What is the difference between a temperature excursion and a runaway? Temperature excursion is defined as a sudden increase in reactor bed or skin temperatures of 15°F (8°C) or when any reactor temperature exceeds the maximum safe reac- tor temperature limit. An increase of 15°F corresponds to a doubling of the reaction rate for a zeolitic hydrocracking catalyst system, which has an activation energy of about 50-70 kcal/gmol. As you can imagine, doubling the amount of hydrocrack- ing reactions in a reactor gives a significant increase in heat release, which requires an immediate response to prevent a temperature excursion from becoming a runaway event.
cracking threshold, the amount of heat release increases tremendously, further increasing the reaction rate. The reactor temperatures spiral upward as rising heat causes increasing thermal cracking, which further increases heat release. This upward spiralling of temperatures can happen extremely fast. In the TOSCO runaway incident cited earlier, the reactor went from a high-temperature alarm to ruptur- ing the outlet pipe in approximately 7.5 minutes. It should be noted that thermal cracking by itself is an endothermic reaction, much like the reactions in a coker or FCC. When the newly formed light molecules that result from thermal cracking are hydrogenated, there is an imme- diate and highly exothermic heat release. Stopping hydro- genation by depressuring will prevent a runaway event by taking away the hydrogen that causes the exothermic heat release. It should also be noted that hydrotreaters can have run- away events. Because the activation energy of hydrotreat- ing catalyst is lower than that of hydrocracking catalyst, hydrotreaters tend to ‘walk away’ rather than run away. However, once reactor temperatures approach the point where thermal cracking occurs, a hydrotreater will run away just like a hydrocracker. Therefore, the bottom line is to stop a temperature excursion emergency before it turns into a runaway emergency. Emergency actions – qualitative ranking Refiners employ a variety of actions to mitigate a temper - ature excursion and prevent it from developing into a reac- tor runaway. Some of the common emergency actions are listed in Table 1 . Table 1 also lists the effect of each emergency reaction – whether it affects the entire reactor or just a localised area of one or two beds. Also listed is the relative ease of recovery from the emergency back to normal operation. Recovery for these actions ranges from an easy recovery that takes place within a shift to a difficult recovery that can take two days or more for two-stage hydrocracking units. On the qualitative rating basis shown in Table 1, the most desirable emergency actions have the biggest effect and the easiest recovery. However, these emergency actions should not be considered independent of each other. Therefore, the most desirable outcome would be to select
Stopping hydrogenation by depressuring will prevent a runaway event by taking away the hydrogen that causes the exothermic heat release
A reactor runaway is defined as any time reactor bed or skin temperatures climb beyond control. In this case, immediate emergency action is required to prevent loss of containment. In both hydrocracking and hydrotreating units, as reactor temperatures approach 850°F (450°C), thermal cracking begins to occur. Thermal cracking is very different than normal hydrocracking in that a hydrocarbon molecule fully disintegrates into methane rather than just cracking into two or three pieces. You can imagine what would happen if you suddenly changed the feed from vacuum gasoil to unsaturated light gasoline or LPG at the same temperature. The amount of heat release would increase substantially. The hydrogenation of the cracked molecule ‘parts’ that follow thermal cracking is extremely exothermic compared to normal hydrocracking or hydrotreating reactions. It is estimated that thermal cracking and subsequent hydro- genation evolve heat at 8-15 times the rate of normal hydrocracking. Therefore, once a reactor temperature excursion occurs and temperatures in the reactor approach the thermal
Common emergency actions used to mitigate temperature excursions
Emergency action
Immediate effect – whole reactor
Immediate effect
Ease of
– local only
unit recovery
Cut back on makeup hydrogen flow to sag system pressure by 200-300 psig Open feed/effluent exchanger bypass to remove heat from reactor inlet
X
Easy Easy Easy Easy
X X X X X X
Chop reactor feed furnace to minimum fires (or pilots) to remove heat from reactor inlet
Add additional quench to hot bed or beds
Cut out cracked stocks from feed to reduce exothermic heat produced in reactor Chop reactor feed furnace completely to remove heat from reactor inlet
Moderate
Difficult Difficult Difficult
Stop feed to unit to reduce reactions in reactor Fully depressurethe unit to stop exothermic reactions
X
Definitions: Easy Recovery = Within a shift, Moderate Recover = Within a Day, Difficult Recovery = Within 2 Days
Table 1
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PTQ Q3 2023
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