Jan-2007

Gamma scanning fractionators

Gamma scanning is a powerful tool that can be used to determine the cause of flooding and regain column efficiency. Debutaniser capacity and performance were improved after tower scanning technology pinpointed mechanical and hydraulic defects

Brian Kirmse and Kevin Krase, Tesoro Refining & Marketing Company
Dave Ferguson, Tracerco Inc

Article Summary

Many problems can cause a loss of efficiency in a distillation column, but one of the most common is flooding. Determining the exact location and cause of the flooding allows operations to chose the best course of action to alleviate it. Gamma scanning is a powerful tool that can be utilised to locate the origin and cause of the flood.

The Tesoro refinery in Martinez, California, had been experiencing higher than normal differential pressure, and the overhead product was off specification on the debutaniser in the fluid coker unsaturates gas plant. Refinery personnel suspected flooding above the feed point, which is located in the diameter transition section of the column. Several theories on the origin of the flooding were assumed. The two leading ones were that the tower capacity had been limited by the presence of fouling material in the column, or there was a design limitation.

It was suspected that the pipe downcomers, located on the tray above the feed point, might have been restricted so that they could not handle the normal amount of liquid from the tray above the feed location. The refinery personnel were unable to determine the cause of the flooding with instrumentation. Hot tapping a bypass around the restricted section of the tower or bypassing the tower for a water wash were considered to be the least expensive options to correct the flooding. However, if the chosen option failed to resolve the problem, the additional cost of then entering the column would be a burden on the profitability of the refinery. Refinery personnel asked Tracerco to gather more information with a proprietary Tru-Scan gamma scan so they could make a more informed decision on the most cost-effective approach to resolve the problem.
The gamma scans identified the location and cause of the problem as multiple tray and downcomer fouling, which had resulted in a maldistribution of liquid and vapour traffic through the tower. This additional information gave Tesoro personnel the confidence to chose the least expensive corrective action, which resulted in a return to normal performance.

Meeting specifications
Tesoro Refining & Marketing Company operates a 168 000 barrel per day facility in Martinez, California. The debutaniser in the fluid coker unsaturates gas plant had been experiencing problems in meeting specification on the overheads product, and the differential pressure across the upper section of the column was 5.2 psi when it normally ran at about 2.5 psi. The feed rate was 48 000 bpd, with a reflux ratio of 1.0. The feed rate was 70% of design and the reflux rate was 52% of design. Figure 1 shows a schematic of the top portion of the column.

Operations and engineering personnel had been able to determine that the problem was in the top half of the debutaniser. However, they had not been able to pinpoint the exact position and extent of the flooding, nor the cause, with the instrumentation on the column. There was concern that the pipe downcomers in the diameter transition area could have been restricted and could not handle the desired rate. However, the column also had a history of fouling, although this had not restricted its operation during previous runs. In addition, they did not know whether the column internals had suffered any mechanical damage. Several courses of action were possible to correct the problems, some of which would involve a tower shutdown, including:
- Hot tap to install a liquid bypass line around the restriction
- Bypass the tower for water wash or chemical clean
- Bypass the tower, isolate and enter to clean and repair
- Bypass the tower, isolate and enter to clean or revise the downcomer pipes.

Each corrective action had an escalating negative economic impact, in both expenditures and loss of production. The worst-case scenario, however, was to try the least expensive method, restart the unit and then find that the chosen course of action was unsuccessful. Plant personnel needed more information about the location and cause of the problem so they could choose the most appropriate and cost-effective course of action. A gamma scan was determined to be the most cost-effective and easiest way to gather the needed information.

Gamma scan
Over the past 16 years, Tracerco Services Inc has employed its proprietary gamma scan technology as an online means to identify the causes of poor column performance due to mechanical and operational problems. Figure 2 shows a schematic of a trayed distillation column, an idealised gamma scan and the scan line orientation for the Tesoro debutaniser.

A gamma scan is performed by positioning a radiation source on one side of the column and a detector on the opposite side. The source and detector are aligned at the top of the column and synchronously lowered down the outside of the column. The orientation is normally chosen to scan through an active area of the tray or longitudinally through one of the downcomers. The amount of radiation that passes through the column is strictly dependent on the density of the material between the source and detector. The major factor affecting the amount of radiation passing through the column is the presence of liquid on the trays and the absence of liquid in the vapour spaces.

