PTQ Q3 2025 Issue

Comparison of pressure drop in the crude distillation column

Section

Pressure drop after Pressure drop after 1 month column implementing split operation, kg/cm 2 stream, kg/cm2

Light kerosene PA

Bypass

Light kerosene

HN draw to LKPA return LKPA return to LK draw LK draw to HKPA return Column top to flash zone

0.14 0.13 0.08 0.70

0.07 0.05 0.05

Heavy kerosene PA

0.5

Table 2

Heavy kerosene

in volatile phosphorus compounds that can accumulate in the crude column. Although the study indicated that fouling was the possible root cause of the increased pressure drop across the col- umn, a gamma scan of the crude column was performed to pinpoint the exact location of flooding. Four scan lines were used to evaluate the column’s hydraulic performance, with one of these scan lines represented in Figure 2 . The scan results provided the following interpretations: • Liquid accumulation and flooding were observed in the top section of the column down to the HN draw section, predominantly in the mid-zone. The bottom of this section showed very little liquid, indicating that the top section trays were holding the liquid while the bottom was drying up. • Liquid accumulation and flooding were also noted in the mid-section between the HN draw and LKPA return. • The LKPA section trays were found to be flooded at the bottom, with a significant pile-up of liquid. • The trays in the LK draw to HKPA return section appeared to be flooded at the top, with less liquid observed at the bot - tom of this section, indicating a liquid build-up at the top. The column could provide stable operation if: • The vapour-liquid traffic in the LK section is reduced. Reducing internal traffic in the LK section was necessary to overcome flooding caused by fouled trays in that zone. • Operating the column at a lower throughput until the turnaround. • Considering a short shutdown for rectification. An immediate shutdown to carry out modifications was not feasible. Although the pressure drop across the top sections was higher, the pressure drop across the HKPA section was lower compared to other upper sections. This indicated the possibility of transferring the load from the high pressure drop LK section to the HK section. By doing so, the loadings in the LK zone and the overall pressure drop could be reduced, thereby allowing the column to handle additional load. To achieve this without shutting down the unit, an uncon- ventional method of routing a slip stream from the LKPA to the HKPA return was devised by NEL. This approach aimed to partially bypass the internal reflux from the LK to the HK zone. The slip stream of LKPA to HKPA return was imple- mented without affecting the column profile and with mini - mal adjustment to pumparound duties. The main challenge was to complete the modification on the operating column and perform a hot tap. A process scheme was developed by NEL, which included taking a hot tap to route a portion of LKPA from the immediate LKPA

Figure 3 Arrangement of routing LKPA to HKPA

pump discharge to the outlet of the last HKPA pumparound exchanger. This scheme was executed and commissioned within a few days (see Figure 3 ). Post commissioning of the slip stream scheme included the following improvements: • Pressure drop across the column top sections came down to normal values. • Instability in product qualities disappeared. LK product qual- ities such as colour, flash point, and freezing point improved. • Crude throughput to the unit could be regained. Table 2 presents a comparison of the pressure drop in the crude distillation column before and after the implementa- tion of the split stream from the LK section to the HK section. A detailed analysis of operating data, simulation studies, and field activities conducted by NEL led to the timely iden - tification and resolution of crude column underperformance. This approach provided a short-term remedy that main- tained production levels until the next turnaround. Turnaround observations After about one year of operation, during the refinery turna - round in 2018, the crude distillation column was inspected, revealing pitting and fouling of trays across different sections of the column. • In the HN-LK fractionation section, top trays have observed severe pitting in the tray deck and downcomer area, along with minor to moderate fouling. Bottom trays observed severe fouling, though pitting was minor or insig- nificant. Valve openings were plugged, potentially restricting vapour flow and increasing pressure drop during operation. Cleaning required significant effort using a wire brush. • In the LKPA zone, fouling was noticed on the tray deck, with deposits increasing from top to the bottom tray. • In LK-HK fractionation and HKPA zones, hard and severe fouling was observed on the tray deck and downcomer area (see Figure 4 ). Most of the valve openings were blocked by deposits, with fouling more prevalent in the mid-section of the tray (see Figure 5 ). All trays were removed, cleaned with a hydro jet followed by grit blasting, and reinstalled. The LK-HK fractionation trays were replaced with new, identical trays. Laboratory analysis of tray deposits revealed signifi - cant quantities of phosphorus, along with iron and sulphur. • In the HK-LGO fractionation section, all trays were found to be clean, intact, and free from corrosion or fouling. • In the LGO PA, LGO-HGO fractionation, and HGO PA

PTQ Q3 2025