Frequent level loss in the kero- sene side stripper bottom and kerosene product draw insta- bility were observed during the operation. Troubleshooting Kerosene draw tray leakage and/or the kerosene side strip- per tray fouling were suspected through initial troubleshooting. Multiple column scans of both the crude atmospheric column and the kerosene side stripper were conducted to confirm restricted areas. However, the scanning did not support initial suspicions. The scans did not indicate any liquid entrainment above the stripper top tray or flooding symptoms across the kerosene side stripper trays. No kerosene draw tray leakage was shown in the crude atmos- pheric column scans. A local pressure survey between the crude atmos- pheric column and the kerosene side stripper was arranged to review the stripper hydraulics. The survey results are illus- trated in Figure 4 . Measured pressure drop values across the kerosene side stripper were in the normal range. Meanwhile, the survey disclosed extremely high pressure drop measure- ments across the kerosene side stripper overhead vapour line. Scrutinising the hydrau-
O gas
Cold re ux
Sour water
Top pumparound
Unstabilised light naphtha
Heavy naphtha
Stripping steam
Kerosene
Diesel pumparound
Stripping steam
Diesel
Desalted crude
Stripping steam AGO
Crude heater
Stripping steam
ATB
Figure 5 Case study 2 crude distillation unit configuration
earlier, the reboiled stripper only generated the stripped hydrocarbon vapour amount. On the other hand, the hybrid configuration required the presence of the stripping steam, which was not considered in the original kerosene stripper vapour return line sizing. The amount of stripping steam increased the pressure drop across the kerosene stripper vapour return line and limited the kerosene draw rate. Optimisation solutions Operation step testing was arranged to identify optimum stripping steam rates without hydraulic restrictions. Table 2 summarises the results. Reducing stripping steam helped reduce the pressure drop across the kerosene side stripper overhead line and increase kerosene draw. Meanwhile, a downgraded kerosene flash point was still within the target ranges. Case study 2: Background The CDU for the second case study was commissioned
lics around the kerosene stripper revealed a root cause. Hydraulic balance was constructed based on the kerosene stripper draw and return geometries. CDU flow sheet topol - ogies in commercially available steady-state process simu- lators did not cover the hybrid configuration of the kerosene side stripper. Therefore, a modified simulation topology was developed to represent the given configuration and define pertinent process conditions for reliable hydraulic calcula- tions. The calculations verified extremely high pressure drop across the kerosene stripper vapour return line and matched measured tray pressure drop values. The measured and sim- ulated values for current operations were summarised and compared to the original design conditions in Table 1 . The kerosene draw rate and the stripper tray pressure drops were similar between the two cases. However, the simulated stripper vapour return flow was extremely high during the current operations. The kerosene side stripper vapour return line sizing was based on the original kero- sene stripper configuration (reboiled stripper). As discussed
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