For a normal operating tray, the radiation readings decrease when froth, liquid and internals appear in the path between the source and detector. As the source and detector move below a tray, the radiation readings rapidly increase due to the fact that vapour is trapped beneath the tray. The shape and depth of the valleys created by the aerated liquid on each tray indicate the mechanical and operating condition of the trays.

Flooded trays
The first proprietary Tru-Scan of the Tesoro debutaniser column was performed at a pressure differential of 5.2 psi. This scan was performed across the southeast active area of the trays. The scan results are displayed in Figure 3. The results show a typical pattern of high radiation readings in the vapour spaces of trays 40 through 34 and low radiation readings on the trays. Trays 33 through 21, however, showed very low radiation readings, both on the trays and in the vapour spaces above the trays. This indicated that the disengagement zones above the trays were full of aerated liquid. Such a condition is defined as flooding. Flooding occurs when there is either a restriction in the downcomers and/or the vapour passing through the trays has too high a velocity. Flooding due to a restriction is known as liquid stack flooding. When the vapour velocity is too high, the flooding is referred to as jet or entrainment flooding. In both cases, tray performance is affected on the flooded trays, resulting in a loss of tray efficiency.

The gamma scan showed that the flooding was obviously starting at tray 21 and stacking liquid up the column. This scan also removed the suspicion that the pipe downcomers in the diameter transition were too small and causing the flooding. The scan clearly shows that trays 20 and 19 were operating normally.

Since the flooding in the first scan was so severe, the mechanical condition of the flooded trays was unclear. It was possible that tray debris from the damaged trays could have restricted the liquid flow by blocking off one or more of the downcomers. The Tesoro personnel were asked to lower rates until the pressure differential across the column was reduced. With the pressure differential reduced to 3.0 psi, the southeast active area was scanned again. The results are overlaid with the first scan in Figure 4.

The second scan of the southeast active area revealed that all of the trays had reasonable froth heights. This indicated that all the trays were mechanically sound. To gain further insight into the cause of the flooding, scans were also performed to measure the liquid levels in the downcomers. Figure 5 shows the results of the centre downcomer scan.

To avoid confusion from metal and liquid on the active areas of the trays, downcomer scans are performed longitudinally through each downcomer. The centre downcomer scan revealed a normal pattern for trays 40 through 29. The scan of the vapour space above tray 28 showed liquid flooding. This flooding was in the centre downcomers and extended down to the centre downcomer of tray 22. The vapour space above tray 20 and the centre downcomer of tray 20 were both normal.

A longitudinal scan of the southeast side downcomers was then performed. The results are overlaid with the results of the centre downcomer scan and shown in Figure 6. The southeast side downcomers were revealed to be full from trays 31 through 21.

The last scan was of the northwest side downcomers. The results of this scan are overlaid with the results of the centre and southeast side downcomers in Figure 7. The northwest side downcomer scan revealed that the downcomer of tray 33 was full of dense liquid. The downcomers of trays 31 through 21 were also full of liquid, but the liquid was less than half as dense as the liquid in the southeast downcomers of the same trays. The only explanation for this is that a significant portion of the vapour was unable to pass through the holes of the tray decks, but was forcing its way up the northwest side downcomers.

This pattern of vapour passing up the northwest downcomers indicated that the fouling causing the flooding in the column was not preferential to downcomers. It must have been closing off some of the holes in the tray decks as well to force vapour up the northwest downcomers.

Corrective action
With this information, Tesoro personnel decided to schedule a short shutdown of the tower to perform a water wash of the column. The wash water was analysed following the wash. The chloride ion concentration was high, indicating the presence of large quantities of chloride salts in the tower. Restart after the water wash showed the column to be operating at high rates with normal differential pressure and an overheads product that met the desired specification.

Conclusion
The Tracerco gamma scans, which cost $11 138 to perform, provided information that gave Tesoro personnel the confidence to choose the lowest-cost option for remediation of the column’s problem. When compared to entering the column to clean and make repairs, this approach saved them five days of tower downtime and approximately $1.4 million in expenditures and lost production.