PTQ Q2 2026 Issue

Water cooler

Gas

Heat recovery

Air cooler

Separator

Wash water

Wild naphtha-reux pump

Crude charge pump

.Naphtha frac

(In case of Pre-frac.)

Crude

SS1

Heavy naphtha Steam

Sour water pump

PA1

Crude column

Kero frac

Steam

SS2

PA2

Preheat-1

Steam

Demister

Kerosene

Diesel frac

PA3

(In case of Pre-fra sh )

Vac. diesel

SS3

PA4

Steam

Gas

Diesel

Vacuum column

Desalter

Pre-ash drum/ Pre-fractionator

M T section

CDU Over ash

Hot well

(Absence of Pre-ash/Pre-frac.)

LVGO

PA5

Stripping sec

Steam

Sour water pump

Slop oil pump

HVGO

PA6

Wash bed

VDU Over ash

Feed inlet device

Fired heater

Coil steam

Stripping sec

Preheat-2

RCO pump

(In case of Pre-ash/Pre-frac.)

Figure 1 Typical CDU-VDU configuration

occasionally requires cutting and replacing Monel-clad col- umn top shell sections. In the VDU, operators face additional difficulties when processing diverse crudes beyond design specifications, particularly with overflash management. Prolonged adjust - ments to the coil outlet temperature (COT) and unreliable overflash flow meters lead to poor wetting from inconsist - ent liquid distribution. This results in zonal coking at the wash bed base, where dry spots promote heavy hydrocar - bon adhesion and coke deposit formation. Uneven wetting triggers channelling and premature flooding, while tem - perature spikes of 54-90°F weaken metal sheets, causing coke lumps and scrambled sections. These issues require premature bed replacement every two to three years, com - promising HVGO quality with colour defects and escalating downtime costs. Additionally, managing entrainment requires drawing substantial unwanted slops above the flash zone to limit metals and colouring compounds in HVGO. In conventional crude and vacuum distillation processes, overflash and the slippage of light fractions into the bottoms, such as diesel slip or slop generation, are commonly practised to address these risks. Operators handling opportunity crudes with uncertain COT from fired heaters often maintain high over - flash levels to mitigate diesel colouring issues, which also involves higher slippage of diesel into the RCO. This practice subsequently overloads the vacuum distillation unit’s rectifi - cation section, particularly the vacuum diesel circuit. Similarly, in vacuum distillation, an increased flash zone vaporisation rate (flash rate) requires proportionally higher slop draws to prevent contamination of the HVGO with metals, asphaltenes, and colour bodies. Due to the limited

presence of lighter components in the vacuum residue (VR), these lighter fractions must be withdrawn as slop to control the light-end content of the VR and maintain its suitability as feed for the delayed coker unit (DCU). Vacuum distillation units further encounter persistent challenges when handling RCO and heavier fractions, aggravated by their reliance on steam to limit lighters in the vacuum residue. This steam, traditionally used to strip lighter components, generates excessive vapour traffic, straining overhead ejectors, compounded by unintentionally stripped vacuum diesel components from the VDU top section. In the stripping sections, sudden surges of medium-pres- sure (MP) stripping steam can mechanically stress the internals, potentially dislodging valves and tray panels. This not only disrupts vapour-liquid contact but also degrades stripping efficiency, leading to slippage of lighter distillates into the bottom residue. Since realignment or replacement of these tray panels can only be performed during a total plant shutdown, operators require due care to avoid steam pressure fluctuations. Low- pressure (LP) steam, while gentler, is avoided because its condensation within the column can wet light distillates and trigger pressure instabilities in the rectification zone. Furthermore, as the name implies, the flash zone oper - ates on a flash principle rather than true fractionation. Ideally, flashing separates lighter distillate fractions from the bottom residue; however, inefficiencies in the flashing process, exacerbated by entrainment from the feed inlet device, compromise this separation and contribute to the aforementioned issues. In contrast, dry distillation methods, which lack a ded - icated stripping mechanism, require maintaining an even

PTQ Q2 2026