Temperature excess

Factors such as reaction temperature, excess of oxygen, water addition, addition of other minor reactants, eg, AlCl to promote the formation of mtile, mixing conditions inside the reactor, and many others influence the quaUty of Ti02 pigment. In general, titanium white pigments produced by the chloride process exhibit better lightness than those produced by the sulfate process. 

The packaging (qv) requirements for shipping and storage of thermoplastic resins depend on the moisture that can be absorbed by the resin and its effect when the material is heated to processing temperatures. Excess moisture may result in undesirable degradation during melt processing and inferior properties. Condensation polymers such as nylons and polyesters need to be specially predried to very low moisture levels (3,4), ie, less than 0.2% for nylon-6,6 and as low as 0.005% for poly(ethylene terephthalate) which hydrolyzes faster. 

Plants are constantly subject to adverse environmental conditions such as drought, flooding, extreme temperatures, excessive salts, heavy metals, high-intensity irradiation and infection by pathogenic agents. Because of their immobility, plants have to make necessary metabolic and structural adjustments to cope with the stress conditions. To this end, the expression of the genetic programme in plants is altered by the stress stimuli to induce and/or suppress the production of specific proteins which are either structural proteins or enzymes for specific metabolic pathways. 

Since NO is a precursor in NDMA formation and high combustion temperatures (usually from 1500 to 1800 0) yield high reaction rates between oxygen and nitrogen, a decrease in NDMA formation can also be achieved by lowering the flame temperature. Excess air seems to be the most economic way to reduce flame temperature and NO synthesis. In a new type of burner developed on this principle the resulting air had only 0.05 - 0.1 mg NO /m as compared with 14 mg/m in conventional burners. Accordingly, malt dried with such burners contains only 1 to 3 mg/kg NDMA, a 15-30 fold reduction of the NDMA concentration. 

Under the Food Quality Protection Act (FQPA), the U.S. EPA evaluates the potential for people to be exposed to more than one pesticide at a time from a group of chemicals with an identified common mechanism of toxicity. As part of the examinations, to clarify whether some or all of the pyrethroids share a common mechanism of toxicity, a comparative FOB (functional observational battery) studies with 12 pyrethroids were carried out under standardized conditions [15]. The FOB was evaluated at peak effect time following oral administration of non-lethal doses of pyrethroids to rats using com oil as vehicle. Four principal components were observed in the FOB data [22], Two of these components described behaviors associated with CS syndrome (lower body temperature, excessive salivation, impaired mobility) and the others described behaviors associated with the T syndrome (elevated body temperature, tremor myoclonus). From the analysis, pyrethroids can be divided into two main groups (Type I T syndrome and Type II CS syndrome) and a third group (Mixed Type) that did not induce a clear typical response. Five other pyrethroids were also classified by an FOB study conducted in the same manner [16]. The results of these classifications are shown in Table 1. The FOB results for all non-cyano pyrethroids were classified as T syndrome, and the results of four ot-cyano pyrethroids were classified as CS syndrome however, three of the ot-cyano pyrethroids, esfenvalerate, cyphenothrin, and fenpropathrin, were classified as Mixed Type. 

Diaryl ditellurides (general procedure) The aryltellurium trichloride is poured into 15 mol equiv of Na2S-9H20 heated (and melted) at 95-100°C in a beaker, with strong manual stirring. An exothermic reaction occurs. After 10 min at the same temperature, excess HjO is added. By cooling the ditelluride solidifies, and is collected by decantation or filtration. The yields are near to quantitative. 

Similarly, the barbituric acids 40a-f (1.00 mmol) were coground with five portions of 291a (1.00 mmol in total). The mixture was transferred to a 100-mL flask which was then evacuated. MegN (0.5 bar) was let in. After 12 h at room temperature, excess gas was recovered in a remote trap at 77 K. The salt was washed away with water (20 mL) and the residual solid dried. The yields were 100,100,100,100,98, and 99%, respectively. 

