PTQ Q4 2026 Issue

the temperature and pressure of the overhead vapours. This hot, compressed vapour then serves as the heating medium for the column’s reboilers. The compressed vapour flows through the tube side of two horizontal thermosi - phon reboilers (Reb-A and Reb-B). In these reboilers, the hot vapour condenses and sub-cools, releasing its latent heat to the liquid propane on the shell side, which is drawn from the bottom of the C3 splitter column. This heat exchanger vaporises the liq-

C splitter

Downstream of FCC

Feed from deethaniser bottom

HPC suction drum

Heat pump compressor

C splitter reux pump

REB-A

REB-B

Overheads

Thermosyphon reboiler

Bottoms

Figure 1 Schematic of C3 splitter and associated units

within the column, providing an independent verification of the level measurement. • Reboiler early anomaly detection: A model to predict upset conditions in the critical thermosiphon reboilers, which are essential for column operation. By deploying these models online and presenting the outputs through an intuitive dashboard and email alerts, the system provides operators with actionable intelligence and significant lead time to intervene and stabilise the pro - cess, thereby preventing costly unplanned shutdowns. The following discussion will provide a detailed account of the process unit and incident details. Furthermore, the development and validation of predictive models, as well as the transformative impact of this data-driven approach on operational reliability, will be discussed. C 3 splitter process description and incident analysis The C 3 splitter column is designed to separate a feed of mixed C 3 hydrocarbons into high-purity propylene (over- head product) and propane (bottoms product). The unit’s operation is a finely balanced interplay of fluid dynamics, heat transfer, and pressure control (see Figure 1 ). The overhead loop is dominated by a heat pump compres- sor (HPC). Vapours from the top of the C3 splitter column are drawn into the HPC suction drum and then compressed by the HPC. The critical function of this compression is to raise

uid on the shell side, providing the necessary vapour traffic (boil-up) for the distillation process inside the column. The subcooled liquid from the tube side is then used as reflux, returned to the top of the column to control product purity. The level of liquid in the bottom of the column is a crit - ical process variable. It is maintained by a level indicating controller (LIC), which modulates the flow of the bottoms product (propane) out of the unit. A stable bottom level is essential to ensure a consistent liquid head for the thermo - siphon reboilers, which rely on the natural circulation driven by density differences. Incident chronology The unplanned shutdown on the day was not a sudden event but a creeping failure that unfolded over several hours. The detailed chronology, reconstructed from process data, reveals a clear causal chain. The following two terms are used in the description of the HPC trip incident date: ‘Trip-day’ is the date of the HPC trip; ‘Trip-previous-day’ is the day before trip-day.  LT malfunction (initiating event): At approximately 16:00 on trip-previous-day, the column bottom level trans - mitter began to malfunction. Its signal, which should have reflected the true liquid level, started to show a steady, non-characteristic rise, despite the actual level beginning to drop. This was the latent, unrecognised fault. A local level

gauge that could have pro- vided a manual cross-check was also known to be faulty.  Loss of column level: Due to the faulty LT reading, the control system was unaware that the actual liquid level in the column bottom was decreasing. The bottoms product withdrawal rate, as indicated by the flow control - ler, began a steep and con- tinuous increase from 18:00 onwards, effectively draining the column (see Figure 2 ).  Loss of thermosiphon in

Level transmitter Column bottoms

LT malfunction (unknown at time of incident)

Column bottoms withdrawal increases

06-09 12

06-09 14

06-09 16

06-09 18

06-09 20

06-09 22

06-10 00

Timestamp

Figure 2 LT malfunction and consequent increase in bottoms withdrawal

PTQ Q4 2026