Autocatalytic nature of the

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

Product separation by vaporization is not a good option because of the thermal sensitivity of the product. In addition, because of the autocatalytic nature of the methacrolein-phosphine adduct, it is imperative that its concentration be controlled. 

To see the autocatalytic nature of the reaction, we can compare the simulation to one made assuming a constant amount of catalyst. Taking mMn(OH)(s) to be 4.5 xlO-4 molal, its value when oxidation in the previous run is complete, the procedure is... 

A thermal stability study was first carried out to determine the following information (1) the solidification temperature as a function of the concentration of the sulfonate (2) the enthalpy of decomposition by DTA (3) the autocatalytic nature of the decomposition by Dewar flask (4) kinetic data for decomposition by Dewar flask (5) the time to maximum rate by ARC, and (6) the heat generation as a function of temperature, also by ARC. In addition, the enthalpy of dilution was determined for various potential water leak rates. These data were useful in defining emergency response times. 

Figure 12.9 First-order model fitted to an autocatalytic peak in a dynamic experiment. The apparent activation energy is 280kJ mol showing the autocatalytic nature of the reaction.

Temperature-programmed DSC, or DTA measurements, can only suggest the autocatalytic nature of the decomposition. Neither the influence of the thermal history and contamination can be detected by them, nor can the kinetic parameters be determined from a single experiment. 

Since this thermogram shows a steep peak, the autocatalytic nature of the decomposition is likely. Thus, two isothermal DSC experiments were performed at 240 and 250 °C, in order to confirm this hypothesis and to evaluate the probability of triggering the decomposition (Figure 12.13). The results can be summarized as follows at 240°C the initial heat release rate is 8.5 Wkg-1 and the maximum heat release rate 260 Wkg-1. At 250°C, the measured heat release rates are 15 and 360Wkg 1, respectively. 

A number of different kinetic attempts and strategies have been employed to obtain a closer understanding of the Soai reaction. The first efforts were made by Blackmond et al. using microcalorimetric studies [77]. The autocatalytic nature of the reaction was confirmed by the observation of the maxima of the reaction heat flows as a function of time. In particular, it was reported that the reaction rate depends on the enantiomeric purity of the initially added pyrimidyl alkanol, where the rate in the presence of the enantiopure alkanol was roughly twice of that using the racemic alkanol. 

Sato et al. confirmed the autocatalytic nature of the Soai reaction by showing the sigmoidal time evolution of the product formation and the acceleration of the reaction by initial addition of the pyrimidyl alkanol [19]. The authors also presented a modeling attempt by using fast pre-equilibrium... 

It cannot be stressed too strongly that without exception, all known cellular life possesses an autocatalytic metabolism, even if the cells are het-erotrophic. Thus for the autocatalytic nature of the whole metabolic network it is not necessary to be able to identify a smaller autocatalytic core as the reductive citric acid cycle or the Calvin cycle. Imagine the following thought experiment. Take away all metabolites from a cell but leave all the water and the informational macromolecules in place. Can the network be recreated from the food materials only, or not Let us be generous and provide enough ATP also for the supposed kick-start. The fact is that no contemporary cell could resume its activity in this experiment. Consequently, all cells today possess a distributive autocatalytic network that cannot be seeded from outside, because some of its seed components cannot be taken up from medium. 

Both calculations and measurements have indicated that it is possible to develop very concentrated metal chloride solutions within occluded sites. For example, stainless steel pits would be expected to contain 5 N Cl", 4 N Fe2+, 1 N Cr3+, 0.5 Ni2+, and 0.007 N Na+ and have a pFl of 0.5. The low sodium ion concentration develops as the Na+ migrates out of the crevice due to the electric field but is not replaced by any reaction in the crevice. Recent measurements (24) of active crevice sites on Type 316L stainless steel showed the following concentrations 2.9M Fe2+, 0.77 M Cr3+, 0.24 M Ni2+, and 0.06 M Mn2+. As was stated above, stainless steel will not remain passive in such a solution and can dissolve at a high rate. The autocatalytic nature of the process stabilizes the environment by exceeding the rate at which diffusion can disperse the concentrated solution. Initial dissolution rates of the order of 1 A/cm2 ( 440 in./yr) can be reached. 

Nowadays the autocatalytic nature of the reaction is thought to be due to the decomposition of the hydroperoxides (Denisov, 2000 Al-Malaika, 2003) ... 

Autocatalytic Nature of the Reduction. The data in Figure 5 show the reduction rate to increase with time, indicating that the reduction is autocatalytic. This is most likely caused by the reduction of the ferric porphyrin by the intermediate radicals generated by Reaction 12, which are expected to be more potent reducing agents than the cyanide ion. Hence any detailed interpretation of the rates will be severely limited. 

