rate vs the strip-out ratio. 5 However, actual stripping per - formance does not always match the theoretical trend because stripping steam rarely influences the column oper - ating pressure due to its effect on condenser duty.⁶ Adequate pre-commissioning and start-up guidelines are necessary to prevent side stripper equipment damage, including the reboiler. Poor stripping steam conditions and inadequate pre-commissioning and start-up procedures increase the chance of damage and result in loss of pro- duction. It is more emphasised when the CDU is started up from cold temperature conditions. Small amounts of water remaining in the unit can vaporise rapidly and gener - ate massive unsteady-state forces. Complete de-watering, slow operation moves, frequent temperature monitoring, and sequential stripping steam line heat-up strategy dur - ing unit start-up can minimise the chance of damage. The aforementioned superheated stripping steam conditions are also important to ensure damage-free operations. Case study 1: Background The CDU under discussion was designed to process light crude slates. The unit equips three side strippers: (1) heavy naphtha, (2) kerosene, and (3) diesel. The kerosene side stripper was originally configured as the reboiled side strip - per, while the other side strippers used steam as the strip - ping medium. The type of kerosene side stripper reboiler was a thermosyphon, and the bottom of the stripper was configured as ‘recirculation with baffle’. The diesel pump - around stream was used as the kerosene side stripper reboiler heating medium. During the operation, rapid fouling of the kerosene side stripper reboiler was experienced due to the presence of phosphorus. The stripping steam injection point was added to the kerosene side stripper in order to retain the desired kerosene stripping performance. The hybrid configuration, which uses a reboiler and stripping steam simultaneously, was adapted for kerosene side stripping. In day-to-day operations, the stripping steam rate was adjusted to meet the desired kerosene flash point, while the side stripper reboiler was running at fouled conditions. The CDU con - figurations, including the side strippers, are described in Figure 3 , where reboiler return and additional stripping steam nozzle locations are also illustrated. Problem description The CDU often experienced operation instabilities after the kerosene side stripper adapted the hybrid configurations.
XX
Pressure (psig) Vapour space
30
42
Stripping steam
43.5
Diesel pumparound
Kerosene
Kerosene pumparound
Figure 4 Local pressure survey result
operating conditions of the liquid distributor also down- grades the packed side stripper. Operations Overall stripping steam rates for the multiple steam strip- pers should be evaluated and optimised along with target product specifications and CDU constraints, especially the crude distillation overhead circuit. Excessive stripping steam consumption not only limits CDU capacity but also impacts the refinery steam balance. Since the kerosene flash point is one of key product specifications to be controlled, increased stripping steam consumption is often observed in the kerosene side stripper operations. The importance of kerosene flash control is emphasised when the kerosene product is routed to the mercaptan removal unit. On the other hand, if the kerosene product is blended into the diesel hydrotreating unit, controlling the kerosene flash point may not be critical. The diesel hydrotreating product stripper can control the kerosene flash point. Reducing stripping steam for the kerosene side stripper may be helpful to unload the unit constraints without sacrificing product specifications. Evaluating and optimising stripping steams for the heavy naphtha, diesel or atmospheric gasoil side strippers is helpful to unload the crude atmospheric column overhead condenser duty. The author has seen that unnecessary amounts of strip - ping steam rates are introduced to these strippers during the operation. Connecting actual stripping steam to prod - uct recovery is helpful in gauging the current performance and predict the improvement accurately. Watkins, R. shows the theoretical stripping steam trend as the unit stripping
Kerosene side stripper step test results
Operating parameter
Unit
Base 10.2 Base Base Base Base Base
Step 1
Step 2
Kerosene side stripper unit stripping steam ratio
lb steam /BBL of kerosene product
9.3
8.5
Kerosene product draw
BPD
+ ∆ 2% - ∆ 0.7 - ∆ 0.1
+ ∆ 3% - ∆ 1.7 - ∆ 0.1
Kerosene side stripper vapour return line ∆ P
psi psi
Kerosene side stripper tray ∆ P Kerosene side stripper ∆ T
ºF ºF
- ∆ 2
- ∆ 3
Kerosene flash point
- ∆ 0.6
- ∆ 1.4
Table 2
18
PTQ Q3 2025
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