in Wiley InterScience (www.interscience.wiley.com).The aim of this work was to investigate an influence of different alcohols on the behavior of a draft tube airlift reactor. The main hydrodynamic and mass transfer parameters in 1 wt % aqueous solutions of five aliphatic alcohols were examined. The results showed that alcohol addition enhanced gas holdup and mass transfer coefficient, but led to notable reduction of liquid velocity and hence elongation of circulation time, relative to the values determined for water. By introducing the surface tension gradient as a relevant independent variable, we defined a simple empirical correlation for overall gas holdup, circulation time, and mass transfer coefficient. The proposed empirical correlations have shown a satisfactory agreement between the calculated and the experimental data.

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Fundamentally based model for pressure drop in gas-flowing solids-fixed bed contactors is presented, together with a phenomenological semiempirical model for prediction of dynamic holdup. These simple models do not require any parameters that need to be determined by measurements in the actual system of interest. The predictions are compared with all available data and give good agreement for a wide range of experimental conditions, different constructions, types, and dimensions of packing and for a variety of flowing solids properties.Countercurrent flow of gas and fine solids through packed beds was patented as an idea in 1948, 1 and the first recorded industrial use occurred in 1965 (Compagnie de Saint Gobain) 2 in heat-transfer applications. The fluid dynamics of such systems received considerable attention over the years, 3-14 and this included heat-and mass-transfer studies. 8,[15][16][17] The interest in exploiting the unique features of the countercurrent gas-fine solids systems was enhanced by the studies of Westerterp and colleagues, 18,19 who proposed the use of fine solids as a regenerative adsorbent flowing through the bed of catalyst for methanol synthesis. Additional reactor-oriented studies included catalytic oxidation of hydrogensulfide 20 and regenerative desulfurization of flue gases. 17,21 The application of this type of gas-flowing solidsfixed bed contactors would be enhanced if the fluid dynamics in these systems could be fully quantified, at least in the macroscopic engineering sense. Reliable prediction of pressure drop, flowing solids holdup, residence time distribution, back mixing, and so forth, are some of the quantities needed when assessing the applicability of the "flowing" or "trickling solids" systems in a variety of processes.Previous studies resulted in a semiquantitative description of the fluid dynamics of the system and empirical correlations for determination of some quantities of interest. Three flow regimes were observed, similar to gas-liquid systems: preloading, loading, and flooding. Gas-flowing solids interaction increases with the increase in gas superficial velocity. When the terminal velocity of gas relative to flowing particles is approached, a sudden increase in pressure drop and fine solids holdup occurs, together with accumulation of solids at the top of the bed and unstable operation, which is characteristic for flooding.The complexity of the fluid dynamics of these systems did not permit, so far, a unique pressure drop equation to emerge without empirical constants. The problem is further complicated by the lack of consensus in accounting for the effects of particle shape, size, roughness, bed porosity distribution, and so forth. The models presented were often developed by fitting the data of a few studies and were not extensively tested, thus lacking in predicting ability. Moreover, the approach used was such that data on the system of interest were always needed to complete the correlation (i.e., one had to have data to predict them!). 6,10 In our earlier s...

The influences of alcohol adding and three different gas spargers (single orifice, perforated plate, and sinter plate) on the gas holdup in an external-loop airlift reactor were studied. The experimental results show that the gas holdup can be increased by mutual influence of both the alcohol adding and the sparger type. The presence of small amounts of normal aliphatic alcohols caused an increase of the gas holdup compared to the air-water system, due to their coalescence-inhibiting nature. The sinter plate was the most efficient sparger, followed by the perforated plate and the single orifice. A proposed artificial neural network (ANN) has the potential to predict gas holdup values in airlift reactors for various alcohols and types of gas spargers.

In this work, the influence of addition of normal aliphatic alcohols (from methanol to n-octanol) and the gas sparger type (single orifice, perforated plate, and sinter plate) on the volumetric mass transfer coefficient in a draft tube airlift reactor was investigated. The results showed that the addition of alcohols from methanol to n-hexanol led to an increase in the volumetric mass transfer coefficient, in comparison to water, while n-heptanol and n-octanol had the opposite effect. The influence of the gas sparger was dominant at low superficial gas velocities. At higher gas velocities, the liquid-phaseproperties and the sparger type, being opposite in effect on mass transfer, annulled one another's influence. Also, simple correlation was proposed to predict the volumetric mass transfer coefficient. The analysis of the parameters in the proposed correlation showed that the liquid-phaseproperties, expressed through the surface tension gradient, and the gas sparger type, mainly through initial bubble size, had a marked influence on the mass transfer.

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The equations for prediction of pressure drop in countercurrent gas-flowing solids-packed bed contactors are proposed. The correlations are based on all available data for various constructions, packing elements, and gas and flowing solids properties. The equations give good predictions and are given separately for preloading and loading zones, together with the correlation for the determination of the loading point.