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Targeted liquid injection improves shed deck performance

06.01.2014  |  Riley, C.,  Valero Benicia Refinery, Benicia, CaliforniaWicklow, C.,  Valero Benicia Refinery, Benicia, CaliforniaHanson, D.,  Valero Energy Corp., San Antonio, TexasBrown Burns, J.,  Valero Energy Corp. , San Antonio, Texas

This case study shows how the installation of an unique piping and lance design achieved targeted liquid injection, quenched vapor hot zones and reduced resid entrainment into the heavy coker gasoil product.

Keywords: [coker] [distillation] [mass tranfer] [heat transfer] [gasoli] [resid]

Targeted liquid injection from a secondary pumparound (PA) effectively compensated for a shed-deck vapor distribution problem in a fluidized-bed coker. This case study shows how the installation of an unique piping and lance design achieved targeted liquid injection, quenched vapor hot zones and reduced resid entrainment into the heavy coker gasoil (HKGO) product.

THE REFINERY

The Valero Benicia refinery, owned by Valero Refining Company—California, operates a 28,000-bpd fluidized-bed coker.a The coker-reactor-effluent vapor passes through a single cyclone discharging into a scrubber that contains several rows of multi-pass shed decks. The scrubber-bottom liquid is pumped both as a recycle stream to the reactor and as PA liquid to the top of the sheds that feed through a slotted-ladder-type pipe distributor. The top section of the scrubber contains structured-grid packing washed with HKGO. After passing through the reactor and scrubber, the HKGO and light gasoil (LGO) products are separated by the fractionator, as shown in Fig. 1.

 
  Fig. 1.  Scrubber and fractionator PFD.



In a coker, the scrubber quenches the thermally cracked-reactor products and condenses the heaviest product fractions for recycling back to the reactor. The void space between the cyclone outlet and the bottom shed row acts as a spray chamber with good liquid distribution. It can provide a significant part of the PA’s heat transfer and solids removal. The remaining heat transfer and solids scrubbing occurs in the shed decks.

While coking and fouling of the scrubber occurs over a typical cycle, at the midpoint of the run, HKGO product quality should be maintainable without undercutting. In this case history, halfway into the cycle, significant residue carryover into the HKGO was observed, along with accelerated increases in the scrubber-grid pressure drop. Fig. 2 illustrates the steep slope and rapid degradation of the HKGO product quality from resid contamination. The impact on downstream units’ catalyst life and conversion would have required an early turnaround for the coker. A troubleshooting effort focused on identifying and resolving the entrainment problem.

 
  Fig. 2. Scrubber grid DP growth over time
  as compared to previous run.


ROOT CAUSE FOR RESID CARRYOVER

Resid is routinely entrained from the scrubber PA section. The resid carryover into the fractionator is typically measured at less than 5 vol% of the HKGO. As the HKGO becomes more contaminated with resid, downstream GO-filter change outs increase. Additionally, the higher resid percentages can negatively impact runs on the downstream cat-feed hydrotreater and fluid catalytic cracking (FCC) units. The elevated resid content level of the HKGO carries both operational and economic penalties.

The scrubber’s structured packing acts as a guard against resid carryover. The grid provides a surface for de-entrainment of liquid carried up from the scrubber PA; it also enables heat transfer between HKGO wash and the scrubber vapor. Efficiency loss of the grid packing can be a function of either vapor maldistribution into the grid or liquid maldistribution over the grid.

Structured packing is very sensitive to liquid maldistribution. Skillful designers pay careful attention to liquid distributor details. Liquid maldistribution of the HKGO wash can result in poor contact of the vapor and liquid through the grid and failure of the scrubber grid to properly wash the upflowing vapor.

Structured packing is also very sensitive to vapor distribution. In this coker scrubber, high localized C-factors (Cs) created channels through the structured packing and entrained liquid from the top of the bed. Both potential root causes of resid entrainment were investigated.

