Organic Mean Residence Time

Example 3.5 A 1-in i.d coiled tube, 57 m long, is being used as a tubular reactor. The operating temperature is 973 K. The inlet pressure is 1.068 atm the outlet pressure is 1 atm. The outlet velocity has been measured to be 9.96 m/s. The fluid is mainly steam, but it contains small amounts of an organic compound that decomposes according to first-order kinetics with a half-life of 2.1s at 973 K. Determine the mean residence time and the fractional conversion of the organic. 

The truncated part of the integral can be obtained by numerical integration (e.g. by means of the trapezium rule) of the function rCp(r) between times 0 and T. The mean residence time MRT is an important pharmacokinetic parameter, especially when a substantial fraction of the drug is excreted or metabolized during its first pass through an organ, such as the liver. 

Denmark 1.5 days after the explosion. Air samples collected at Roskilde, Denmark on April 27-28, contained a mean air concentration of 241Am of 5.2 pBq/m3 (0.14 fCi/m3). In May 1986, the mean concentration was 11 pBq/m3 (0.30 fCi/m3) (Aarkrog 1988). Whereas debris from nuclear weapons testing is injected into the stratosphere, debris from Chernobyl was injected into the troposphere. As the mean residence time in the troposphere is 20-40 days, it would appear that the fallout would have decreased to very low levels by the end of 1986. However, from the levels of other radioactive elements, this was not the case. Sequential extraction studies were performed on aerosols collected in Lithuania after dust storms in September 1992 carried radioactive aerosols to the region from contaminated areas of the Ukraine and Belarus. The fraction distribution of241 Am in the aerosol samples was approximately (fraction, percent) organically-bound, 18% oxide-bound, 10% acid-soluble, 36% and residual, 32% (Lujaniene et al. 1999). Very little americium was found in the more readily extractable exchangeable and water soluble and specifically adsorbed fractions. 

In soils, lead concentrates in organic-rich surface horizons (NRCC 1973). In one instance, only 17 mg of soluble Pb/kg was found in soils 3 days after the addition of 2784 mg of lead (as lead nitrate)/kg (NRCC 1973). The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades (Nriagu 1978a). In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years (Turner et al. 1985) to more than 500 years (Friedland and Johnson 1985). 

Because of its relatively long mean residence time, ocean water has a constant isotope composition. Corals are about l%c and foraminifera are about 4.5%o lighter than ocean water. Thus significant Mg isotope fractionations occur during biomineralization of carbonate secreting organisms which is larger than for Ca isotopes (see Section 2.11). 

Municipal waste water treatment plant. Consider a municipal water treatment plant for a small community (Fig. Pl.l). Waste water, 32 000 m3/day, flows through the treatment plant with a mean residence time of 8 hr, air is bubbled through the tanks, and microbes in the tank attack and break down the organic material... 

These changes in soil C stocks are relatively rapid, due to rapid turnover of soil organic matter (SOM) in these tropical soils. Using radiocarbon derived from atmospheric testing of nuclear weapons in the 1960s as a tracer, Trumbore et al. (1995) estimated that the mean residence time of C in the top 10 cm of soil is about 3 years for 30% of the SOM and 10-30 years for 60% of the SOM. Only about 10% of the C in the top 10 cm of soil cycles on a millennial time scale. This very old C fraction increases to 40-80% in the... 

Sanderman J., Amundson R., and Baldocchi D. D. (2003) Application of eddy covariance measurements to the temperature dependence of soil organic matter mean residence times. Global Biogeochem. Cycles, 17(2), 1061. 

TABLE 7. Mean Residence Times of Organic Matter in Some Typical Soils... 

TABLE 8. Mean Residence Times (MRTs) for Different Organic Matter Fractions of a Chernozemic Black Soil (Melfort Silt Loam) as Reported by Campbell et al. (1967)... 

In Fig. 19 are presented the spectra of inlet and outlet of a large treatment plant characterised by a rather short mean residence time in the aeration basin (few hours). This process, working with a high-load F/M ratio, does not lead to the complete biodegradation of organic matter as it is shown with the outlet spectrum shape. The efficiency is quantified by removal yields calculated from different aggregate parameters (Table 3). 

The accumulation of carbon in the organic layer after fire points at a basic problem of the simplified flux scheme of Schulze and Heimann (1998), namely, that intermediate pools exist at each level with different mean residence time, and depending on the level of spatial integration these pools may or may not average out. The problem is illustrated in Figure 7, where an inventory-process type approach (in contrast to eddy covariance flux measurements) was chosen to calculate the carbon sink capacity of European forest ecosystems. If NPP was plotted against C-mineralization, then NPP minus C-mineral-ization would represent NEP at the plot scale (Schulze et al.,... 

Lead is known to be fairly immobile in organic soils, with a mean residence time... 

Campbell (1965) made an extensive study of the carbon-dating method as applied to the study of problems in soil organic matter investigations and found it to be very satisfactory. The mean residence times of humus ftom the Ap-horizon of 5 Saskatchewan soils ranged from 250 years for a gray-wooded podzolic soil to 1000 years for black chernozemic soils the humus from chernozems from Alberta was even older. The calcium humates and non-hydrolyzable humic acids of the chernozems had an age of 1,400 years, whereas the age of the humic acid hydrolysate was only 25 years. This hydrolysate, which was a small fraction of the total humus, accounted for 80% of the nitrogen released per annum the major portion of the soil humus was found to be inert. In general, the humus of podzolic soils was much more labile than that of chernozems. 

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