Solid‐liquid mass transfer in packed beds with cocurrent gas‐liquid downflow

Abstract

Local instantaneous solid-liquid mass transfer coefficients were measured in two-phase gas-liquid downflow through packed columns for 3 X 3 mm and 6 X 6 mm cylinders. An electrochemical technique was used. Liquid flow rates from 3.0 to 26.6 kg/m2.s and gas flow rates from 0.07 to 1.16 kg/m2*s covered the gascontinuous, ripple, and pulse flow regimes. Timeaveraged mass transfer coefficients in trickle flow and in pulse flow for the pulse proper and the base (outside the pulse) were found to increase with increasing gas and liquid rates. Correlations are presented in terms of liquid phase Reynolds numbers and in terms of Kolmogoroff numbers. The mass transfer coefficients in the pulse were found to correspond closely to the coefficients that would be attained in the dispersed bubble flow regime. V. G. RAO and A. A. H. DRINKENBURG Department of Chemical EnglneerlngRljksunlversltelt Gronlngen 9747 AG Gronlngen, The Netherlands SCOPE Trickle-bed reactors, when operated in the gas-continuous trickle flow regime, are generally restricted to relatively slow reactions since the rate is often controlled by the mass transfer resistance. Operation of the reactor at higher gas and liquid rates produces pulse flow in which high transfer rates can be realized (Hirose et al., 1976). The pulse flow regime is better suited for fast reactions than the trickle flow regime. For design, the resistance to absorption of gaseous components into the liquid and the transfer of the reacting species through the liquid film to the catalyst surface must be known, which necessitates the study of solid-liquid mass transfer. The many reported studies on solid-liquid mass transfer generally have utilized one of two methods: measuring the dissolution rate of some soluble packing or packing coated with a soluble dye, or measuring the electrical current in water under diffusion-limited transport conditions.The electrochemical method has certain advantages over the other. It facilitates direct and instantaneous measurements of solid-liquid mass transfer and is thus very useful to measure mass transfer fluctuations, especially under pulsing flow conditions. Chou et al. (1979) first measured such fluctuations in pulse flow. The pulses were found to play an important role in enhancing mass transfer coefficients, Not much is known about the mass transfer coefficients in the pulses, their dependence on flow rate of the phases, and the packing properties. No study other than Chou's has been reported in this direction. All earlier studies employed the dissolution method, from which only time-averaged values were found.The present study was conducted to understand the fundamental nature of the pulses with respect to mass transfer, the extent of mass transfer fluctuations, and their qualitative and quantitative influence on mass transfer. The instantaneous mass transfer from a single active particle (brass cylinder electrochemically coated with nickel) in the bed was measured. Beds with particles of 3 X 3 mm and 6 X 6 mm cylinders were used. The active particl...

Trickle flow liquid–solid mass transfer and wetting efficiency in small diameter columns

“…The reagent concentration for the anode reaction is ten times that needed for the cathode reaction to ensure rate limitation at the cathode. The standard electrochemical method described by Rao and Drinkenburg (1985) is used to measure the liquid-solid mass transfer at the surface of the electrodes. The current through the system, for an applied potential difference of 1000mV, is measured using a NI PXI-4071 Digital Multimeter with 7 ½ digit accuracy supplied by NI-Solutions.…”

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

“…As one can see in Fig. 8 the experimental data are largely underestimated by the correlation of Rao and Drinkenburg (1985). The correlation of Lakota and Levec (1990) predicts liquid/solid mass transfer with a better accuracy at high value of liquid interstitial velocity but the discrepancy increases for Reynolds number based on liquid interstitial velocity lower than 100.…”

“…The validation of this correlation is thus restricted to a particle diameter between 2.5 and 3 mm and illustrates mainly the effect of liquid interstitial velocity on liquid/solid mass transfer. It must be noticed that the exponent of Reynolds number is larger than those proposed by existing correlations for f · k LS (Satterfied et al, 1978;Rao and Drinkenburg, 1985;Lakota and Levec, 1990). Nevertheless, in these previous correlations, partial wetting was not taken into account, the whole solid surface being referred as the transfer surface, and only water/air system was used.…”

“…Furthermore, these correlations express wetting efficiency mainly as a function of gas/liquid flow hydrodynamics but none of them include the effect of solid intrinsic wettability. The liquid/solid mass transfer in trickle bed reactors has been largely studied and numerous correlations have been proposed for liquid/solid mass transfer coefficient k LS (Goto and Smith, 1975;Specchia et al, 1978;Satterfied et al, 1978;Rao and Drinkenburg, 1985;Lakota and Levec, 1990). The validity domain of these correlations remains limited since they are all issued from experimental data obtained with aqueous solution as liquid phase and air as gas phase.…”

Selective Hydrogenation of 1‐Butene Rich Cuts: the Impact of the Intraparticle Diffusion Limitations on the Selectivity

“…/s, respectively. The values of k j were calculated from a correlation proposed for trickle-bed reactors [11] assuming that the liquid superficial velocity is 0.028 m/s, which is a representative value in industrial practice [12].…”