The influence of polymer additives upon the gas-liquid mass transfer in stirred tanks is examined. While the addition of sodium carboxymethylcellulose brings about a reduction of the overall liquid side mass transport coefficient which is in line with the predictions by earlier works, the reduction of kLa in polyacrylamide solutions of equal viscosity is much stronger. This is explained by the viscoelasticity of the PAA solutions, and an empirical correlation is proposed. SCOPERecent research has shown that the addition of small amounts of water soluble polymers can be beneficial in certain biochemical processes. Some of these polymer additives, like polyox and poly acrylamide, which are known to be drag reducing agents, can reduce the power requirement in stirred tanks by about 20yo and in gas-liquid contactors by as much as 7001,. Lucrative incentives thus exist for the addition of polymers in various processes.Mixing is, however, rarely carried out for its own sake, and the influence of polymer additives in stirred tanks must be examined with respect of processes like heat and mass transfer. Further, some polymers will not only influence the viscosity of the aqueous solutions, but they will also make the solution viscoelastic. The manifestations of viscoelasticity can be numerous, ranging from recoil in concentrated solutions to a phenomenon known as drag reduction in very dilute solutions.In a recent paper, Quaraishi et aI. (1977) reported that drag reducing polymers added to stirred aqueous dispersion have no influence upon the performance of the impeller to suspend solid particles and only a limited negative influence upon the liquid phase mixing. The question of the gas-liquid mass transfer was, however, left open in their work. An attempt to find the answer to this question is the scope of this work. CONCLUSIONS AND SIGNIFICANCEPolyacrylamide and sodium carboxymethylcellulose polymers were added in small quantities to the gas-liquid dispersion in a stirred tank, and the overall liquid side mass transfer coefficients were measured using a slow chemical reaction technique.The overall gas-liquid mass transfer coefficient 1cr.n is reduced in diluted aqueous solutions of polyacrylamide far more than one would expect on the basis of increaied viscosity of the solution. Parallel experiments with solutions of sodium carboxymethylcellulose do not show such a drastic reduction. It is argued in this work that this is due to the increased viscoelasticity of the PAA solutions. Following the arguments of Seyer and Metzner (1967), one of the manifestations of the viscoelasticity of diluted polymer solution is the increase of the elongation viscosity. This, in turn, prevents highly deformed gas bubbles from reaching the final stage when they break up. An indirect support for this model is found in the considerably reduced gas holdup in PAA solutions. The consequence of this is a reduction of the specific interfacial area a.When selecting polymer additives for any particular process, one has to bear in mind that the addi...

The residence time distribution in a helically coiled tubular reactor is computed for laminar Row of inelastic non-Newtonian liquid at small Dean number. Erect of pseudoplasticity on the residence time distribution is explored. I t was found that pseudoplasticity of the liquid makes the residence time distribution narrower. The efect of dialatant fluid behavior is opposite but very small in magnitude. The results of numerical integralions are correlated with wwer law index, allowing re dictions over a wide range of the index. The --.results could be used to predict conversion of reactions in a coiled reactor, such as continuous polymerization o r fermentation reactors, where the reaction mixtures are likely to be non-Newtonian in nature. Results will be also useful in characterization of coiled chromatographic columns.

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The turbulent flow of mildly viscoelastic liquids in curved tubes is analyzed using boundary layer approximations. The momentum integral approach is used and the governing equations are solved numerically by the Runge-Kutta-Merson technique. The influence of Deborah number on the velocity distribution and the boundary layer thickness has been studied for the first time. The theoretically developed expression for frictional losses was checked with available experimental data and sound agreement was found. The results of this analysis are likely to find application in the biomedical fluid mechanics and in the analysis of the flow with so-called "drag reducing" additives.