Abstract
Films and rivulets are the two basic forms of dynamic liquid in a three-phase fixed bed (trickle bed), which determines the wetting efficiency of the catalyst. This paper is devoted to the conflicting wetting performance observed between non-porous glass beads and less wettable porous alumina, and a parallel zone model is applied to resolve the complex liquid flow texture. It shows in both cases of glass beads and aluminium pellets, the pressure drop, film flow and rivulet flow fractions all display pronounced multiplicities along with the liquid flow rates in increasing and decreasing branches, although the rivulet flow fraction is reduced to 0 in the liquid decreasing branch started from pulsing flow in both cases. Different from the glass beads, there is almost no wetting efficiency difference for the alumina pellets with respect to liquid flow rate increasing or decreasing, which is in agreement with the dynamic liquid holdup measurements. The liquid is significantly more uniformly distributed over the crosssection in the Al2O3 bed since rivulet flow is much reduced than in the case of glass beads.
Highlights
A three-phase fixed-bed or trickle-bed reactor has been successfully applied in the traditional field of hydrogenation of petroleum fractions [1,2], waste organics oxidation [3,4], etc., and its application in the new area of biomass conversion is growing rapidly, as is shown in sugar hydrogenation [5,6], the hydrodeoxygenation of palm oil [7], aqueous phase reforming [8] and biological methanation [9]
Since ηce is generally defined as the fraction of catalyst surface covered by flowing liquid, while according to Zimmerman and Ng [12], the flowing liquid includes the film and rivulet, decomposition of the complex liquid flow texture into individual components is key to the determination of wetting efficiency
The measured static liquid holdups for glass and alumina pellets are shown in Table 1, which are 0.0499 for glass and 0.0745 for alumina pellets
Summary
A three-phase fixed-bed or trickle-bed reactor has been successfully applied in the traditional field of hydrogenation of petroleum fractions [1,2], waste organics oxidation [3,4], etc., and its application in the new area of biomass conversion is growing rapidly, as is shown in sugar hydrogenation [5,6], the hydrodeoxygenation of palm oil [7], aqueous phase reforming [8] and biological methanation [9]. In designing a trickle-bed reactor, the liquid–solid contacting condition, which is characterized by the external wetting efficiency, ηce , is of central importance for the safe and efficient operation of the reactor [10,11]. Since ηce is generally defined as the fraction of catalyst surface covered by flowing liquid, while according to Zimmerman and Ng [12], the flowing liquid includes the film and rivulet, decomposition of the complex liquid flow texture into individual components is key to the determination of wetting efficiency. In the past few decades, visualization techniques have been developed to provide direct observation of liquid wetting morphology. Similar results were observed by Sederman and Gladden [14]; they found the number of rivulets significantly increased with the increase in liquid velocities by using magnetic resonance imaging (MRI). With the high spatial resolution of X-ray CT, van der Merwe et al [15]
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