Delayed Coking – Operations

[Mrityunjay SinghParticipant]

Operations Breakout Group
Leads / Facilitators
Dale Wilborne Chevron
Danny Ollre Foster Wheeler
Tom Meek Shell
For even more comprehensive notes with color illustrations: [link]http://www.coking.com/seminar/2011CokingCom_Breakout-Operations.pdf

Topics

Drum Check List Layer of Protection
There needs to be a Drum Check off list for each step of the Cycle that should be checked off and turned into the unit Supervisor or Team Lead
Completed for every Cycle noting any issues
Done regardless of Interlocks

Drum Maintenance on the Structure during the Cycle
Should have in Place a Procedures or Administrative Standing Instruction for Maintenance on the Structure during the Drum Cycle.
Specific Time when Non-Essential Personnel should not be on the Drums
1. Switch some include Steam Test and Warm Up
2. Draining the Drum
3. Deheading Drums
4. Cutting the Drum Pilot Cut, Cutting Cone out and Top 20 to 40 Feet. Basically when the Drum Cut looks good!
5. No one should be allowed on the Top Deck or Derricks during any part of the Cut.

Drum Warm Up
Normal Operations
SP-3 Backwarming Valves are typically used during Warm Up. This Valve should be Installed and Operational!
Best Practice is control the Backwarming Valve from the DCS – Used to Mitigate Foaming – apply back pressure to drums.

Initial Start Up
Increase Fractionator Pressure for better Delta P – WGC Suction Controller or Vac. Breaker Fuel gas controller
Drum routed to CCD (Warm Up Sump) with CCD Vapors routed to Blowdown and Flare Gas Recovery OR Drum Bottom routed directly to Blowdown and FGR (Depending on Licensor design).

Hot Drums / Blow Backs (Quenching)
Every Location has had Hot Drums and Blowouts Consensus is that it is related mostly to Feed changes, however can also be related to Quench Procedure and/or Instrument problems.
Most Locations Track the Amount the amount of Water Added to the Drum and the total Flashover to Blowdown to ensure consistent Quenching.
Everyone should have Guidelines for Deheading which are Drum Conditions of Amount of Water added, Temperatures, Pressure, Valve Line ups.
Regulation for California is to vent the drums at < 2 PSIG. This is coming nationwide at some point due to EPA Regulations. Sites must be looking forward to this and start installing facilities and procedures to accomplish.
Vent Routing is an issue at some locations. Discussion of Routing
Blowback/Blowout Preventor Should be Vented with proper size if used. Eng Calculations should be done for Blowback Preventor Vent and Drum Vents. Some have Open Top.

Antifoam Usage
50/50 usage of 600,000 and 60,000 Antifoams being used
Most use LCGO for Carrier, however some switching to HCGO to mitiage flashing in the overhead.
Nozzles are 180 degrees from Overhead Vapor Line.
Control using Drum Levels APC Advance Process Control.
Silicon protection in HTU many have Guard Beds in Naphtha HTUs.

Half Switch
Most are doing Half Switch – Only two locations doing immediate switch over with no stopping
Half Switch Definition 5 to 10 minutes to 20 min to 1 Hour.
Benefit
a.) Ensures there is a Clear Pathway into the Drum before completing the Switch.
b.) Helps greatly with Fractionator Recovery 2 drums units especially but also 4 Drum units.
c.) New FW unit are set up with Half Switch Capability

Drum Levels
Drum Level Detectors Top Vendors – Ohmart Vega (Gamma) and Thermo Fisher (Nuetron Backscatter and Gamma)
Point levels which are Typically NBS, neutron Backscatter, 3 to 4 per drum, are typical.
Some Drums have Continuous Gamma either Ohmart Vega or Thermo Fisher.
Redundant Levels at the Top portion of the Drum Meaning Point Detector and Gamma all the way from the Top level to the Top Tangent is being installed at some locations. PSM Reasons 1.) Mitigation of Foaming Carryover Risk, and 2.) Overfilling During Quench

Relief Systems
1. Coke Drums
Routings – three different locations – Blowdown, Fractionator, or Flare. If to flare then must be able to handle high temperature and load. Blowdown is typical routing by Licensors.
Steam Purge should be used both Upstream and Downstream of the PSV
PSVs should have Discharge Isolation Valves which are closed after the vents are opened. (Blowdown closed prior to Venting)
2. Heater Outlet
Licensors – Not required by Foster Wheeler. Conoco Newer units do not require as well.
Some locations have installed due to risk of Heater Bottle Up especially two Drum single heater units, and severe coking due to stoppage of flow, and potential bottle up. Question is there a history of blocking in the SP-6 Feed Line isolation Valve? Can the Valve Actuator indicate open when in fact it is closed due to failure?
Some have eliminated due to Coking. With those with a good steam purge system is required both inlet and outlet of PSVs.

Abnormal Drum Operations – Short Tarry Drums
Discussed the following Procedures and Required Guidelines
Administrative – All Locations needs to have Procedures in place for Abnormal Drum Operations so the Operators and Support Staff know how to respond!
Need to have Guidelines on how long Operations can continue feeding a Coke Drum at less than normal Operating Temperature. Typically is if over an hour then Put the Drum Pair in Circulation!
Procedures which have been done by the Companies present in the Group or by others in the Industry

1.) Operator Procedures and Guidelines a.) some use < 4 Hrs at normal Temperatures then the Drum is treated as a Tar Drum. If > 4 Hrs then drum is treated as a normal Drum. b. ) Some use <, > 3 Hrs Time at Normal Conditions. c. ) Original Equilon procedures were <, > 6 Hrs which is very conservative. Typically if the Drum Overhead reaches 800 F then drum is typically pretty good shape but if <3 or 4 then do extended Steaming to harden up tarry material to avoid Check Affect during drum draining.

2.) Switch back into the Drum Need to have specific Guidelines – a.) Process conditions prior to losing Feed – 1. ) Amount of Feed, is there room? Can all feed be converted. b.) Temperatures. Some have used 400 F overhead temp as a guide. Some us Natural Gas as an Alternate Purge which does not take the Heat out of the Bed as quickly saturated steam.

3.) Alternate Puge – Some use Alternate Purge media to ensure the Feed line and Bed stay open if there is a Utility problem. Natural Gas or Nitrogen is used as Alternate Purge which does not take the Heat out of the Bed as quickly saturated steam.

4.) Drum Draining to a Process Vessel – Ensure Tarry drum with low level of Coke fines. (Process Review needed). Inject Cutter into drum typically Antifoam Carrier and Agitate mixing with Steam or Alternate Purge. Drain to 1.) CCD Drum, or 2.) Fractionator.

5.) Superheated Steam – Cooking The Tar Online Spalling conditions. 900 F, to as high as 1100 F – however need to review Heater Outlet line Design conditions and get approval from Engineering on high temps.

6.) Typical Water Quench If open in the Bottom BTU Balance is critical and void overfilling if Tarry Check Affect is expected.

Additional Topics

Feed Quality and Composition Correlation to Hot Drums

Coke Drum Movement During Drum Cycle especially during Quench

Operational Effects Bulging

Heater Spalling –

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