Concentration reduction

Gombnstible concentration reduction can also be used to prevent deflagrations and detonations in process equipment and piping. The combustible concentration is reduced below the lower flammable limit (LFL) by means of ventilation (air dilution). 

NFPA 69 (NFPA 1997) contains information on basic design considerations, design and operating requirements, and instrumentation requirements. Appendix D presents methods for ventilation calculations, including the time required for ventilation to reduce the concentration to a safe limit, the number of air changes required for reaching a desired 

Overview of Deflagration and Detonation Prevention and Proteotion Praotioes 

NFPA 69 stipulates that the combustible concentration be maintained at or below 25% of the LFL with the following exceptions  

When automatic instrumentation with safety interlocks is provided, the combustible concentration may be maintained at or below 60% of the LFL. 

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Figure 6-9 Concepts for Stress Concentration Reduction around Holes...

Combustible Concentration Reduction The technique of maintaining the concentration of combustible material in a closed space below the lower flammable limit. 

The treatment performance data for the 19 completed projects presented in Table 24.9 show that SVE (either alone or in combination with other technologies) has been used to remediate MTBE in groundwater from concentrations >100,000 to <50 pg/L and has achieved MTBE concentration reductions >99%. The median project duration for the 19 completed sites ranged from 3 months to 5 years. 

Membrane depolarization typically results from an increase in Na+ conductance. In addition, mobilization of intracellular Ca2+ from the endoplasmic or sarcoplasmic reticulum and the influx of extracellular Ca2+ appear to be elicited by ACh acting on muscarinic receptors (see Ch. 22). The resulting increase in intracellular free Ca2+ is involved in activation of contractile, metabolic and secretory events. Stimulation of muscarinic receptors has been linked to changes in cyclic nucleotide concentrations. Reductions in cAMP concentrations and increases in cGMP concentrations are typical responses (see Ch. 21). These cyclic nucleotides may facilitate contraction or relaxation, depending on the particular tissue. Inhibitory responses also are associated with membrane hyperpolarization, and this is a consequence of an increased K+ conductance. Increases in K+ conductance may be mediated by a direct receptor linkage to a K+ channel or by increases in intracellular Ca2+, which in turn activate K+ channels. Mechanisms by which muscarinic receptors couple to multiple cellular responses are considered later. 

Asphyxiation and Toxicity Hazards An asphyxiant is a chemical (either a gas or a vapor) that can cause death or unconsciousness by suffocation (BP, Hazards of Nitrogen and Catalyst Handling, 2003). A simple asphyxiant is a chemical, such as N2, He, or Ar, whose effects are caused by the displacement of 02 in air, reducing the 02 concentration below its normal value of approximately 21 vol %. The physiological effects of oxygen concentration reduction by simple asphyxiants are illustrated in Table 23-18 (BP, Hazards of Nitrogen and Catalyst Handling, 2003). 

The data in Table IV and Figure 1 indicate that most of the paraffin concentration reduction took place in the Ce to Ce range and involved monomethyl paraffins. The blending octane numbers for 2-methyl and 3-methyl hexanes are reported as 40 to 56 by ASTM ( ) and are lower than the blending numbers for light (Cs to C ) olefins and Ce to Cii aromatics. Thus, reduction in concentration of these branched paraffins is expected to improve the research octane number of the gasoline. 

As it turns out, the influence of an exponential input rise of 10% per year keeps C(t) close to the steady-state concentration (Fig. 21.4). In contrast, an exponential increase of the input of 270% per year leads to a concentration reduction of 80% relative to the hypothetical steady-state. To find such an enormous rate as a longterm trend is hardly realistic. Thus we conclude that a possible trend in the input does not really put the steady-state assumption into question for this particular case. 

Figure 5.6 shows the same data plotted as a function of cM0 6S to test the low concentration reduction scheme based on c rf] with a typical value of the Mark-Houwink exponent for good solvents. The data have been shifted vertically to achieve superposition at high molecular weights. It is clear that the cM variable produces a better superposition of data at all molecular weights and concentrations. The apparent variation in the values of cM at the intersections in Fig. 5.4 (Table 5.1) is largely due to a lack of data to define the limiting behavior at low molecular weights at some concentrations. The intersection on the superposed plot in the composite Fig. 5.5 is cM = 30000, giving Mc = 30600 for undiluted polystyrene (q = 0.98 at T = 217° C, in good agreement with the value 31200 reported by Berry and Fox (16).

Under the ribs, the species diffusion in the direction orthogonal to the cell plane, i.e. cross-plane diffusion, is obviously impossible. Hence, the reactant concentrations on the electrode/electrolyte site under the ribs are driven only by in-plane diffusion. Due to the strong diffusion property of H2, in-plane diffusion allows H2 to penetrate under the ribs, while in the case of O2, a relevant concentration reduction is noticed. The effect of the ribs is significant at all operating conditions, but it becomes predominant at high fuel utilization (not shown in the figures). 

Let us now have a closer look at the theory endorsed by J. Bailer,54 W. Wegner,49 and G. Wellers,55 that only a small amount of HCN was used for the killings. In such a case, the concentration reduction due to the respiration of the victims is no longer a negligible quantity. 

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