Simultaneous measurement of interfacial area and mass transfer coefficients for a well—mixed gas dispersion in aqueous electrolyte solutions

Abstract

A new measuring technique involving concurrent chemical absorption of carbon dioxide and desorption of oxygen is developed for simultaneously evaluating the liquid-phase mass transfer coefficient k~, and the specific area a of sparingly-soluble gas dispersions in stirred tanks containing an aqueous solution of inorganic electrolytes. The method ensures that k L and a are evaluated under consistent hydrodynamic conditions.Results from three different nonviscous systems show that at high agitation power, such that the average bubble diameter is between 0.2 and 2 mm, k L decreases with increasing power input and is dependent on the bubble diameter. This behavior is in contrast to the results of others at lower agitation levels or in nonelectrolytic liquids, but in general agreement with previous results for bubbles of the same diameter range produced in viscous, nonelectrolytic solutions. In the absence of a sufficiently fast chemical reaction in the liquid phase, the gas absorption rate capability of a particular stirred tank design customarily has been characterized by the overall volumetric mass transfer coefficient KLa. It would seem preferable, however, to be able to predict K L u from separate correlations for its constituent parameters K L and a since their values are predominantly dependent upon distinctly different phyqicochemical parameters of the system. In previous studies k L has been shown both theoretically (Higbie, 1935; Danckwerts, 1951) and experimentally (Calderbank and Moo-Young, 1961) to be dependent primarily upon liquidphase diffusivity and viscosity. On the other hand, a is dependent primarily upon interfacial tension for pure liquids or nonelectrolyte solutions (Calderbank, 1958) or upon ionic strength in solutions of inorganic electrolytes exhibiting Newtonian rheological behavior (Robinson and Wilke, 1971). CAMPBELLDirect measurement of k L in stirred tank gas dispersions is not possible. However, given separate evaluations of K L u and a for sparingly soluble gases wherein the major resistance to mass transfer resides in the liquid phase, the liquid-phase mass transfer coefficient k, may be computed No general correlations for either JcL or a applicable to stirred tank gas dispersions in aqueous electrolyte solutions have yet been developed. The purpose of the present study was to develop and to conduct preliminary experimental tests of a new technique by means of which k L and a could be simultaneously evaluated under consistent physico-chemical and hydrodynamic conditions in aqueous electrolyte solutions having a wide variation in both composition and ionic strength.Experimentally, the new technique involves unsteady state desorption of oxygen from an aqueous electrolyte solution containing a controlled, low concentration of hydroxyl ion accompanied by concurrent, pseudo steady state absorption with chemical reaction of carbon dioxide from the sparged gas. The oxygen desorption rate is measured by a dissolved oxygen probe, the response of which, after correction for the diffusional l...

Effect of hydrodynamics on mass transfer in a gas–liquid oscillatory baffled column

“…Combining this equation with k L a data, it is possible to evaluate the liquid-side mass transfer coefficient (k L ), according to A similar increase in k L with the Sauter mean diameter has been observed previously, from mass transfer measurements in stirred tanks [26,27], bubble columns [28,29], reciprocating-plate columns [30,31] and pulsed columns [32]. Calderbank [33] suggested the existence of two bubble size ranges ("small" and "large"), within which the values of k L are essentially independent of bubble size and are controlled by the turbulence of the liquid phase [34].…”

Gas-Liquid Mass Transfer in an Unbaffled Vessel Agitated by Unsteadily Forward-Reverse Rotating Multiple Impellers

“…As a result, favorably comparable k L a L values were obtained in the unbaffled vessel agitated by the forward-reverse rotating impellers. Presence of electrolytes in liquid phase is known to decrease the rate for gas bubbles to coalesce (Marrucci and Nicodemo, 1967;Zieminski and Whittemore, 1971) and to decrease the size of gas bubbles dispersed in liquid phase (Linek et al, 1970;Robinson and Wilke, 1973;Robinson and Wilke, 1974;Van't Riet, 1979;Hassan and Robinson, 1980;Linek et al, 1987). Decreased size of gas bubbles in liquid phase containing electrolyte causes increase of the gas-liquid interfacial area, a L , which is further enhanced by the tendency for the gas hold-up to increase with decrease of the bubble size.…”

“…Decreased size of gas bubbles in liquid phase containing electrolyte causes increase of the gas-liquid interfacial area, a L , which is further enhanced by the tendency for the gas hold-up to increase with decrease of the bubble size. On the other hand, decrease of the bubble size causes decrease of the liquid-phase mass transfer coefficient, k L , and then k L is often considered to be a function of the bubble size (Robinson and Wilke, 1974;Hassan and Robinson, 1980). That is, the volumetric coefficient that is the product of a L and k L suffers the two counter influences.…”

“…By measuring the major and minor axes for at least 100 bubbles photographed, the volume-surface mean diameter, d vs , was calculated. The overall gas holdup, φ gD , based on the gassed liquid volume was determined using the manometric technique (Robinson and Wilke, 1974). The manometer reading was corrected for the difference of dynamic pressure, namely, that of the reading measured in ungassed liquid.…”

“…Presence of inorganic electrolytes is known to decrease the rate for gas bubbles to coalesce because of the electrical effect at the gas-liquid interface (Marrucci and Nicodemo, 1967;Zieminski and Whittemore, 1971). In many cases, the electrical effect creates different gasliquid dispersion characteristics, such as decreased size of gas bubbles dispersed in liquid phase without practical changes in their density, viscosity and surface tension (Linek et al, 1970;Robinson and Wilke, 1973;Robinson and Wilke, 1974;Van't Riet, 1979;Hassan and Robinson, 1980;Linek et al, 1987). The present work assesses the mass transfer characteristics in aerated electrolyte solutions, following assessment of those in the air-water system, for the forward-reverse agitation vessel.…”