Influence of polymer additives on the gas‐liquid mass transfer in stirred tanks

Abstract

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...

Absorption of carbon dioxide into aqueous colloidal silica solution with different sizes of silica particles containing monoethanolamine

“…It may be concluded that the exponent, m, in the relationship k L a L ∝µ −m for stirred tanks is between 0.35 and 0.6 based on the previously reported values of m of 0.4 by Yagi and Yoshida [4], 0.41 by Ranade and Ulbrecht [5], and 0.38 by Moo-Young and Kawase [32]. The value of n 1 of 0.37 in this study compares well with the findings of the papers mentioned above.…”

“…As shown in Table 2, the values of A increased with increasing C s and decreasing d p , which means that the aqueous colloidal silica solution exhibits elastic behavior [5].…”

Pilot-scale production of bacterial cellulose by a spherical type bubble column bioreactor using saccharified food wastes

“…Also, on the addition of agar to the saccharified food wastes medium (5.8 g/L), even the k L a value decreased to 10.7 hr −1 , but the viscosity and BC productivity were increased ( Table 2). It could be supposed that the BC productivity was increased due to the decreasing conversion rate of the cel − mutant by decreasing the shear stress and the prevention of BC coagulation due to the increased viscosity on the addition of a polysaccharides, such as agar [16,17]. As a result, it was concluded that the influence of the viscosity was higher than that of the k L a on the BC productivity.…”

Absorption of carbon dioxide into aqueous colloidal silica solution with diisopropanolamine

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