Innovative grassroots CDU-VDU configuration
Decoupling stripping and rectification sections via a dedicated flash drum to enable steam-free rectification for energy efficiency and net-zero pathways in refineries
Ramanayya Gorle, Grandhi Srivardhan, Narendra Kumar Paladugu, and Anil Kumar Engineers India Ltd
I n the evolving landscape of petroleum refining, where net-zero emissions targets are reshaping design par- adigms, the crude distillation unit (CDU) and vacuum distillation unit (VDU) remain foundational, accounting for more than 50% of distillate yields, including naphtha, kero - sene, diesel, and vacuum gas oils (VGO). This article intro - duces a CDU-VDU configuration tailored for grassroots refineries, decoupling stripping and rectification sections via a dedicated flash drum to enable steam-free rectifica - tion. By integrating a closed-loop high-temperature wash oil recirculation system, sourced from condensed stripper vapours, this design eliminates hydrocarbon dilution, boosts tray temperatures for superior heat integration, and miti - gates coking risks, achieving up to 40% steam savings and 16% power reductions without compromising throughput or product quality. Validated on a 172,000 barrels of oil per day (bopd) Bombay High crude facility, this innovation yields annual Opex savings of $2.05 million, a CO₂ footprint reduction of 24,000 metric tons/year (equivalent to 52.9 million pounds/ year), and enhanced flexibility for variable crude slates. For new-build projects, capital efficiencies arise from down - sized columns (up to 4% weight reduction) and optimised ejectors. This configuration not only addresses energy-in - tensive legacy challenges but positions grassroots refiner - ies as leaders in sustainable processing. Detailed process flows, thermodynamic insights, and economic analyses underscore its transformative potential as a technology for market expansion. The global refining sector, pivotal to energy security, pro - cesses more than 100 million bopd of crude into fuels and feedstocks that power economies. Yet CDU and VDU oper - ations, responsible for initial fractionation, consume roughly 30% of a refinery’s total energy, primarily through fired heaters, steam generation, and compression. As refiners target net zero by 2050, grassroots designs must embed efficiency from inception, countering rising crude diversity, margin pressures, and regulatory scrutiny on emissions. In a typical CDU-VDU, the CDU separates lighter frac - tions at atmospheric pressure, while the VDU processes heavier residues under vacuum to produce VGO and vac - uum residue (VR) without thermal cracking, as shown in Figure 1 . Crude oil is preheated to 482-572°F via a heat
exchanger network using heat from product streams and pumparound circuits, then heated to 644-698°F in the CDU furnace before being flashed in a single column, where vapours rise, and atmospheric residue settles. The integrated rectification zone refines vapours into naphtha, kerosene, and diesel with reflux, while side strip - pers remove lighter components. Steam injection at the bottom strips lighter hydrocarbons from the residue, boost - ing diesel yield but increasing vapour traffic. The CDU res - idue (reduced crude oil [RCO]) is heated to 716-752°F in the VDU furnace and processed under vacuum in a similar single-column design with an integrated rectification zone, using steam to strip lighters and produce vacuum diesel, VGO, and VR. Overflash in both CDU and VDU, generated by vapourising a portion of the bottom product in the flash zone due to feed heating, supplies liquid for washing above the flash zone, returning as liquid to the bottom section to ensure clean diesel and heavy vacuum gas oil (HVGO), respectively. The global refining sector, pivotal to energy security, processes more than 100 million bopd of crude into fuels and feedstocks that power economies Need for innovation The use of steam in conventional CDU-VDU systems, while essential for stripping lighter components in the strip- ping zone, introduces significant hurdles when it migrates upward into the rectification section. This migration dilutes hydrocarbon vapours, requiring higher reflux rates and larger column diameters, which elevate both capital and operational expenses. In the CDU, the presence of steam lowers the dew point at the column top, rendering it sus - ceptible to corrosion due to water condensation (promot - ing hydrochloric acid [HCl] corrosion from sodium chloride [NaCl] decomposition), particularly with temperature fluc - tuations. This necessitates frequent top tray replacements during maintenance and inspection (M&I) cycles and
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