8.6.1. General
This unit is equipped with certain emergency shutdown controls which will automatically place the unit in a non-hazardous status should a major failure occur. The actions of the emergency shutdowns are responsible for protecting the personnel, the catalyst and equipment from heavy coking or irreparable damage.
Personnel and equipment protection also results from:
- Personnel having a satisfactory knowledge of the safe operating and shutdown procedures.
- A compliance with the safety rules in plant construction i.e. safety distances, adequate orientation, etc.
- The installation of adequate fire and gas detection devices and fire fighting equipment.
- Adequate operator safety awareness and procedures training.
The following special considerations are highlighted for operator awareness of the isomerization process.
- The unit contains benzene and tetrachloroethylene which are known health hazards.
- The unit contains HCl downstream of the first isomerization reactor inlet. The concentration is particularly high in the stabilizer overhead and in the LPG separator drum. For this reason, operator shall bypass LPG recovery section, for any upstream upsets, such as loss of H2, loss of feed, upset in stabilizer operation.
- The reactions are highly exothermic, mostly due to the benzene saturation and the
C7+ components hydrocracking. If the levels of these components in the feedstock
increase significantly, the hydrogenation reactor temperature could raise beyond a controllable range.
- A reactor temperature excursion could occur if the flow of liquid through a hot
reactor is too low or if the reactor temperatures are too high. Reactor feed rate should not decrease below 75% of design capacity (FT 1101).
- During an emergency situation in the stabilizer section, the stabilizer must not depressurize below the pressure of the scrubber. There is a check valve in the stabilizer overheads gas line to the scrubber and a static height to overcome but despite this water and caustic could back up into the stabilizer causing a potential corrosion problem.
- In the event a reactor temperature excursion occurs and the reactor temperatures
increase above 190°C, the reactors must be depressurized to avoid excessive hydrocracking reactions and to prevent mechanical damage. If for some reason the reactors cannot be depressurized through the stabilizer, only as a last resort should they be depressurized to the flare through the hydrocracking drain lines. It is recommended to block the reactors in prior to depressurizing and to start a cold nitrogen purge if necessary. Depressurization is started once the LPG recovery
section has been bypassed on off gas side (bypass is activated by I-101 emergency shut down interlock).
Concerning the catalyst preservation, operators must avoid:
- An excessive catalyst grain temperature which can change the structure of the alumina (> 700°C). To avoid damaging the catalyst structure, bulk temperature must never exceed 500°C. Note that the reactor design temperature (under design pressure) is much lower.
- The hydrocarbons presence without a sufficient hydrogen quantity which would lead
to a rapid coke deposit and the possible agglomeration of catalyst particles.
The following sections cover most of the emergencies that may have to face by the operator. All operating personnel must study and fully understand the steps to be taken in such situations prior to the unit start-up.
Some of these situations are handled by automatic shutdown trips. These trips must always be operational, bypassing must be kept to a minimum e.g. during start-up, transient periods etc…
The following procedures include all the actions to be undertaken by the operator assuming no action by the automatic shutdown devices.
8.6.2. Loss of Feed
Loss of feed to the isomerization reactors is an emergency situation. When feed flow stops, the feed in the reactor circuit will pass slowly through the reactors swept by the low flow of make-up gas. For a sudden decrease in feed flow, the temperature controller on the inlet will not act fast enough and the unit pressure will decrease causing the back pressure controller to close. The net effect is high severity in the reactors, over conversion and soaring reactor bed temperatures.
- Cut steam immediately from the reactor feed heater and close the bypass around feed/effluent exchanger. Bypass LPG recovery section on off gas side.
- Increase the make-up hydrogen flow to maximum (2nd hydrogen make up line).
- Stop the isomerization feed pumps at hydrogenation reactor flash drum.
- Slowly depressurize the reaction section to stabilizer (via control valve PV 1901).
Depressurize at a rate which allows stable pressure control of the stabilizer.
