Temperature onset

The reaction onset temperature is the temperature where it is assumed significant fuel consumption begins. The onset temperature is expressed by [7] 

A = rate constant (pre-exponential factor from Arrhenius equation k = A exp (-E /RT), sec (i.e., for a first order reaction) B = reduced activation energy, K C = liquid heat capacity of the product (J/kg K) 

Tonset = onset temperature, K Xq = initial mass fraction AH[ = heat of decomposition, J/kg 

T = bulk heat-up rate driven by an external heat source, °C/sec TIME-TO-MAXIMUM RATE 

An onset temperature can be selected based on an arbitrary time-to-maximum-rate from the relation 

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As commercially pure materials, the ethyleneamines exhibit good temperature stabiUty, but at elevated temperatures noticeable product breakdown may result in the formation of ammonia and lower and higher mol wt species. This degradation becomes mote pronounced at higher temperature and over longer time periods. Certain contaminants, such as mineral acids, can lower the onset temperature for rapid thermal decomposition. The manufacturer should be contacted and thermal stabiUty testing conducted whenever ethyleneamines ate mixed with other materials. 

The evaluation of thermal stabihty requires the determination of the temperature at which an exothermic reaction occurs, the rate of such a reaction as a function of temperature, and the heat generated per unit mass of material by the reaction. In many cases, data on the increase of pressure during a reaction are also required, especially for vent sizing. The term onset temperature Toa%et is used in two contexts ... 

The second context is the process reac tor. There is a potential for a runaway if the net heat gain of the system exceeds its total heat loss capabihty. A self-heating rate of 3°C/day is not unusual for a monomer storage tank in the early stages of a runaway. This corresponds to 0.00208°C/min, 10 percent of the ARC s detection limit. ARC data for the stored chemical would not show an exotherm until the self-heating rate was 0.02°C/min. Therefore, onset temperature information from ARC testing must be used with considerable caution. 

Onset-of-reaction temperatures reported by the DSC are higher than the true onset temperatures, so the test is mainly a screening test. 

Differential scanning calorimetry (DSC) Onset temperature of exotherms, heat of reaction... 

Thermogravimetric analysis (TGA) Onset temperature of weight loss... 

If the decomposition reaction follows the general rate law, the activation energy, heat of decomposition, rate constant and half-life for any given temperature can be obtained on a few milligrams using the ASTM method. Hazard indicators include heats of decomposition in excess of 0.3 kcal/g, short half-lives, low activation energies and low exotherm onset temperatures, especially if heat of decomposition is considerable. 

Equation 12-5 gives an onset temperature that eorresponds to a time-to-maximum rate t. (min) using a sueeessive substitution solution proeedure. An initial guess of T = 350 K for the right side of Equation 12-5 will give a solution value of on the left side of Equation 12-5 within 1% or on an absolute basis 3°C. Con-vergenee is reaehed within several sueeessive substitution iterations. 

Onee the onset temperature is determined, it is then possible to obtain the maximum reaetion temperature by the adiabatie temperature rise and any eontribution due to external heat input. The theoretieal adiabatie temperature inerease is... 

Onset temperature The temperature at which the heat released by a reaction can no longer be completely removed from the reaction vessel... 

Fig. 7.10. The solid state reactivity of shock-modified zirconia with lead oxide as studied with differential thermal analysis (DTA) shows both a reduction in onset temperature and apparent increase in reaction rate. The shock-modified material has a behavior much like the much higher specific surface powder shown in B (after Hankey et al. [82H01]).

High-resolution dilatometric measurements have revealed the appearance of anisotropy in the cubic-phase thermal strain in the precursive temperature region for the soft-mode martensitic transformations in VaSi/ Ni-Al, In-Tl/ and SrTiOa In the case of Ni-Al and SiTiOa, the onset temperatures for the strain anisotropy are close to those at which the appearance of central peak behaviour occurs. 

