Effective quench rating of common emergency actions used to mitigate temperature excursions on hydrocracker first-stage reactor
Emergency action
First stage equivalent
Total quench to reactor
Assumptions and
quench effect in 5 minutes
in 5 minutes
comments
Cut back on make-up hydrogen flow to sag system pressure by 200-300 psig Open feed/effluent exchanger bypass to
8°F on 5 beds (each)
40°F
Lower pressure reduces reactions on all beds Can open bypass 20%
90°F on 1st bed
90°F
remove heat from reactor inlet
within 5 minutes
Chop reactor feed furnace to minimum fires (or pilots) to remove heat from reactor inlet
30°F on 1st bed
30°F
Reduce furnace heat to 25%
of normal
Add additional quench to 1 bed Add additional quench to 2 beds
25°F on 1 bed 25°F on 2 beds 30°F on 1st bed
25°F 50°F 20°F
Apply maximum quench to 1 bed Apply maximum quench to 2 beds Reduce olefin saturation in 1st bed
Cut out cracked stocks from feed to reduce exothermic heat produced in reactor Chop reactor feed furnace completely to
10°F on 1st bed
10°F
Further reduce furnace heat to zero
remove heat from reactor inlet
Stop feed to unit to reduce reactions in reactor
Neutral
0°F
Top bed cools down – other beds heat up
Table 2
the combination of emergency actions that are most effec- tive and facilitate the easiest recovery. Emergency actions – quantitative ranking There are quantitative rankings of emergency actions that can be taken prior to automatic depressuring. Although the qualitative ranking of emergency actions is valuable, it does not help to understand whether each action or combina- tion of actions will be adequate to stop a reactor tempera- ture excursion before it becomes a reactor runaway event. Therefore, it becomes necessary to rate the effectiveness of each emergency action based on how many degrees of ‘quench’ is provided by each action. The reactor inlet temperature is controlled by the outlet temperature of the reactor feed furnace. The inlet temper- atures of other beds in the reactor are controlled by the amount of quench gas applied between each bed. Although the amount of quench is frequently described by the flow of quench gas, it can also be described by how much temper- ature change occurs due to the application of quench gas to each bed. As an example, a typical two-stage hydrocracker unit is used as the basis for quantifying the effect of each emer- gency action. The unit can be described as follows: Two-stage hydrocracker: • One five-bed reactor in each stage • Reserve compressor capacity exists to double-quench on nomore than two beds • Sum of bed DT = 250°F in each stage, reactor inlet/outlet DT = 150°F, total quench = 100°F (25°F/bed) • F irst-stage feed furnace DT = 40°F, first-stage feed/effluent DT = 450°F, second-stage feed furnace DT = 40°F, second- stage feed/effluent DT = 350°F Feed = 30% LCO
Note that for the typical hydrocracker assumed in this example, the total quench supplied to each reactor is about 100°F (56°C) during normal operation. Tables 2 and 3 show the list of emergency actions that can be taken along with an estimate of the equivalent quench provided by each action within five minutes. Since these actions are intended to prevent a temperature excur- sion from turning into a runaway event, effective actions are only those actions that can be affected within five minutes. Experience shows preventative actions must be taken within five minutes to be effective in stopping a temperature excursion from progressing to a runaway event Experience shows that these preventative actions must be taken within five minutes to be effective in stopping a temperature excursion from progressing to a runaway event. In other words, if you cannot stop a severe reactor temperature excursion within five minutes, you will likely end up depressuring the unit. Please note that the ratings in Tables 2 and 3 apply to the example unit. Each unit should be ranked using the methodology described in the ‘Assumptions’ and ‘Comments’ columns of both tables. Quantitative quench rating of emergency actions The most effective emergency actions (in order of descend- ing quench) are shown in Table 4 . Other observations to consider when evaluating the most effective emergency actions include: • Not all hydrocrackers have a bypass on the feed/effluent exchangers with a control valve to open the bypass from the control room. The analysis in this discussion suggests
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PTQ Q3 2023
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