Contact approximation quenching

Figure 3.62. The light dependence of the Stem—Volmer constant K, i (/, i for diffusional quenching with given exponential rate (Wq — 103 exp[—2(r — cr)/L] ns-1), but at different diffusion in pairs containing the excited molecules (a) D =0.1 Dd, (6) D = D= 10 5cm2/s (dashed line—the same, but in contact approximation), (c) D = WD. Other parameters a = 5A, L = 1.0 A, i = 10ns, ko = f Wq(r)d3r = 1.9 x 105 A3/ns. (From Ref. 200.)...

In the same figure we also demonstrate the sensitivity of the results to the ratio of the diffusion coefficients in the stable and excited pairs, D Q and D Q, which increases from (a) to (c). The difference in the diffusion coefficients ignored in the convolution recipe affects the field dependence no less than the contact approximation. This is the unique advantage of IET, that it is free of any limitations of this sort and can be used for quantitative investigations of field effects, at any space dispersion of quenching rate and at arbitrary diffusion coefficients. In fact, this is diffusion of the excited molecule that affects the field dependence of k0. By slowing down this diffusion, one increases the encounter time x,/ and thus enhances the field acceleration of quenching. 

As before, the analytical solution becomes possible only in the contact approximation (3.40). It is inappropriate for the dipole-dipole transfer rate (3.44) but can be considered as a reasonable model for the exponential exchange quenching carried out by the dipole-forbidden triplet-triplet energy transfer [204]. 

In liquids the static kinetics precedes the diffusion accelerated quenching, which ends by stationary quenching. The rate of the latter k = AkRqD has a few general properties. In the fast diffusion (kinetic control) limit Rq — 0 while k —> ko. In the opposite diffusion control limit Rq essentially exceeds a and increases further with subsequent retardation of diffusion. As the major quenching in this limit occurs far from contact, the size of the molecules plays no role and can be set to zero. This is the popular point particle approximation (ct = 0), which simplifies the analytic investigation of diffusional quenching. For the dipole-dipole mechanism the result has been known for a very long time [70] ... 

However, not only the contact lines themselves are beneath all the points but their slope is also too small to explain the true concentration dependence. Perhaps these difficulties stimulated Stevens to propose the original finite-sink approximation instead of the existing theory [103,250]. The inconsistency of this model is seen from the very fact that it does not discriminate between the Stem-Volmer and the steady-state constants, k and k = lim, >rXj k(t), and ascribes to the latter the concentration dependence, which is not inherent to k in principle. At the same time the model predicts the linear dependence of the results in coordinates 1/k versus c1/3, and this linearity was confirmed experimentally a number of times [6,248,250,251], According to their finite-sink model [248], the straight lines representing this dependence for different temperatures should intersect at a common point where the static quenching limit is unambiguously located. ... 

The reaction takes place under fuel-rich conditions to maintain a nonflammable feed mixture. Typical feed composition is 13% to 15% ammonia, 11% to 13% methane and 72% to 76% air on a volumetric basis. Control of feed composition is essential to guard against deflagrations as well as to maximize the yield. The yield from methane is approximately 60% of theoretical. Conversion, yields, and productivity of the HCN synthesis are influenced by the extent of feed gas preheat, purity of the feeds, reactor geometry, feed gas composition, contact time, catalyst composition and purity, converter gas pressure, quench time and materials of construction. 

Kurcha crude oil steam cracking technology was developed jointly with Union Carbide for the manufacture of ethylene (see Section 2.13.4). By operating at very high temperature and with very short. contact times (0.003 to 0.010 s), approximately equal amounts of acetylene and ethylene can be produced from a number of crude oils. This is illustrated by Table 5.2 for Indonesian and Arabian crudes, cracked in the presence of steam at 2000°C, in a steam to feed weight ratio of about 3, and with residence time of 0.005 s. In these conditions, the temperature at the reactor exit before quench reaches 11S0-C. ... 

Conditions for the direct oxidation of propylene to acrolein include use of a catalyst of cuprous oxide deposited on granular silicon carbide, catalyst temperature of 375 C, feed stream composition by volume of 20 per cent propylene, 20 per cent air, and 60 per cent steam, and contact time of 1 sec. Recovery and primary purification of the acrolein from the reaction product are effected by quench scrubbing the reactor effluent with water and wth liquid propylene. The composition of the carbonylic compounds in the product is, approximately, acrolein, 90 per cent by weight acetaldehyde, 6 per cent propionaldehyde, 2 per cent and acetone, 2 per cent. At reaction temperatures of about 300 C and conversions of about 50 per cent, a selectivity to acrolein of about 40 per cent is reported for 10 per cent propylene-in-air mixtures. ... 

Cyclization of 2 in concentrated sulphuric acid [14-16] predominantly leads to p-ionone (17). The reaction proceeds rapidly even below room temperature and, to avoid secondary reactions, is carried out continuously. The precooled streams of sulphuric acid and the solution of 2 in petroleum ether or liquid CO2 are mixed in a reactor and then quenched with cold water. Small amounts of a-ionone (18) can be separated off by distillation during isolation of the product. In the cyclization step large amounts of approximately 40% aqueous sulphuric acid are produced. Treatment to deal with this is expensive but is essential for environmental reasons. Organic impurities are broken down to carbon dioxide in a cracking furnace with heavy oil burners. In the course of this process, sulphuric acid is thermally converted into sulphur dioxide, which is reoxidized in the contact plant. 

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