Residence time sources

Figure 9.29 Reduction of reaction times by as given in Ref. [119]) as compared to standard several orders of magnitude using a falling film organic laborato processing with a laboratory microreactor (FFMR) or micro bubble columns bubble column (LBC). x residence time. Source (MBC I and II, denoting different dimensions, By courtesy of I MM.

Residence times may vary slightly from those in Table 2 because of the differences in the primary source. Ref. 8... 

Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

The reaction rates for oxidation of atmospheric SO2 (0.05-0.5 d ) yield a sulfur residence time of several days, at most this corresponds to a transport distance of several hundred to 1000 km. The formation of HNO by oxidation is more rapid and, compared with H2SO2P results in a shorter travel distance from the emission source. H2SO4 can also react with NH to form NH HSO or (NH2 2S04 aerosols. In addition the NH NO aerosols are in equihbrium with NH (g) and HNO (g). 

Product (raw materials) Type Reactor phase Catalyst T c(2 P, atm Residence time or space velocity Source and page ... 

Figure 8-38. Residence time distributions of some commerciai and fixed bed reactors. The variance, equivaient number of CSTR stages, and Peciet number are given for each reactor. (Source Wales, S. M., Chemicai Process Equipment—Seiection and Design, Butterworths, 1990.)...

The main stationary sources of NO are gas turbines, fired heaters, power generation plants, and, of course, the FCC. The amount of NO produced is a function of residence time and combustion temperature. Combustion temperature is influenced by fuel composition. 

Typical results for these three collision mechanisms are shown in Figure 3 where the relative intensities of the primary, secondary, and tertiary ions are plotted against N, the concentration of molecules in the source. In deriving these curves, the parameters used were kp = 2.0 X 10 9 cc./molecule-sec. k8 = 1.0 X 10 9 cc./molecule-sec. tp = 8.5 X 10 7 sec., (the residence time of the ion (jn/e — 33) in a field of strength 9.1 volts/cm. in the Leeds mass spectrometer). In applying this analysis to a system in which the tertiary ion reacts to form quaternary and higher order ions, ITtotal represents the sum of tertiaries, quaternaries, etc. 

Gas Principal biological source Residence time in the atmosphere... 

The residence time is the time spent in a reservoir by an individual atom or molecule. It is also the age of a molecule when it leaves the reservoir. If the pathway of a tracer from the source to the sink is characterized by a physical transport, the word transit time can also be used. Even for a single chemical substance, different atoms and molecules will have different residence times in a given reservoir. Let the probability density... 

Thus, the chemical reactivity of the elements in seawater is reflected by the residence time. It is important to note, however, that while residence times tell us something about the relative reactivities, they also tell us nothing about the nature of the reactions. The best source of clues for imderstanding these reactions is to study the shape of dissolved profiles of the different elements. When we do this we find that there are six main characteristic types of profiles as described in Table 10-8. Notice that most of these reactions occur at the phase discontinuities between the atmosphere, biosphere, hydrosphere, and lithosphere. 

The present sources to the ocean are the weathering of old evaporites (75% of river flux) and CP carried by atmospherically cycled sea-salts (25% of river flux). Loss from the ocean occurs via aerosols (about 25%) and formation of new evaporites. This last process is sporadic and tectonically controlled by the closing of marginal seas where evaporation is greater than precipitation. The oceanic residence time is so long for CP ( 100Myr) that an imbalance between input and removal rates will have little influence on oceanic concentrations over periods of less than tens of millions of years. 

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