It is well known that halide anions lower the thermal stability of ILs due to their nucleophilic nature and their ability to decompose by S l or 5 2 nucleophilic decomposition [20]. ILs that are not thoroughly purified and examined using ion chromatography for the presence of trace levels of halide anions will produce significant colunm bleed at relatively low-column temperatures. Excessive decomposition/volatilization of [C4QIm]Cl has been observed starting at 145°C [21]. 

After the autoclave was allowed to cool to room temperature, excess hydrogen was released carefully. The resulting mixture was evaporated, and the residue was purified by flash column chromatography over silica gel (eluent ethyl acetate/hexane= 1/20-1/5) to give (5)-3-methyl-A-(p-toluenesulfonyl)indoline... 

A reactor charged with 50 ml triethyl phosphite was heated to 140°C and then treated with the dropwise addition of 2,7,9-tribromofluorene (30.63 g) dissolved in 100 ml of hot o-xylene. The mixture was heated to 159°C for 30 minutes while distilling off ethylbromide and o-xylene. The mixture was then cooled to ambient temperature, excess triethyl phosphite removed under reduced pressure, and a light orange color residue obtained. The product was isolated after recrystallization in cyclohexane as colorless crystals, mp = 119.0-119.5°C. 2... 

Volatility or readiness with which a substance vaporizes, is an undesirable characteristic for military explosives. Explosives must be no more than slightly volatile at the temperature at which they are loaded or at the highest storage temperature. Excessive volatility often results in the development of pressure within the rounds of ammunition and separation of mixtures into their constituents. Volatility also affects chemical composition of the explosive resulting in the marked reduction in stability leading to an increase in the danger of handling. 

The evolution in time of the concentration of the species A and of the temperature rise AT, for the example data in Table 4.1, is shown in Fig. 4.1. The behaviour is in many ways similar to that of the isothermal cubic autocatalysis model of the previous chapters. The concentration of the precursor P decreases exponentially throughout the reaction. The temperature excess jumps rapidly to approximately 80 K, from which value it begins to decay approximately exponentially. At the same time, the concentration of the intermediate A rises relatively slowly to values of the order of 10"i mol dm-3. After approximately 15 s, the concentration of A and the... 

Fig. 4.1. Computed concentration and temperature histories for the thermokinetic model with parameters given in Table 4.1 showing monotonic decay of precursor reactant p but oscillations in the concentration of intermediate A and the temperature excess A7 (a) p(r), (b) a(t), and...

In this form, therefore, the reaction rate constant increases simply by a factor of 2.718 for every unit increase in the dimensionless temperature excess. If we take the data from Table 4.1, K7 a/E = 8K, so an 8K temperature rise causes an increase in the rate by e. 

FIG. 4.2. Dependence of dimensionless pseudo-steady-state intermediate concentration and temperature excess on reactant concentration for the model with the exponential approximation (a) three-dimensional representation of universal locus (b) projection showing dependence of a5S on n (c) projection showing... 

The pseudo-stationary temperature excess is clearly linearly proportional to the reactant concentration n and hence would show an exponential decrease in time ... 

Unstable stationary states can only be found therefore if the reaction rate constant kt(Ta) < e-2/rN. For any k less than this, eqn (4.49) has two solutions for the stationary-state temperature excess 0. For example, with k = 0.05 the roots can be found numerically and are 0 = 1.1594 and 4.1399. 

Fig. 4.4. The change in stability and growth of oscillatory solutions in the intermediate concentration and temperature excess as functions of the reactant concentration showing Hopf bifurcations at fi and n (parameter details as given in Table 4.1 except for y = 0) (a)...

Fig. 4.7. Stationary-state loci for the intermediate concentration and temperature excess for model with full Arrhenius temperature dependence (a) y = 0.175, showing maximum and minimum in aM(p) (b) the disappearance of extrema in the a (p) locus with increasing y. y = 0,...

