Sep-2020

ThruVision™ scans are the next step to investigate liquid maldistribution

ThruVision™ technology allows for a complete understanding of liquid distribution, saving days of production losses and massive costs of a unit shutdown.

Nicola 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 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 was experiencing process deficiencies that suggested inadequate liquid distribution characteristics after a previous power loss. The unit had a temporary power failure that resulted in a six hour loss of liquid flow to the vessel. Once flow was restored and the unit was back to normal operating conditions, they could not regain product quality.

The Analysis
A Tru-Grid™ Scan, consisting of four independent equal distance cords, was conducted on the vessel. The Tru-Grid™ Scan concluded that there was a need for further investigation of liquid distribution characteristics within a 180-inch (4.5 m) bed. The customer elected to perform three ThuVision™ scans within the packing at 12, 92 and 156 inches (0.3, 2.3 and 3.9 m) into the bed.

The first scan (Figure 1) conducted in the upper portion of the bed revealed a significant amount of high-density material within the north and west quadrants of the packing ranging between 25.0 and 30.0 lb/ft3 (400 and 481 kg/m3). The average combined process/packing density of 24.4 lb/ft3 (391 kg/m3) was identified at this elevation.

The second scan (Figure 2), conducted in the middle of the bed, showed low density zone in the southeast and central core of the packing. The average combined process/packing density of 20.1 lb/ft3 (322 kg/m3) was identified at this elevation.
 
The Conclusion
After relaying this information to the customer whilst onsite, it was determined that this high density material in the upper north to west quadrant of the packing was solids material that has formed during the power loss (Figure 4). The low density readings in the southeast to central core of the packing, seen on both the middle and lower scans, was operating at or near dry packing density of 15.6 lb/ft3 (250kg/m3). It was determined that the vapour inlet nozzle entering on the southeast orientation of the vessel had dried the packing during the power loss. The elevated density readings around the circumference was process material flowing down the walls due to the solids accumulations blocking its appropriate flow path.

The third scan (Figure 3) conducted in the lower portion of the packing showed lower density material in the southeast and central core of the packing when compared to the middle scan. The also appeared to be increased densities around the packing perimeter along the walls. However, the average combined process/ packing densities were around 19.5 lb/ft3 (312 kg/m3), which was only 0.6 lb/ft3 (9.6 kg/m3) lower than the middle average density.

The customer determined that the solids accumulation could be eliminated by increasing operating temperatures by 20-25°F along with increasing tower pressure. They effectively regained full production rates after only a few hours by liquidifying the solids accumulation online, saving them days of production losses and the massive costs of a unit shutdown.