Air-Water Mass Transfer in the Field

The principles of air-water mass transfer are often difficult to apply in field measurements and thus also in field predictions. The reasons are that the environment is generally large, and the boundary conditions are not well established. In addition, field measurements cannot be controlled as well as laboratory measurements, are much more expensive, and often are not repeatable. 

The value of has not been attempted in the field to date. So, how do we determine Kl for field applications The determination of dynamic roughness, zo, has also been difficult for water surfaces. The primary method to measure Kl and Kq is to disturb the equilibrium of a chemical and measure the concentration as it returns toward either equilibrium or a steady state. Variations on this theme will be the topic of this chapter. 

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Chapter 9 Air-Water Mass Transfer in the Field. The theory of interfacial mass transfer is often difficult to apply in the field, but it provides a basis for some important aspects of empirical equations designed to predict interfacial transport. The application of both air-water mass transfer theory and empirical characterizations to field situations in the environment will be addressed. 

The hydrodynamics of the experimental system can be described theoretically. Such approach is very important for correct interpretation of the experimental results, and for their extrapolation for the conditions not attainable in the existing experimental system. With the mathematical model the parametric study of the system is also possible, what can reveal the most important factors responsible for the occurrence of the specific transport phenomena. The model was presented in details elsewhere [2]. It was based on the equations of the momentum and mass transfer in the simplified two-dimensional geometry of the air-water-surfactant system. Those basic equations were supplemented with the equation of state for the phopsholipid monolayer. The resultant set of equations with the appropriate initial and boundary conditions was solved numerically and led to temporal profiles of the surface density of the surfactant, T [mol m ], surface tension, a [N m ], and velocity of the interface. Vs [m s ]. The surface tension variation and velocity field obtained from the computations can be compared with the results of experiments conducted with the LFB. 

Jahne, B. 1991. Heat as a proxy tracer for gas exchange measurements in the field Principles and technical realization. In Air-Water Mass Transfer. S. C. Wilhelms and J. S. Gulliver (Eds). American Society of Civil Engineers, Reston, VA, 582. 

Typical mass balance methods to measure the air-sea gas transfer have one major drawback the response time is of the order of hours to days, making a parameterisation with parameters such as wind forcing, wave field, or surface chemical enrichments nearly impossible. The controlled flux technique uses heat as a proxy tracer for gases to measure the air-sea gas transfer rate locally and with a temporal resolution of less than a minute. This method offers an entirely new approach to measure the air-sea gas fluxes in conjunction with investigation of the wave field, surface chemical enrichments and the surface micro turbulence at the water surface. The principle of this technique is very simple a heat flux is forced onto the water surface and the skin-bulk temperature difference across the thermal sublayer is measured. 

Because the application of carbon-based monoliths forms an emerging field in (bio)chemical engineering, new applications and process developments can be expected in the future. One might think of applications of monoUth-CNF combinations in water-air filter systems [66], carbon precnrsors with tailored porons texture for production of integral/coated monoliths, ultrathin coatings on ACM monoliths for higher mass-transfer rates, and carbon monoliths in adsorption processes or as high-surface-area electrodes. 

There is considerable information available in the hterature on the design of ejectors (steam jet ejectors, water jet pumps, air injectors, etc.) supported by extensive experimental data. Most of this information deals with its use as an evacuator and the focus is on ejector optimization for maximizing the gas pumping efficiency. The major advantage of the venturi loop reactor is its relatively very high mass transfer coefficient due to the excellent gas-liquid contact achieved in the ejector section. Therefore, the ejector section needs careful consideration to achieve this aim. The major mass transfer parameter is the volumetric liquid side mass transfer coefficient, k a. The variables that decide k a are (i) the effective gas-hquid interfacial area, a, that is related to the gas holdup, e. The gas induction rate and the shear field generated in the ejector determine the vine of and, consequently, the value of a. (ii) the trae liquid side mass transfer coefficient, k. The mass ratio of the secondary to primary fluid in turn decides both k and a. For the venturi loop reactor the volumetric induction efficiency parameter is more relevant. This definition has a built in energy... 

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