Sep-2020

Managing vacuum tower wash beds

ThruVision™ scans monitor the rate of coking in vacuum tower wash beds to get the most out of bed productivity.

Nicole Porter
Tracerco

Article Summary

The Approach - When properly operated, wash beds can help to maximise operating profits and increase HVGO yields. Should vapour temperatures be too high or the bed not receive enough “wetting”, problems can begin to arise. The bed may begin to coke up and drastically reduce its efficiency, potentially leading to shortened intervals for unit shutdown and maintenance, which can be incredibly costly and can reduce unit productivity. Tracerco’s ThruVision™ scanning technology is used to monitor wash bed density throughout the bed lifecycle. This allows the customer to determine operational conditions to achieve the highest efficiency and longest useful life of the wash bed.

The Field Test
A customer established a schedule to monitor the coking progress throughout the expected life of their wash bed using ThruVision™. The scans were conducted with a frequency of approximately every six months, starting with a baseline scan at the beginning of the run cycle then rescanned at the cycle mid-life and end of run.

You may be asking yourself “What exactly is a ThruVision™ scan?”. A ThruVision™ scan consists of a series of gamma transmission measurements with 360-degree coverage performed at a fixed elevation to generate a detailed cross-sectional density profile as illustrated in Figure 1.

The Analysis
At the beginning of the run cycle a ThruVision™ scan was performed to establish a baseline as shown in Figure 2. You can see that the density throughout the bed is uniform and are slightly above dry packing density, which is a result of the scan detecting the wash oil flowing through the bed. The vast majority of the bed was operating in the density range of 8.8-10.6 lbs/ft³ (141-1701 kg/m3). The high-density area in the southeast quadrant is due to two manways at that elevation.Approximately 75% of the bed is now operating with a density range of 12.5-18.1 lbs/ft³ (200-290 kg/m3). This scan was performed just before the end of run cycle and is indicative of a wash bed that had been fully utilised throughout its life cycle.

Figure 3 shows the scan conducted at mid-life through the run cycle. It should be noted that there are areas of higher density now present along the edge of the vessel walls and the density of the interior of the wash bed is increasing as well. Fifty percent of the bed is now operating with a density range from 10.6-14.4 lbs/ft³ (170-231 kg/m3). Since operating conditions of the column had not changed, the increase in density across the bed can be attributed to coke beginning to build within the packing.

When you look at the end of run scan results shown in Figure 4, the areas of higher density that were noted in the previous Figure 3 have continued to increase in density and have spread further throughout the packing.

Figure 5 is a representation of the density distribution data plotted on a line graph. As the density starts to increase you will no longer see the sharp peak with densities all compact. The bulk densities start to move to the right on the graph the longer the run takes place. This is another visual tool used to help with the understanding of fouling/ coking taking place in the bed.

The Conclusion
ThruVision™ scans are used to monitor the rate of coking within vacuum column wash beds. Monitoring the rate at which the bed is coking allows for the overflash rate to be adjusted to control the rate of coking. This will ensure maximum profits by squeezing the most productivity out of the wash bed and allowing it to operate at the highest possible efficiency throughout the units run. Avoiding any unnecessary shutdowns or early ends to unit run cycle will save significant amounts of money.