 



LIQUID DISTRIBUTION

The structured packing is washed by HKGO through a spray header.b To meet the unique anti-fouling requirements of the coker, the spray header uses three separate laterals, each with its own column nozzle. Each lateral has several large orifice spray nozzles. There are no 90° elbows on the laterals to allow for external flexible-lancing should they become plugged.

Operating data review

While the overall slope of the scrubber grid pressure drop was trending higher during this cycle than in the previous cycle, a distinct step change was observed at the approximate time that the resid content in the HKGO increased (Fig. 3). An event with the loss of HKGO flow to the grid spray headers could result in coke formation within the bed. Operating data were evaluated to determine whether any instances of a loss in wash-oil rate preceded the higher resid entrainment or step change in the scrubber-grid pressure drop. There were no obvious losses in wash oil that may have created a dry-out situation.

 
  Fig. 3.  Scrubber grid DP and resid in
  HKGO step change occurring at the end
  of August 2012.

Failure of a spray nozzle or full lateral can leave areas of the grid bed unwetted, resulting in localized dry, hot zones that are subject to coking. Plugged or damaged nozzles would be evidenced by a deviation in pressure drop from design. A pressure survey was conducted to determine the actual nozzle pressure drop. Comparison of field data against the calculated pressure drop indicated that the spray header was operating normally.

VAPOR DISTRIBUTION

The scrubber PA section consists of several rows of multi-pass sheds. The liquid pool at the bottom of the scrubber vessel is a mixture of cooler pitch feed and condensed heavy components from the reactor vapors entering the scrubber through the single cyclone. The pool liquid is pumped and distributed over the top of the sheds.

As shown in Fig. 4, the vapor from the cyclone discharges to one side of the vessel, and this encourages maldistribution into the shed rows. Pressure drop is generated as the vapor winds its way through the column and passes through the shed-deck liquid curtains. Low pressure drop through shed decks exceeds that of packing. Yet, it still challenges any vapor redistribution.

 
  Fig. 4.  Scrubber snout and shed tray
  orientation in plane view.

The directional flow from the cyclone snout, compounded by the loss of any sheds, could result in vapor-flow channeling. The performance of the PA return-liquid distributor (H-header in this case) and uniform cascade of the liquid showers is consequently critical to maximizing vapor redistribution.

VAPOR DISTRIBUTION FROM SHED PA

Several thermocouples (TIs) are installed in the scrubber shed section. Readings from the TIs seem to converge at the time the grid ΔP escalated. Both indications are located in the Northeastern quadrant. Based on the elevations under previous normal operating conditions, the 9014 TI reads vapor temperature and the 9015 TI reads the cooler PA liquid temperature. After the incident, both TIs converged and yielded similar readings (Fig. 5). The data indicated that the TIs are now reading the same phase, either liquid or vapor, indicating a maldistribution problem.

 
  Fig. 5.  Scrubber shed temperature
  profile between TI 9014 and TI 9015.

The TI readings were confirmed using a series of skin temperatures at four elevations. Cutouts were made in the insulation so that a thermo-scanning “gun” could be used to read skin temperatures at approximately 15° increments around the vessel circumference.

The skin temperatures had reasonable repeatability, and they indicated that a hot zone existed at the northwest quadrant. This position was located at the orientation of the TIs and confirmed the deviation in readings (Fig. 6).

 
  Fig. 6.  Scrubber shed TI 9014 and TI 9015
  from previous turnaround inspection.

The H-header design is a simple, ladder-type slotted-pipe distributor with laterals positioned over each of the top layer of sheds. Discussions of the hydrodynamics of liquid flow from a shed deck are found in literature.1 The up-flowing vapor can cause atomization of the liquid as it flows through the curtain, and impingement of the shower on surfaces or other showers can also cause liquid stream breakup.

