REFINING INDIA 2025

refining india 2025

Application of CFD as a diagnostic tool to resolve flow-induced failures in process units

Pruthviraj Nemalipuri, Arun Kumar, Abdul Quiyoom, Pranab K Rakshit, and Prashant Nandanwar Bharat Petroleum Corporation Limited

Background: Case 1: Reactor effluent air cooler (REAC) tube plugging issue in DHDS unit The diesel hydrodesulphurisation (DHDS) unit was commissioned in 2000 with a design capacity of 2.0 MMTPA, later revamped in 2005 to increase capacity to 2.54 MMTPA. The REAC, DDE4 A-D, is constructed using duplex metallurgy and consists of four bays: A, B, C, and D. Monthly thermography is carried out to monitor its thermal performance and detect any abnormalities. Since June 2023, recurring tube plug- ging incidents have been observed in bays DDE4A and DDE4B, as indicated by ther- mography. To mitigate salt deposition in the upstream section of the REAC, the hot separator temperature was increased by 14°C. Wash water maximisation also did not improve tube plugging phenomena. During the scheduled catalyst replace- ment shutdown in August 2023, the REAC tubes were cleaned. Despite the cleaning, tube plugging reappeared in November 2023 and increased in subse- quent months, with plugging consistently confined to bays A and B. No issues were detected in bays C and D. Another shut- down was taken in April 2024 to perform tube cleaning. Deposits collected during both shutdowns were analysed in the lab- oratory, revealing that the primary com- ponent was ammonium chloride salts. The recurring plugging pattern necessitates a thorough investigation to identify the root cause and implement an effective solution. Case 2: Hydrogen blistering issue in rich amine flash drum The vacuum gas oil hydrodesulphuri- sation (VGOHDS) unit, commissioned in 2011 with a design capacity of 1.7 MMTPA, includes a rich amine flash drum which receives rich methyl diethanolamine (MDEA) from the recycle gas scrubber and cold flash drum off-gas scrubber. This ves- sel operates at a pressure of 7-8.5 kg/cm² and a temperature of approximately 40°C, with pressure maintained via split-range control using make-up hydrogen. Excess pressure is relieved through a knockout drum (KOD), and the MDEA level is regu- lated to the amine regeneration unit. The vessel is protected by two pressure safety valves (PSVs) set at 10.5 kg/cm². During the 2024 turnaround inspec- tion, significant hydrogen blistering was detected on the internal surface of the flash drum, particularly in shell courses two and three, covering an area of approx- imately 6 m² across four zones. Around 300 hydrogen blisters were identified below the amine inlet nozzle. Ultrasonic thickness measurements showed localised wall thinning, with remaining metal thick- ness ranging from 10 to 11 mm. These findings point to evolving corrosion pat- terns, warranting a detailed analysis and design review to prevent recurrence.

Plugging (red dots) in DD E4A tubes

Plugging (red dots) in DD E4B tubes

Figure 1 Plugging of the REAC tubes

Case Study 2: CFD analysis to identify and mitigate wall corrosion in rich amine column During a turnaround inspection of the VGOHDS unit, severe hydrogen blister- ing was observed on the internal surface of the rich amine flash drum ( Figure 3b ). Preliminary analysis suggested that flow- induced corrosion, possibly due to improper nozzle orientation, was the root cause. To investigate further, a CFD study was conducted using the actual drum geom- etry. Simulations evaluated internal flow patterns for three nozzle orientations: 180°, 90°, and 45°. The results showed that the existing 180° orientation led to direct jet impingement of H₂S-rich amine on the vessel wall ( Figure 3c ), causing localised turbulence and corrosion. In contrast, the 90° orientation redirected the incoming flow away from the wall, resulting in smoother flow patterns and significantly reduced wall impact ( Figure 3d ). Based on these insights, the nozzle ori- entation was modified to 90° and imple- mented during the plant turnaround, which is expected to minimise wall erosion and wet H₂S corrosion, thereby extending the equipment life and improving overall relia- bility of the rich amine flash drum. These studies demonstrated the effec- tiveness of CFD as a diagnostic and design optimisation tool in refinery oper- ations. By visualising internal flow pat- terns and predicting the impact of design changes, CFD enables informed decision- making, reduces trial-and-error modifi- cations, and significantly improves the reliability and safety of critical process equipment. By visualising internal flow patterns and predicting the impact of design changes, CFD enables informed decision-making

D2 D1 C2 C1 B2 B1 A2 A1

Figure 2a Snapshot of plant REAC manifold (existing); 2b Exiting manifold geometry and predicted phase contours; 2c Modified geometry and predicted phase contours

1 0.889 Velocity Magnitude (m/s)

0.778 0.667 0.556 0.444 0.333 0.222 0.111 0

Figure 3a Snapshot of amine column; 3b Snapshot of H₂ blusters on the wall; 3c Predicted velocity contours on the wall with existing inlet nozzlen; 3d Predicted velocity contours on the wall with modified inlet nozzle

inlet manifold configuration as a poten- tial contributor to this issue. To validate these concerns and explore improvement opportunities, a detailed CFD study was conducted. Multiphase CFD simulations were per- formed to compare the gas-liquid velocity profiles and wash water dispersion pat- terns in the existing vertical inlet mani- fold ( Figure 2b ) with a proposed horizontal arrangement ( Figure 3b ). Results from the simulation revealed flow maldistribution in the current vertical configuration, with poor phase mixing. In contrast, the horizon- tal configuration achieved a more uniform flow distribution and enhanced gas-liquid mixing, ensuring better wash water disper- sion ( Figures 2a and 1b , predicted phase fraction contours). These findings aligned with the licensor’s assessment and provided strong technical justification for modifying the REAC inlet manifold design. The CFD-driven insights supported the implementation of the hor- izontal manifold configuration, expected to improve wash water effectiveness, min- imise corrosion risks, and significantly reduce the likelihood of future tube plug- ging in REAC.

The application of computational fluid dynamics (CFD) as a diagnostic and trou- bleshooting tool is valuable for diag- nosing complex flow issues in refinery process units, offering detailed insights into internal fluid flow dynamics that are difficult to measure experimentally. In this work, CFD was applied to address two critical challenges: flow maldistribu- tion in the DHDS unit’s REAC and hydro- gen blistering in the rich amine flash drum inlet nozzle flow impingement on the vessel wall of the VGOHDS unit. Through simulation and design evaluation, CFD enabled root cause identification and guided effective design modifications to improve the reliability and performance of both systems. Case 1: Mitigation of REAC tube plug- ging through CFD analysis In the DHDS unit, plugging of the REAC tubes, particularly in DDE4A and DDE4B ( Figure 1 ), raised operational concerns. Investigations pointed towards uneven flow distribution and poor wash water dis- persion as likely causes. Corrosion stud- ies, along with recommendations from the process licensor, identified the REAC