Using in-line static mixers to intensify gas–liquid mass transfer processes
Introduction
Process intensification is a relatively new concept that has been gaining strong momentum because of its ability to significantly reduce capital and operating costs while improving process yield/selectivity and enhancing the inherent safety of many CPI operations (Stankiewicz and Moulijn, 2002). Such desirable benefits are mainly based on the ability to match the fluid dynamics of mixing to the chemical, biological and/or physical requirements of the process. This investigation, which focuses on enhancing inter-phase mass transfer between gases and industrial liquids, provides an example of such a situation.
The approach presented in this investigation is based on the fact that most industrial liquids contain varying quantities of secondary compounds (alcohols, organic acids, electrolytes, amines, glycols, proteins, finely divided particles and emulsions) the presence of which is known to strongly affect the hydrodynamics of gas/liquid contacting. They also adversely impact the volumetric inter-phase mass transfer coefficient in a fashion that is dependent on the type and concentration of the contaminant, as well as the type of gas/liquid contactor used (Jamialahmadi and Muller-Steinhagen, 1992, Shen et al., 1999, Al-Masry, 1999, Vazquez et al., 2000a, Vasconcelos et al., 2002). This observation is mainly attributed to the reduction of the liquid phase mass transfer coefficient due to the presence of contaminant at the interface. On the other hand, the presence of these contaminants can significantly reduce bubble coalescence rate (Camarasda et al., 1999, Zahradnik et al., 1999), a phenomenon that can be taken advantage of to achieve large interfacial area of contact at low energy consumption rates.
The effect of contaminants on the gas/liquid behaviour of industrial streams can be attributed to the observation that the bi-polar contaminant entities tend to adsorb at the interface and depict interfacial characteristics that are very similar to those of dilute surfactant-containing aqueous solutions (Al Taweel et al., 2005), including the development of Marangoni elastic interfacial forces. When the bubbles move relative to the surrounding liquid, the surface active contaminant entities get convected to the bubble's tailing end where they accumulate forming a stagnant cap. This reduces internal circulation and the value of the inter-phase mass transfer coefficient, , decreases from that of a circulating bubble (Higbie's penetration theory) to that of a rigid sphere (Frössling equation). This represents up to 7-fold reduction in the case of 1 mm bubbles (Bischof et al., 1993, Vasconcelos et al., 2002, Vazquez et al., 2000a, Alves et al., 2004). Additional reduction is caused by the ability of the Marangoni elasticity to dampen the hydrodynamic disturbances near the gas–liquid interface (Kastánek et al., 1993, Walter and Blanch, 1986; Vazquez et al., 1997, Vazquez et al., 2000a, Vazquez et al., 2000b; Shen et al., 1999), a factor that plays an important role in determining the magnitude of inter-phase mass transfer (Davies, 1972).
In order to understand the effect of surface active agents, SAA, on bubble breakage and coalescence, it is necessary to consider their effect on the counteraction processes of bubble dispersion and coalescence (Fig. 1), an approach which forms the basis of the widely used population balance analysis. The presence of surface active contaminants in industrial gas/liquid systems (or systems in which trace concentrations of SAA are present e.g., industrial and municipal wastewaters) strongly affects the rate of bubble coalescence as well as the rate of bubble breakage and, thereby, influences the size distribution of the bubbles present in the gas/liquid contactor. Walter and Blanch (1986) found that the presence of SAA in low concentrations increases the maximum stable bubble size, a phenomenon that was attributed to the ability of the Marangoni elastic forces to resist the surface stretching encountered during bubble breakup. On the other hand, the ability of SAA to retard bubble coalescence is much more pronounced and order of magnitude reductions in coalescence rates have been observed at very low surfactant concentrations (Machon et al., 1997, Luo, 2002).
A novel static mixer design (screen-type static mixers) that is capable of taking advantage of the aforementioned phenomenon was developed. It has been successfully used to generate narrowly-sized liquid–liquid dispersions in an energy efficient fashion (Al Taweel and Chen, 1996, Al Taweel et al., 2003). When used in conjunction with gas–liquid systems, interfacial area of contact as high as were obtained in the regions adjacent to the screens where local energy dissipation rate is highest (Chen, 1995). The fine bubbles formed in these regions tend to coalesce as they migrate to regions of lower energy dissipation rates (further downstream) and the coalescence hindrance phenomenon encountered in industrial streams is taken advantage of to maintain high interfacial area of contact for relatively longer times. The formation of such high interfacial area of contact is expected to enhance inter-phase mass transfer particularly considering the very intense micro mixing levels encountered in the regions close to the screens.
The objective of this investigation is to determine the operational characteristics of this novel gas–liquid contactor and to establish how the volumetric mass transfer coefficient is affected by the interfacial characteristics of the system.
Section snippets
Experimental
In this investigation, small quantities of surface active agents (SAA) were added to the aqueous phase in order to simulate the behaviour of industrial streams (Al Taweel et al., 2005) as well as wastewaters in which SAA are present (Stenstrom and Gilbert, 1981, Wagner and Popel, 1996). Consequently, a physical technique for measuring volumetric mass transfer (oxygen desorption method) was selected in order to eliminate the influence of the reagents needed for the various chemical techniques.
Results and discussion
In this investigation, the effect of various designs and operational parameters, as well as the interfacial characteristics of the system, were investigated with the objective of intensifying the rate of gas–liquid mass transfer and enhancing the efficiency by which energy is utilized while accomplishing this objective.
Conclusions
Based on the results obtained in this investigation, it is possible to conclude that the presence of surface active impurities in most wastewater (and the presence in most industrial streams of bi-polar compounds such as alcohols, amines, salts, fine particles, etc.) affects the static and dynamic interfacial characteristics of the solutions. This, in turn, strongly influences gas–liquid contacting behaviour and inter-phase mass transfer rates. The value of was thus found to increase with
Notation
a interfacial area, m2/m3 A surface area, m2 b screen wire diameter, m bulk surfactant concentration, kmol/m3 equilibrium surface concentration, kmol/m3 oxygen concentration in liquid phase, kmol/m3 oxygen concentration in gas phase, kmol/m3 maximum bubble diameter, m equivalent jet size, , m D pipe diameter or tank diameter, m E energy dissipation rate, kW specific energy consumption rate per unit mass liquid processed, kW/kg the amount of oxygen transported to/or from liquid
Acknowledgements
The financial support of the Environmental Science and Technology Alliance Canada (ESTAC) and the Natural Sciences and Engineering Research Council is gratefully acknowledged.
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