Runaway reactions accelerating

Changing process or mechanical conditions to reduce the potential for runaway reactions, accelerated corrosion or erosion, or other possible causes of undesirable events... 

Dropwise addition of the chloride caused a steady exotherm to 60-65°C when a runaway reaction accelerated to explosion. 

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. 

Thermal runaway reactions are the results of chemical reactions in batch or semi-batch reactors. A thermal runaway commences when the heat generated by a chemical reaction exceeds the heat that can be removed to the surroundings as shown in Figure 12-5. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase, and subsequently accelerates the rate of heat production. Thermal runaway occurs as follows as the temperature rises, the rate of heat loss to the surroundings increases approximately linearly with temperature. However, the rate of reaction, and thus the... 

The released energy might result from the wanted reaction or from the reaction mass if the materials involved are thermodynamically unstable. The accumulation of the starting materials or intermediate products is an initial stage of a runaway reaction. Figure 12-6 illustrates the common causes of reactant accumulation. The energy release with the reactant accumulation can cause the batch temperature to rise to a critical level thereby triggering the secondary (unwanted) reactions. Thermal runaway starts slowly and then accelerates until finally it may lead to an explosion. 

The most recent major expln in a US TNT plant occurred in May 1974 at the Radford Army Ammunition Plant. The accident completely destroyed one of the three continuous nitration lines at the plant. According to the AMC News, Sept 1974, the investigation board reported that an operator inadvertently introduced a 5 to 6-foot rubber hose to clean out unwanted material that had collected in a transfer line leading to the nitrator, when the hose was pulled from his hands into the nitrator. This resulted in a rapid temp rise and subsequent explosion. The hose was commonly used in this manner . The material causing the blockage in the transfer line was believed to be an oxidation product of TNT, 2,2 -dicarboxy-3,3, 5,5,-tetra-nitroazoxybenzene, also referred to as White Compound. The introduction of the rubber hose caused a rapid, exothermic oxidation reaction between the hose material and the mixed acid present. The heat generated by this reaction caused a local acceleration of the normal nitration/oxidation reactions which occur in the nitrator until a critical temp was reached, at which point rapid oxidation of DNT/TNT proceeded as a runaway reaction, igniting the material present in the vessel. 

Finally, the oxidation reaction has to been run under strict conditions of temperature, which are impossible to be operated in a batch reactor. Indeed, utility stream in the Shimtec reactor was heated to 47 °C, which first initiates the reaction, accelerates its kinetics, and then controls the temperature when the heat of the reaction is too important. In a batch reactor, working with such UF temperature is impossible because of security constraints. It would certainly lead to a reaction runaway. We now consider this question in the next section. 

The molten material, after holding for 4 h at 78°C in a stainless steel vessel, underwent a thermal runaway reaction and 500 kg erupted through the vent line. It was later found that addition of 0.1% of rust to the hot material led to an accelerating self-condensation Friedel-Craft reaction, catalysed by iron(III) chloride, which led to formation of poly-benzyls accompanied by evolution of hydrogen chloride. 

Typical pressure versus time curves for runaway reactions are illustrated in Figure 8-2. Assume that an exothermic reaction is occurring within a reactor. If cooling is lost because of a loss of cooling water supply, failure of a valve, or other scenario, then the reactor temperature will rise. As the temperature rises, the reaction rate increases, leading to an increase in heat production. This self-accelerating mechanism results in a runaway reaction. 

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 investigation team found that the reaction accelerated beyond the heat-removal capacity of the reactor. The resulting high temperature led to a secondary runaway decomposition reaction, causing an explosion that blew the hatch off the reactor and allowed the release of the contents from the vessel. 

Runaway reactions can be triggered by a number of causes, but, in most cases., their resultant features after initiation are similar [31]. Whenever the heat production rate exceeds the heat removal rate in a reaction system, the temperature begins to rise and can get out of control. The runaway starts slowly but the rate of reaction accelerates, and the rate of heat release is very high at the end. Most runaways occur because of self-heating with the reaction rate (and reaction heat output) increasing exponentially with temperature, while the heat dissipation is increasing only as a linear function of the temperature. 

Information gained from simulations can reveal key insights that explain gaps or contradictions in information. The time line is a useful tool in this development. For incidents of unexpected chemical reactions, it is common to attempt a lab scale simulation of the conditions involved in the exotherm or explosion. Many chemical processes can be modeled and duplicated dynamically by computer algorithms. Accelerated rate calorimeters (ARC) have proven to he highly useful tools for studying exothermic or overpressure runaway reactions. 

See Lead(IV) oxide Carbon black, Chlorinated paraffin, Manganese(IV) oxide See AGITATION INCIDENTS, CATALYTIC IMPURITY INCIDENTS, RUNAWAY REACTIONS See also ACCELERATING RATE CALORIMETRY, REACTION SAFETY CALORIMETRY... 

Adiabatic conditions may be achieved either by a thermal insulation or by an active compensation of heat losses. Examples are the Dewar calorimeter, achieving a thermal insulation [2-4] or the Accelerating Rate Calorimeter (ARC) [5] or the Phitec [6], using a compensation heater avoiding the heat flow from the sample to the surroundings. These calorimeters are especially useful for the characterization of runaway reactions. 

Thanks to its versatility, this model has proved to describe a great number of autocatalytic reaction systems [5]. Systems with a slow initiation reaction are called strong autocatalytic. Because the rate of the initiation reaction is low, product is formed slowly, leading to a long induction time under isothermal conditions. For such systems, the initial heat release rate is low or practically zero. Consequently, the reaction may remain undetected for a relatively long period of time (Figure 12.4). When the reaction accelerates, such an acceleration appears suddenly and may lead to runaway. A strong autocatalytic reaction is formally equivalent to a Prout-Tompkins mechanism. 

Accelerating Rate Calorimeter (ARC). The ARC is sturdily constructed for the main purpose of simulating runaway reaction conditions on a small scale, typically using a 2 to 5 g sample. The sample is heated to a predetermined starting temperature in a spherical metal bomb. The sample is allowed to incubate at this temperature while the instrument control system scans for initiation of an exotherm. If no exothermic activity is found, the sample temperature is raised, and the wait-exotherm search routine is... 

Some materials are naturally unstable and can spontaneously decompose, polymerize, or undergo other reactions. These reactions can be initiated or accelerated by promoters such as light, heat, sources of free radicals or ions, or catalysts such as metal surfaces. These reactions can sometimes be retarded by adding inhibitors or diluents. Reactions of this kind are usually exothermic and if allowed to proceed will lead to a runaway reaction with serious consequences. 

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