Runaway adiabatic

Accelerating Rate Calorimeter (ARC) The ARC can provide extremely useful and valuable data. This equipment determines the self-heating rate of a chemical under near-adiabatic conditions. It usu-aUy gives a conservative estimate of the conditions for and consequences of a runaway reaction. Pressure and rate data from the ARC may sometimes be used for pressure vessel emergency relief design. Activation energy, heat of reaction, and approximate reaction order can usually be determined. For multiphase reactions, agitation can be provided. 

Transient Studies in an Adiabatic Packed-Bed Reactor was the title of a publication by Berty et al (1972). This was in connection with thermal runaway of reactors. The pertinent subject will be discussed in a following chapter in which the interest is focused on how to avoid the onset of a runaway. Here the object of the experiment was to see what happens after a runaway has started. 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

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. 

The temperature counterpart of Q>aVR ccj-F/R and if ccj-F/R is low enough, then the reactor will be adiabatic. Since aj 3>a, the situation of an adiabatic, laminar flow reactor is rare. Should it occur, then T i, will be the same in the small and large reactors, and blind scaleup is possible. More commonly, ari/R wiU be so large that radial diffusion of heat will be significant in the small reactor. The extent of radial diffusion will lessen upon scaleup, leading to the possibility of thermal runaway. If model-based scaleup predicts a reasonable outcome, go for it. Otherwise, consider scaling in series or parallel. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

There is a potentially dangerous reaction of carbon tetrachloride with dimethylformamide in presence of iron. The same occurs with 1,2,3,4,5,6-hexachlorocyclohexane, but not with dichloromethane or 1,2-dichloroethane under the same conditions [1], A quantitative study of the reaction by DSC and ARC techniques shows that in a 1 1 wt. mixture with carbon tetrachloride in absence of iron, an exothermic reaction sets in below 100°C. Under adiabatic conditions, the heat release (207.6 J/g) would take a runaway reaction to over 240°C. In presence of 3% of iron powder, the same mixture shows 2 exotherms, one at 56°C (108 J/g) and the second at 94°C (275 J/g), a final adiabatic temperature exceeding 285°C being possible [2], Dimethylacetamide behaves similarly but more so. 

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. 

Not even traces of strong acids should be added to high strength solutions of the hydroperoxide [1], The thermal stability and mechanism of cleavage of mixtures with 4-toluenesulfonic acid have been studied under adiabatic conditions, and there is potential for development of a thermal runaway [2]. 

After a thermal runaway reaction dining chlorination in DMF solution, investigation revealed that saturated solutions of chlorine in DMF are hazardous, and will self-heat and erupt under either adiabatic or non-adiabatic conditions. Principal products are tetramethylformamidinium chloride and carbon dioxide, with dimethylammonium chloride and carbon monoxide in small amounts. A detailed account of the mechanism is to be published. 

DTA shows that the reaction mixture from sulfonation of the nitro compound in 20% oleum, containing 35 wt% of 2-chloro-5-nitrobenzenesulfonic acid, exhibits 2 exothermic stages at 100 and 220° C, respectively, the latter being violently rapid. The adiabatic reaction mixture, initially at 89° attained 285°C with boiling after 17 h. At 180° the induction period was about 20 min [1], Sulfonation of 4-chloronitrobenzene with 65% oleum at 46°C led to a runaway decomposition... 

Several commercial calorimeters are available to characterize runaway reactions. These include the accelerating rate calorimeter (ARC), the reactive system screening tool (RSST), the automatic pressure-tracking adiabatic calorimeter (APTAC), and the vent sizing package (VSP). Each calorimeter has a different sample size, container design, data acquisition hardware, and data sensitivity. 

The runaway reactor is treated as entirely adiabatic. The energy terms include (1) energy accumulation resulting from the sensible heat of the reactor fluid as a result of its increased temperature due to overpressure and (2) the energy removal resulting from the vaporization of liquid in the reactor and subsequent discharge through the relief vent. 

Minimum exothermic runaway temperature Establish minimum temperature Adiabatic Dewar Adiabatic calorimetry ARC... 

Consequence of runaway reaction Temperature rise rates Gas evolution rates Adiabatic Dewar Adiabatic calorimetry Pressure ARC VSP/RSST RC1 pressure vessel... 

These tests can also be used to evaluate the induction time for the start of an exothermic decomposition, and the compatibility with metals, additives, and contaminants. The initial part of the runaway behavior can also be investigated by Dewar tests and adiabatic storage tests. To record the complete runaway behavior and often the adibatic temperature rise, that is, the consequences of a runaway, the accelerating rate calorimeter (ARC) can be used, although it is a smaller scale test. 

Figure 2.6D shows the temperature curve from a typical "heat-wait-search" operation of adiabatic calorimetry. The sample was held adiabatically at three temperatures without detecting self-heating. At the fourth step, selfheating was detected and, after a wait of 20 minutes, a runaway occurred. 

To get an idea of the possible effects of a runaway, it is useful to calculate or to determine experimentally the adiabatic temperature rise, and to consider the effect of this temperature increase on the system. An adiabatic temperature rise of 150°C or above is considered a strongly exothermic situation that could result in loss of containment. 

Techniques such as adiabatic calorimetry (Dewar calorimetry) were by then well established [2, 118, 119]. All these techniques can be used for obtaining data to design for the prevention of runaway reactions, that is, to design for inherent plant safety. 

The Reactive System Screening Tool (RSST), marketed by Fauske and Associates, is a relatively new type of apparatus for process hazard calorimetry [192, 196-198]. The equipment is designed to determine the potential for runaway reactions and to determine the (quasi) adiabatic rates of temperature and pressure rise during a runaway as a function of the process, vessel, and other parameters. 

For small vessels and slow reactions, corrections must be made because of the heat content of the reaction vessel itself. For large-scale reaction vessels and for rapid reactions, the system will be close to adiabatic operations. This aspect must be taken into account in scale-up. In effect, the extrapolation of data obtained in small-scale equipment has limitations as discussed in [193]. In case of a runaway, the maximum temperature in the reaction system is obtained from the adiabatic temperature rise, that is, Tmax = (Tr + ATad). In reality, the adiabatic temperature rise is significantly underestimated if other exothermic reaction mechanisms occur between Tr and (Tr + ATad). Therefore, a determination must be made to see if other exothermic events, which may introduce additional hazards during a runaway, occur in the higher temperature range. This can determine if a "safe operating envelope" exists. 

In most bench-scale reaction instruments, it is also possible to perform adiabatic experiments, although precautions have to be taken to avoid an uncontrollable runaway in the final stages. From these types of experiments, the temperature constraints at which, for example, side reactions or decomposition reactions start, together with the possible control requirements, can be obtained. If the adiabatic temperature rise may exceed, say, 50 to 100°C, it is safer to use other methods to obtain similar information, such as the DSC, ARC, or Sikarex, because these instruments use relatively small amounts, thereby decreasing the potential hazard of an uncontrollable runaway event in the test equipment. 

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