Revamps 2024 Issue

showed taller and denser froths than the NE side (blue pen). The NE sides of trays 6, 7, and possibly 9 looked dry, while the SW sides looked normal and not heavily loaded. The ‘normal operation’ scan showed flooding on tray 10 only. Trays 11 and higher were not heavily loaded, with little difference between the NE and SW chords. This veri- fies that the flooding was due to a restriction near tray 10. The taller and denser froths observed on the SW of trays 6-9 in the flooded scan persisted on tray 9 in the ‘normal operation’ scan but became much less pronounced on trays 6-8. Only tray 6 in the ‘normal operation’ scan NE side approached drying. In search for the condition at which the downcomers began to fill up, the centre downcomers of trays 6-15 were scanned (see Figure 3 ) two weeks following the active area scans. The crude feed rate was set at 28,400 BPD and the flash zone temperature at 651ºF, similar to those in the flooded active area scans. To unload the downcomers and keep them out of the flood, the diesel pumparound flow rate was lowered to 2,700 BPD compared to 3,800 BPD in the active area scans. For the ‘more loaded’ scan (blue pen), the pumparound return temperature was set the same as during the active area scans, 211ºF. For the ‘less loaded’ scan (red pen), this temperature was raised to 244ºF. The pressure drop at both tests was about 2.4 psi and stable. Figure 3 shows no flooding in the tower, as confirmed by the low dP. The active area above the tray 10 centre downcomer approached flood, but the active areas above the centre downcomers of trays 12 and 14 were not heavily loaded. This again verifies that the first active area to flood is that above tray 10. Figure 3 shows that the froth height in the downcomer from tray 10 in the more loaded scan was about double that in the less loaded scan and approached the top of the downcomer. The froth in the more loaded scan was also much denser than in the less loaded scan. In contrast, the loading difference had little effect on the froth heights and froth density in the tray 12 downcomer (which also serves the pumparound), and its froth did not approach the tray above. This strongly supports a restriction in the tray 10 downcomer, plugging on tray 10, or both. The tests established that the flood was both vapour-sen- sitive and liquid-sensitive. The vapour sensitivity was established by the observation that raising the flash zone temperature by as little as 5ºF while keeping the crude rate, pumparound rate, diesel draw rate, and pumparound return temperature constant, induced flood. The liquid sensitivity was established by a large increase in froth height and den- sity in the tray 10 downcomer, resulting from cooling the pumparound return and the experience that increasing the diesel pumparound rates can bring about flood while keep- ing other variables constant. These scans established that the tower bottleneck is a downcomer back-up flood initiating in the downcomer from tray 10 and then building up and flooding tray 10 and above. The scans also provided evidence for uneven drying up of some of the wash section trays when the liquid rate below tray 9 dropped off. The drying occurred preferen- tially on the NE.

Jet 452˚F

PA return 5700 BPD 308˚F

From diesel stripper

Diesel/PA 528˚F

8 7 6

Overash 500 BPD

Crude feed 30,000 BPD 6 6 5˚F

Steam

Resid

and the diesel pumparound flow rate was 3,800 BPD. The key parameter changed was the coil outlet temperature. In the flooded scan, the flash zone temperature was 653ºF, lowered to 646ºF to give the ‘normal operation’ scan. The pressure drop at the flooded scan ranged from 2.8 to 3.7 psi, compared to 2.3 psi and stable in the ‘normal operation’ scan. The flooded scans (see Figure 2 , blue and red pens) showed flooding propagating from tray 10 upwards to trays 17-19 (Figure 2 was cut off at tray 15, but the scans continued all the way up the tower). In the flooded region (tray 10 and up), the scans show little difference between the northeast (NE) and southwest (SW) chords. On the unflooded trays below (6 through 9), the SW side (red pen) Figure 1 Tower schematic with typical rates and tempera- tures before October 2021

Revamps 2024