End of conversion

In slow pyrolysis by the dynamic method, the temperature domain is generally large and in this case, the main parameter is the heating rate. Table 10.6 indicates the temperature interval for the decomposition of the PE defined between the temperatures Ts and T95. These two temperatures are defined respectively as the temperature at which the conversion starts T5 at 5% conversion) and Tg (close to the end of conversion, at 95%). The maximum rate of conversion occurs between T5 and Tgs, at Tmax-... 

Theoretical work has also been devoted to examine the influence of reactions that interrupt the often repeated cycles of radical formation (activation) and cross-termination to dormant species (deactivation). For the nitroxide- and the cobalt-mediated systems, such reactions are the formation of R(—H) and YH by a usual radical disproportionation, which competes with the coupling of R and Y or by a direct fragmentation of R—Y to the hydroxylamine or a hydridocobalt complex and the alkene.22-33a-35-47-51 57 Even rather small fractions of these processes limit the maximum conversion and stop the polymerization prematurely in nearly indistinguishable ways, because they lead to an exponential decay of the dormant species. Before the end of conversion this does not affect the linear dependence of An on conversion and causes only minor increases of the polydispersity.57 To some extent the deteriorating effect of these reactions can be compensated by the rate enhancement through an additional initiation.50... 

Control signals for fast ADC units usually consist of a start command and an end-of-conversion or status signal. Other control signals are possible. In units with output storage-buffers, controls may be available to load or store in the buffer. 

The relations in this section enable one to determine the particle number and particle size distribution, n and rate of polymerization. The particle size at the end of conversion, which is frequently a quantity of interest, is trivially related to the number of particles by... 

From an industrial point of view, this process has many advantages. The monomer is converted into polymer without the need to eliminate byproducts. Continuous production of large amounts of polymer is possible. As polymerisation is exothermic, the polymer is usually in a molten state at the end of conversion, and its direct transformation into extruded products is easy. 

This process has many advantages, as thermal control is excellent in water and the viscosity of the medium remains low and constant. Each droplet of monomer is converted directly into a polymer bead. Provided that the size of the droplets is well controlled, polymer beads of defined size, e.g. in the range 10-1000 pm, are obtained at the end of conversion. This permits easy storage and feeding of moulding machines for transformation into objects. An example of an industrial process for suspension polymerisation is presented in Figure 3.9. 

Let us consider that monomer A is bound faster than B to active centres. The result is that the concentration of A in the monomer mixture decreases when conversion increases. Consequently, the composition of copolymers formed at the beginning and at the end of conversion is very different when nothing is done to compensate for the preferential consumption. From an industrial point of view, such heterogeneity in the composition of a batch of copolymer can be a real drawback. 

End of the 1980 s first stage in the introduction of heavy ends conversion... 

The propagation of polymer chains is easy to consider under stationary-state conditions. As the preceding example illustrates, the stationary state is reached very rapidly, so we lose only a brief period at the start of the reaction by restricting ourselves to the stationary state. Of course, the stationary-state approximation breaks down at the end of the reaction also, when the radical concentration drops toward zero. We shall restrict our attention to relatively low conversion to polymer, however, to avoid the complications of the Tromms-dorff effect. Therefore deviations from the stationary state at long times need not concern us. 

The preheated gases react exothermically over the first-stage catalyst with the peak temperature ia the range of 330—430°C, depending on conditions and catalyst selectivity. The conversion of propylene to waste gas (carbon dioxide and carbon monoxide) is more exothermic than its conversion to acroleia. At the end of the catalyst bed the temperature of the mixture drops toward that of the molten salt coolant. 

Residual monomers in the latex are avoided either by effectively reacting the monomers to polymer or by physical or chemical removal. The use of tert-huty peroxypivalate as a second initiator toward the end of the polymeri2ation or the use of mixed initiator systems of K2S20g and tert-huty peroxyben2oate (56) effectively increases final conversion and decreases residual monomer levels. Spray devolatili2ation of hot latex under reduced pressure has been claimed to be effective (56). Residual acrylonitrile also can be reduced by postreaction with a number of agents such as monoamines (57) and dialkylamines (58), ammonium—alkali metal sulfites (59), unsaturated fatty acids or their glycerides (60,61), their aldehydes, esters of olefinic alcohols, cyanuric acid (62,63), andmyrcene (64). 

Some control over the spHt between methyl radical oxidation (to HCHO) and dimerization in heterogeneous oxidation can be achieved by varying conditions (116). For homogeneous oxidation, an efficiency of 70—80% to methanol has been claimed at 8—10% conversions (110). This is the high end of the reported range and is controversial. Even so, such technology appears unlikely to be competitive for regular commercial use until further advances are made (117). The critical need is to protect the products from further oxidation (118). 

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