Pressure drop and liquid holdup for two-phase concurrent flow in packed beds

Long chain polymers have a unique ability to become highly extended in elongational flow fields. The forces developed along the backbone give rise to scission of the chains near their center. Recently, this unique property of polymers has been adopted to explore new chemical transformations by embedding structural elements into the backbone designed to undergo site-specific bond cleavage, termed mechanophores. Experimental techniques to generate elongational flow fields exist in a variety of different arrangements and have been used to study polymer mechanochemistry in solution. This tutorial review will discuss progress in the field of polymer mechanochemistry as well as survey the techniques used to generate elongational flow fields. Ultrasonication will be highlighted as the technique that has been widely adopted to screen mechanophore reactivity in solution.

Polymer mechanochemistry: techniques to generate molecular force via elongational flows

Data on design and operation of trickle beds at elevated pressures are scarce. In this study the influence of the gas density on the liquid holdup, the pressure drop, and the transition between trickle and pulse flow has been investigated in a tricklebed reactor operating up to 7.5 MPa and with nitrogen or helium as the gas phase. Gas-liquid interfacial areas have been determined up to 5.0 MPa by means of CO2 absorption from CO2/N2 gas mixtures into amine solutions. 
A comparison of the results from nitrogen as the gas phase to those of helium shows that at equal gas densities the hydrodynamic states are the same. The gas-liquid interfacial area increases when operating at higher gas densities. When the determined dimensionless interfacial areas agl/as are all within the range 0.25-0.8, the trickle-bed reactor is suggested to operate in the trickle-flow regime. The gas density has a strong influence on the liquid holdup. Due to the higher pressure gradients at elevated gas densities, the liquid holdup decreases noticeably. Besides, the boundary between the trickle-flow and pulse-flow regime shifts toward higher liquid throughputs: the region for trickle-flow operationg becomes larger. For the liquid holdup and the pressure gradient in the trickle-flow regime, correlations derived based on dimensionless numbers can be applied to high-prssure trickle beds.

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Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures

Packed beds are typically tall cylindrical vessels filled with pieces of inert solid that have been dumped or stacked to leave considerable space for fluid between them (Leva 1 95 1 ). They are used to bring flowing gas and liquid into the intimate contact needed for rapid exchange of dissolved substances and, often, for chemical reaction as well. This use began nearly two centuries ago and is common in large-scale chemical processing; there are other applications too. Flow in packed beds is justly regarded as complex, irregular, even capricious, and scaleup from laboratory and pilot­ plant models to commercial sizes is notoriously risky (Ng & Chu 1 987). Despite this, packed beds have had relatively little fluid-mechanical study (Westerterp et a1 1 984, NRC Panel Report 1 986). This article reviews what appears most significant from what has been done. The point of view we take began to emerge in the mid-1970s in the course of research into how immiscible fluids flow together in the much finer, yet no less irregular, pore space of sedimentary rocks. The key idea was H. T. Davis's-that the way to approach macroscopic two-phase flow in porous media is with the statistical physics of network processes, then blooming in applications of so-called percolation theory (Larson et al 1977, 1981, Levine et al 1977). The basic ingredient was the essence of accumulated knowledge of the fluid mechanics of microscopic behavior in the individual passages, enlargements, and junctions of the pore space (Ng

Mechanics of Gas-Liquid Flow in Packed-Bed Contactors

Multiphase caulytic packed-bed reactors are used in chtmid, petrocbemid, as well as in the petroleum Lndustrg for contactirrg gaseau and liquid reactaxzzs with 2 solid ucalyst. Xydrodesulfurszltion of petroleum fractions, liquid-phase catalytic hydrocracking, and brrryaadiol synthesis fmm forrmldehyde and acetylene (one of the main steps of the Reppe-Buna synthesu) am well-bwn mrrltipnase caralytic packed-bed pmcesses. Multpharse catalytic packed-oed reactom have also been used in liquid waste treazmens recemly. h the case of absoqzion followed by homogeneous catalyrlc reacsions LO the liquid phe. The Sxed bed is formed by inert packing which enables an inmnsiw mixing between the gas and liquid phases, and thus producar a large gas-liquad interface.

B.E.T. Hutton

Papers

Cocurrent gas—liquid flow in packed columns

On flooding in packed columns