Another method was used to evaluate the scrubber shower “liquid throw” and was back-checked against internal geometries to confirm that the cascades from the parallel shed decks are not impinging on each other.2 The liquid throw averaged only 15% of the shed spacing (very low weir loadings). In a perfect design, the H-header liquid would be distributed proportionally through the H-header according to the design open area, and each side of the shed should have a weir loading proportional to the vapor space that the liquid needed to cover. Shed decks are not ideal mass-transfer devices, and they are subject to maldistribution (Fig. 7). Accordingly, the actual liquid throw could be slightly different than the calculated value.

 
  Fig. 7.  Scrubber nozzle and internals orientations in plane view.

The exit velocity of the H-header end slots is below the target design guidelines for fouling systems. It is possible that fouling or coking could have obstructed liquid flow to a section of the header, resulting in maldistribution of liquid between the sheds or along the length of an individual shed. Damage occurring at a lateral or possibly at a flange could similarly change the header flow hydraulics and alter distribution. With the low weir loadings, this could result in one section of the scrubber having very sparse liquid coverage. Other than the observed temperature deviations, it was not possible to confirm the H-header performance while the scrubber was in operation (Fig. 8).

 
  Fig. 8.  Scrubber primary PA distributor
  orientation in plane view.

A high C value could contribute to liquid entrainment from the sheds. The vapor rate to the sheds is approximated by blending the product stream compositions and estimating the steam from the reactor side. This does not account for the heavier components that are condensed within the shed PA section. However, it should offer a fair approximation of whether vapor superficial velocity is outside the normal design parameters.

Vapor velocity was calculated two ways. The first method calculates the conventional superficial velocity through the column cross-sectional area, and the second method uses the window velocity of the gas.1 The cross-sectional column area and the window area are very similar in this column, making the two Cs similar. These Cs were then compared against the Kister and Olsson Eq. 3 to determine that this particular shed section operates significantly away from the flood point.3 Table 2 summarizes the results from these two methods.

 



Operational adjustments

The resid entrainment was initially shown to improve with the use of the startup header, which is an internal open-pipe tee, positioned above the H-header. When the startup bypass is in service, the total PA rate and duty increases. Higher duty results in undercutting GO, thereby improving its quality. However, the liquid from the startup pipe is poorly distributed and drops across the central sheds only. If the duty is held constant, then the side shed TIs in the hot zone are unresponsive when using the startup header. While using the startup pipe can increase overall PA flow, it was ineffective at controlling resid entrainment without major undercutting and economic penalties.

LIQUID/VAPOR DISTRIBUTION INVESTIGATION

After reviewing the equipment design, operating data and unit impacts to operational adjustments, the factor contributing to excessive resid entrainment was a malfunction of the H-header PA return distributor. It reduced liquid coverage over the northwest quadrant of the column. The liquid maldistribution promotes vapor maldistribution along with high localized vapor velocity and Cs by creating a lower pressure drop pathway, thus leaving the upcoming vapor unwashed.

ENGINEERING A SECONDARY PA

From using the startup header, it was clear that a non-targeted application of PA liquid would not resolve the localized high temperatures. The solution had to direct PA liquid to the hot zone in a more precise way and be installed without a unit shutdown.

Several existing nozzles on the scrubber vessel were identified at several feet above the PA zone. These nozzles were out of service, and had been part of a decommissioned project to create a spray chamber that could partially compensate for a fouled grid section during a previous cycle. This project had used the HKGO wash oil as a spray media. These existing nozzles were repurposed to inject PA liquid into the scrubber as a supplement to the H-header flow. The nozzle orientations around the tower allowed the liquid flows to be targeted to identified regions that were observed as hot zones by the temperature surveys.

Engineering proceeded to route PA liquid to the nozzles. The piping to the new secondary injection location was designed with the hydraulic capacity to operate as the primary PA, should H-header liquid distribution problems become more limiting. A branch was installed on the main PA line, and routed to a new ring header at the existing nozzle elevation. The ring header design is fed at two locations and allows for continuous flow circulation with the flexibility to block and unblock specific nozzles without creating deadlegs.

