Troubleshooting a heavy crude topper
Thorough troubleshooting revealed the primary cause of a heavy crude topper’s poor availability: desalter operation. A comprehensive revamp resolved matters.
DARYL HANSON, MIKE WESSELS and ADAM SMITH, Valero Energy
ISIS MEJIAS, Consultant
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Historic refinery utilisation rates have caused refiners to critically review the availability of each unit. For many operating companies, under-utilisation of one unit will result in a resultant under-performance of downstream units, which affects the overall utilisation rates. In order to maximise gains, operating companies need to maximise utilisation rates.
The heavy crude topping (HCT) unit at the Valero Corpus Christi refinery was revamped in early 2017. The HCT unit operates with a very heavy feedstock (typically <20° API), primarily a mix of heavy crude, vacuum tower bottoms, and coker gasoil. The feedstock is highly opportunistic. The feed is heated, desalted, and then fed to a stabiliser tower. The stabiliser tower removes the naphtha and lighter components. The heavy bottoms stream is fed to an atmospheric residual hydrotreating desulphurisation (ARHDS) unit which removes feedstock impurities before being fed to the fluid catalytic cracking (FCC) unit.
Feedstock variability is very high for this unit. Before the revamp, the unit struggled to meet performance (including production) expectations reliably. The HCT unit was routinely plagued by loss of desalter water control, unpredictable stabiliser overhead water production, flooding in the stabiliser, and unit heater hydraulic limitations. Problems typically started in the desalter system but would progress throughout the unit. During problems, the unit would ‘wind down’ until a unit rate cut was required. Operations had identified key variables in the unit that would signal the onset of problems and would make moves including production cuts of 30% before returning to stable operations.
Extensive troubleshooting revealed that the primary unit problem was the desalter operation. Old desalter technology, poor level control technology, and low wash water rates were identified as problem areas. Even though the desalter utilised low wash water rates, the unit was troubled by loss of water inventory. During troublesome times, the preflash drum performance would degrade and was identified as the primary contributor to the unit wind down. Unit wind down always resulted in loss of water inventory, loss of preheat temperature, and overall loss of unit capacity.
A fast track project was initiated to improve unit performance and availability. The project replaced outdated technology in the desalter and preflash drum. These modifications were successful and the unit’s performance (availability and operability) has improved significantly. Unplanned production cuts due to the unit wind downs has decreased from one per month to one in the last 24 months. Upset severities that resulted in wind downs have decreased dramatically and that single event only required a 10% reduction in capacity to get the unit back on track. The unit’s ability to handle lighter feedstocks has been greatly improved.
Heavy crude topping unit
The HCT is fed from the tank farm with a prescribed blend of feedstocks that optimises the FCC yields and economic performance. Feedstock is heated in several product cross-exchangers up to the desalter temperature range of 260-290°F (126-143°C). The desalted crude is heated by an additional product exchanger before entering the flash drum. The flash drum removes the light components at 35 psig and 325°F (163°C). Vapour from the flash drum is sent to the stabiliser column, while the flashed crude (liquid) is pumped through the heater to a 550°F (260°C) outlet temperature goal and is fed to the stabiliser. Stabiliser bottoms cutpoint is nominally 400°F (204°C) and is steam stripped before being used for preheat and feed to the ARHDS unit. Stabiliser overhead is naphtha boiling range material that is stabilised before being fed to the naphtha hydrotreater (NHT, see Figure 1).
The desalter is an atypical three-stage system. The majority of industry experience is with two-stage desalting, with the authors estimating that there are less than 10 three-stage systems in the world to date and likely more one-stage desalters in operation than three-stage desalters.
Unit operation and design prior to revamp.
Before the revamp, the desalter system was plagued by operating upsets. In order to optimise the unit’s capacity, operations performed troubleshooting on the unit. Operations would notice:
• Loss of second stage water rate to the first stage
• Flash drum would pressure up
• More loss of water balance
• Stabiliser overhead water rate would increase.
During this time, operations would reduce the water rate to the desalters which was thought to reduce the severity of the upsets. Theories abounded about the cause of the operating upsets, but all indications before a thorough unit troubleshooting study was executed seemed to lead back to crude instability (see Figure 2).
Chemical vendors were contacted to assist with the review. Desalter internals design (older style), crude incompatibility, feedstock tank switching, and crude types were identified as contributing factors. Extensive laboratory testing was performed to identify the missing link to solve the incompatibility issue. Feedstock elimination and changing the blend ratios seemed to mildly assist unit availability, but no definite rules could be put into place to eliminate the issues. Chemical solutions provided mild improvements, but the missing link was never identified that could improve unit operability and reliably meet productivity goals.
If you ask any operator the question “What makes a unit a good unit to operate?”, invariably there are likely two answers. Firstly, a good unit does not require a lot of outside work in hot or cold ambient conditions. Secondly, a good unit is easy to operate; that is, it can run by itself. This unit did not satisfy either of these standards. Like a car that is at end of life, this unit required frequent attention, intervention, and frequent changes to optimise its operation.
