Volumetric mass transfer coefficients in slurry bubble columns operating in the churn-turbulent flow regime

We report the results of an extensive experimental study of the gas hold-up, ε , and volumetric mass transfer coefficient, k<SUB>L</SUB>a, in bubble columns operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range 0-0.4 m/s, spanning both the homogeneous and churn-turbulent flow regimes. Air was used as the gas phase in all cases. Three different measurement campaigns were carried out. In the first campaign the influence of liquid properties (viscosity, surface tension) were investigated in a column of 0.1 m diameter, equipped with a sieve plate distributor with 0.5 mm holes. Four different liquids were investigated: water, tetradecane, paraffin oil and Tellus oil, with viscosities ranging from 1 to 75 mPa s. The gas hold-up ε<SUB>G</SUB> in these systems vary significantly. The volumetric mass transfer coefficient, k<SUB>L</SUB>a, closely follows the trend in gas hold-up. For the churn-turbulent regime of operation, i.e. U&gt; 0.08 m/s, the value of k<SUB>L</SUB>a/ε<SUB>G</SUB> is found to be practically independent of U; the value of this parameter is found to depend on the liquid phase Schmidt number, showing a Sc<SUP>−1/3</SUP> dependence. In the second campaign, we investigated the influence of catalyst particles addition (porous silica, mean diameter=38 μm) to the liquid phase (water and tetradecane), with slurry concentrations varying up to 25 vol.%. With increasing slurry concentrations, ε<SUB>G</SUB> is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations, the "small" bubbles are almost completely destroyed. For U&gt;0.08 m/s, the value of k<SUB>L</SUB>a/ε<SUB>G</SUB> is again found to be practically independent of U; this parameter is however significantly lowered with increased catalyst concentrations, due to increase in the size of the "large" bubbles. In the third campaign, the influence of increasing column diameter D<SUB>T</SUB> was investigated by experiments with water and Tellus oil. For both systems, k<SUB>L</SUB>a/ε<SUB>G</SUB> shows a slight increase with D<SUB>T</SUB> in the churn-turbulent regime. This increase is due to increased liquid circulations with increasing scale, leading enhanced bubble split up. Our studies provide a simple method for estimation of k<SUB>L</SUB>a in industrial size slurry bubble columns operating in the churn-turbulent flow regime

Large bubble sizes and rise velocities in a bubble column slurry reactor

The results are reported of an experimental study of the gas holdup, ϑG, large bubble diameter, dLb, and large bubble rise velocity, VLb, in a 0.1m wide, 0.02m deep and 0.95m high rectangular slurry bubble column operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range of 0-0.2m/s, spanning both the homogeneous and heterogeneous flow regimes. Air was used as the gas phase. The liquid phase used was C9-C11 paraffin oil containing varying volume fractions (ϑS = 0, 0.05, 0.10, 0.15, 0.20 and 0.25) of porous catalyst (alumina catalyst support, 10% &lt; 10μm; 50% &lt; 16μm; 90% &lt; 39μm). With increasing slurry concentrations, ϑG is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations the "small" bubbles are significantly reduced in number. By the use of video imaging techniques, it was shown that the large bubble diameter is practically independent of the gas velocity for ϑS &gt; 0.05 and U &gt; 0.1m/s. The measured large bubble rise velocity VLb agrees with the predictions of a modified Davis-Taylor relationship

Gas-liquid mass transfer in a slurry bubble column at high slurry concentrations and high gas velocities

The volumetric mass transfer coefficient &#954;_La in a 0.1  m-diameter bubble column was studied for an air-slurry system. A C₉-C₁₁n-paraffin oil was employed as the liquid phase with fine alumina catalyst carrier particles used as the solid phase. The n-paraffin oil had properties similar to those of the liquid phase in a commercial Fischer-Tropsch reactor under reaction conditions. The superficial gas velocity U_G was varied in the range of 0.01 to 0.8  m/s, spanning both the homogeneous and heterogeneous flow regimes. The slurry concentration &#949;_S ranged from 0 to 0.5. The experimental results obtained show that the gas hold-up &#949;_G decreases with an increase in slurry concentration, with this decrease being most significant when &#949;_S &#60; 0.2. k_La/ &#949;_G was found to be practically independent of the superficial gas velocity when U_G &#62; 0.1 m/s is taking on values predominantly between 0.4 and 0.6 s^–1 when &#949;_S = 0.1 to 0.4, and 0.29 s^–1, when &#949;_S = 0.5. This study provides a practical means for estimating the volumetric mass transfer coefficient &#954;_La in an industrial-size bubble column slurry reactor, with a particular focus on the Fischer-Tropsch process as well as high gas velocities and high slurry concentrations

Studies on the iron-catalyzed Fischer-tropsch process in a laminar flow slurry column reactor

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Experimental investigation of the liquid volumetric mass transfer coefficient for upward gas-liquid two-phase flow in rectangular microchannels

The gas-liquid two-phase mass transfer process in microchannels is complicated due to the special dynamical characteristics. In this work, a novel method was explored to measure the liquid side volumetric mass transfer coefficient kLa. Pressure transducers were utilized to measure the pressure variation of upward gas-liquid two-phase flow in three vertical rectangular microchannels and the liquid side volumetric mass transfer coefficient kLa was calculated through the Pressure-Volume-Temperature correlation of the gas phase. Carbon dioxide-water, carbon dioxide-ethanol and carbon dioxide-n-propanol were used as working fluids, respectively. The dimensions of the microchannels were 40 µm×240 µm (depth×width), 100 µm×800 µm and 100 µm×2000 µm, respectively. Results showed that the channel diameter and the capillary number influence kLa remarkably and that the maximum value of kLa occurs in the annular flow regime. A new correlation of kLa was proposed based on the Sherwood number, Schmidt number and the capillary number. The predicted values of kLa agreed well with the experimental data