Hydrocracker Emergency Depressuring System

Depressuring is crucial for the safe operation of chemical plant and protection of its equipment. This is particularly so for high pressure hydroprocessing units like hydrocracker and hydrodesulfurizer. Typical hydroprocessing unit operating conditions (measured at reactor outlet) range from 150 to 190 bar in ARDS and HCK , 30 to 90 bar for DHT and Temperatures is around (315 - 427 oC).

Depressuring may be automatic or manually initiated; and may be caused by a variety of triggering events and initial conditions. Some reasons to depressure are:

• fire in the unit - Manually opened
• Reactor temperature excursion (Manual Opened in DHT and Automatic in HCK and ARDS by TAHH of reactor bed)

Depressuring is the process of lowering the pressure in the reactor loop by venting the loop content to a safe location, usually the plant flare system for disposal.

The purposes are to shutdown the process unit following a potentially hazardous situation and to limit a temperature excursion in the reactor. This can cause localized catalyst bed plugging by coke formation, which in turn leads to flow mal-distribution, control difficulty, reduced performance, and problems with catalyst dumping. Thermal excursions can also damage the reactor and downstream equipment and piping.

The picture shows Depressuring system for HCK and is provided by a Depressuring valve located at the suction line of RGC (Recycle gas compressor) .

. Depressuring is initiated when the Depressuring valve is opened. The valve bleeds the contents of the high pressure reactor loop to the flare.

 Valve OPEN/CLOSE positions reflected in the control room through annunciations ZLC (valve closed) and ZLO (valve opened) via proximity switches (ZSC for Close Position and ZSO for Open Position).

What happen after activation of the depressurizing valve is different from one licensor to another as the following :

there is a huge difference between CLG and UOP and Axens in logic philosophy

In UOP philosophy : If the depressurizing valve opened (Feed pump trip, RGC trip, Furnace totally trip , Injection water trip , amine Pump trip)

In Axens philosophy: If the depressurizing valve opened (Feed pump trip, RGC trip, Furnace partially trip, Injection water trip , amine Pump trip)In Chevron Philosophy: : If the depressurizing valve opened (Feed pump will not trip and feed is still flowing to quench the reactor, Injection water will not stop and water will flowing to flush the reactor effluent air cooler, amine Pump will not stop as the field operator will go to isolate the absorber and open the bypass around the absorber – Heater only will partially trip) - After the bed temperature (Maximum one ) become 300C We can stop manually injection water pump  and amine pump and and after system pressure become 80 bar we can stop feed pump manually.

Due to the seriousness of the consequences, such interlock system is usually activated as a last resort, i.e. in an emergency situation where there is a risk of lives or injury to personnel or damage to equipment. There is usually restricted access to the ESD (emergency shutdown) pushbutton.