Gas liquid mass transfer in agitated vessels for newtonian and non-newtonian liquids

Abstract

increase was observed in ki_a values when the agitation was increased using the small impeller. The effect o-f impeller geometry and sise has been shown to affect the dependence of the ku.a on the dimensionless groups representing the various processing conditions. When these differences in impeller geometry Are neglected the average values -for these dependencies, i.e. exponents, were -Found to agree with the literature values. On the other hand, when these geometric differences were considered to be significant it was observed that the values of exponents of the Power Number and of the superficial gas velocity show a decreasing trend with respect to increasing impeller area. In the second part of this study, the influence of polymer addivites on the gas liquid mass transfer in the sparged agitated vessel was examined. The results showed that the addition of carboxyl methyl cellulose results in a reduction of the overall liquid side mass transfer coefficient compared with the ki_a values obtained in water at the same condition,i.e, the agitator speed,the gas flow rate and the temperature. ki_a values determined in the CMC solution were between S.StlO``3 <s~a) and 4.1*10~3B (s-1) as compared to 7.7#10`a: (s~*) and 7.3*10``3 <s-1) in water under similar operating conditions. These observations are in agreement with results of earlierIV ABSTRACT In this study, the effects of impeller geometries and process variables on the volumetric coefficients of liquid phase mass transfer, ki_a, in a sparged agitated vessel were investigated by the dynamic method. The process variables chosen for investigation were air flow rate, stirrer speed, impeller diameter, number of blades of the impeller and viscosity of the liquid. The stirrer speed was varied between 350 rpm and 1225 rpm, air flow rates were 2.75 lt/min and 5.05 lt/min. 5 cm. and 7.5 cm diameter impellers having four and six blades for each size were used for agitation. Dissolved oxygen concentrations were measured by a dissolved oxygen analyzer. Distilled water was used as the Newtonian liquid and a carboxyl methyl cellulose water solution (1 `/. wt CMC ) was used as the non Newtonian liquid. It was observed that ki_a values increase linearly with increasing impeller speed. The experimental results also showed that better mixing is obtained in the vessel with the larger diameter impeller even at low speeds above the critical speed, and ku.a values were not highly affected by the increase in the agitator speeds. On the other hand, the smaller impeller does not seem to cause as good mixing at lower impeller speeds. Considerable

IV ABSTRACT In this study, the effects of impeller geometries and process variables on the volumetric coefficients of liquid phase mass transfer, ki_a, in a sparged agitated vessel were investigated by the dynamic method. The process variables chosen for investigation were air flow rate, stirrer speed, impeller diameter, number of blades of the impeller and viscosity of the liquid. The stirrer speed was varied between 350 rpm and 1225 rpm, air flow rates were 2.75 lt/min and 5.05 lt/min. 5 cm. and 7.5 cm diameter impellers having four and six blades for each size were used for agitation. Dissolved oxygen concentrations were measured by a dissolved oxygen analyzer. Distilled water was used as the Newtonian liquid and a carboxyl methyl cellulose water solution (1 `/. wt CMC ) was used as the non Newtonian liquid. It was observed that ki_a values increase linearly with increasing impeller speed. The experimental results also showed that better mixing is obtained in the vessel with the larger diameter impeller even at low speeds above the critical speed, and ku.a values were not highly affected by the increase in the agitator speeds. On the other hand, the smaller impeller does not seem to cause as good mixing at lower impeller speeds. Considerableincrease was observed in ki_a values when the agitation was increased using the small impeller. The effect o-f impeller geometry and sise has been shown to affect the dependence of the ku.a on the dimensionless groups representing the various processing conditions. When these differences in impeller geometry Are neglected the average values -for these dependencies, i.e. exponents, were -Found to agree with the literature values. On the other hand, when these geometric differences were considered to be significant it was observed that the values of exponents of the Power Number and of the superficial gas velocity show a decreasing trend with respect to increasing impeller area. In the second part of this study, the influence of polymer addivites on the gas liquid mass transfer in the sparged agitated vessel was examined. The results showed that the addition of carboxyl methyl cellulose results in a reduction of the overall liquid side mass transfer coefficient compared with the ki_a values obtained in water at the same condition,i.e, the agitator speed,the gas flow rate and the temperature. ki_a values determined in the CMC solution were between S.StlO``3 <s~a) and 4.1*10~3B (s-1) as compared to 7.7#10`a: (s~*) and 7.3*10``3 <s-1) in water under similar operating conditions. These observations are in agreement with results of earlierIV ABSTRACT In this study, the effects of impeller geometries and process variables on the volumetric coefficients of liquid phase mass transfer, ki_a, in a sparged agitated vessel were investigated by the dynamic method. The process variables chosen for investigation were air flow rate, stirrer speed, impeller diameter, number of blades of the impeller and viscosity of the liquid. The stirrer speed was varied between 350 rpm and 1225 rpm, air flow rates were 2.75 lt/min and 5.05 lt/min. 5 cm. and 7.5 cm diameter impellers having four and six blades for each size were used for agitation. Dissolved oxygen concentrations were measured by a dissolved oxygen analyzer. Distilled water was used as the Newtonian liquid and a carboxyl methyl cellulose water solution (1 `/. wt CMC ) was used as the non Newtonian liquid. It was observed that ki_a values increase linearly with increasing impeller speed. The experimental results also showed that better mixing is obtained in the vessel with the larger diameter impeller even at low speeds above the critical speed, and ku.a values were not highly affected by the increase in the agitator speeds. On the other hand, the smaller impeller does not seem to cause as good mixing at lower impeller speeds. Considerable