First explosion limit

In step (1) and step (2) there is an increase from one to two chain carriers . (For brevity, step (x) is used to refer to equation (A3.14.V) tliroughout.) Under typical experimental conditions close to the first and second explosion limits (see section A3.14.2.3). step (2) and step (3) are fast relative to the rate detemiining step (1). 

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

GP 11] [R 19] An impressive example of the impact of miniaturization on the explosion limit is given in [9], For a conventional reactor of 1 m diameter, explosive behavior sets in at 420 °C at ambient pressure (10 Pa). In turn, an explosion occurs at about 750 °C when the reactor diameter is decreased to about 1 mm. A further reduction to 100 pm shifts the explosive regime further to higher pressures and temperatures. Even the first explosion limit is above ambient pressure. Now, explosive behavior can be excluded and so the reaction becomes inherently safe. 

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

In the design of P3 explosives the first consideration is again that of power. Approximate limits, above which ignition of gas is likely to occur in unstemmed tests, are given in Table 7.4. 

As the pressure in the reaction vessel increases, the mean free path of the gaseous molecules will decrease and the ease with which radicals can reach the surfaces of the vessel will diminish. Surface termination processes will thus occur less frequently fst will decline and may do so to the extent that fst + fgt becomes equal to fb oc — 1). At this point an explosion will occur. This point corresponds to the first explosion limit shown in Figure 4.1. If we now jump to some higher pressure at which steady-state reaction conditions can again prevail, similar... 

It should be evident from this discussion that the first explosion limit will be quite sensitive to the nature of the surface of the reaction vessel and its area. If the surface is coated with a material that inhibits the surface chain termination process, the first explosion limit will be lowered. Inert foreign gases can also have the effect of lowering the first explosion limit, since they can hinder diffusion to the surface. If something like spun glass or large amounts of fine wire are inserted, one can effect an increase in the first explosion limit by changing the surface/ volume ratio of the system. 

Even though there have been appreciably more studies of CS2, COS is known to exist as an intermediate in CS2 flames. Thus it appears logical to analyze the COS oxidation mechanism first. Both substances show explosion limit curves that indicate that branched-chain mechanisms exist. Most of the reaction studies used flash photolysis hence very little information exists on what the chain-initiating mechanism for thermal conditions would be. 

This is of the same form as Equation 30, but involves the mixed diffusion coefficient, Jci9, instead of the thermal conductivity of the mixture. However, as seen from the kinetic theory of gases, the thermal conductivity is proportional to the diffusion coefficient. Therefore agreement of experimental results with either Equation 30 or 53a is not an adequate criterion for distinguishing between first explosion limits in branching chain reactions and purely thermal limits. It has been reported (52), that, empirically,... 

Kach method suffers from one or more inherent sources of error. Method 1 is not readily adaptable to the determination of second explosion limits. If temperature equilibrium is reached very quickly by the gas flowing into the vessel, as the continued flow causes the pressure to increase, the system must first intersect the lower explosion limit. Method 2 can lead to large errors if explosion is preceded by an induction period. In the carbon monoxide-oxygen reaction, for example, it was found that the heating rate could considerably affect the results owing to the existence of a zone of slow reaction adjacent to the second limit and inhibition of the reaction by the product, carbon dioxide... 

The lower, or first explosion limit occurring at low pressures. Below the pressure limit, i.e. along a, b, the reaction is in the steady state where branching is balanced by surface termination. As the pressure increases, termination at the surface becomes less effective because diffusion to the surface is progressively inhibited, but steady reaction is still maintained until the lower limit is just reached. At this pressure, surface termination just fails to balance branching, and the concentration of radicals can then build up very rapidly to a very, very large value from the steady state concentration. The reaction rate increases dramatically and explosion can occur. It does so with the appearance of a very sharp boundary, which in actuality is... 

A detailed mechanistic investigation of the explosion limits revealed [59] that the first and the third explosion limits of the reaction are dependent on the reactor dimensions (see Figure 2.27). The first explosion limit is reached when the mean free path of the molecules becomes smaller than the reactor dimensions, which... 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

Tank No. 5, which was not being fire-water sprayed because of limited fire-water supplies, catastrophically exploded with a large BLEVE fireball. One section of Tank No. 5 rocketed over 1500 ft. from its original location. This was followed by more explosions as first Tank No. 7 and then Tank No. 6 broke up in pieces. One section ofTank No. 7, which weighted 52 tons, also rocketed over 1800 ft. [11]... 

It is generally found that the first explosion limit of a branching chain is shifted to lower pressures by the decrease of surface/volume ratio (for example, a system becomes more explosive in larger vessels) or by the addition of an inert gas (for example, N2 or Ar). While the surface/volume behavior is similar to that found in thermal explosions, the effect of added... 

The feature which is unique to the chain-branching system is the paradoxical, upper, or second explosion limit. Plere one observes that a reaction proceeding with explosive speed at pressures below the limit is effectively (picnched on raising the pressure. In addition, the pressure limit increases if the temperature increases, just opposite to the behavior at the first and third limits. It is the existence of this limit that is the real evidence of the branching chain. It is observed that the limit is much less sensitive to surface-volume effects than is the first limit, while added inert gases always tend here to lower the limit (i.e., quench the explosion). 

At very low pressures, kbP - V and (C)ss Ky P Jlci However, as the pressure increases, the branching grows more important relative to wall termination, and as the two terms approach equality, (C)ss—> >, that is, the reaction proceeds to explode." From Eq. (XIV.5.2) we find for this first explosion limit... 

It is the requirement that chain branching exceed chain termination at the first explosion limit and lead to explosion which makes it necessary that the branching reaction be of the same or a higher kinetic order in free radi( al concentration than the termination reaction. If it were of lower order, the termination reaction would increjise in rate relative to branching as the radical concentration grew and quench the reaction. 

As pointed out previously, an upper limit is placed upon (C) by second-order recombination processes, so that the radical concentration cannot grow without limit. At pressures near the first explosion limit this restriction is unimportant. Thus if we assume that the reaction H -f- H -f M proceeds at every tenth triple collision (that is, A = 3 X 10 liters2/mole -sec) then the lifetime of an II atom at 750 K when M = 8 mm Hg and H = 0.8 mm Hg ( ) is about 0.2 sec [t = l/fc(M)(H)]. This is much slower than the rate of branching in the H2 + O2 system (see page 457). 

At regions near the junction of the first and second explosion limits this is a poor approximation and the more exact equation defining both the first and second limits is given by the positive root of the equation — kbP ktJi/V = 0. At thc turn-... 

It is difficult to postulate without considerable strain a branching process of higher kinetic order in total pressure and first or higher order in radicals which could account for the third explosion limit. Much attention has been given to this problem without a decisive answer appearing. The chief reason lies in the appearance of the thermal explosion limit in this region of pressures above the explosion peninsula. In the absence of specific evidence to the contrary it is usually safer to interpret the third explosion limit as a thermal explosion. 

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