Unit performance definitely pointed towards the desalters as being the prime problematic area of its operation. Justly or unjustly, the unit problems seemed to be blamed on the desalter’s ability to handle the crude types that were purchased. The desalter was performing poorly primarily because it utilised old desalter technology, had poor level control technology, and utilised lower than design water injection rates.
The first generation, vertical flow desalter technology that was installed in the existing desalter system was an old style design used for light crude oils. Grid design was primarily based on maximising the oil residence time by placing the grids below the vessel centre line. All of the feed (water and oil) was introduced with a non-optimised distributor more suited to a distillation column. Having the oil/water fed below the grid bank, in the narrower section of the vessel, induces turbulence in the water phase and results in poor water resolution in heavy oil service. Light crude oils separate much better from water than do heavy crudes, due to the droplet size and physical properties. It was obvious that the desalter internals required upgrading to reduce the water carry-over problems.
For heavy crude oils, however, separation is more difficult. The water-oil fluid entering the vessel induces turbulence in the water phase. This turbulence makes it more difficult for water droplets to coalesce in heavy oil service, and water starts to carry over with the desalted crude to downstream units. Thus, for heavy oil service it is more effective to separate the bulk of the water in the high intensity electric field between the electrodes, rather than below the electrodes. The desalter internals, then, required upgrading to reduce the water carry- over problems.
Wash water injection was identified as another operating problem in the desalter. When processing heavy crudes, it is critical to pay attention to the water droplet population formed prior to reaching the desalter with the wash water injection. Efficient desalter designs reduce the carry-over of water soluble inorganic contaminants by coalescing water droplets with electrostatic forces. Efficiently contacting these inorganic contaminants with water prior to the desalter, however, is key to creating a water droplet population that enables separation. With the adequate droplet population for a heavy crude, these droplets can coalesce easier with electrostatic forces and settle by gravity. When the water droplet population is low in heavy crudes, separation becomes more difficult; it is much harder to wash a highly viscous heavy crude with high salt content than it is to wash a lighter crude. Typically, wash water is introduced at a rate of 3-6% by volume for light to medium crudes and 6-10% for heavier crudes.
In addition to the desalter’s design and wash water injection limitations, it utilised older style capacitance probes and AGAR probes to control water/oil levels. With certain types of blends, level control and performance became unreliable.
The flash drum was a horizontal drum that provided vapour/liquid separation. Vapour generated was fed to the stabiliser tower in the feed zone. Liquid was inventoried and pumped through the heater. Without the drum, pressure drop through the heater would require feed reductions due to hydraulic limitations caused by vaporisation.
The revamp included replacing the existing desalter internals with Edge II desalter internals (see Figure 3). The Edge II was designed specifically to treat heavy feedstock or emulsion sensitive crudes. An Edge II desalter has a distributor designed to increase residence time for the interface emulsion and effluent water, provide the flexibility to handle upsets, yield decreased chemical consumption, and provide low oil carry-under in the effluent water. It had several design improvements over the first generation design.
The Edge II crude inlet distributor has risers that introduce the flow horizontally between the electrode grids. Improved laminar flow provides an enhanced water droplet environment for emulsion sensitive crudes. The dual horizontal flow distribution provides quick, complete coalescence in the high intensity electric field, thereby ensuring larger droplets and faster settling. High level distribution between the grids allows for two to three times the volume of water/oil emulsion, compared to the first generation design. Furthermore, the distribution improves the control of the interface (rag) emulsion and minimises oil carry-under for improved effluent water quality (see Figure 4).
The improved Edge II electrode and transactor design also enhance the desalting process for heavy or emulsion sensitive crudes. Its solid steel rod electrodes ensure long life and ease of maintenance. The independently energised electrode grid design provides flexibility to handle upsets that can come with heavy or emulsion sensitive crudes and facilitates long run cycles for maintenance. Independently energised grids also allow for continuous desalting even upon loss of a grid due to loss of level control or other upsets. Five transactor secondary voltage levels provide the flexibility to optimise performance at varying operating conditions. In high conductivity environments, such as with very conductive crudes, the lower voltages provide longer service life for electrical components.
In addition to modifying the desalter internals, the level control instrumentation was upgraded to a nuclear type technology. The refinery maintained the existing instrumentation and added a Vega multi-point density array (MDA) desalter interface profiler, nuclear type, for level indication (see Figure 5). Valero has positive experience with this technology in many desalters while processing light crude, upgraded crude, and heavy crudes. In addition to raw oil/water level information, the new instrumentation provided additional data to understand the quality of separation in the emulsion zone (see Figure 6). Identifying operational changes needed to mitigate the growth of the interface emulsion is critical. Understanding the quality of separation of the emulsion enables more stable operation and prevents previously unseen emulsion growth upsets. These upsets tend to be very severe and result in overhead corrosion spikes due to chloride carry- over and fouling of equipment due to solids migration.
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