Packed beds holdup, measurement

Inglezakis, V.J., Zorpas, A.A., Grigoropoulou, H.P., Arapoglou, D., Liquid holdup measurements using tracing techniques in zeolite packed beds, 5th International Scientific and Technical Conference Water Supply and Water Quality, Poznan, Poland, 430-440 (2002)... 

Achwal and Stepanek1 recently measured the holdup profile in a packed bubble-column by a method based on measuring thermal conductivity. They found that the gas holdup in a packed bed increased with height and related this increase to the change in pressure. Two separate correlations for the average gas holdup were derived. One, based on the homogeneous flow model, was expressed as... 

For abscissa values less than 0.02, which usually indicate the operation is under vacuum, special pressure drop equations can be developed from the packing dry line. In such operations, the liquid rate is low, and the liquid holdup is small. When holdup is only a few volume percent, the void fraction reduction of the packed bed is slight. Actual measurements indicate the pressure drop in operation can be lower than that predicted from Figure 1-15 using an abscissa value of 0.02. (See Chapter 8 for further discussion.)... 

Piret et al. measured liquid holdup in a column of 2J-ft diameter and 6-ft packed height, packed with graded round gravel of lj-in. size, the total voidage of the bed being 38.8%. The fluid media, air and water, were in countercurrent flow. The liquid holdup was found to increase markedly with liquid flow rate, but was independent of gas flow rate below the loading point. Above the loading point, an increase of liquid hold-up with gas flow rate was observed. 

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. 

The measured cross-sectionally averaged liquid holdup (i.e. measured liquid saturation times the bed voidage) are consistent with previously reported results [4], For a similar packing structure, Illiuta and Larachi [4] found liquid holdups, as calculated by their mechanistic model and compared with the reported experimental results, slightly lower than the values obtained in this study. However, there are some differences between the two studies this work was performed in a smaller diameter column ( 30 cm versus... 

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. 

The models for catalyst effectiveness in trickle bed reactors developed in this paper require explicit measurements or predictions of external contacting, ncE pore fill-up, rii In laboratory conditions this can be accomplished by tracer techniques (22, ). Fractional pore fill up may be determined by the difference in first moments of the impulse response tracer tests performed on two beds of same particle size and shape when one bed consists of porous the other of nonporous particles. Fractional pore fill-up can also be assessed from the measured volumetrically static holdup. External contacting is measured by adsorbable tracer tests on beds of nonporous particles ( ). In industrial conditions ncE hj would have to be evaluated from correlations. Unfortunately at present the existing correlations for ncE unsatisfactory since they were developed for fixed bed adsorbers with larger packing and correlations for m ate nonexistent but may be developed in the future. 

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