, can be also directly measured in reactive or nonreactive systems (see, e.g., Char-pentier, Advances in Chemical Engineering, vol. 11, Academic Press, 1981, pp. 2-135). Mass-transfer coefficients, interfacial areas, and liquid holdup typical for various gas-liquid reactors are provided in Tables 19-10 and 19-11. 

The liquid-phase holdup data in the three-phase systems were correlated in terms of Froude, Reynolds, and Weber numbers as... 

The liquid-phase holdup is expressed as a fraction of bed volume, i.e., volume of liquid present per volume of empty reactor. This is then subdivided into external holdup, liquid contained in the void fraction of the bed outside of the catalyst particles, and internal holdup, liquid within the pore volume of the catalyst. There is an even further subdivision of the external holdup into a static holdup —the amount of liquid in the bed that remains after the bed has been allowed to drain freely—and dynamic holdup which depends on a number of factors but is most simply defined as the difference between total holdup and static holdup. ... 

Liquid-liquid mass transfer depends on whether the transfer is from the continuous to the dispersed phase or vice versa. The liquid-liquid interfacial area an can be estimated from an = 6/iL/do where /il is the dispersed liquid phase holdup and c/q is the average size of the dispersed droplets which can be determined from a correlation given by Okufi et al. (1990). 

The design of these reactors requires the knowledge of both the hydrodynamic parameters (flow patterns, liquid phase holdup and two-phase pressure drop,. ..) and the interfacial parameters (a, kj a, k a, kga). 

As for pressure drop, many workers C5, 12, 16, 29, 23, 34) have proposed different correlations for predicting total liquid phase holdup in two-phase concurrent downward flow. The liquid holdup depends on the nature and the flowrates of fluid phases, on the type of packing and on the eventual distribution or redistribution of the liquid phase. It is interesting to note that, both liquid holdup and two-phase pressure drop are mutually dependent, that is why many authors tried to correlate them in function of 6 and parameters. We present in Fig. (5 ) as an example, a comparison between our experimental results of two-phase pressure drop and total liquid holdup with those predicted by the correlations proposed by Midoux et al. (19)... 

It is also to note that the two-phase pressure drop and the liquid phase holdup necessary for the use of this correlation may be estimated from equations (7) and (8) respectively. This representation is only suitable for weak gas-liquid interaction flow in which the eventual anticoalescent characteristics of solutions do not intervene. Besides it may be noticed that the results obtained with ionic solution for glass beads do not fit with this correlation, and this occurs all the more as the anticoalescent characteristics are more pronounced (NaOH then Na2S0 ). 

One advantage of microstructures for photooxygenation reactions lies in the formation of thin liquid layers, which allows higher spatial illumination homogeneity, an irradiation of the complete solution inside the reactor, and also a shorter distance between the irradiation source and the solution. In addition, microfluidic reactors exhibit a low liquid phase holdup of explosive intermediates such as peroxides that are formed during photooxygenations. Furthermore, these products can immediately be quenched after leaving the photoreactor. 

The operating pressure has an indirect effect on the liquid-phase residence time. Increasing its value suppresses the vaporization of low-boiling fractions, thus increasing the liquid-phase holdup and the liquid-phase residence time. In addition. 

As for hydrodynamics, liquid-phase residence time is affected by pressure, since an increase of this variable suppresses the vaporization of low-boiling fractions, increases the liquid-phase holdup, and, conseqnently, the liqnid-phase residence time. In commercial visbreakers, the snperficial gas velocity is two to five times that of the superficial liquid velocity, and this ratio may change with residue conversion, operating pressure, and amount of steam injected in the coil, and consequently the operation regime may change. That is why for proper modeling of the hydrodynamics of visbreaking reactor, all these effects should be taken into consideration for accurate prediction of the liqnid-phase residence time. Pressure drop is also another process parameter that is vital to predict. 

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