We may view eqns (4.71)—(4.73) in another way. Choose a system with y < Next choose the dimensionless rate constant k. If k is less than (1 — 4y)e-2, eqn (4.71) can be solved to yield two positive roots 9 and 9. From these values for the stationary-state temperature excess we calculate the reactant concentration required for Hopf bifurcation from eqn (4.72) whilst (4.73) gives the stationary-state concentration of the intermediate A. 

The pre-oscillatory period, during which the temperature excess decreases and the concentration of A increases, will last at least until i has fallen from H0 to the value n, i.e. until the time tf given by... 

The rate at which the temperature excess d changes on the new timescale is given by dd/dT = g(cc, d)/c. Because e has been defined as a small parameter, this will be a large quantity unless the function g(oc, d) is close to zero. Thus we can expect that the temperature excess will try to adjust very quickly (i.e. while a changes very little) until. 

However, the temperature excess is large here, certainly 0B > 6C y 2, so the exponent on the right-hand side reduces to e 1/r. Using this we then have for point B... 

Fig. 5.11. Excitability in a chemical system, (a) The nullclines /(a, 0) = 0 and g(a,0) = 0 intersect just to the left of the maximum. A suitable perturbation must make a full circuit, as shown by a typical trajectory, before returning to the stable stationary state, (b), (c) The corresponding evolution of the concentration of intermediate A and the temperature excess in time showing the large-amplitude excursion.

For temperature, we can again recognize that we are more interested in the temperature rise rather than in T itself. We can use the same dimensionless temperature excess 9 as that introduced in chapters 4 and 5, with a slight modification. Instead of basing 9 on the ambient temperature Ta we will base... 

This says simply that at complete reaction (ass - 0), the temperature excess attains its adiabatic value for 50 per cent reaction, 9SS = 0ad and so on. In fact, in this special adiabatic case the relationship between the extent of conversion and the temperature rise implied by eqn (7.20) holds for all conditions and not just at the stationary state. We will examine the adiabatic behaviour in some detail later, but first return to the more general case. 

Equation (7.28) only has real roots, and hence ignition and extinction can only occur if the discriminant under the square root sign is positive. This then makes a requirement on the size of the dimensionless adiabatic temperature excess 0ad. In particular, tangency only occurs if the reaction is sufficiently exothermic such that... 

The line retains the same slope as that given above, but its intercept moves up the 0ad axis as y increases, tending to infinity as y approaches The equation for the cusp is slightly more complex and is again most easily expressed parametrically. The appropriate values for the adiabatic temperature excess must be obtained from a quadratic equation before it can be used to determine tn. Thus, for any given y [Pg.196]

Here F represents the functional form of the left-hand sides of the various stationary-state equations, x is the stationary-state solution such as the extent of reaction, the temperature excess, etc., and rres is the parameter we have singled out as the one which can be varied during a given experiment (the distinguished or bifurcation parameter). All the remaining parameters are represented by p, q, r, s,. For example, in eqn (7.21) the role of x could be played by the extent of reaction 1 — ass, with p = 0ad and q = tN for isothermal autocatalysis, x can again be the extent of reaction, with p = P0, q = k2, and r = jcu. 

The dimensionless temperature excess and activation energy have the same forms as those used in chapter 4 6 = (T — TJE/RT2 and y = RTJE, where Ta is the ambient (reservoir) temperature. The dimensionless concentration X is simply c/c , where c0 is the reservoir concentration of the gaseous reactant. The group 3 is given by... 

We now consider the dependence of the stationary-state solution on the parameter d. To represent a given stationary-state solution we can take the dimensionless temperature excess at the middle of the slab, 0ss(p = 0) or 60,ss-With the above boundary conditions, two different qualitative forms for the stationary-state locus 0O,SS — <5 are possible. If y and a are sufficiently small (generally both significantly less than i), multiplicity is a feature of the system, with ignition on increasing <5 and extinction at low <5. For larger values of a or y, corresponding to weakly exothermic processes or those with low temperature sensitivity, the hysteresis loop becomes unfolded to provide... 

A different set of boundary conditions is that for which the concentration and temperature excess at the edge of the slab are determined by the two fluxes from the pellet. Thus, these Robin conditions have the form... 

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