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Jeff Lewellen (HollyFrontier)

In 2009, we completed a group of projects at El Dorado involving larger coke drums and a deep cut vacuum tower. The coke drum project limited the scope to drum replacement with minimal changes to the balance of the unit. Some of the more significant changes to the unit included:

Fractionation/Wet Gas Systems -

• Higher wet-gas volumes due to higher coke yields.

• Larger swings in gas rate due to drum switching activities. Unit Operators reported this was a significant change in operation.

• Some shift in liquid product yields affected by slightly lower drum pressures, lower drum outlet temperature, and longer drum cycles.

Charge Heater -

• Lower outlet temperature and fired duty requirements to achieve target VCM/coke quality. This is mostly influenced by longer drum cycle times, and less HGO in the feed.

• Increased tube fouling rate leading to shorter times between heater tube decokings.

Drum and Drum Cycle -

the drum volumes approximately doubled with required cycle length extending from 12 to 18 hours.

• Drum superficial velocity decreased with drum diameter, which improved carry-over and entrainment issues (no significant pressure changes).

• The reduced velocity also decreases foam height. However heavier feed and lower drum temperature created more stable foam that can be difficult to collapse.

• The large drums also lengthen drum air freeing, pressure test, and warm-up cycles.

Quench/Blow-down Systems - Our modifications to this system were limited to additional quench pump and air cooler/condenser capacity. The quench tower was also reconfigured with limited success.

• More quench water needed leading to more storage (surge capacity)

• Higher sour water production – proportional with coke yield

• Higher off-gas rates

• More quenched oil/slop from the system

• Marginally sized quench system struggles.

Coke Storage and Handling - We replaced railcars with a feeder-breaker/conveyor system due to logistics of the additional rail and cars. The heavier feed also shifted our coke to much more shot coke production leading to more drum fallouts that are difficult to manage with a railcar system.

• Greater surge volumes are needed in the

o Pit/slab/bin storage area

o Crusher/slurry pump, feeder-breaker/conveyor, or bridge crane/loader systems

o Shipping logistics – railcars, truck, barge

Jet Pump/Cutting system – We operated for several months with the original low pressure/volume jet pump system until the new unit was delivered and installed.

• The smaller jet pump/cutting system led to a significant increase in coke fines to the jet pump system due to extended cutting times. Cutting times more than doubled from previous drums. Extended cycle times or unexpected feed changes led to even greater cutting durations.

o Significant reliability problems developed from erosion issues in the decoking valve, cutting tools, and eventually the jet pump.

o Restrictions in instrumentation and small-bore piping from the additional coke fines.

• After the jet pump/cutting system upgrade was complete, the reliability and the operation of this system improved greatly. However, some of the changes impacting this operation include:

o Decoking rates are limited by bottom unheading device/dump chute plugging limit. Cutting rates are higher in tons per hour, but duration remained longer than the smaller drums.

o the longer cutting times (and potentially changes in feed and coke handling) led to more fine's generation than original drums. However, much improved over the previous 6 months.

o Retrofitted additional settling/fines separation capacity to improved water quality.

Gary Gianzon (Marathon Petroleum Company)

The larger drum can take longer to quench and cut assuming that your blowdown system and cutting water system are not designed for the larger drums.

Eberhard Lucke (Commonwealth E&C)

The extra capacity gained by installing larger diameter coke drums can be used by either increasing the unit throughput or by taking advantage of a longer drum cycle time at the same unit charge rate. Due to the larger diameter, steam stripping and quenching can be more challenging and the probability of hot spots may increase. Before installation of larger diameter coke drums special attention has to be paid to the jet pump pressure. If the jet pump discharge pressure is lower than the recommended pressure for the new coke drum diameter, coke cutting will take a lot longer and will create significantly more coke fines due to the grinding effect of the broader water jet. Jet pump and drill/cutting tool may have to be replaced or revamped to ensure proper coke cutting. Also, the capacity of the coke handling system – pit, pad, attached breaker or feeder-breaker system, slurry system – receiving a larger amount of coke has to be checked for adequate capacity.

Gary Gianzon (Marathon Petroleum Company)

Coke drum velocity limit depends on the capability of the coker main fractionator and downstream equipment to handle entrained coke. One of MPC’s units, processing much lighter feed than design and at a higher charge rate, currently operates with a drum velocity above 0.80 ft/sec. At these velocities, the flash zone gasoil strainer needs to be cleaned twice per week and the backwash interval on the HCGO filters is less than ideal. Raising coke drum pressure will reduce velocity and coke entrainment. Two of our cokers with no coke removal capabilities in the fractionator, operate with a drum velocity of around 0.40 ft/sec. At this low velocity, accumulation of coke in the main fractionator is minimal and only requires cleaning every two years.

Jeff Lewellen (HollyFrontier) 

The El Dorado facility targets coke drum velocities in the 0.5 ft/sec range. In troubleshooting coke carryover issues, a frequently overlooked contributor to high drum velocity can be unaccounted (or under accounted) sweep steam sources. Wet steam caused by steam system upsets or faulty condensate traps can also unknowingly increase drum (and fractionator) velocities.

Eberhard Lucke (Commonwealth E&C)

As a general design guideline, the vapor velocity in the open section above the coke bed should be in the range of 0.5 to 0.6 ft/s while the vapor velocity in the inlet of the vapor line should not exceed about 60 ft/s.

