Three-phase contacting is often carried out in gas-slurry stirred vessels. The influence of the solids concentration on the gas-liquid interfacial area has to date always been determined by chemical methods. This method has many drawbacks, including the fact that the mass transfer coefficient kL itself is dependent on the bubble diameter and interfacial area. Therefore a mass-transfer-independent means was used to evaluate the gas-liquid interfacial area. Also the solids concentration and sizes were varied and a coalescence frequency determined to evaluate the influence of the size of the solids on bubble coalescence. Experimental ProcedureThe experimental apparatus, shown in Figure 1, consisted of a 0.44 m dia. by 0.46 m high fully baffled stirred tank agitated with a single six-blade, flat-disk impeller. Two impellers having diameters of 0.23 and 0.15 m, respectively, and an impeller width-to-diameter ratio of 1/5 were used at an impeller location of one impeller diameter off of the bank bottom.Dispersions of air in water slurries of glass beads, having average sizes of 25,70, and 200 pm, were studied at various concentrations. The method of light scattering (Calderbank, 1958) was used to determine the gas-liquid interfacial area. Details of the equipment can be found elsewhere (Nagaraj, 1984). In each case the gas rate was maintained at a constant rate of 0.0283 m'/min and the light intensity readings were taken at an optical path length of 0.0508 m. Measurements in liquid, gas-liquid, solid-liquid and gas-liquid-solid systems were made for calibration purposes.The concentrations of solids were kept low to limit the effect of multiple scattering. McLaughlin and Rushton (1973) and Weinstein and Treybal ( 1973) have discussed conditions under which multiple scattering is negligible, and McLaughlin and Rushton have considered that when less than 10% of the inciThe prcsent address of N. Nagaraj is Ames Research Center. Moffett Field, CA 94035 dent intensity is received by the photocell, multiple scattering becomes important. This was further confirmed by two methods.1. Since the solids are much smaller than the gas bubbles, it could be expected that any multiple scattering, if present, would largely be due to the solids. Absence of multiple scattering in a solid-liquid system was confirmed by the method of Van de Hulst (1957), whereby a linear relationship was obtained between light attenuation and solids concentration.2. The effect of multiple scattering on the data obtained was found to be negligible by comparing the results obtained for the cases where the photocell received less than 10% of the incident light, using the linear correlation of Mclaughlin and Rushton (1973), and also using the multiple scattering calculations of Langlois et al. (1954). The work of Langlois et al. does not cover a gas-liquid-solid system, but does cover a liquid-solid system (glass beads in liquid) and a gas-liquid (air-glucose) system. By extrapolating these results, a value of , f 3 = 1.4 was used for the present system. This va...
To control the emissions from the diesel engines of modern automobiles, it requires the development of adequate and advanced exhaust gas aftertreatment devices. Selective Catalytic Reduction (SCR) is a method that can be used in mobile diesel engine aftertreatment systems to reduce harmful NOx emissions. Due to the toxicity and handling problems of ammonia, currently injection of a liquid Urea-Water Solution (UWS) into the exhaust stream approach is used. The water evaporates and the urea undergoes thermal decomposition producing ammonia that reacts with the NOx in the exhaust gas inside a SCR catalyst to produce nitrogen and water vapor. This work presents the study of UWS injection spray using commercial available CFD code, Fire v8.3. The evaporation of water from a single droplet of UWS is investigated theoretically and droplets are treated with Lagrangian particle tracking. Simulation study at different exhaust gas temperatures and injector locations is carried out and compared with experimental values. Thus, the present study results predict the local distribution and the conversion of the reducing agent.
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