Gas Transfer in Rivers

Rivers are generally considered as a plug flow reactor with dispersion. Determination of the dispersion coefficient for rivers was covered in Chapter 6, and determination of the gas transfer coefficient is a slight addition to that process. We will be measuring the concentration of two tracers a volatile tracer that is generally a gas (termed a gas tracer, C) and a conservative tracer of concentration (Cc). The transported quantity 

Of course, the typical objective is the K2 value for oxygen to be used in such things as total daily maximum load calculations, and we just have the Ki value for the gas tracer. We can use equation (8.62) to get us from the transfer of one compound to another, because Prms is similar for all compounds  

Moog and Jirka (1998) investigated the correspondence of a number of equations with the available data, using the mean multiplicative error, MME  

Tsivoglou Wallace (1972) Q Flint river O South river A Jackson river A Patuxent river Chattahoochee river 

The MME is the geometric mean of Kp jKm. Thus, a given equation is on average, in error by a factor equal to the MME. 

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The physical transport of mass is essential to many kinetic and d3mamic processes. For example, bubble growth in magma or beer requires mass transfer to bring the gas components to the bubbles radiogenic Ar in a mineral can be lost due to diffusion pollutants in rivers are transported by river flow and diluted by eddy diffusion. Although fluid flow is also important or more important in mass transfer, in this book, we will not deal with fluid flow much because it is the realm of fluid dynamics, not of kinetics. We will focus on diffusive mass transfer, and discuss fluid flow only in relation to diffusion. 

Mass transfer phenomena govern the rate of dissolution of a gas due to the exposed water surface, local turbulence, and the degree of air and water mixing. Consequently, large water surfaces under turbulence (as the rapids in a river) favor gas dissolution in this way, a turbulent cold river is richer... 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

As an example of global land-sea transfers involving gases and the effect of human activities on the exchange, consider the behavior of CO2 gas. Prior to human influence on the system, there was a net flux of CO2 out of the ocean owing to organic metabolism (net heterotrophy). This flux was mainly supported by the decay of organic matter produced by phytoplankton in the oceans and part of that transported by rivers to the oceans. An example... 

The actual rate of volatilization of plasticizers from water apparently has not been experimentally measured. The rates of volatilization, however, estimated using the Sonth-worth Method to estimate gas-phase and liqnid-phase mass transfer coefficients, were reported in HSDB, and summarized in Table 18.10. VolatiUzation was estimated nnder two scenarios (Figure 18.1) a shallow (1-m deep) river moving at a rate of 1 m/sec below an air mass moving at 3 m/sec. The second scenario was a shallow (1-m deep) lake moving at 0.05 m/sec below a breeze of 0.5 m/sec. The estimated volatilization half-lives of the plasticizers followed the trend established by their Heniy Law constants. Consequently, the predicted half-lives of ditridecyl phthalate, di-(2-ethylhexyl) adipate, di-(2-ethylhexyl) azelate, diundecyl, diisooctyl, dihexyl, dinonyl phthalate were less than 6 days. The same plasticizers would likely be more persistent in the lake scenario. The volatilization rates of the other plasticizers appeared to be much slower, and volatilization from water may not be a significant environmental pathway. Furthermore, the Southworth estimation technique does not take into acconnt sorption of dissolved chemicals by suspended particles. Sorption would increase the residence time of dissolved plasticizers. The predicted volatilization rates, however, were based on Hemy s Law constants which are themselves only estimates. 

Oil and gas production systems, drainage networks, supply (delivery) networks, evacuation networks, flows of information on the Internet towards a common destination, wireless networks transferring information from Wi-Fi access points to a wired access point that coimect to the Internet, root network of plants, river basin systems, water distribution systems, the blood vessel system of animals, the morphology of the human limgs, certain data collection networks, etc. are all examples of flow networks with merging flows. In an oil and gas production system or a drainage system for example, the branches correspond to the components characterised by flow capacities while the nodes are notional, used to represent the topology of the network where the streams flow into another stream. 

We must next formulate an expression for the mass transfer rate N g and here we encounter the same difficulty we had seen in Illustration 2.2 dealing with the reaeration of rivers. In both cases the interfacial area is unknown and we must therefore resort again to the use of a volumetric mass transfer coefficient Kji where the unknown interfacial area a (m /m column volume) is lumped together with If the gas phase is assumed to be controlling, we can write... 

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