Induction period Inerting

Inhibitors and retarders differ in the extent to which they interfere with polymerization, and not in their essential activity. An inhibitor is defined as a substance which blocks polymerization completely until it is either removed or consumed. Thus failure to totally eliminate an inhibitor from purified monomer will result in an induction period in which the inhibitor is first converted to an inert form before polymerization can begin. A retarder is less efficient and merely slows down the polymerization process by competing for radicals. 

Inhibitors or retarders that give inert products are called ideal .173 The term ideal inhibitor has also been used to describe a species that stops all polymerization until such time as it is completely consumed (i.e. the induction period) and then allows polymerization to proceed at the normal rate. However, in many cases the products formed during inhibition or retardation are not inert. Four... 

Although most handbooks give its expin temp as 60°, more recent work (Ref 7) has shown that when 0,02g samples of NH4Mn04 were heated in air, they expld at 96° only after heating (induction period) for 600—700 seconds, while at 117° it required from 80 to 110 seconds. No exptn at 60° was reported. When smail samples were heated under confinement (immersed in an inert oil), they expld at 99.5° after about 10 minutes, but required 250 mins to expld at 70°... 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

The perchlorate was found to sublime in vacuo to some extent at all temperatures. A small pressure of inert gas was found to reduce the sublimation. The sublimate was found to be ammonium perchloratej with traces of H and NOf ions. It decomposed with a reduced induction period at a slightly higher rate. It was found that sublimation continued after decomposition had stopped. It is curious that the vapour, which may or may not be dissociated, is not decomposed at these temperatures. 

This is in marked contrast to the higher aldehydes which oxidize much more readily, at temperatures as low as 100° C.—e.g., acetaldehyde (55). The mechanism by which formaldehyde reduces the induction period in hydrocarbon oxidation below 300° C. is not evident. An inhibiting effect is explicable on the basis of removal of free radicals, as Lewis and von Elbe (32) have pointed out. This could occur by CH20 -f R — RH -f HCO. HCO is then oxidized to the relatively inert CH03 which diffuses to the wall and is destroyed. Above 300° C. formaldehyde is oxidized more rapidly, giving rise to free radicals, and it is not surprising to find that the induction period in some hydrocarbon oxidations is shortened. 

Effect of Addition of Inert Diluents. The addition of inert gases to an explosive mixture will have two major effects. It will increase the heat capacity of the mixture, and depending upon the nature of the added gas, it will change the mixture thermal conductivity. Equation 26 shows that an increase in the heat capacity of the mixture will tend to increase the induction period. The addition of a high thermal conductivity gas such as helium will increase the limiting pressure. Rearranging Equation 18 shows that for a given vessel diameter, reactant concentration, and furnace temperature, the ratio... 

In concomitance with the displacement observed by i.r., an evolution of the catalytic activity has been observed while studying the liquid-phase epoxidation of cyclohexene in the presence of (EGDA)- Mo(VI), freshly prepared or after four months of conditioning at room temperature under inert atmosphere. As usual, the appearance of epoxide was followed by gas chromatographic analyses or by direct titration of oxirane oxygen and the disappearance of hydroperoxide was monitored by iodometric titration. In figure we report concentration-time for typical runs in ethylbenzene at 80°C obtained with the experimental procedure already described (ref. 9). It may be seen that with a freshly prepared catalyst an induction period is observed which lowers the initial catalytic activity. Our modified Michaelis-Menten type model equation (ref. 9) cannot adequately fit the kinetic curves obtained due to the absence of kinetic parameters which account for the apparent initial induction period (see Figure). 

Methanol is somewhat less reactive than its higher homologues and slow combustion takes place at a conveniently measurable rate only above 390 °C. In uncoated pyrex vessels [7], or vessels coated with boric acid or potassium chloride [8], reaction begins immediately without a true induction period and accelerates to a maximum rate. This maximum is increased by the addition of inert gas and is proportional to the square of the initial methanol concentration, (in boric acid coated vessels this power is about 2.5) but independent of oxygen concentration over a wide range of conditions. The overall activation energy (calculated from the effect of temperature on the maximum rate of pressure change) is about 40 kcal.mole" in coated vessels and about 53—61 kcal.mole in uncoated ones. 

The point has recently been made that the kinetic data which indicate that the cyclic intermediates are involved are also compatible with mechanisms in which the true reaction is a bimolecular one and the complexes of reactants are inert. In support of this reinterpretation is the observation of an induction period in the reaction with propane-1,2-diol. Although the point is valid, there are two reasons why the Bunton mechanism of Figure 6.65 should be maintained. The first is the rapid-reaction work, which clearly showed the kinetic competence of an absorbing complex. The second is Occam s razor, which would reject the parasitic equilibria plus unspecified bimolecular process in favour of the simpler Bunton proposals. 

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