- Stop the C2Cl4 injection when the lead reactor temperature reaches 105°C.
- Monitor the reactor bed temperatures. If the hydrogenation reactor temperature is
below 190°C and stable, continue cooling with make-up gas and cool down to 65°C.
- If the reactor bed temperatures are still climbing, cut the hydrogen make-up, block in
the reactors and depressurize to flare through the stabilizer and the Scrubber. Purge with dry nitrogen.
8.6.3. Loss of Make-up Hydrogen
When make-up hydrogen flow is lost, reactor pressure will decrease immediately, no hydrogen will be available for hydrogenation or isomerization reactions. A hydrocracking increase may occur, possibly causing a temperature excursion and a coke deposits increase.
Actions:
- Cut steam immediately from the reactor feed heater.
- Cut the feed to unit.
- Increase ti maximum the recycle flowrate in the recycle air cooler. Close the recycle
air cooler bypass line. Start all fans at the air cooler.
- Bypass LPG recovery section on off gas side.
- Stop C2Cl4 injection when the isomerization lead reactor reaches 105°C.
- Stop the isomerization reactor pump at the hydrogenation reactor flash drum.
- Slowly depressurize the hydrogenation reactor via the associated flash drum.
- Slowly depressurize the reaction section to the Stabilizer to keep the back pressure
- Reduce the stabilizer pressure as necessary to maintain pressure control on the column and flow of bottom to Deisohexaniser column.
- If the reactor temperatures cannot be controlled and increase above 190°C during
this procedure, interlock I-101 must be activated (emergency shut down). 8.6.4. High Reactor Temperature
High isomerization reactor temperature could be the result of a high content of C7+ in the
feed or high catalyst severity due to low feed rates or poor temperature control of the reactor inlet feeds. In order to avoid a reactor temperature excursion:
- Avoid liquid feed rates less than 75% of design (FT 1101)
- Avoid larger than design quantities of benzene or C7+ in the feed. It is also important
to be aware of changes in these contents due to crude changes or variations in the upstream units.
- Avoid overheating the reactor charge.
Actions
- Increase the recycle flowrate to maximum.
- Cut steam immediately to the reactor feed heater.
- Increase to maximum the recycle flow rate in the recycle air cooler. Close the recycle
air cooler bypass line. Start all fans at air cooler.
- Maximum hydrogen make-up flow in the isomerization reaction section (2nd
hydrogen make up line)
- Stop C2Cl4 injection when the isomerization reactor reaches 105°C.
- Hold the reactor inlet temperatures at 100°C if conditions are stable.
- Modify the fractionation towers upstream of the unit to restore the design quantities
of benzene and C7+ in the feed.
- Or check out the controllers to determine the cause of the poor control.
- When the faults have been corrected, increase the reactor temperatures slowly back
- If any of the reactor temperatures exceeds 190°C, cut the liquid feed and make-up hydrogen. Activate I-102 / 103, emergency shut-down interlock in order to depressurize the reaction section.
If the reactor temperature excursion is due to low liquid feed rate and this cannot be corrected quickly, cut out the remaining feed and follow the procedures for loss of feed. 8.6.5. Steam Failure
If steam is lost in the unit, the Reactor Feed Heater, the Reboilers of the stabilizer, the Deisohexaniser and the LPG stripper, the Caustic Recycle Heater and the Regenerant Vaporizer will shut down.
Critical equipment is the reboiler of the stabilizer as unstabilized product containing HCl could be sent through the downstream equipment to tankage, making the storage equipment susceptible to corrosion.
Actions:
- Bypass the LPG recovery section on off gas side. The stabilizer off-gases are sent to
fuel gas via scrubber.
- Increase the hydrogen make-up flow to maximum rate in the isomerization reaction
section (2nd hydrogen make up line)
- Stop the C2Cl4 injection.
- When the reactors cool down to 100°C, cut C5/C6 fresh feed to 50% of design rate by
steps of 10%.
