Residence-time parameter, critical

The gas flow rate is usually presented as a deposition parameter however, it is much more instructive to report the gas residence time [6], which is determined from the flow rate and the geometry of the system. The residence time is a measure of the probability of a molecule to be incorporated into the film. The gas depletion, which is determined by the residence time, is a critical parameter for deposition. At high flow rates, and thus low residence times and low depletion [303], the deposition rate is increased [357, 365] (see Figure 39) and better film quality is obtained, as is deduced from low microstructure parameter values [366],... 

In practical combustion systems, such as CO boilers, the flue gas experiences spatial and temporal variations. Constituent concentration, streamline residence time, and temperature are critical to determining an efficient process design. Computational fluid dynamics (CFD) modeling and chemical kinetic modeling are used to achieve accurate design assessments and NO, reduction predictions based on these parameters. The critical parameters affecting SNCR and eSNCR design are listed in Table 17.4. 

The choice of the above three modes of catalyst placement relative to the membrane can significantly affect the reactor performance. From the analysis of catalytically active and passive (inert) membrane reactors [Sun and Khang, 1988], it appears that the critical parameter determining the choice is the reaction residence time. At low residence times, the difference between a catalytically active and a catalytically passive membrane is not significant. However, as the reaction residence time becomes high, the catalytically active membrane shows a higher reaction conversion. 

Provided r, is small, then the critical inflow concentration for this branching-termination model under CSTR conditions differs slightly from the so-called pool chemical result which is obtained by assuming [A] = constant. For typical chemical systems the residence time will be such that k, > 1/fres, SO the two results are not significantly different but the extra influence of the flow is clearly evident in the above forms. In neither of the analyses above, however, is there a discontinuous jump in the steady-state response as the parameters are varied. 

If some other parameter of the system, such as the adiabatic temperature excess ad is varied, so the shape of the steady-state locus may deform. For low values of B a in this model, corresponding to weakly exothermic processes, then the hysteresis loop is unfolded, as indicated in Fig. 5.6(c, d), and a simple smooth variation of the steady-state temperature excess with the residence time is observed. Thus, systems can lose criticality as other experimental parameters are changed. 

There are a number of drawbacks to using continuous processes. Resources are needed to develop the process the appropriate residence time to reach a level of suitable reaction completion must be determined under the desired conditions of temperature, flow rate, and any other critical parameters. The reaction system may have limited flexibility for running other reactions. Pressure drops occur when using tubular flow reactors, and these can be calculated [18]. Once the conditions have been developed, time is necessary to reach steady-state conditions. What happens to material produced while the conditions are approaching steady state Such material is not produced under the desired conditions and hence is atypical of the majority of the batch. Effective control equipment is mandatory for large-scale operations otherwise expensive material is at risk and may need to be reworked. 

As polynuclear aromatic hydrocarbons show a high tendency towards undesired gas-phase nucleation and soot formation, they should be avoided by choosing a suitable critical residence time and other appropriate processing parameters. 

Gas hold-up is a critical parameter in characterizing the hydrodynamic behavior and hence the performance of a bubble column reactor. It determines a) the reaction rate by controlling the gas-phase residence time and b) the mass-transfer rate by governing the gas-liquid interfacial area. It is mainly a function of the gas velocity and the liquid physical properties. 

The sample residence time in the flow manifold, often associated with the expression sample incubation time, is an important parameter in flow-based analytical procedures involving relatively slow chemical reactions and/or physicochemical processes, e.g., dialysis, gas diffusion or liquid—liquid extraction. This parameter may become a limiting factor in the system design, especially if sensitivity is critical. Moreover, the susceptibility to biased results is less pronounced when the chemical reactions and /or the involved physico-chemical processes tend towards completion. Fig. 1.4 refers to a hypothetical situation where biased results are obtained when chemical equilibrium is not reached. 

The reaction is carried out in a 0.478 cm i.d. Inconel 625 reactor at 960 to 1,100 bar and 380° to 450°C with approximately one minute of residence time. Treatment at these conditions is often called supercritical water oxidation (SCWO) however, since the critical parameters of the PBX-9404 hydrolysis product mixture are unknown, the treatment may or may not be above the critical parameters for the hydrolysate. Therefore, the more general term hydrothermal treatment is used in this case. 

Some pharmacokinetic software packages perform noncompartmental analysis without fitting the entire response curve. These programs compute the elimination rate constant (k) for the terminal elimination phase of the data, and then use a trapezoidal rule with this elimination rate constant to compute AUC and AUMC. With these terms, the total body clearance, the steady-state volume of distribution, and the mean residence time in the body can be calculated. Without C , it is not possible to calculate the volume of distribution of the central compartment or the mean residence time of the sampling compartment. The latter term is therefore critical in accurately determining these parameter values and depends on an unbiased and close fit of the data to Equations 13.2 and 13.6. 

Critical Process Parameters. In addition to temperature, residence time is also a critical process parameter for SNCR. Longer residence time allows a higher degree... 

These two expressions for residence time, after integration, can be equated in a critical particle trajectory analysis similar to that described in Section 8.2.2. When considering only 50% capture of particles the volumes processed now have to be related to the area of a ftustum (a truncated cone), the start radius being that value that has equal volumes of the section between each disc above and below it. The resulting expression for sigma (machine parameter) is ... 

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