Effective quench rating of common emergency actions used to mitigate temperature excursions on hydrocracker second-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
10°F on 5 beds (each)
50°F
Lower pressure reduces reactions on all beds Can open bypass 20%
70°F on 1st bed
70°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 0°F on 1st bed
25°F 50°F
Apply maximum quench to 1 bed Apply maximum quench to 2 beds
Cut out cracked stocks from feed to reduce exothermic heat produced in reactor Chop reactor feed furnace completely to
0°F
Reducing cracked stocks will not affect 2nd stage
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 3
this is one of the most important emergency features every hydrocracker should have in place • Not all hydrocrackers have the ability to easily cut back on the make-up hydrogen flow, as described in Table 4. Since this is one of the few actions that affect the entire reactor, this feature should be installed in all hydrocrackers • It has been taught that chopping the reactor feed furnace fires to a minimum was a very important emergency action to take in a temperature excursion. Analysis in this article suggests that chopping the feed furnace to minimum fires is important. However, it is not the most important and effec - tive action that can be taken to stop a temperature excursion. • Many refiners have already automated some or all the emergency actions taken before or concurrently with depressuring. Automating the initial actions can be espe - cially valuable to aid operators in responding to a tempera- ture excursion. Stopping a temperature excursion How much quench is necessary to stop a temperature excursion? The main question to be answered is how much quench is needed to prevent a temperature excursion from becoming a runaway incident. Although it is likely that a temperature excursion will occur only in one bed initially, let
us assume the worst-case scenario of a temperature excur - sion occurring in all five beds simultaneously. If a temperature excursion = 15°F increase in a bed and we have an excursion in five beds, the total temperature increase will be approximately 15 * 5 = 75°F. Let us also assume that all the actions listed in Table 4 are not taken quickly enough, and we only get half of the effec - tive quench from those actions. This means that the effective quench from emergency actions will be 230/2 = 115°F in the first stage and 200/2 = 100°F in the second stage. Even half of the effective quench in both reactor stages is more than enough to quench the temperature excursion in all five beds (115° > 75°) and (100° > 75°). Therefore, the temperature excursion will be stopped in both stages. Automating emergency actions Since we have concluded that the recommended emer - gency actions are more than adequate to stop a temper - ature excursion (even in a worst-case scenario), a refiner must then consider how to ensure a board operator will be able to detect a temperature excursion and be sure to take the right actions in a timely manner. It is possible that the combination of appropriate alarms, effective procedures, and well-trained board operators can
Effective emergency actions
Emergency action
1st stage effective quench ( º F)
2nd stage effective quench ( º F)
Open the feed/effluent bypass
90 40 50 30 20
70 50 50 30
v Let the pressure sag by cutting back make-up hydrogen w Adding quench to 2 beds (above and below the hot spot) x Chopping the reactor feed furnace to minimum fires
y Cutting out cracked stocks from the unit
0
Total effective quench applied
230
200
Table 4
36
PTQ Q3 2023
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