Kerosene yield Unit stripping steam consumption
Time
Figure 7 Operation step test result
Kerosene yield
Figure 6 Kerosene yield and flash point trend
kerosene side stripper capacity reached the limit, while the crude atmospheric column retained ample capacity. Operating step testing was arranged to verify the root cause analysis. The unit performance was monitored while the kerosene draw rate was increased with small steps. Figure 7 plots the step testing trend results. The test results reveal the kerosene draw rate could not be increased unless stripping steam rate was reduced. This trend clearly confirmed that the kerosene side stripper was maxed out. Tray hydraulics showed the kerosene side stripper diameter was not big enough to handle the target vapour and liquid traffic. Any tray upgrade/modification could not resolve the limitation. Solutions/modification The aforementioned heavy naphtha stripper was idle and could be repurposed as an additional kerosene side strip- per. However, it was found that the liquid head between the crude atmospheric column kerosene draw and heavy naphtha stripper feed was insufficient for gravity hydrau - lics. All four side strippers were stacked and erected as one single column shell, with the heavy naphtha stripper located on top. The kerosene cannot be withdrawn and introduced to the heavy naphtha stripper using gravity hydraulics. The process evaluation for the root cause analysis also identi- fied poor AGO side stripping performance. Simulation modelling indicated less than one theoretical stage across the existing structured packing. CDU diesel recovery was reviewed to cover the case of the AGO side stripper being eliminated. A process review indicated that the diesel recovery was not impactful without AGO side stripping. Gravity hydraulics between the crude atmos- pheric column kerosene draw and the AGO stripper were confirmed.
with four side strippers: (1) heavy naphtha, (2) kerosene, (3) diesel, and (4) atmospheric gasoil (AGO) services. The heavy naphtha side stripper is out of service since all naph- tha boiling range materials are run down as overhead dis- tillate. All side strippers were originally designed as trayed strippers. Original trays in the AGO side stripper were later converted to structured packing through the previous mod- ification. Stripping steam is used as the stripping medium for all side strippers. Figure 5 illustrates the CDU with the side strippers. Problem description The CDU experienced poor kerosene flash point when the kerosene product rate was approaching the target yield value. Operating trends of kerosene yields and flash points are plotted in Figure 6 . Downgraded kerosene flash points required additional naphtha stripping actions through the downstream kerosene hydrotreating unit. This poor perfor- mance increased Opex. Moreover, since stripped naphtha was routed to the other CDU overhead circuit, the increased stripped naphtha amount unnecessarily loaded the other unit capacity. Root cause analysis A dedicated process evaluation was conducted to identify the root causes of kerosene yield limitations. Validated pro- cess simulation model and detailed tray hydraulic reviews were used. The evaluation indicated that the crude slate pro- cessed in recent operations contained a much higher per- centage of kerosene boiling range materials compared to the original design crude slate composition. The kerosene side stripper capacity was identified as the root cause. Calculated
Case study 2: performance comparison
Operating parameter
Unit
Pre-mod operation
Pre-mod simulation
Post-mod simulation
Kerosene yield
BPD
Base Base
Base Base
+ ∆ 10% + ∆ 6ºF
Kerosene flash point
ºF
Kerosene side stripper unit stripping steam ratio Crude atmospheric column top temperature Crude atmospheric column overhead condenser duty
lb steam /BBL of kerosene
5.9
5.9
10.1
ºF
Base
Base Base Base
- ∆ 9ºF + ∆ 2% + ∆ 8%
MM BTU/hr MM BTU/hr
- -
Top pumparound duty
Table 3
20
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