Globe valves were added to each arm leading off the ring header to provide throttling flow to individual nozzles. Instrumentation was installed to monitor total flow to the lances, along with pressure and temperature at the H-header and at each lance location. PA material, by its heavy nature, has elevated fouling and erosive properties. Accurate, reliable flowmetering is problematic, and sufficient pressure and temperature instrumentation was included in the design to approximate flow as a backup to the flowmeters (Fig. 9).

 
  Fig. 9.  Scrubber primary and secondary
  PA designs.

The only means to create back pressure and throttle flow on the primary line feeding the H-header was an existing block valve (gate), which has poor control range. A bypass line with a globe valve was installed with a hot tap around the existing gate valve to force liquid to the higher elevation where the ring header was located.

Design of the nozzle inserts

The lances or nozzle inserts were designed at various lengths, so that each nozzle would be able to target a specific zone of the vessel by dumping liquid onto a particular shed deck in the top row. In this manner, the PA liquid could be applied to the northwest quadrant hot zone to supplement the H-header poor performance.

An insertable lance would be installed in each of the six nozzles on the run, avoiding a unit shutdown. The nozzles were cleared of wall coke using a hot-tap machine, and the lances inserted through a packing gland. The nozzles were equipped with a steam purge to be used when oil was not flowing through the lance to reduce fouling conditions.

The nozzle insert design was intended to accomplish two key objectives: to allow large droplets of liquid to spray in a contained zone from an elevation of 9 ft, and to allow hydrolancing on the run should they become plugged. Several nozzle designs were bench tested with water to determine the spray profile, all with high line and exit velocities to prevent plugging. The best performing design incorporated attributes for anti-fouling and created a tight cone without generating mist (Fig. 10).

 
  Fig. 10.  Nozzle design iterations.

Test No. 1: Rectangular slot at end of capped pipe. The initial flow test showed that the material retained more forward velocity than expected, despite the end plate acting to deflect the liquid downward. The pattern was insufficient to wet sheds directly below the nozzle, making a hollow arc 4°-ft outward and 8°-ft to each side. This design would be ineffective in reaching the known hot zone near the wall when used in the existing nozzles.

Test No. 2: Rectangular slot shifting inward from end of capped pipe. The exit point was moved further away from the end cap to create a turbulent chamber above the nozzle that would direct more flow downward. This test nozzle showed a similar hollow arc with an even further forward throw of material.

Test No. 3: Internal sleeve. An internal sleeve was fitted into the exit orifice to force the liquid to change direction inside the lance, giving the fluid vertical velocity before it exited the slot. This design showed a more uniform pattern, centered only one foot outward from plumb.

Test No. 4: Internal weir and sloped end plate. An internal weir would force the same fluid directional change inside the lance, and would be easier to fabricate. The weir covered the full width of the exit slot to minimize side-throw of the liquid, and the end plate was modified to slope backward. This design showed a wide oval cone, but it greatly reduced the forward throw of material.

Test No. 5: Final design. For the final test nozzle, the slot’s arc was narrowed to shrink the wide oval shape of the bulk flow into a tighter full cone. This design was installed.

Commissioning the secondary PA. The six nozzles and corresponding lances (A, B, C, D, E and F) were installed, as shown in Fig. 11. All six lances were inserted and set on steam purge. Oil was commissioned to the lances that most closely targeted the hottest areas identified by the skin temperature scans (nozzles A and F). Once the secondary PA was commissioned, operating data indicated an almost instant improvement, and the charge rate was increased without deterioration in HKGO product quality (Tables 3 and 4).

 
  Fig. 11.  Secondary PA nozzle orientation.

 

 

The skin temperature survey was repeated to determine the impacts on the “hot zone” from the secondary PA liquid application. The local temperatures in the northwest quadrant decreased by 30°F–50°F.

