Overhead
Top re ux
O - gas
1
Re ux
9
Light naphtha
Feed
15
Top pumparound
21
100%
65%
45
LV
Heavy naphtha side str i pper
0%
46
Side reboiler
47 48
49
Bottom reboiler
Kerosene side stripper
Kerosene pumparound
Bottom
Figure 5 Operational moves for process optimisation
Figure 6 DeIsoButaniser (DIB) configuration
Scrutinising the heavy naphtha draw circuit revealed another root cause. The heavy naphtha draw circuit geom- etries and pressure measurement values are summarised in Figure 4 . The hydraulic balance was constructed based on the draw geometries and measured pressure values. The simplified hydraulic balance is also illustrated in Figure 4. Heavy naphtha flow to the side stripper was controlled by the side stripper level. The static inlet pressure of the level control valve was 35 psig. If the upper draw was full, pressure at the upper draw location would be 10 psi higher. A static pressure of 45 psig was ample to maintain driving force. Meanwhile, measured pressure at the heavy naphtha lower draw was not matched to possible head pressure at the tie-in point unless the upper draw was completely empty. In this hydraulic balance, the lower draw would be pushed back into the crude atmospheric tower by the upper draw liquid. This undesired reverse flow caused seven underloaded fractionation trays and downgraded fractionation performance. Liquid from the heavy naphtha upper draw bypassed seven fractionating trays. It resulted in low tray efficiency. Kerosene pumparound instability was also reviewed. The kerosene pumparound configuration was previ - ously depicted in Figure 1. Three different draw nozzles were available. Operators thought a single nozzle was not large enough to draw both product and pumparound streams. Kerosene product and pumparound streams
were withdrawn at different trays. The configuration was also believed to maximise fractionation between kerosene and diesel. A detailed tower internal evaluation revealed that sub- cooled pumparound liquid could be continuously sucked into the kerosene product draw nozzle. This undesired flow behaviour caused less pumparound return liquid, con - tributing to heat transfer. Pumparound heat removal could be reduced by less condensation. 1 Reduced pumparound flow due to recent circuit fouling exaggerated pumparound instability. Case study 1: Optimisation solutions Operational moves illustrated in Figure 5 were imple- mented to prevent the heavy naphtha reverse flow and
Crude atmospheric tower performance
Operating parameter
Pre-optimisation Post-optimisation
Crude charge, b/d
Base Base Base Base
Base
Heavy naphtha yield, b/d
+ ∆ 5%
Kerosene yield, b/d
Base
Kerosene flash point, ºF Gap (kerosene T10- heavy naphtha T90), ºF
+ ∆ 2ºF
Base
+ ∆ 5ºF
Heavy naphtha tail (EP-T90), ºF
Base
- ∆ 15ºF
Table 1
91
PTQ Q4 2023
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