- When 50% is reached, open the reactor bypass line and quickly remove the remaining C5/C6 feed from the reactors.
- Continue the reactors cooling with hydrogen make-up gas, while maintaining pressure profile in the reaction section and stabilization section.
- Cut the C5/C6 feed circulation and maintain the recycle flow rate around the
hydrogenation reactor.
- If at any time temperature begins to increase rapidly in any one of the reactors, begin
- Stop the heavy product rundown as soon as possible. 8.6.6. Cooling Water Failure
This refers to cooling water failure or air cooler failure. If the recycle of the C5/C6 feed
fails, the dryers will be operating at a higher temperature significantly reducing the dryers water retention capability. Any water breakthrough would be detrimental to the isomerization catalyst.
Loss of cooling around the Deisohexaniser, stabilizer and LPG stripper will cause an over pressuring of the column leading to possible relief through the PSV. There is also the risk of sending hot isomerate or LPG product to tankage causing a fire hazard. If cooling water or air cooling fails for an extended period of time, shutdown the unit using the following procedure:
- Cut steam immediately from the Reactor Feed Heater.
- Bypass the LPG recovery section on off gas side. The stabilizer off-gases are sent to
the scrubber.
- Increase hydrogen make-up to maximum rate to the isomerization reactors.
- Increase to maximum the recycle flow rate in the recycle air cooler. Close the recycle
air cooler bypass line. Start all fans at air cooler.
- When the isomerization lead reactor reaches 105°C, cut the C2Cl4 injection.
- Reduce steam to deisohexaniser reboiler, stabilizer reboiler and LPG stripper Reboiler, but maintain the pressures.
- Follow the procedure as above for steam failure.
8.6.7. Power Failure
If a power failure occurs, all electrically driven equipment i.e. pumps; air coolers and compressors (make-up hydrogen) will stop.
This is a severe condition as loss of flow through the reactors where exothermic reactions occur will lead to a temperature runaway situation.
Actions:
- Immediately bypass LPG recovery on off gas side and simultaneously begin depressurizing the reactors to flare (through the stabilizer and scrubber for the isomerization section.
- When the pressure is low begin purging with cold dry nitrogen to cool the reaction
beds and displace the hydrocarbons.
- Shut off steam to the Reactor Feed Heater, the Reboilers of stabilizer, deisohexaniser
and LPG stripper.
8.6.8. Instrument Failure: Air or Power
It is too early to prepare a procedure on this type of failure without knowledge of the system back-up and level of redundancy, etc, preferred by the unit owner. This section will be fully defined during the detailed engineering stage based on the strategies developed for other emergency situations.
8.6.9. Major Leak or Fire:
The following is only an overview of the steps to be taken during the discovery of a major leak resulting in a fire. The following steps are from a process view mainly aimed at avoiding runaway reactions and protecting the equipment and catalyst.
Incase of fire, depressurization of the reactors shall be actuated as per API 521— 3.19. As the safety valves at the reactors have set pressure corresponding to supercritical conditions, protection against fire exposure by safety valves is not sufficient. It is mandatory to depressurize the corresponding vessels whenever a fire is detected.
Actions:
- Shut off both C5/C6 feed and hydrogen make-up to the unit and block in the product
- Shut off steam to the Reactor Feed Heater, Stabilizer Reboiler, deisohexaniser reboiler and LPG stripper reboiler.
- Immediately bypass LPG recovery on off gas side and simultaneously begin depressurizing the reactors to flare, through the stabilizer and scrubber.
- If it is possible, isolate the problem area and block in the reactors and purge with
nitrogen to flare.
- The objectives of the following steps which should be developed by the client/engineering contractor are to drain hydrocarbons from all the vessels, purge the unit with inert gas in particular the reactors if they cannot be accessed. Leaving the reactors with hydrocarbon inside and no flow will eventually lead to a temperature excursion and mechanical failure, so all effort must be made to purge these vessels.