Several optimizations of H-header, startup header and lance nozzles were explored to determine the configuration, resulting in the most even radial skin temperatures and lowest resid entrainment into HKGO product. The test with the H-header and lance nozzles A, F and E in service provided the optimal results (Tables 5 and 6). Half of the PA flow was directed to the H-header, while the other half was directed to the nozzles. Nozzles A and E were throttled to allow the majority of the flow to be directed to nozzle F.

 

 



FUTURE OPPORTUNITIES

Although confirmation of the nature of the maldistribution cannot be made until a future outage, there are several engineering optimizations that should be considered as lessons learned in this example.

H-header distributor. The design of the H-header allows for low velocity at the end of the laterals and low exit velocity through the slots furthest from centerline. This is a function of the pipe size and slot area. Redesigning the H-header with eccentric reducers along the length of the laterals will allow velocity to be maintained to minimize fouling. Because this system operates with a low shed weir loading, orifice sizes must be balanced against velocities suitable for fouling protection.

De-entrainment grid. There are multiple options for structured grid/packing styles that may balance de-entrainment ability against antifouling characteristics (open area for larger capacity). A limited number of fluidized-bed cokers are operating that set a reference for using an alternate grid type.a There may be opportunity to optimize the bed height and grid type to provide longer cycle lengths with adequate entrained resid removal.

Fractionator. While the primary issue caused by resid entrainment is contamination of the HKGO product, any liquid carried by vapor up to the trays from the inlet nozzle is subject to coke formation. These trays are notorious for fouling problems, both with standard coke and with polymeric coke. Providing adequate means to break the inlet momentum can prevent liquid entrainment onto the trays, in addition to optimizing the tray design (or considering alternatives to trays). HP

ACKNOWLEDGMENT

This is an upgraded version of a presentation given at the 2013 AIChE Spring National Meeting, San Antonio, Texas, April 29, 2013.

NOTES

a The fluidized coker is the Fluid Coking processed licensed and owned by ExxonMobil Research and Engineering.
b FLEXIGRID is a registered trademark owned by Koch Engineering Co., Inc.

LITERATURE CITED

1 Fair, J. R., “How to design baffle tray columns,” Hydrocarbon Processing, May 1993, pp. 75–80.
2 Bolles, W. L., Chapter 14, Design of Equilibrium Stage Processes, Ed. B. D. Smith. New York, McGraw-Hill, 1963, p. 498.
3 Kister, H. Z. and M. Olsson, “Don’t let baffle tray flood baffle you,” the AIChE Spring Meeting, Distillation Topical Conference, Chicago, March 2011. Jill Brown Burns is a principal process engineer at Valero Energy Corp. She is involved in troubleshooting, operations, and design of the many columns and crude and vacuum units at the 14 Valero refineries. Her previous experience included positions at Marathon Petroleum and Sulzer Chemtech. Ms. Burns has a BS degree in chemical engineering from the University of Oklahoma.

The authors

Daryl Hanson is a technology advisor for Valero Energy Corp. and is focused on distillation/fractionation/separation issues. He is responsible for design, troubleshooting, operation issues at the 14 refineries and Valero’s ethanol plants. His previous experience included positions at Glitsch, Koch-Glitsch and Process Consulting Services. Mr. Hanson has a BS in chemical engineering from Texas A&M University.

Chris Riley is the staff process engineer at Valero’s Benicia refinery. He has supported Benicia in various technical and operational roles since hiring on with Valero in 2008. His previous experience included positions at Chevron and Genentech. Mr. Riley has a BS degree in chemical engineering from Brigham Young University.


Cameron Wicklow is the process engineering manager at Valero’s Benicia refinery. He has supported Benicia in various technical, operational, and planning roles since hiring on with Exxon in 1999. Mr. Wicklow earned his BS degree in chemical engineering from the University of Illinois at Urbana Champaign, Illinois. 



 



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