Acceleration period

The terminal velocity in the case of fine particles is approached so quickly that in practical engineering calculations the settling is taken as a constant velocity motion and the acceleration period is neglected. Equation 7 can also be appHed to nonspherical particles if the particle size x is the equivalent Stokes diameter as deterrnined by sedimentation or elutriation methods of particle-size measurement. 

An explicit expression should be obtained for the distance through which a particle will settle in a given time if it starts from rest and if the resistance force is proportional to the square of the velocity. The acceleration period should be taken into account. 

Near the end of the dormant period, the rate of cement hydration increases sharply. The onset of the acceleration period has been attributed to the following effects [125] ... 

The Setback Mechanism. The duration of the setback acceleration period is long compared with the transit time for a compression wave in the material, but short compared with the time required for significant heat transfer. Thus the compression that results may be considered to be essentially adiabatic. As soon as the physical mechanism of setback is understood to be adiabatic compression, it becomes clear how explosives can be initiated by setback. Hot spots are caused by the adiabatic compression of minute air spaces within the explosive (See also under Hot Spots in Vol 7, HI 70 ff)... 

A typical conversion-time curve exhibits a short period of acceleration followed by a nearly constant rate up to 50% conversion or more, then followed in turn by a diminishing rate. The acceleration period is sometimes reported to persist to quite high conversion. Although some... 

Acceleration period Monomer exponent in rate expression Initiator exponent in rate expression Energy of activation... 

Many supported highly active catalysts show behaviour similar to case B in Figure 3.13 the polymerisation rate may also start at a maximum value and then decrease more or less rapidly with time. Such kinetic behaviour is also characteristic of some homogeneous catalysts. Other polymerisation systems show no acceleration period but have a polymerisation rate that remains almost constant with time this is a rare case and relates, for instance, to 4-methyl-l-pentene polymerisation with MgCl2-supported catalysts containing phthalate esters as well as to ethylene polymerisation with the Cp2TiCl2—[Al(Mc)0]x catalyst (apart from a short settling period in the latter case) [240],... 

WL/kv = force required to support the weight of material in the vertical section of the pipe, where W is the weight of material conveyed in tons per hr, k is a constant to allow for acceleration period of grain in the pipe, and v is the velocity of the material in m per sec given by the equation... 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

The effect of using too large a volume of solution in the dilatometer is to extend the acceleration period, resulting in too high a rate. This is symptomatic of nonisothermal conditions. 

The 7.6 value for the NO2 ratio, obtained over the period of 397-492 hours of run No. 2 indicates that the decomposition of NOCIO4 (Reaction 2) was occurring 10 times faster than the rate of NOCIO4 production by Reaction 1, and rapidly diminishing the excess produced during the accelerated period of the N02C104 decomposition. 

If diffusion phenomena are not involved, the formation and deactivation of polymerization centers should reflect in rate-time dependences, other conditions being constant. Rate acceleration period of very widely differing lengths is often observed, followed either by a more or less steady rate or by a deceleration (rate decay) period. As for the polymerization center deactivation, it is quite important to know whether a macromolecule or a metal-polymer bond is formed due to this reaction (see Sect. 4). 

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