Jeff Lewellen (HollyFrontier)

The ideal coke drum level indication would provide accurate and reliable information to the unit operator for all of the following:

• The foam front level for anti-foam/silicone addition control

• Coke/hydrocarbon level to optimize drum use

• Quench water level to verify (along with pressure/temperature/flow totalizer) adequately quenched drums This is also a demanding environment for instrumentation.

The process conditions are highly fouling, high temperature, changing pressure and composition, and high velocity hydraulic decoking every cycle. Most common technologies use “non-contact” methods with a radiation source and detector system. These include:

• Neutron backscatter

• Gamma point source (density and/or level switch)

• Gamma continuous level

Neutron backscatter

These devices use a common neutron source and sensor housing that directs fast (high energy) neutrons from typically Americium-Beryllium (AmBe) or similar neutron emitter source through the vessel wall into the vessel interior. If hydrogen bearing material is present, the fast (high energy) neutrons are converted into slow (low energy) neutrons which are scattered back to the neutron sensor in direct proportion to the hydrogen density.

Due to significant differences in hydrogen density, the technology is very effective in detecting changes from clear vapor to foam (including light to heavy foam densities) to coke level and detecting water level. The limitations of this technology are:

• Point detection only.

• Measures only the area immediately adjacent to the vessel wall.

• Historically has experienced difficulty with thick wall vessels. Trials at our facility in the 1980s were unsuccessful. However, these detectors are used very successfully throughout the industry.

Gamma point source

This technology utilizes a gamma emitter source (usually Cesium-137) shielded to emitting gamma radiation only across the diameter of the drum with a detector located opposite of the source. The gamma radiation reaching the detector is inversely proportional to the mass of material between the source and sensor. The detector output can be reported as either:

• An analog signal representing density changes in vapor to foam to coke/liquid.

• Or a digital signal between clear vapor and the foam/coke level.

Our previous drums were equipped with this technology utilizing 5 detectors per drum at various levels. The indications were fairly reliable; however, lightening/electrostatic discharge from thunderstorms did cause some problems.

This technology has (and continues to be) used throughout the industry in multiple applications.

However, there are some significant disadvantages:

• Does not detect foam height. Addition of anti-foam may cause the level to drop below the detector, but how far and for how long?

• Predicting drum outages and switch times are difficult with changing rates or feed composition. Extrapolating drum fill rate between level detectors may not provide adequate time to make unit adjustments.

• We have experienced low density foam fronts that did not activate the level indicators resulting in a drum “foam-carryover” event. The foam was confirmed by a contractor scanner, and detector sensitive were adjusted to account for the change.

Gamma Continuous Level

Similar to the point source, this method utilizes a gamma emitter source that is shielded to project a fan shape beam diagonally across the drum. The detector is a lengthened scintillation tube or (more recently) a fiber-optic scintillation bundle. Multiple sources and detectors can be used to expand the range of the level indication to provide a continuous indication for the drum.

Items to note on this technology:

• The level is interpreted from the amount of gamma radiation blocked. It does not directly detect the vapor/foam/coke interface. However, a handheld detector can be used to find this interface if verification is needed.

• Level indication is normally the combined coke and foam level. The foam portion is determined by collapsing the foam front using the silicone anti-foam and monitoring the change in level.

We have installed this technology in our El Dorado facility. In addition to the fiber optic continuous levels, the system utilizes bottom of range point detector to reset zero and a density point detector at top of range for both vapor density correction and the LAHH redundancy to the continuous level.

We have noted a deviation between the level indication at the end of the drum cycle and the level determined by drill stem gauging the coke outage at the beginning of the drum decoking. This deviation at times has been several feet. At these times, we have confirmed coke to be higher on the walls with a depression toward the center of the drums. We have attributed this, in part, to the bed collapsing during the quench/draining steps of the drum cycle.

The advantages of the continuous level technology are best seen when utilized in combination with the vapor density and low-level detectors as a packaged system.

• Foam level is inferred by the change in level with the introduction of antifoam/silicone

• Coke/hydrocarbon level is continuous for the full cylinder length of the drum.

• Redundant high-level indication Probably the most significant disadvantage of this system is the complexity. Although this equipment has been very reliable, it has been in service for less than 3 years.

Gary Gianzon (Marathon Petroleum Company

MPC currently uses both neutron backscatter and gamma continuous level detection and here are the pros and cons based on our operating experience: Gamma continuous level detection.

Pros:

1. Measure the level continuously throughout relevant levels of the drum. We use the continuous level measurement to optimize antifoam usage. The level can also be used to determine the height of the foam front, and this information can be used to adjust furnace outlet temperature.

2. Continuous level detection measures across the drum so it can detect the peak level in the drum.

3. Measures the vapor density across the top of the coke drum, which we use to optimize steam-out time to the fractionators and can indicate foam carryover to the main fractionators. 4. Smaller source than the neutron backscatter which is easier to permit.

Cons:

1. Difficult to calibrate and easily gets out of calibration. This is quite problematic if this is your only source of drum level measurement.

2. Only measures the total level, cannot distinguish between foam, coke, and water. Neutron Backscatter

Pros

1. The neutron backscatter can distinguish between foam, coke, and water.

2. The level is exact and does not drift.

Cons

1. Level can be difficult to detect on very thick drums.

2. Point source only detects about a foot inside the drum, level measurement is not continuous across the drum.

3. Long half-life responsibility