Cyanide Ion Concentration. Increasing the cyanide ion concentration speeds up the autoreduction, as shown in Figure 5. Because of the autocatalytic nature of the reduction, the exact cyanide ion dependence has not yet been defined. Also it is not yet clear whether the cyanide ion is involved mechanistically in the autoreduction. The cyanide ion concentration can affect the observed rate via the autocatalytic mechanism or by competing with the complexed cyanide ion for hydrogen bonding with trace amounts of water. 

The autocatalytic nature of the reaction, described by Hinshelwood and Williamson [1], is in sharp contrast with the effect of water on the surface reaction at lower temperatures, which is poisoned by steam, and also with the inhibiting effect of water vapour on the second limit explosions. The autocatalysis has been studied in some detail by Chirkov [36], who used a reaction vessel of Durobax glass with diameter 5 cm and volume 200- 250 cm. For hydrogen oxygen ratios of about 2 1 at 550 torr initial pressure and 524 °C, he found the reaction rate w (torr sec ) to be given in terms of the initial pressure p and the amount of gases reacted x by... 

First, since competition between reactions (xi) and (v) or (vb) is excluded by the second limit behaviour referred to in Sect. 4.2 immediately above, the peroxide must be formed by mutual interaction of two HO2 radicals either at the surface by reaction (va) or in the gas phase by reaction (x) below. Second, the autocatalytic nature of the reaction can only be attributed to the dissociation of H2 Oj by reaction (vii) or the alternative (viia). Competition between the dissociation and a surface destruction of H2O2 would introduce a diameter dependence of the rate which is contrary to the results. A second function of the ageing of the surface therefore must be to eliminate surface destruction of the peroxide. Third, if H2O2 always dissociates by (vii) or (viia), the formation of HO2 by reaction (iv) always leads to a chain propagating cycle... 

Fig. 2.2. Pressure vs time profiles for products from C3H6 + O2 at 480°C, showing the autocatalytic nature of the reaction. Initial conditions C3H6 = 12, O2 = 30, N2 = 18 Torr. O, CO X, HCHO , C2H4 , CH3CHO V, CH2=CHCHO , hexadiene (x5) O,...

The autocatalytic nature of the reaction (see Chapters 1 and 5), which may be a limitation on the validity of the underlying assumption about a constant reaction stoichiometry (i.e., constant y and z), requires there to be a specific reference point at which the rate of pressure change may be compared. The usual reference is taken to be the maximum rate of pressure change in each experiment (pmax)- This approach has been exploited extensively over the years, and there are very many other examples to be found in the literature (Table 6.1). For example, Wu et al. [15], investigating the gaseous oxidation of butane in the temperature range 470-810 K, were... 

The autocatalytic nature of the reaction is attributed to slow tautomerism of the pentavalent phosphonate (2a) to the more reactive phosphite (2b), which reacts rapidly with (I). The tautomeric shift is catalyzed by a trace of hydrogen bromide formed by hydrolysis of the phosphoryl bromide (4). With methanol as solvent, methanolysis of (4) with production of acid reduces the induction time to 30 seconds. 

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-Isooctane, and methanol-water-lsooctane. The resulting decrease In the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As Indicated by the decrease In properties, reaction with methanol Is Initially faster, but the reaction rate with water Increases with time — presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy Indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. 

A plot depicting the effect of aging In water on the tensile strength of polymer A Is shown In Fig. 1. The plot Is characterized by an Initial slow decrease In strength with time which accelerates after 10 days. This Increase In the rate of tensile strength loss Is consistent with the autocatalytic nature of the hydrolysis reaction. The reaction generates an acid which In turn catalyzes further hydrolysis of the polyurethane XLL ... 

Assuming co-reaction, the cure reaction of a mixture of bis(4-maleimido phenyl) methane and BACY was followed by FTIR [221]. The reaction kinetics, studied by DSC, suggested dependency of cure mechanism on blend composition. The apparent activation energy computed by the Prime method increased with BMI content. The rate maximum at a fractional conversion range of 0.32-0.33 indicated an autocatalytic nature of the reaction. The different pattern of activation energy with fractional conversion for two different blend compositions indicated non-identical cure mechanisms for the two compositions. The cyclot-rimerization of BACY occurred during the cure of a 1 2 molar ratio of BMI and BACY. Since activation parameters derived from DSC method are generally not consistent, and since the cyanate cure can be catalyzed by impurities present in BMI, which was not taken into consideration, the authors conclusions on the cure mechanism based on DSC kinetics can be erroneous. 

Calorimetric studies of radiation-induced bulk polymerization also confirmed the autocatalytic nature of the process (10, 11). 

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