Gas concentration profiles

The resulting velocity profiles and the flow pattern inside and around the jet are shown in Figs. 29 and 30 for a jet velocity of 32.6 m/s and with two different aeration flows. The jet boundary at Vz = 8 m/s shown in Figs. 29 and 30 was calculated from Tollmien similarity. The boundary where the tracer gas concentration becomes zero, C = 0, was determined from the normalized experimental tracer gas concentration profiles shown in Figs. 

Specific Remarks. The established dependence of the microkinetics on the oxidation state of the catalyst make clear that a) results of kinetic investigations at lower temperatures are different in respect to the mechanistic scheme from those obtained at higher temperatures, b) in a distributed catalytic system in the steady state a distribution of the catalytic steps is possible as a direct consequence of the ambient gas concentration profile and the axial temperature distribution in an extreme situation it is conceivable that at the reactor inlet, another mechanism dominates as at the reactor exit. These two facts can perhaps explain some contradictory results about the same reaction scheme which have been reported in the past by different authors. As stated recently by Boreskov (19) in a review paper, this conclusion holds true for the most catalytic systems under the technical operating conditions. 

As noted earlier, air-velocity profiles during inhalation and exhalation are approximately uniform and partially developed or fully developed, depending on the airway generation, tidal volume, and respiration rate. Similarly, the concentration profiles of the pollutant in the airway lumen may be approximated by uniform partially developed or fully developed concentration profiles in rigid cylindrical tubes. In each airway, the simultaneous action of convection, axial diffusion, and radial diffusion determines a differential mass-balance equation. The gas-concentration profiles are obtained from this equation with appropriate boundary conditions. The flux or transfer rate of the gas to the mucus boundary and axially down the airway can be calculated from these concentration gradients. In a simpler approach, fixed velocity and concentration profiles are assumed, and separate mass balances can be written directly for convection, axial diffusion, and radial diffusion. The latter technique was applied by McJilton et al. 

In this paper the gas mixing in a CFB biomass gasifier is discussed on basis of radial and axial gas concentration profiles measurement in the pilot plant of ECN (100 kglhr fitel). The standard dispersion model is applied to interpret the experimental results. 

Actual data of gas concentration profiles in a pilot riser reactor are reported by Jiang et al. They studied ozone decomposition in a catalytic circulating fluidised bed. By applying an UV detection technique they were able to measure strong radial and axial ozone concentration gradients in their experimental set-up. It was concluded that the ozone concentration profiles are consistent with the trend exhibited by the voidage distribution in both axial and radial direction. 

To study the gas mixing capacity of circulating fluidised bed (CFB) biomass gasifiers, radial and axial gas concentration profiles have been measured and interpreted in both a hot pilot scale biomass gasifier (100 kg/hr fuel) and a cold-flow set-up. The presented pilot plant data are unique in their sort and provide new insight in radial gas mixing in a CFB riser. 

In the pilot plant, strong radial gas concentration profiles, up to a factor 3 higher concentrations near the wall compared to the centre, have been measured. These profiles are obtained during gasification conditions (d50 °C, and X=0.30). However, measured CO profiles are found to be flat under certain process conditions. In case of only primary air, the CO profiles are flat On the other hand secondary air and combustion process conditions yield important radial CO gradients. This indicates that the shape of the radial CO profile is related to the oxygen to combustibles ratio. 

Moonen R.H.W, Radial and axial gas concentration profiles in a CFB biomass gasifier - measurements and analysis (1999), MSc. thesis University of Twente... 

For a volatile liquid reactant or a volatile product, these steps are essentially reversed. For a nonvolatile liquid reactant or product, only the reaction and diffusion in the liquid take place. Figure 7-15 describes the absorbing gas concentration profiles in a gas-liquid system. 

Evidence for the two-phase model came from measurements of the gas concentration profile in a commercial catalytic cracking regenerator 40 ft in diameter with a 15-ft bed [8]. The exit gas had 1% O2, but samples drawn from different bed depths had only 0.1-0.4% O2. The bed samples also showed 12-14% CO2, compared to 10% CO2 in the exit gas. Although most of the gas flow was in the bubbles, the probe saw mainly dense-phase gas, where the conversion was higher than in the bubbles. Samples taken very rapidly showed wide fluctuations in oxygen content, since the probe was sometimes in a bubble and sometimes in the dense bed. 

The equations are developed in the paper by Ishida and Wen [9]. The gas concentration profile during the first stage is found by solving Eq. 42-9 accounting for Eq. 42-12, and the boundary condition Eq. 42-S. This leads to... 

Figure 2.6 Axial temperature, mass deposition and gas concentration profiles used in the derivation of the sharp front approach for the capture step (a) and recovery step (b)...

In general the combustion of most of the sulfur tends to take place towards the feed end of the sinter machine, and the concentration of SO2 in gas is much lower towards the tip end of the machine. A typical gas concentration profile is shown in Figure 4.5 for a machine with ten windboxes. Average SO2 levels as shown are about half the maximum and need to be above five per cent for sulfuric add production by the standard contact plant technology. 

Figure 4.59 Snapshots of the tracer gas concentration profile inside the bubble at different times after bubble injection (snapshots are taken every 0.01 s with an injection velocity of 17 m/s). CO2 concentration is presented in mol/l. Reprinted from Dang et al. (2013) with permission from Elsevier.

Gas exchange between the jet and the outside emulsion phase was studied by tracer gas injection and by integration of gas velocity profiles in the jet at various heights above the jet nozzle in a 28.6 cm diameter bed with a 3.5 cm jet using polyethylene beads as bed material (Yang et al., 1984a). The concentration profiles obtained at different elevations were found to be approximately similar if the local tracer concentration is normalized with the maximum tracer concentration at the axis, C/Cm, and plotted against a normalized radial distance, r/(ri/2)c, where (ri/2)( is the radial position where the tracer concentration is just half the maximum tracer concentration at the axis. Thus in a permanent flamelike jet in a fluidized bed, not only the velocity profiles in the jet but also the gas concentration profiles are similar. 

Continuous He injection into a FB. Gas concentration profiles measured... 

Figure 6.16 shows typical hydrogen and oxygen gas concentration profiles in the electrodes and in the adjacent gas channels at a given cross section of the cell. 

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