Pulsing flow liquid 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. 

In concurrent downward-flow trickle beds of 1 meter in height and with diameters of respectively 5, 10 and 20 cm, filled with different types of packing material, gas-continuous as well as pulsing flow was realized. Residence time distribution measurements gave information about the liquid holdup, its two composing parts the dynamic and stagnant holdup and the mass transfer rate between the two. 

Relations of the rate of mass transfer between gas and liquid and the influence of the stagnant and dynamic holdup were not researched intensively, until the present work, although papers on the general subject have been presented (3-6). Lately an interesting paper about mass transfer from liquid to solid in pulsing flow was presented by Luss and co-workers (7 ). 

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.

A model is presented to predict flow transition between trickling and pulsing flow in cocurrent downflow trickle-bed reactors. Effects of gas and liquid flow rates, particle size, and pressure on the transition are studied. Comparison of theory with published transition data from pilot-scale reactors shows good agreement. Since the analysis is independent of reactor size, calculations are extended to include large-scale columns some interesting observations concerning flow transition and liquid holdup are obtained. 

Figure 2 shows the influence of particle size on flow transition. It is interesting to note that the 4mm and 6mm transition curves intersect each other. This phenomenon can be explained by considering the effect of particle size on the interstitial gas velocity needed to induce pulsing and the liquid holdup of the bed. 

It should be noted that the liquid holdup is a time averaged value. In practice, for flows in the pulsing regime at a given column depth, the holdup will vary with time due to the alternating passage of liquid and gas rich slugs. 

In this paper correlations presented by Sato et al. for liquid holdup and pressure drop in trickle bed reactors were used to examine the characteristics of large-scale columns. The trickling-pulsing transition relationship given by Ng was also employed to determine the flow regime present. Some interesting phenomena were observed, specifically ... 

Specchia and Baldi90 presented separate correlations for the dynamic liquid holdup in the poor interaction regime (i.e., gas-coirtinuous-flow regime) and the high-in teraction regime (i.e., pulsed and spray flow).In the poor-interaction regime, they presented a relation... 

This correlation was in reasonable agreement with the low gas flow (Gg < 0.01 g cm-2 s1) data of Sato et al.74 for a benzoic acid-water system with 5.5- and 12.2-mmparticle diameter. Hirose et al.38 made extensive measurements in trickle-flow, pulsed-flowrand bubble-flow regimes and correlated the enhancement factor in Ks due to parallel gas flow with the liquid velocity. They found that this enhancement factor (ratio of Ks in the presence of gas flow to Ks in the absence of gas flow) was inversely proportional to the total liquid holdup and to a first approximation has the value 2. Their data for Ks as a function of liquid velocity... 

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. 

Actually Sato et al. expressed their particle mass-transfer coefficients in terms of an enhancement factor representing the ratio of with two-phase flow to ks at the same liquid flow rate in single-phase flow. For pulsing and dispersed bubble flow this enhancement factor was found to be inversely proportional to liquid holdup j3, which in turn is a function of the two-phase parameter A or A (see Section IV,A,3,a). For comparison, the data for single-phase liquid flow are best represented by an equation of the same form as Eq. (115) but with a constant of 0.8. 

Xiao, Q. Anter, A.M. Cheng, Z.M. Yuan, W.K. Correlations for dynamic liquid holdup under pulsing flow in a trickle-bed reactor. Chem. Eng. J. 2000, 78, 125. 

Liquid holdup, which is expressed as the volume of liquid per unit volume of bed, affects the pressure drop, the catalyst wetting efficiency, and the transition from trickle flow to pulsing flow. It can also have a major effect on the reaction rate and selectivity, as will be explained later. The total holdup, h, consists of static holdup, h, liquid that remains in the bed after flow is stopped, and dynamic holdup, h, which is liquid flowing in thin films over part of the surface. The static holdup includes liquid in the pores of the catalyst and stagnant packets of liquid held in crevices between adjacent particles. With most catalysts, the pores are full of liquid because of capillary action, and the internal holdup is the particle porosity times the volume fraction particles in the bed. Thus the internal holdup is typically (0.3 — 0.5)(0.6), or about 0.2-0.3. The external static holdup is about... 

It is well known that a variation of the liquid flow rate at the reactor inlet results in pulses of liquid traveling through the catalyst bed and a corresponding variation in the liquid holdup. For the liquid holdup oscillation a mathematical... 

Fig. 2. Liquid holdup inside and in between pulses as a function of gas and liquid flow rates.

---