Conversion front

With respect to the considerations above, research is split into three parts. The first is connected to the kinetic description of the release of ammonia from the biomass as function of temperature. This research employs infrared spectroscopy using a tunable diode laser. Here very small biomass particles are used that are heated up very rapidly in a small reactor, which ensures that transport effects are virtually excluded from the kinetic release effects. Since ammonia is released in very small quantities it is quite hard to detect. Therefore, we first measure CO release, which is easier. In the second part we investigate the propagation of a conversion front in biomass layers. Here we perform experiments and try to establish a modeling approach for the propagation by analytical and numerical approaches. In the third part the gas-phase conversion processes are described in terms of... 

To validate the numerical work and to study the phenomena that play a role in fixed bed combustion, experiments with a fixed bed reactor (Fig. 8.8) were performed [15]. Essentially, the reactor consists of an insulated metal tube filled with biomass. The biomass is ignited at the top, while air is supplied at the bottom of the reactor. The conversion front can be tracked with thermocouples. A mass balance is used to record the conversion of the biomass. As the results of the previous section are for coal conversion, the two sets of results can not yet be compared directly. 

The raw data of the thermocouples consist of the temperature as a function of time (Fig. 8.9, left). In the raw data, the passing of the conversion front can be observed by a rapid increase in temperature. Because the distance between the thermocouples is known, the velocity of the conversion front can be determined. The front velocity can be used to transform the time domain in Fig. 8.9 (left) to the spatial domain. The resulting spatial flame profiles can be compared with the spatial profiles resulting from the model. The solid mass flux can also be plotted as a function of gas mass flow rate. The trend of this curve is similar to the model results (Fig. 8.9, right). 

The experimental setup will be tested and improved. In the raw thermocouple data, the temperature of the conversion font decreases strongly when the conversion front has passed a thermocouple. This is caused by heat losses through the... 

The conversion process occurs both on macro- and micro-scale, that is, on single particle level and on bed level. In other words, the conversion process has both a macroscopic and microscopic propagation front. One example of the macroscopic process structure is shown in Figure 10. The conversion front is defined by the process front closest to the preheat zone, whereas the ignition front is synonymous with the char combustion front. 

The transport of heat to the conversion zone controls the conversion rate in regime n. Regime II exists in the mid range of the volume flux of primary air and is characterised by a conversion zone without extension and an off-gas with relatively high contents of combustibles. Regime II is a consequence of macroscopic conversion front rates being equal to the overall conversion rate. Consequently, the conversion zone has no thickness and no distinct bed process structure. 

To approach the analysis of, and to be able to comprehend, the complex phenomena of thermochemical conversion of solid fuels some idealization has to be made. For a simplified one-dimensional analysis, there is an analogy between gas-phase combustion and thermochemichal conversion of solid fuels, which is illustrated in Figure 41. Both the gas-phase combustion and the thermochemical conversion is governed by a exothermic reaction which causes a propagating reaction front to move towards the gas fuel and solid fuel, respectively. However, there are also some major differences between the conversion zone and the combustion zone. The conversion front is defined by the thermochemical process closest to the preheat zone, which is not necessarily the char combustion zone, whereas for the flame front is defined by the ignition front. In practice, many times the conversion zone is so thin that the ignition front and the conversion front can not be separated. 

Figure 9.6 shows the evolution of temperature and conversion profiles when the specimen thickness is reduced i.e., for W, =0.1. Again, the material located close to the wall polymerizes at a fast rate, originating thermal and conversion fronts that travel to the core and to the wall. The maximum temperature is obtained at an intermediate location and is higher than that attained in the previous case T0 + ATad < Tmax < Tw + ATad. 

Typical yields in conversion front carbon dioxide to elemental carbon 90%... 

X. (2015) Engineering sugar utilization and microbial tolerance toward lig-nocellulose conversion. Front. Bioeng. Biotechnol., 3, 17. 

If Dg in regions where B protmdes into A, rapid A diffusion will lead to conversion of H to 5 leading to front... 

Dual-Pressure Process. Dual-pressure processes have a medium pressure (ca 0.3—0.6 MPa) front end for ammonia oxidation and a high pressure (1.1—1.5 MPa) tail end for absorption. Some older plants still use atmospheric pressure for ammonia conversion. Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance. Platinum losses are significantiy lower and production mns are extended by a longer catalyst life. Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance. Due to the spHt in operating conditions, the dual-pressure process requires a specialized stainless steel NO compressor. 

Pressure is defined as force per unit of area. The International System of Units (SI) pressure unit is the pascal (Pa), defined as 1.0 N /m. Conversion factors from non-SI units to pascal are given in Table 1 (see also Units and conversion factors front matter). An asterisk after the sixth decimal place indicates that the conversion factor is exact and all subsequent digits are 2ero. Relationships that are not followed by an asterisk are either the results of physical measurements or are only approximate. The factors are written as numbers greater than 1 and less than 10, with 6 or fewer decimal places (1). 

This, as is shown by the theory, is due to the evolution of the heat of absorption, during solute adsorption at the front part of the peak. Conversely, the back of the peak is eluted at a lower temperature than the surroundings throughout the length of the column due to the absorption of the heat of solute desorption. As a result, the distribution coefficient of the solute at the front of the peak, and at a higher temperature, will be less than the distribution coefficient at the back of the peak, at the... 

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

It is unlikely that a single mechanism suffices to cover all conversions of organometallic compounds to alkyl halides. In a number of cases the reaction has been shown to involve inversion of configuration (see p. 762), indicating an Se2 (back) mechanism, while in other cases retention of configuration has been shown, implicating an Se2 (front) or SeI mechanism. In still other cases, complete loss of configuration as well as other evidence have demonstrated the presence of a free-radical mechanism. ... 

Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.12). The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride enrichment of the uranium-235 isotope conversion of the enriched product to uranium oxides and fabrication into reactor fuel elements. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

At places where the front is concave toward the unburnt gas, the heat flux is locally convergent. The local flame temperature increases and the local propagation velocity also increases, see the red arrows in Figure 5.1.5. The converse holds for portions of the front that are convex. The effect of thermal diffusion is to stabilize a wrinkled flame. 

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