Hydrodynamic behaviour of a gas—solid counter-current packed column at trickle flow
Trickle flow of a more or less fluidized catalyst through a packed column is a promising new gas—solid counter-current operation. The hydrodynamic, behaviour of such a column, filled with dumped PALL rings, has been investigated, while some results have been obtained with RASCHIG rings and cylindrical screens as packing. The solid used was a microspherical catalyst carrier. Pressure drop, hold-up, loading and flooding were evaluated and compared with literature data for gas—liquid systems. The behaviour is analogous although the absolute magnitude is different. 
 
Pressure drop is low, up to 50% of the solid being carried by the packing. A correlation for the pressure drop, which is mainly caused by suspended particles, has been derived. At low gas velocities particle velocity is constant, whilst near flooding the slip velocity between gas and solid reaches a constant value. Using empirical values for particle velocity and slip velocity, hold-up, loading and flooding can be predicted. Scaling-up problems still need to be investigated. Results on mass transfer, axial dispersion of both phases and solid spread factors will be published later.
A small scale regularly packed circulating fluidized bed. Part I: Hydrodynamics.
The present investigation is based on the idea of intensifying the gas?solids contact in a circulating fluidized bed by introducing obstacles into it. Such obstacles may effectively suppress radial inhomogeneities in the solids flux and concentration, increase the dynamic solids hold-up, and break up solids clusters. This article (Part I) deals with the hydrodynamics (pressure drop and solids hold-up) investigated at ambient conditions, for cocurrent upward flow of air and microsize solid particles (FCC, 70 µm diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × 0.06 m2 and contains regularly-stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas?solids contact. Slide-valves mounted above and below the packed section can be used to trap the solids inventory and determine the (dynamic) solids hold-up. Gas and solids mass fluxes have been varied in the range of 0.7 < Gg < 4.4 and O < Gs < 15 kg m-2s-2, respectively. Part II will report on the results of gas?solids mass transfer measurements, which have been carried out in the same set-up at comparable experimental conditions. Results of this work show that: (i) the pressure gradient over the packed section increases linearly with increasing solids mass flux, but faster than linearly with increasing applied gas mass flux, (ii) the dynamic solids volume fraction can be described quite well by the correlation s dyn = 0.0084 GsGg-1.22 for almost the entire range of applied gas and solids mass fluxes, (iii) the value for the solids friction factor derived for the gas flux range 0.7 < Gg < 3.7 kg (m-2s-1) varies from 1.4 to 2.5 and is linear with the solids volume fraction. These fs values are about 2 to 3 decades higher than those obtained from fs correlations derived for dilute-phase pneumatic conveying lines operated under the same experimental conditions.
Synthesis of 2,4-dideoxy-.beta.-D-erythro-hexopyranosyl nucleosides
The synthesis of 1-(2,4-dideoxy-β-D-erythro-hexopyranosyl)thymine (12) 1 was accomplished using two different synthetic routes. It was obtained starting either from 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose or from tri-O-acetyl-D-glucal. The other modified nucleosides, with either a cytosine, guanine, or adenine moiety, were synthesized using the second reaction scheme. Deoxygenation reactions were accomplished with the (2,4-dichlorophenoxy)thiocarbonyl derivatives generated in situ