Chemistry /chemical residence time

The processes through which rainfall is turned into runoff, together with the nature of the material through which water moves, control the chemical characteristics of streamflow. Specific runoff mechanisms operating in a landscape control the flowpaths by which water moves through the landscape. Flowpath-depen-dent differences, such as the total time that water spends in contact with different soil horizons or bedrock (residence time), can strongly influence runoff amounts and timing, the relative contribution of event (new) versus stored (old) water, and runoff chemistry. 

A number of studies have documented that concentrations of some of the directly emitted species found in outdoor atmospheres can be quite high indoors if there are emission sources present such as combustion heaters, gas stoves, or tobacco smoke. In addition, there is evidence for chemistry analogous to that occurring outdoors taking place in indoor air environments, with modifications for different light intensities and wavelength distributions, shorter residence times, and different relative concentrations of reactants. In Chapter 15, we briefly summarize what is known about the chemical composition and chemistry of indoor atmospheres. 

For the no reaction state ass = a0, the relaxation time given by eqn (8.10) is simply equal to the residence time. In terms of the eigenvalue, we have A = - l/tres, which is negative. The stationary state is always stable, irrespective of a0 and kl. Chemistry makes no contribution (formally we have l/tch,ss = 0, so the chemical time goes to infinity) the perturbation of a does not introduce any B to the system, so no reaction is initiated. The recovery of the stationary state is achieved only by the inflow and outflow. 

Figure 7 further shows that, as gaseous C02 moves up the absorber, phase equilibrium is achieved at the vapor-liquid interface. C02 then diffuses through the liquid film while reacting with the amines before it reaches the bulk liquid. Each reaction is constrained by chemical equilibrium but does not necessarily reach chemical equilibrium, depending primarily on the residence time (or liquid film thickness and liquid holdup for the bulk liquid) and temperature. Certainly kinetic rate expressions and the kinetic parameters need to be established for the kinetics-controlled reactions. While concentration-based kinetic rate expressions are often reported in the literature, activity-based kinetic rate expressions should be used in order to guarantee model consistency with the chemical equilibrium model for the aqueous phase solution chemistry. 

Patents provide valuable technology information for designers. Firstly, information about process feasibility may be collected with respect to chemistry, catalyst, safety and operation conditions. Qualitative data regarding the reaction engineering, such as conversion and selectivity, as well as the productivity and residence time are useful for the selection of the chemical reactor. Even more important are data regarding the reaction-mixture composition for the assessment of separations, namely with respect to byproducts and impurities. 

The residence time for retention of compounds within the stratosphere increases very rapidly from a low and highly variable quantity in the region just above the tropopause, to a period approaching a decade in the lower-middle stratosphere. Thus the altitude at which CFG photolysis occurs can have important consequences concerning the transfer of halogen compounds into the chemical inventory of the stratosphere, which is in turn a measure of the effectiveness of converting industrially produced halogen into active participants in the stratospheric chemistry. 

Recently, we have shown that non-Flory distributions cannot arise from the higher solubility of larger olefins because thermodynamic equilibrium between the two phases requires that the fugacity, chemical potential, and kinetic driving force for each component be the same in the two phases (4,5,14,40,41,44). Transport restrictions, however, can lead to higher intrapellet concentrations and residence times of a-olefins, a feature of FT chemistry that accounts for the non-Flory distribution of reaction products and for the increasing paraffin content of larger hydrocarbons (4,5,14,40,... 

Finally, we must somehow resolve the vexing problem of the chemical speciation of trace elements in seawater what is the chemical nature of the various metal chelators whose existence has been demonstrated by electrochemistry Is the chemistry of several metals in surface seawater really controlled by metallophores released by prokaryotes Or are dissolved metals chiefly present as parts of metalloproteins in the process of remineralization How do metal chelators affect the residence times of metals (in particular, scavenged elements such as iron and cobalt) and in turn how do those chelators influence the global carbon cycle via changes in marine primary productivity ... 

Our understanding of natural water systems has, until recently, been seriously limited by a lack of kinetic information on critical reactions in water, in sediments, and at interfaces. Earlier in atmospheric chemistiy (Seinfeld, 1986) and more recently in aquatic chemistry (Brezonik, 1993), a considerable growth of information on rates and mechanisms for reactions central to environmental chemistry has taken place. As a result, we are now better able to assess the characteristic time scales of chemical reactions in the environment and compare these with, for example, residence times of water in a system of interest. Schematically, as shown here, for chemical vs. fluid time scales... 

The average residence time of an air parcel in the upper stratosphere (above approximately 16 km) is approximately 2 years. The stratosphere should not be regarded as fully mixed, however. Upwelled layers of air, their exact chemical composition changing with season, have been shown to retain an identity for many months. Another example of stratospheric heterogeneity is the air in polar stratospheric vortices (see Section 4.6.4, Fig. 4-41 later) these vortices may have a trace gas chemistry that differs from that of stratospheric air in the tropics. 

Antal, M. J. The Effects of Residence Time, Temperature and Pressure on the Steam Gasification of Biomass , American Chemical Society Division of Petroleum Chemistry, Honolulu, 1979. 

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