Mass Transfer Of Multi-Phase Stirred Reactors At Elevated Temperature

Multi-phase mechanically stirred reactors are widely applied in biochemical engineering, petrochemical engineering, minerals and processing, fine chemical engineering and environmental engineering. They can supply an even concentration field and energy field for the reactions of catalytic hydrogenation, halogenation, polymerization and oxidization in the industrial processes, such as oil refining, coal liquefaction, mineral processing, metallurgy, pharmacy, food processing, aerobic fermentation, and sewage treatment. In the above reactions, reactants should be mixed well to prevent the uneven reactant concentration and the consequent abnormal reactions. The fluid hydrodynamics and the mass transfer are two important factors influencing on the multiphase reaction. Especially for the fast reaction, the gas-liquid mass transfer is often controlling the reaction rate, and the volumetric mass transfer coefficient, kLa, is considered as a key parameter in the performance, design, and optimization of stirred tank reactors. In the past decades, many researchers focus on the mass transfer at ambient temperature and little attentions have been paid on the system operated at elevated temperature. Results and rules obtained at ambient temperature are often used to guide the design and scale-up of the reactors at hot conditions, which would cause error inevitably because previous works showed that the hydrodynamics characteristics in hot systems are obviously different from those in cold systems. Therefore, we carry out the research on mass transfer performance at elevated temperature to provide helpful guidance for the industrial design and operation of multi-phase stirred reactors at high temperature.Experiments were carried out in non-coalescent and coalescent system, respectively. Firstly, sodium sulfite aqueous solution was used as a non-coalescent liquid phase. A triple-impeller combination consisted of a half-elliptical disk turbine (HEDT) below two up-pumping wide-blade hydrofoils (WHU) was used as impellers. kLa was obtained by catalyzed sulfite oxidation method at various power inputs, superficial gas velocities, and temperatures. The results show that kLa increases with increasing power consumption, superficial gas velocity, and temperature. A kLa correlation as function of power consumption, superficial gas velocity, and temperature was proposed based on a regression analysis of the experimental data. The exponent for temperature is the biggest, and the exponent for power consumption is 0.63, higher than that for gas superficial gas velocity with value of 0.34. This indicates that increasing temperature can increase kLa evidently, and increasing power consumption is more effective than increasing superficial gas velocity in order to increase kLa.Deionized water, representing a coalescent batch, was used as the liquid phase to investigate effects of temperature on gas dispersion and mass transfer in gas-liquid two phase system, to investigate effects of temperature on gassed power number and mass transfer in gas-liquid-solid three phase stirred reactors. The results show that the gas hold-up decreases obviously with the increasing temperature and the gas hold-up at 80? is only 58.8% of that at 25?. However, the temperature has little effect on the volumetric mass transfer coefficient (kLa). Experimental kLa at 80? increases only 2% compared with that at 25?. The correlations of gas hold-up and kLa with power consumption, superficial gas velocity, and temperature were obtained and in good agreement with the experimental values of kLa with error in ±10%. The exponent for these independent variable show that gas hold-up is affected by temperature the most, then superficial gas velocity, and power consumption. However, the effect sequence of these three parameters reverses for kLa. The exponent for power consumption is twice of that for superficial gas velocity and four times of that for temperature. In gas-liquid-solid three phase system, kLa increases with the increase of temperature. At ambient temperature (25?), kLa decreases significantly with increasing Cv, but this decreasing effect becomes weaker as the temperature increases. At 54?, kLa is almost independent on CV; At 80?, kLa increases with increasing Cv. But the whole variation range of kLa is within 10% when the temperature ranges from 40 to 80?.In order to satisfy the need for diversity of industrial impellers, various triple-impeller combinations were used to investigate kLa at different operation conditions. The impellers used included six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W). The results show that kLa differences for different impeller combinations are not obvious at low superficial gas velocity (uG). However when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass transfer performance in all impeller combinations and operating conditions. At high uG and a given power input, the impeller combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, we obtained the regressed correlations between gassed power number and Froude number and gas flow number, and between kLa and power consumption and superficial gas velocity for five different impeller combinations, which could be used as helpful guidance for the industrial design.Models for kLa and kL in non-coalescent and coalescent system at elevated temperature were obtained based on multi-variable method. Models for kL was expressed in the form of "eddy" model and was used to describe the rules of kL at hot conditions., the exponent for DL in kL model at elevated temperature is similar to 0.5 in "eddy" model; but the exponent for PTm in kL model at elevated temperature decreases from 1/4 to 1/6, which is due to the increasing temperature weakening the effect of power dissipation on kL. In non-coalescent system, the exponent for DL in kL model is less than that in coalescent system. Accuracy testing of kL model was carried out by comparing the theoretical value of kLa with the experimental results. Theotical analysis based on kL model was carried out on calculation for the ratios of gas holdup, bubble size, gas-liquid interfacial area, diffusion coefficient, mass transfer coefficient, and volumetric mass transfer coefficient at different temperatures to that at 25? in coalescent gas-liquid system and to that at 50? in non-coalescent gas-liquid system. Results of theoretical analysis are in agreement with the experimental results, which indicates the accuracy of kL model.