Brosse, N. Pinto, M-F. Jamart-Gregoire, B. J. Chem. Soc., Perkin Trans. 1 1998, 22, 3685. Under nitrogen atmosphere, the thermal potential is slightly lower (AH = 2150 J/g), and the onset temperature is 150 °C. 

The next best in the rank solely based on the HjS release onset temperature is CeOj. Notice in Figure 2, however, that there is practically no take-off in HjS release over Sample E as long as the temperature remains at 530 C. This represents a situation where SOj tiapp on the surface or stored in the bulk [10] is released very slowly, thus creating a condition where the rate of SO emission control is limited by the number of SOj trapping sites. 

Considerably trailing behind VjOj but only slightly behind CeO is FejOj, which shows an onset temperature for HjS release of approximately 520 C. This means Fe Oj is not expected to be able to adequately catalyze step 3 in the FCCU riser environment because of short contact time [5], even though the temperature at the very bottom of the riser exceeds 530 C. In fact, the result of our pilot plant test of such a catalyst is in agreement with this assessment. Thus, it is quite clear that the onset temperature for HjS release is more critical than the rate of take-off in determining the catalyst efficiency for step 3. 

The result obtained from a Hj (5%)/Ar (95%) - TPR/MS in a soak-ramp mode test is shown in Figure 3 for a sample of DESOX. The onset temperature found for H S release in this case, approximately 580°C, is substantially higher than 450°C, the typical onset temperature found in the propane-TPR/MS test. The result was essentially identical in terms of the onset temperature for H S release even when undiluted was used as the reactant. Unlike the propane-TPR/MS tests, where the reaction products are essentially HjS only with virtually negligible amounts of SOj, Hj-TPR/MS tests always showed both SOj and HjS. These data, notably the pattern of change in the rates of SOj and H2S released with temperature in Figure 3, clearly demonstrate, as expected, that the reduction of S to S in step 3 is a consecutive reaction. 

The result obtained from a CH -TPR/MS test of the same DESOX sample is shown in Figure 4. As in Hj-TPR, the H S release is preceded by the evolution of a large amount of SOj. The onset temperature (580°C) of HjS evolution is also comparable to that observed in Hj-TPR. It has been presumed that syngas formation [11] is associated with the generation of HjS in this case. 

As we have already pointed out, the onset temperature for the reduction of SO, stored in any given catalyst varies with the source of hydrogen. This suggests that the rate of step 3 is limited by the supply of active hydrogen. Since hydrocarbons are the primary source of active hydrogen in the FCCU riser and can be readily reduced to [12], step 3 can be depicted as consisting of the following scheme of events ... 

Many kinetic data can be collected from ARC experiments the exothermic onset temperature, the rate of temperature rise, the rate of pressure rise, and the apparent activation energy. The basic data obtained are, however, thermodynamic properties the adiabatic temperature rise, the maximum pressure potential, the quantity of gaseous products generated, and the heat of reaction can be obtained in one run. The heat of reaction is estimated from ... 

Fig. 5.4-66 outlines the probability and consequences of a thermal runaway in case of a plant incident. For the solvent process, failure results in a temperature rise from 27 °C to 119 °C. This is far from the onset temperature of secondary processes, which only start at 150 °C or higher. Consequently, the solvent process can be considered safe. A failure of the water process can cause a temperature rise from 50 to 95 C, i.e. higher than the onset temperature (90 °C) of the secondary decomposition of the di-nitro compound. The decomposition would start before the reaction mixture started boiling. Hence, the water process cannot be considered inherently safe. 

By surface science techniques, Mullins and Overbury showed that the presence of reduced ceria might create new active sites for NO dissociation [82], The degree of decomposition is increased and the onset temperature for decomposition is reduced when Rh is supported on reduced ceria (Rh/CeOj compared to Rh on oxidized ceria (Rh/Ce02) NO dissociation being self-inhibiting. 

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