PTQ Q2 2026 Issue

Water cooler

Gas

Heat recovery

Air cooler

Separator

Wash water

Wild naphtha-reux pump

Crude charge pump

Reux

Sour water

Naphtha frac

(In case of Pre-frac.)

Crude

SS1

Ejectors/Eductors

Heavy naphtha

PA1

MP steam

Crude column

Kero frac

Pre-ash drum/ Pre-fractionator

Preheat-1

Demister

PA2

SS2

Vac. diesel

Kerosene

PA4

(In case of Pre-f l a sh )

Gas

Diesel frac

PA3

M T section

SS3

Vacuum column

Steam

Desalter

Feed inlet device

Diesel

LVGO

PA5

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

Sour water pump

Slop oil pump

CDU over ash pump

HVGO

PA6

Wash bed

Heat recovery

CDU ash drum

Separator

Feed inlet device

VDU ash drum

VDU Over ash pump

Stripping sec

CDU stripper

Ejectors

Steam

Sour water Coil steam

Feed inlet device

RCO pump

VDU wash oil pump

Heat recovery

Preheat-2

Stripping sec

Fired heater

Can be used as heater purging steam/top ejector

Fired heater

VDU stripper

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

Figure 2 Novel CDU-VDU configuration for grassroot refinery

higher COT to prevent lighters from slipping into the bottom residue. This higher COT demands increased fired heater duty and elevates the risk of thermal cracking. As a result, effective fractionation across the flash zone remains elusive, perpetuating these operational risks and inefficiencies. Structural and operational innovations The novel CDU-VDU configuration is engineered to redefine traditional distillation through structural and operational innovations. It decouples the stripping and rectification sections using a flash zone, allowing each to operate under optimised conditions. This means that the stripper can operate at a different pressure profile to optimise stripping efficiency, unlike the conventional design where the pres - sure below the flash zone is linked with the rectification section top pressure and pressure drop. This configuration combines the advantages of utilising steam, considered one of the best stripping media, for stripping purposes, alongside the adoption of a dry column configuration in the rectification section. Additionally, wash oil recirculation is integrated into the design. Moreover, steam optimisation strategies are implemented, such as condensing stripping vapours in the VDU stripper overhead to separate hydro- carbons, enabling its use as pure steam to save stripping steam in the CDU-VDU strippers. Process flow and integration The process flow is depicted in Figure 2 . Crude oil enters the CDU, preheated via heat exchanger network using product streams and pumparound circuits, then heated in

the furnace, flashed in the flash zone, with bottom liquid stripped in a separate stripper and vapours routed to a steam-free rectification column. The CDU residue (RCO) is sent to the VDU using a similar steam-free rectification approach. In the VDU, the stripper overhead is connected to a separate ejector system to isolate hydrocarbons from the stripper overhead steam, which can be re-utilised as strip - ping steam. The closed-loop wash oil recirculation system is strategically deployed above the flash zone in the VDU wash section. This system repurposes condensed stripper vapours as high-temperature wash oil to ensure uniform wetting, enhancing reliability and preventing coking. By eliminating steam in the rectification zones of both the CDU and the VDU and integrating this closed-loop system, the design mitigates coking, enhances heat integration, and reduces energy consumption. Additionally, the con- densed hydrocarbons from the stripper overheads of both the CDU and the VDU contain minimal metal and colouring compounds, improving product quality, with further purity enhancement in the CDU stripping section through wash - ing with stripping steam condensate. As illustrated in Figure 2, with a dedicated flash drum, wash oil from the CDU stripper column can serve as a motive fluid in the last-stage ejector of the VDU top overhead. It absorbs C₃/C₄ hydrocarbons from the hotwell off-gas (fuel gas) through a sponging effect. It is then returned to the CDU wash zone for hydrocarbon recovery. Figure 2 builds on the conventional CDU-VDU scheme in Figure 1, with additional details marked in red to emphasise the changes to be considered in a grassroots refinery CDU-VDU.

PTQ Q2 2026