Temperature run-away

Temperature run-away and ingnition can occur. Gas mixture must lie outside the explosive range... 

Nitration Hazards arise from the strong oxidizing nature of the nitrating agents used (e.g. mixture of nitric and sulphuric acids) and from the explosive characteristics of some end products Reactions and side reactions involving oxidation are highly exothermic and may occur rapidly Sensitive temperature control is essential to avoid run-away... 

Safe operation for many years does not prove that a reaction will not run away. Unknown to the operators, the plant may be close to the conditions under which it becomes unstable, and a slight change in pressure, temperature, or concentration, too small to cause concern, may take it over the brink. The operators are blind men walking along the edge of a precipice, as the following incidents illustrate. 

Use the computer program in Appendix 13.1 to explore thermal runaway. Feed pure styrene at 135°C and maintain a wall temperature of 300 K. At what tube diameter does the reactor run away Repeat... 

Sensitive temperature control is essential to avoid run-away... 

Besides the overall enthalpy of reaction, the rate of heat evolution at various temperatures is needed in order to design a process. It is desirable, though, whenever possible, to have a complete understanding of the kinetics of all of the reactions and to know the heat contributed by each. Extending the temperature of experimental measurements far above the desired and anticipated reaction temperature will give information about additional reactions that can occur if the reaction at lower temperature is allowed to run away. It is also desirable to get data on reaction streams with various concentrations of reactants. 

Both curves show the decrease in runaway temperature as the radius is increased. The plot also indicates that the reaction run at 50°C will not run away in a vessel of 10 cm radius, or probably 20 cm radius, as these points are below both curves. 

When the reactor is scaled up to 60 cm radius, however, the operating point is between the two curves. This means that the reaction can be safely run at 50°C in a well-agitated process vessel of 60 cm radius with the heat transfer coefficient as stated above becauseerating point is below the Semenov curve. In case the agitation is lost, however, the Frank-Kamenetskii curve becomes the better predictor of runaway temperatures, and because the operating point is above this curve, the estimate is that the reaction will run away. The calculation of the Frank-Kamentskii method is available in ASTME-1231 [166]. 

It is obvious, then, that only the H2—Cl2 reaction can be exploded photo-chemically, that is, at low temperatures. The H2—Br2 and H2—12 systems can support only thermal (high-temperature) explosions. A thermal explosion occurs when a chemical system undergoes an exothermic reaction during which insufficient heat is removed from the system so that the reaction process becomes selfheating. Since the rate of reaction, and hence the rate of heat release, increases exponentially with temperature, the reaction rapidly runs away that is, the system explodes. This phenomenon is the same as that involved in ignition processes and is treated in detail in the chapter on thermal ignition (Chapter 7). 

From this analysis one concludes that if one radical is formed at a temperature in a prevailing system that could undergo branching and if this branching system includes at least one chain branching step and if no chain terminating steps prevent run away, then the system is prone to run away that is, the system is likely to be explosive. 

The checkers found that reactions run on up to four tiroes the present scale and rectified using a molecular still (wall temperature 110-120°C, 0.10 mm) gave yields of 89-94%. Warning-. On this larger scale (i.e., four times the present scale) a brief run-away was experienced and some material which escaped from the condenser was caught in a trap however, the yield was still excellent (94%). 

The form taken by the resistivity rise at low temperatures in the ICF metallic phase has not yet been satisfactorily analyzed. Fig.3 shows that a p a exp relation makes fair representation of the data (38) for SmS at 20 kB. For lOkB, SmS is not properly converted at the lowest temperatures (Ref. 38, inset to Fig. 2) and the sigmoid deviation shown could be ascribed to this cause. However none of the data presently available for ICF metals (e.g. SmBg, TmSe), seems to express an exponential run away in resistivity to low temperatures. It is possible that the observed flattening off marks the limit set by surface conduction, via permanently and fully... 

The setpoint of the temperature control SP is ramped from 300 to 340 K over a period of time. The effect of this ramp rate is investigated below. If the ramp rate is too fast, the reactor temperature may run away because the heat removal system may not be able to remove the heat generated with the high initial concentrations of reactant A. If the ramp rate is too slow, the batch time will be long and therefore reduce productivity. 

The effect of controller gain is shown in Figure 4.4. The heat transfer area is 4 times the jacket area, and the ramp time is 60 min. The importance of controller tuning is clearly illustrated. If the gain is too small, temperature control is poor (for Kc = 0.05, there is a 20 K overshoot of the desired temperature for smaller gains, the reactor runs away). If the controller gain is too large, the response is very oscillatory. The Kc = 0.1 response shows a 6 K overshoot. 

Suppose the reactor has been started using the fast-fill-and-hold method and has reached a = 0.65 at 7 = 420 K. Continuous flow is started with ain= 1, Y , = 375 K, and Ts,i = 404K. Figure 14.4 shows the response. The temperature response is very rapid, but the conversion increases slightly during the first seconds of operation. Without temperature control, the reaction would have run away. The concentration is slowly evolving to its eventual steady-state value of about 0.26. There is a small offset in the temperature because the controller has no reset term. 

Parametric Sensitivity and Dynamics The global stability and sensitivity to abrupt changes in parameters cannot be determined from an asymptotic analysis. For instance, for the simple CSTR, a key question is whether the temperature can run away from a lower stable... 

With exothermic equilibrium reactions and endothermic reactions these requirements are less stringent since these reactions cannot run away, although here it has been recognized that a tight and uniform temperature control over the reactor cross section is also of advantage. 

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