Transport fast fraction

A further complication is evident in that there exist different transport channels with different properties. The auxin stream seems to contain a fast fraction of low density which is separate from a main and slower fraction (e.g., Vardar 1964, Newman 1965, Rayle etal. 1969, de la Fuente and Leopold 1972, Kaldewey and Kraus 1972, Patrick and Woodley 1973, Krul 1977, Sheldrake 1979, see also Goldsmith 1977, p452ff.). The fast and slow transport fractions may be associated with different compartments of the cells, possibly the cytoplasm and the vacuole, respectively. This possibility is based on the multiphasic efflux and elution profiles of plant sections supplied with labeled auxin (de la Fuente and Leopold 1970 b, 1972, Davies 1974 see also Goldsmith 1977, p 453 f.). 

The acid-base reaction is a simple example of using the mixture fraction to express the reactant concentrations in the limit where the chemistry is much faster than the mixing time scales. This idea can be easily generalized to the case of multiple fast reactions, which is known as the equilibrium-chemistry limit. If we denote the vector of reactant concentrations by and assume that it obeys a transport equation of the form... 

If the cell is well supplied with nutrients, then the production of activated enzyme is great and this step is relatively fast. If the transport of sulfate into the cell cannot keep up with the reduction of sulfate, the concentration of sulfate within the cell becomes small, and very little of the isotopically fractionated sulfate inside the cell can leak back out of the cell. Thus, the effect of the internal isotopic fractionation on the outside world is minimal and the overall fractionation of the process is small. In a hypothetical extreme case, every sulfate anion entering the cell would be consumed by reduction. This would require a complete lack of isotopic fractionation, because when all S atoms entering are consumed, there can be no selection of light vs. heavy isotopes. The isotopic fractionation of the overall reduction reaction would be equal to that which occurs during the diffusion step only. 

Pyrolysis oil (bio-oil) is produced in fast and flash pyrolysis processes and can be used for indirect co-firing for power production in conventional power plants and potentially as a high energy density intermediate for the final production of chemicals and/or transportation fuels. Gas chromatographic analysis of the liqtrid fraction of pyrolysis products from beech wood is given in Table 3.6 (Demirbas, 2007). Biocmde resrrlts from severe hydrothermal upgrading (HTU) of relatively wet biomass and potentially can be used for the production of materials, chemicals,... 

At initial reaction times, i.e. for the first ca. 100 s, all three phenomena should be controlled by transport considerations. If the induction kinetics are intrinsically fast compared to transport, then the evolution of the system is transport controlled, and most of the precursor cannot be converted to intermediates before 100 s is reached. Furthermore, if both induction kinetics and turnover frequency are intrinsically fast compared to transport, the system may experience only ca. one turnover vithin the first 100 s. Finally, if deactivation kinetics are intrinsically fast compared to transport, a significant fraction of precursor has been degraded to inactive species vithin the first 100 s. The net effect, for better or worse, is that transport effects bias the in situ observations and hence the accessible set of observable species in Eq. (4). 

The influence of various gas pressure conditions within the laser ablation cell on the particle formation process in laser ablation has also been investigated.69 In LA-ICP-MS studies at low pressure (down to 2kPa) a small particle size distribution and a reduction in elemental fractionation effects was obtained. But with decreasing pressure and transport volume of ablated material, a significant decrease in the ion intensities was observed as demonstrated for uranium measurements in the glass SRM NIST 610.69 However, the laser ablation of solid materials at atmospheric pressure in LA-ICP-MS is advantageous for routine measurements due to lower experimental effort and the possibility of fast sample changing in the ablation chamber. Fractionation... 

So we deduce that only one DMB molecule out of 11 will be in the moving ground-water at any instant (Fig. 9.6). This result has implications for the fate of the DMB in that subsurface environment. If DMB sorptive exchange between the aquifer solids and the water is fast relative to the groundwater flow and if sorption is reversible, we can conclude that the whole population of DMB molecules moves at one-eleventh the rate of the water. The phenomenon of diminished chemical transport speed relative to the water seepage velocity is referred to as retardation. It is commonly discussed using the retardation factor, Rfi, which is simply equal to the reciprocal of the fraction of molecules capable of moving with the flow at any instant, ff (see Chapter 25). 

The combustion reaction rate is controlled both by the availability of fuel and oxygen kinetic effects (temperature). In full-scale fire modeling, the resolvable length and time scales are usually much larger than those associated with the scales of the chemical combustion reaction, and it is common to assume that the reactions are infinitely fast. The local reaction rate depends on the rate at which oxygen and fuel are transported toward the surface of stoichiometric mixture fraction, shown in Figure 20.2 as a point where both oxygen and fuel mass fractions go to zero. For almost 20 years, the EBU or eddy dissipation models were the standard models used by the combustion CFD community. With the EBU, in its simplest form, the local rate of fuel consumption is calculated as [3] ... 

The cumulative effect of the instantaneous fractionations given by Equations (6)-(9) is easily calculated if it is further assumed that mass transport processes (e.g., chemical diffusion) are sufficiently fast to maintain chemical and isotopic homogeneity in both the gas and in the condensed phase. There are cases where diffusion in the residue or gas limits mass transport and these effects on isotopic and chemical fractionation have been explored by Richter et al. (2002). Let us consider first the isotopic fractionations associated with condensation in a supersaturated closed system. The change in the moles of isotope 1 of element k in the gas can be written as... 

Despite the extensive photoconductivity data, the nature of photoexcitations in poly(phenylenevinylene), PPV, and its soluble derivatives has remained controversial. In part, the controversy arises from the conflict between the results obtained with fast time-resolved photoconductivity and those obtained by the more familiar steady-state photoconductivity the latter indicate a strong T-dependence for the np product. Recent experiments have resolved the apparent conflict [203]. The idea is rather simple If the sample is sufficiently thin that the photocarriers can be swept out before a significant fraction fall into traps, the steady-state photocurrent will provide information similar to that obtained at short times by transient photoconductivity (in the latter, sample thickness is not important since pre-trapping and trap dominated transport are separated in the time domain). 

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