Runaway reactions onset

In scale-up, runaway exothermic chemical reactions can be prevented by taking appropriate safety measures. The onset or critical temperature for a runaway reaction depends on the rate of heat generation and the rate of cooling, which are closely linked to the dimensions of the vessel. 

To determine the onset temperature of a runaway reaction using calorimetry. 

The onset temperature is required when the. safety of the plant is to depend on preventing the onset of runaway — for example, process control (see Section 6.3, page 112). The rates of heat and gas production, and the maximum pressure developed, are required where safety is to be based on coping with the consequences of runaway. The problems associated with measuring heat generation and heat loss, described in Section 3.4.1 for the normal reaction, also apply to the runaway reaction. Accurate calorimetry is required in each case. 

The minimum temperature at which a runaway reaction will occur is not an absolute value. It is linked to the rate of heat loss from the system and depends markedly on the process conditions and scale of manufacture. Thus, the rate of heat loss due to natural cooling from a 501 reactor is of the order of 0.2 W kg K whereas a typical value for a 20 m- vessel is 0.04-0.08 W kg K . Accurate laboratory assessment of the minimum temperature for onset of runaway reaction requires equipment where the rate of heat loss is the same as it is in the full-scale process. 

In essence, the onset of runaway reactions in such systems depends not only on their size but also on their shape or symmetry. An increase in sample size reduces the heat loss from the reaction zone at the centre of the sample by acting as additional insulation. Since heat loss from the system occurs at the surface this change is proportional to the surface area, whilst the heat generation is proportional to the mass or volume of the substance. Thus the temperature at which decomposition occurs decreases as the size increases. 

While ACOMP gives the comonomer conversions, which those techniques also do, ACOMP additionally and simultaneously evolution of weight average molecular weight and [p], average polymer properties of critical importance in the nltimate characterization, and utilization of the polymers. Additionally, ACOMP can provide immediate detection of nnforeseen or unwanted phenomena such as microgelation, runaway reaction, dead end reaction, onset of tnrbidity, and so on. 

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. 

Freeder, B. G. et al., J. Loss Prev. Process Ind., 1988, 1, 164-168 Accidental contamination of a 90 kg cylinder of ethylene oxide with a little sodium hydroxide solution led to explosive failure of the cylinder over 8 hours later [1], Based on later studies of the kinetics and heat release of the poly condensation reaction, it was estimated that after 8 hours and 1 min, some 12.7% of the oxide had condensed with an increase in temperature from 20 to 100°C. At this point the heat release rate was calculated to be 2.1 MJ/min, and 100 s later the temperature and heat release rate would be 160° and 1.67 MJ/s respectively, with 28% condensation. Complete reaction would have been attained some 16 s later at a temperature of 700°C [2], Precautions designed to prevent explosive polymerisation of ethylene oxide are discussed, including rigid exclusion of acids covalent halides, such as aluminium chloride, iron(III) chloride, tin(IV) chloride basic materials like alkali hydroxides, ammonia, amines, metallic potassium and catalytically active solids such as aluminium oxide, iron oxide, or rust [1] A comparative study of the runaway exothermic polymerisation of ethylene oxide and of propylene oxide by 10 wt% of solutions of sodium hydroxide of various concentrations has been done using ARC. Results below show onset temperatures/corrected adiabatic exotherm/maximum pressure attained and heat of polymerisation for the least (0.125 M) and most (1 M) concentrated alkali solutions used as catalysts. 

A typical chronology for testing is shown on Table 1.2. The tests provide either qualitative or quantitative data on onset temperature, reaction enthalpy, instantaneous heat production as a function of temperature, maximum temperature, and/or pressure excursions as a consequence of a runaway, and... 

While the rationale for the above order remains to be well-understood, the authors speculated that the relative effectiveness and solubility of the SEI as well as the reactivity of these bulk solvents might be responsible.Overall, this study showed that the reaction between the electrolyte and the lithiated carbon could trigger thermal runaway, except at much higher onset temperatures than those of lithium electrodes. 

The seminal work by Maleki et al. in 1999 seemed to provide a direct answer to the question about which of the five possible processes was responsible for the thermal runaway of a lithium ion cell. Using ARC, they first determined the thermal runaway onset temperature in a lithium ion cell based on LiCo02/graphite with LiPFe/EC/DMC/DEC to be 167 °C. The thermal reaction, however, was found to start at 123 °C and continued to self-heat the system to the above onset temperature. Using DSC and TGA, they further determined the heat evolution as well as the thermal profile for the individual components of the cell in the presence of electrolytes, which included cathode, anode, and anode binder (PVdF). 

Differences in sample size and composition can also affect heating rates. In the latter case, this particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low-molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. When working under pressure, it is essential to measure pressure. This can be used for reaction control. If pressures fall beyond acceptable upper and lower limits or the rate of pressure rise exceeds a tolerable value, operating software should automatically shut down the machine. In combination with efficient cooling this approach can avoid thermal runaways near their onset. 

In these equations, e represents the relative volume increase due to the feed and Rh the ratio of the heat capacities of both liquid phases. By representing the reactivity number as a function of the exothermicity number (Figure 5.3), different regions are obtained. The region where runaway occurs is clearly delimited by a boundary line. Above this region, for a high reactivity, the reaction is operated in the QFS conditions (Quick onset, Fair conversion and Smooth temperature profile) and leads to a fast reaction with low accumulation and easy temperature control (see Section 7.6). 

In adiabatically operated industrial hydrogenation reactors temperature hot spots have been observed under steady-state conditions. They are attributed to the formation of areas with different fluid residence time due to obstructions in the packed bed. It is shown that in addition to these steady-state effects dynamic instabilities may arise which lead to the temporary formation of excess temperatures well above the steady-state limit if a sudden local reduction of the flow rate occurs. An example of such a runaway in an industrial hydrogenation reactor is presented together with model calculations which reveal details of the onset and course of the reaction runaway. 

The ultimate purpose of these types of tests is to evaluate two similar (in results) but different occurrences. These are runaway chemical reactions and exothermic chemical decompositions. The first may actually just be a desired reaction out of control while the second is an undesired reaction out of control. Among the purposes which analytical tests serve are the determination of the "onset" of exothermic (endothermic) decomposition. While frequently a specific temperature is cited for such "onsets," one must remember that this temperature is highly dependent on instrument sensitivity, degree of adiabaticity and time-temperature history. It should be stated that tests results are accurate only for the exact conditions under which they were run. Physical factors such as density and geometry can also influence test data. In theory, reaction rates are not a step function but are continuous. 

The difTerence in the mechanism of autoacceleration, depending on the size of the test tubes, changed the character of the PMMA produced in the reaction system. As shown in Figure 3, both M and MJM increased in the smallest test tube after the onset of autoacceleration. This is a typical phenomenon for the Trommsdorff effect, since it is led by the retardation of the termination reaction. Increase of M of the polymer, however, was not observed in the larger test tubes. Thermal runaway led to rapid decomposition of the initiator to produce more radicals transiently. This process produced more polymers having a lower degree of polymerization, lowering M,. 

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