Computational fluid dynamics (CFD) provides an efficient method for investigating highly complex fluid flows in mechanically stirred tanks, but less attention was given to unbaffled stirred tanks generated with axial impeller, which are frequently used in process industries. The present study is intended to evaluate the CFD predictions of key properties related to the mixing against measurements and to provide a detailed insight into the process. A three-blade 70°PBTD (Pitched Blade Turbine Downflow) was used to generate axial flows in a stirred tank, of which the local solid concentration profiles were numerically simulated. The liquid-solid system consisted of spherical silica beads in water. A method using "vector distance" in a Eulerian reference frame was proposed to resolve the impeller and the bottom of the tank of complex geometry. The predicted results of two-phase flows at various impeller speeds and solid holdups are presented. In addition, various Schmidt numbers and laminar viscosity coefficients of dispersion phase were tested to improve the predictions. In the experiments, an optical fiber probe was used to measure solid particle concentration, and the effects of impeller speed on solid holdup were examined. The solid particle distribution was affected with the impeller speed in the range of 113-173 rpm in the stirred tank. The radical concentration of solids particle presents different profiles at different axial cross sections. Good agreement between the experiments and simulations was observed.
Direct coal liquefaction in the heating stage of Shenhua Shendong bituminous coal was carried out in a 0.01 t/d continuous tubular facility with iron catalyst and hydrogenated anthracene and wash oil as solvent at a residence time (t) of 3.5− 6.5 min and a reaction temperature (T) of 340−450 °C. The results show that when t = 3.5 min and T = 340 °C, a cracking reaction of coal occurs, while the oil yield was almost zero. As the residence time and temperature each increase, coal conversion and product yield exhibit different change patterns. Especially when t = 6.5 min and T = 450 °C: under these conditions, the coal conversion and oil yield reached 83.67 and 52.27 wt %, respectively. To investigate the liquefaction kinetics, a 8-lump reaction kinetic model which follow first-order irreversible reactions (r = k i dC/dt) was developed to estimate the rate constants. The results indicated that the model is perfectly valid for the heating stage, and the yield of oil and gas were mainly from coal other than preasphaltene (PAA).
In order to study the direct coal liquefaction in the isothermal stage of Shenhua Shendong bituminous coal, the direct coal liquefaction with iron-based catalyst was carried out in a 0.01t/d continuous tubular facility in the temperature range of 445℃ to 465℃, with hydrogenated anthracene and wash oil as solvent. A 8-lump kinetic model of the isothermal stage was proposed, and the kinetic parameters were estimated. The result showed that in the isothermal stage the oil was mainly obtained from PAA rather than from coal directly. The model was valid for the isothermal stage of direct coal liquefaction.
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