likelihood of water condensation under operating conditions. This promotes corrosion in the column overhead system, particularly under fluctuating operating temperatures, and may necessitate frequent top tray replacements during maintenance and inspection (M&I) cycles, or even replacement of Monel-clad shell sections in severe cases. In the VDU, operators face additional challenges when processing diverse crude slates beyond design conditions,
particularly in overflash management. Prolonged adjustments to coil outlet
temperature (COT) and unreliable overflash measurements lead to inconsistent liquid distribution and poor wetting of wash beds. This initiates a cascade of operational
Figure 2 Structured packing and grid internals severely clogged with coke deposits in the wash bed above the flash zone
from the residue, enhancing diesel recovery but simultaneously increasing vapour traffic in the rectification section, thereby impacting column hydraulics and separation efficiency. The CDU residue (reduced crude oil, RCO) is further heated to 380-400°C in the VDU furnace and processed under vacuum in a similar single- column configuration. Overflash in both CDU and VDU is generated by partial vaporisation of the bottom product in the flash zone due to feed heating. This provides internal liquid for washing above the flash zone, which returns to the bottom section, ensuring improved product quality, particularly for diesel in the CDU and heavy vacuum gas oil (HVGO) in the VDU. Why revamp a conventional CDU-VDU The use of steam in conventional CDU-VDU systems, while essential for stripping lighter components in the stripping section, introduces significant challenges when it migrates into the rectification section. This results in vapour dilution, increased reflux requirements, and larger column diameters, thereby increasing both capital (Capex) and operating expenditure (Opex). In the CDU, the presence of steam reduces the margin to dew point, increasing the
issues, starting with localised dry zones, followed by heavy hydrocarbon adhesion, coke formation, and eventual fouling of structured packing. Uneven wetting triggers channelling and premature flooding, while temperature spikes of 30-50°C weaken metal sheets, causing coke lumps and scrambled sections. These effects often necessitate premature bed replacement, sometimes every two to three years, while also degrading HVGO with colour defects and increasing overall downtime. Additionally, the management of entrainment requires drawing substantial unwanted slops above the flash zone to limit metals and colouring compounds in HVGO. These combined effects significantly compromise the reliability and performance of structured packing in existing refinery vacuum columns, as clearly evidenced by the severe coke deposition observed in the wash bed above the flash zone (see Figure 2 ). Vacuum distillation units also encounter persistent challenges when handling reduced crude oil (RCO) and heavier fractions, aggravated by their reliance on steam to maintain lighters in the VR. This steam, traditionally used to strip lighter components, generates excessive vapour traffic, increasing
Refining India
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