Minimal modifications for maximum gains Figure 3 highlights the minimum modifications needed for a quick payback. It focuses on nozzle additions, tray adjustments, and partition plates in CDU and VDU, which can be implemented during routine annual turnarounds of 20-30 days without major shutdowns. The concept is also independently applicable to the VDU, enabling a low-modification, quick-fix implementation where required. This approach leverages existing infrastructure to deliver immediate Opex savings, avoiding extensive capital-intensive overhauls. In this configuration, no additional flash drums are envisaged, and a minimum ejector system is required to boost steam pressure for the vacuum stripping section. Moreover, the remaining steam is routed to the overhead ejector condenser, ensuring a balanced and stable vacuum circuit. Case study: Revamp of Bombay High unit The proposed configuration is demonstrated for a CDU-VDU processing Bombay High crude in a typical ~8 MMTPA refinery, representing a common operating scenario in Indian refineries. The scheme integrates partition- based segregation of rectification and stripping However, operating a column at low pressure increases the overhead vapour load, which can lead to higher duty requirements for downstream equipment, including the air cooler (AC-02), cooling water heat exchanger (E-02), and chiller (E-03). To address this, simulations were performed at reduced pressure and by adjusting the reflux ratio to achieve proper hydrocarbon separation and meet product specifications, while ensuring that the downstream sections‘ heat duty remained within acceptable limits. The key oper - ating parameters captured during the operating case and low-pressure simulation are tabulated in Table 1 . Results and discussion sections within the existing configuration, enabling improved control over vapour-liquid traffic and separation behaviour. Operating conditions, including pressure, temperature, and flow rates, were maintained in line with the existing base case, with product yields held within ±1% for consistent comparison. The broad product distillation ranges and specifications considered for the base case are presented in Tables 1 and 2 . The configuration incorporates steam-free Simulations were carried out with operating case feed over a range of decreasing stabiliser column pressure from 18 kg/cm²(g) to 14 kg/cm²(g) to evaluate the feasibility of operating the column at reduced pressure and to assess its impact on product quality and steam consumption. Based on the simulation results (Table 1), it was observed that operating the stabiliser column at 18 kg/cm²(g) is pressure of 20 kg/cm²(g) has been maintained for the oper - ating case. A feed with a higher proportion of low-boiling components requires lower stabiliser column pressure for efficient separation, thereby reducing the reboiler duty required to vaporise the lighter fractions. Thus, based on the feed composition analysis, it was observed that there is significant potential for energy saving by optimising the stabiliser column at a lower pressure. Figure 3 Relationship between stabiliser pressure reduc- tion on reboiler steam consumption and its effect on prod- uct quality specification 14 18 16 Pressure (kg/cm g) 21 12 14 Meeting product spec Pressure < 18 kg/cm g >Increased chiller duty >Retro t required to meet product spec 16 20 18 22 0.15 0.19 0.23 0.31 0.27 0.35 Stabili s er reboiler steam Exchanger chiller duty
restore pressure losses across the stripper and associated circuits, enabling its reuse within the CDU-VDU system. Depending on the achieved pressure level, the recovered steam can be reutilised for VDU stripping and, upon further boosting, for CDU stripping requirements, thereby reducing fresh steam demand. The core modification lies in the introduction of the partition and associated nozzle system, where vapour and liquid traffic are hydraulically segregated within the same shell. Vapours from the flash zone rise freely into the rectification section, while liquid streams are directed to the stripping section. Vapours generated in the stripping section are prevented from entering the rectification zone by the partition and are instead routed to dedicated overhead condensation systems. This enables independent control of stripping and rectification, effectively transforming a single column into a dual-zone system without major structural changes or vessel replacement. For revamp scenarios in Indian refineries, this configuration allows existing column shells to be retained, with optional addition of a flash drum where required, enabling implementation within typical 30-45 day shutdown windows without exceeding turnaround constraints. Heat integration is further enhanced by shifting selected pumparound duties to stripper condensers, resulting in improved preheat levels, reduced furnace duty, and lower NOx emissions, while also minimising maintenance requirements of radiant coils under high-sulphur Subsequently, the model predictions were validated against the actual plant conditions (operating case), con- firming the accuracy of DHA lab analysis and the selected thermodynamic fluid package. Further, Figure 2 presents a comparative analysis of the stabiliser feed composition between the base case and the current operating case, with differences expressed as delta wt% (Δ wt%). service. Overall, the configuration achieves seamless integration within existing hardware while delivering enhanced energy efficiency and operational flexibility, making it well-suited for brownfield revamp applications. The analysis indicates that the current stabiliser feed is lighter by approximately 1.5 wt% compared to the base case feed composition. Despite this shift, the stabiliser column detailed hydrocarbon analysis (DHA) along with relevant operating data, including temperature, pressure, flow rates, and steam consumption. • Thermodynamic framework: The Peng-Robinson equa- tion of state was selected as the fluid package for modelling, as it is highly recommended for predicting the pressure-vol- ume-temperature (PVT) behaviour of liquid and gas phases in petroleum processes. This equation of state provides accu- racy for hydrocarbon systems operating across wide pres- sure and temperature ranges typical of refinery operations. • Stabiliser column convergence criteria were established to meet stream specifications: a) C₅+ content in the FG is ≤0.1 wt%; b) HCl content in the stabiliser bottom stream is ~ 0 ppm (zero tolerance for corrosive species). • LPG stripper convergence was performed to ensure compliance with LPG product specifications: a) C₅+ content in the LPG product is ≤2.0 wt%; b) HCl content in the LPG product is ≤6 ppm. Figure 2 Difference of DHA composition from base case stabiliser feed with operating case feed, wt% -10 0 iC4 nC4 iC5 nC5 22 DMB 23 DMB 2MP 3MP nH MCP CH C7+ -5 10 5 15 ∆wt% (base feed-operating feed)
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