Experimental Study On The Influence Of Addition Of Low Surface Tension Substance On Gas Liquid Mass Transfer

Gas-liquid mass transfer is one of the most common phenomena and a keycomponent determining the process efficiency in chemical engineering processes. Theresearch on the enhancement of mass transfer is of great importance. In gas-liquidmass transfer processes, the surface tension plays an important role, and change inliquid surface tension may lead to interfacial instability and even so-called Marangonieffect, a local convection phenomenon induced by surface tension gradient near theinterface. Such an interfacial convection may affect the gas-liquid interfacial masstransfer significantly. In this work, the influence of the addition of low surface tensionsubstances on gas-liquid mass transfer was investigated, aiming at finding a method toenhance gas-liquid mass transfer.First, the effects of additions of three different surfactants with different criticalmicelle concentrations (CMC) on the CO2absorption into aqueous solutions wereinvestigated. The surfactants are n-octyltrimethylammonium bromide (OTABr),sodium dodecyl benzene sulfonate (SDBS) and Tween80. The results showed that,the specific gas liquid mass transfer area was increased by the addition of thesurfactants, but the volumetric mass transfer coefficient and the liquid mass transfercoefficient were decreased because of the micelle phenomenon of the surfactants. Itwas also shown that the mass transfer coefficient depends on the CMC of thesurfactants.Second, the effect of addition of low surface tension substances (ethanol andn-propanol) in the gas phase was investigated. It was shown that local movement onthe liquid surface occurred when it was put into contact with the gas with lowconcentration of the low surface tension components. With the aid of the high-speedphotography technique, such a Marangoni phenomenon was recorded and the velocitywas calculated by software. This experiment demonstrated the possibility of localMarangoni convection generated by the addition of low surface tension substancesinto the gas phase.Then, the effect of the addition of a small amount of ethanol in the gas phase onthe water-CO2absorption process was studied in the bubble column. The experimentsshowed that the bubble size increased and the specific interfacial area became smaller,but the mass transfer coefficient was increased with the addition of ethanol in the CO2.As an ensemble effect, the gas-liquid mass transfer was enhanced in the absorptionprocess. The experiments also showed that, with the increase of ethanol concentration in the gas, negative Rayleigh effect would occur; the mass transfer came to becontrolled by the combined effects of both local Marangoni effect and negativeRayleigh effect. Such a mechanism resulted in the phenomenon that, with the increaseof the ethanol concentration in the gas, the mass transfer coefficient increased forlower concentration of ethanol in the gas, and decreased at higher ethanolconcentration in the gas.In order to prove the occurrence of the Marangoni and Rayleigh effects in theabsorption system, a PIV experiment was conducted. An average velocity distributionalong the direction perpendicular to the interface in the liquid was obtained in the PIVexperiment for the same absorption process but within a simulator for the gas liquidcontacting. It was shown in the experiment that the addition of ethanol into the gasgenerated liquid local convection, propagating from the surface to the deeper area inthe liquid, which demonstrated the occurrence of the Marangoni effect. It was alsoshown that, with further increase of the ethanol concentration in the gas, the highervelocity area of the liquid tended to concentrate to the liquid surface, or theconvection was shown to be confined in a limited area near the interface: a prove ofthe appearance of the negative Rayleigh effect. This experiment explained thevariation tendency of the mass transfer coefficient: increased at lower concentration ofethanol in the gas and decreased at higher concentration of ethanol in the gas.Finally, a mass transfer model was proposed based on Higbie’s penetrationtheory and Kolmogorov’s isotropic turbulence theory to correlate the mass transfercoefficient for the absorption process with the Marangoni and negative Rayleigheffects. The calculated value agreed well with the experimental value. So theinfluence of the Marangoni and negative Rayleigh effects on gas liquid mass transferwas further verified. The model was shown to be promising for predicting the masstransfer behaviors of absorption process with the addition of a lower surface tensioncomponent into the gas phase.