Reactor runaway

Moderate (attenuation and limitation of effects) Use vacuum to reduce boiling point Reduce process temperatures and pressures Refrigerate storage vessels Dissolve hazardous material in safe solvent Operate at conditions where reactor runaway is not possible Place control rooms away from operations Separate pump rooms from other rooms Acoustically insulate noisy lines and equipment Barricade control rooms and tanks... 

Emergency shutdown Describes the procedure used to shut down the equipment if an emergency should occur. This includes major leaks, reactor runaway, and loss of electricity, water, and air pressure. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

By integrating (4.11) and (4.12), the concentration and temperature profiles in the reactor can be obtained, and conditions leading to reactor runaway can be investigated. Numerical solutions are required even for the simple kinetic scheme adopted here because of the nonlinear nature of the Arrhenius term. 

We also can generally state that major temperature control problems can and often do occur when the reactions are both exothermic and irreversible. These systems are not inherently self-regulatory because an increase in temperature increases the reaction rate, which increases temperature even further. The potential for reactor runaways is particularly high if the reactor is operating at a low level of conversion. The large inventory of reactant provides plenty of fuel for reaction runaway. These concepts will be quantitatively studied in later chapters. 

The key element in temperature control of chemical reactors is to provide sufficient heat transfer surface area or some other heat removal mechanism so that dynamic disturbances can be safely handled without reactor runaways. 

The tuning of the temperature controller is another important issue, which is explored below. We will see that a low controller gain leads to reactor runaways and a high... 

The dynamic simulations of the reactor by itself are shown in Figure 7.12. A very small increase in reactor inlet temperature produces a reactor runaway because of the high activation energy and high heat of reaction. Compare these results with those of the base case kinetic parameters shown in Figure 1.9b. 

In this section, we present examples to illustrate the usefulness of multi-mode homogeneous reactor models in predicting micromixing effects on yield and selectivity, reactor runaway, etc. 

Rupture disks are often used upstream of relief valves to protect the relief valve from corrosion or to reduce losses due to relief valve leakage. Large rupture disks are also used in situations that require very fast response time or high relieving load (for example, reactor runaway and external fire cases). They are also used in situations in which pressure is intentionally reduced below the operating pressure for safety reasons. 

For systems where the adiabatic temperature rise is low (as is the case considered here) the thermal spikes introduced by the flow reversals do not dramatically affect the reactor performance. However, the concentration of feed streams to such treatment reactors can fluctuate to a high level which can result in a high temperature thermal spike developing within the reactor. Pinjala, Chen, and Luss characterized this dynamic response and showed that reactor runaway could occur within the single-pass reactor. Their work is directly applicable to the RFR as the forced oscillations in the gas flow direction can result in a thermal spike formation at the beginning of each half cycle. Thus, there is a need to understand thermal stability within these systems. Further complicating the matter is the fact that the temperature spikes are very narrow and are thus difficult to detect using thermocouples or other sensors imbedded within the reactor. 

In the first case, the reactions of interest are those which are intrinsically fast and exothermic, but which are currently limited by the poor heat and mass transfer for rates achievable in a stirred pot. Existing technology routinely entails substantial hazardous process inventories, possible reactor runaway and indifferent product selectivity. Fast response reactors open up the possibility of switching to more severe process conditions which would be prohibited in conventional reactors in view of the tendency to degrade the product. It may therefore be possible to exploit a virtuous circle - short residence time -higher temperature - faster kinetics - smaller reactor - shorter residence time. 

In conventional fixed-bed reactors, catalyst particles of various sizes are often randomly distributed, which may lead to inhomogeneous flow patterns. Near the reactor walls, the packing density is lower than the mean value, and faster flow of the fluid near the wall is unavoidable. As a result, reactants may bypass the catalyst particles, and the residence time distribution (RTD) will be broadened. Moreover, the nonuniform access of reactants to the catalytic surface diminishes the overall reactor performance and can lead to unexpected hot spots and even to reactor runaway in the case of exothermic reactions. 

The rapid heat transfer allows nearly isothermal operation with a defined residence time. Therefore, undesired side reactions can be effectively suppressed. The formation of hot spots within the reactor and reactor runaway during fast, highly exothermic reactions can be avoided. As a consequence, higher operating temperatures are attainable, and the same conversion can be achieved with a smaller reactor volume and less catalyst. The smaller unit size in turn improves the energy efficiency, reducing the operational cost. 

In addition to appropriate mass transfer rates, sufficiently rapid heat transfer is essential to control the behavior of chemical reactors. For example, if the local rate of heat removal does not match the rate of heat produced by the chemical reaction, hot spots may form. Because reaction rates depend exponentially on temperature, reactor performance, product yield, and selectivity are strongly influenced by non-isothermicity. In the case of exothermic reactions, a steep local temperature increase may lead to reactor runaway. 

Scale-up of Stirred-Tank Batch Reactors-Runaway Reactions... 

A sensitivity analysis for different values of A show that a new steady state is reached at every new A but at an incicasmgly higher T. The required A depends on the defirution of reactor runaway. 

However, we can see that the small reactors have small Qma JQ ratios and are more difficult to control. For example, if we could operate a 60-lb-mol reactor at 200°F, we could achieve 72 percent yield of component B. However, the Qm JQ ratio of this reactor is only 1.15, indicating that temperature control will be poor and reactor runaways can easily occur. A 120-lb-mole reactor at 175°F has a Qm JQ ratio of 1.3 and will show better dynamic controllability, but the yield is only 66 percent. 

Criteria have also been presented by many other investigators, for example, Thomas (1961), Dente and Collina (1964), Hlavacek et al. (1969), Hlavacek (1970), Oroskar and Stem (1979), and a detailed review has been written by Morbidelli et al. (1987). In general, all other parameters being fixed, the chances of reactor runaway increase as the reaction order increases, the activation energy increases, or the inlet temperature of the reactants increases. 

Extremely high safety potential peculiar to this type of die RI is characterized by the fact that even when such initial events as containment destruction and primary circuit loss of tightness coincide, neither reactor runaway, nor explosion and fire occur, and the radioactivity yield is lower than that one which requires the population evacuation. 

Manufacturing industries in which batch reactor runaway incidents have been reported during the period 1962-1987... 

The main disadvantage of these screening tests is that the test conditions tend towards being isothermal (whereas the conditions in a reactor runaway are nearer adiabatic). This can mean that the tests are not always sufficiently sensitive, and that the measured onset temperature for thermal decomposition is a function of the sample heating rate. Also, the small sample size may lead to it being unrepresentative of plant materials, and evaporation losses can lead to errors unless sealed test cells are used. 

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