Runaway reactions kinetics

Such calorimeters can successfully be used to test the stability of chemicals at certain temperatures (e.g., for safe storage or transport) or to investigate the runaway reaction kinetics of, say, explosives (Townsend and Tou, 1980). 

Runaway Reactions Runaway temperature and pressure in process vessels can occur as a resiilt of many fac tors, including loss of cooling, feed or quench failure, excessive feed rates or temperatures, contaminants, catalyst problems, and agitation failure. Of major concern is the high rate of energy release and/or formation of gaseous produc ts, whiai may cause a rapid pressure rise in the equipment. In order to properly assess these effec ts, the reaction kinetics must either be known or obtained experimentally. 

Several studies have been published to assess the kinetics of polymerization reactions at high temperatures. (irZ) However, most of these studies only describe experiments conducted at isothermal conditions. Only a few papers are based on adiabatic runaways. This paper is one of the first studies based on "first principles" characterizing adiabatic runaway reactions. 

If a reliable kinetic model and data on cooling capacity are at hand, runaway scenarios can be examined by computer simulations and only final findings have to be tested experimentally. Such an approach has been presented, e.g. by Zaldivar et al. (1992). However, the detailed reaction mechanism and reaction kinetics are rarely known. Therefore, thermokinetic methods with gross (macro-)kinetics dominate among methods for data... 

In most cases, a thermal runaway depends on the balance between heat generation and heat removal. When heat removal is insufficient, the temperature will increase according to the reaction kinetics. Gases may either be... 

The discussions in Sections 3.1 and 3.2 show that the interaction among enthalpies of reaction, reaction kinetics, and surrounding conditions is of paramount importance relative to the existence of potential thermal hazards such as runaways. Whereas valuable information on parameter sensitivity can be estimated by a theoretical approach, it remains of vital importance to evaluate hazards by appropriate and adequate laboratory tests to obtain information on the rates of heat and gas generation, and the maximum quantities of heat and gas involved. Materials which are real to the process should be used in tests to assure that the effects of any contaminants are recognized. 

Noronha, J. A., C. S. Brown, Jr., M. R. Juba, and J. Schmidt, "Kinetics of Runaway Reactions," CEP Technical Manual, Loss Prevention, 13 (1980), American Institute of Chemical Engineers, New York, NY. 

Scientists The QRA team may need specific data on the kinetics of the system being analyzed. For example, what contaminants can trigger a runaway reaction, at what temperature does the substance decompose, and what by-products result when the material is released to the air Chemists, physicists, researchers, and others may be needed to provide data. Expect to commit a few staff-weeks of effort during the project. 

Alternatively, with a knowledge of the reaction kinetics, it may be possible to rule out certain scenarios as being unlikely to produce the worst case. However, the reaction kinetics used to do this must be based on the reaction that actually occurs under runaway, rather than the reaction that is theoretically expected. A degree of testing of the reaction under, runaway is usually needed and, for. the information to be sufficiently reliable, this should normally have been obtained using a purpose built vent sizing calorimeter (see Annex 2). It is essential that personnel who are experienced in carrying out these tests are involved in the assessment. 

Use of sensors to measure gas phase NO2 concentration, electrical conductivity of the reaction mass, and gas phase temperatures at several critical points in semi-continuous nitration reactors permits safe operation of nitration processes [10]. The use of non-aqueous titration analysis in the control of nitration processes in explosives manufacture is discussed [11]. Counter-intuitively, safety of spent acids from nitrate ester production is decreased by lowered nitric acid content. This is because the runaway reaction is oxidation of alcohols, kinetically easier than that of the dissolved nitrate esters from which the alcohols are reformed by hydrolysis [16]. 

Even if this equation implicitly assumes a zero-order reaction, it was initially developed for zero-order kinetics, and may also be used for other reaction kinetics, giving a conservative approximation since the concentration depletion that would slow down the reaction is ignored (see Section 2.4.3). It gives realistic values for strongly exothermal reactions, as decomposition reactions often are. The calculation of TMRad, according to Equation 11.2, requires the specific heat capacity, the specific heat release rate (q 0) at the runaway initial temperature T0, and the activation energy. 

Different criteria have been introduced in the past decades to individuate runaway boundaries in batchwise operated reactors. Most of them can be used to ensure a safe batch operation only when the reaction kinetics is fully known and the hypothesis of perfect mixing is satisfied. These criteria also strongly depend on the mode of operation with respect to heat exchange. Excluding isothermal conditions, the following modes of operation can be considered ... 

As the scale of operation increases, the effect of the heat consumption by the plant typically declines. Therefore, the extent to which the kinetics of the runaway reaction is influenced by the plant is reduced. For plant scale vessels, the ())-factor is usually low (i.e., 1.0-1.2) depending on the heat capacity of the sample and the vessel fill ratio. Laboratory testing for vent sizing must simulate these low -factors. If the laboratory ( )-factor is high, several anomalies will occur ... 

Ahmed, M., Fisher, H. G., and Janeshek, A. M., Reaction Kinetics from Self-heat Data Correction for the Depletion of Sample, Proceeding of the Int., Symp. on Runaway Reactions, AIChE/ CCPS, Boston, MA, March 7-9, 1989. 

In the literature there are numerous runaway criteria with which operating ranges of high parametric sensitivity can be precalculated for known reaction kinetics [16, 59, 60]. In practice, however, these parameters are of only limited importance because they rarely take into account the peculiarities of individual cases. Sensitive reactions such as partial oxidation and partial hydrogenation are therefore generally tested in singletube reactors of the same dimensions as those in the subsequent multitubular reactor. This allows the range of parametric sensitivity to be determined directly. Recalculation of the results for other tube diameters is only possible to a limited extent due to the uncertainties in the quantification of the heat transfer parameters (see Section 10.1.2.4). 

Calorimetry Another category of laboratory systems that can be used for kinetics includes calorimeters. These are primarily used to establish temperature effects and thermal runaway conditions, but can also be employed to determine reaction kinetics. Types of calorimeters are summarized in Table 7-12 for more details see Reid, Differential Microcalorimeters, /. Physics E Scientific Instruments, 9 (1976). 

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