Sep-2024

Maximising MS production from the isomerisation unit within VLI constraints (RI 2024)

At HPCL Mumbai refinery, gasoline (MS) is produced by blending six naphtha streams: Isomerate (from the isomerisation unit), reformate, light cracked naphtha, heavy cracked naphtha, diesel hydrotreating unit naphtha, and straight-run naphtha (sweet).

Akriti Garg, Udayakumar V Unnikrishnan, Sumedh S Shirsat and K Sri Ganesh
HPCL Mumbai Refinery

Article Summary

Each of these streams has distinct specifications. The blending process aims to maximise profit while avoiding quality giveaways and adhering to BS-VI specifications, including research octane number (RON), benzene, aromatics, olefin, sulphur, and vapour lock index (VLI).

Understanding VLI and constraints
The VLI is a measure used to predict the likelihood of vapour lock in gasoline engines. It is influenced by the fuel’s volatility, temperature, and pressure characteristics.

Key factors affecting VLI include:
• Reid Vapour Pressure (RVP): This measures the fuel’s volatility. A higher RVP indicates that the fuel is more prone to vapourisation, increasing the risk of vapour lock.
• Distillation characteristics: Fuel consists of hydrocarbons with different boiling points. The presence of lighter components that vapourise at lower temperatures can contribute to a higher VLI. The distillation curve of the fuel indicates how quickly different components evaporate. Fuels that evaporate too quickly at higher temperatures can lead to vapour lock.

VLI is calculated using vapour pressure in kPa at 100°F and the distillation profile per cent evaporated at 70°C, as follows:

VLI=10(VP)+7(E70)

The VLI for MS varies between summer and winter formulations to ensure optimal fuel performance under different temperature conditions. From April to July, the VLI limit is a maximum of 750, while from August to March, it is a maximum of 950. Achieving a lower VLI with the same RON for summer specifications presents a challenge for MS blending and production, leading to a reduction in MS production. To target lower VLI, the refinery restricts additional straight naphtha blending to MS; this would, in turn, reduce MS production.

Analysis
Table 1 depicts the various MS blending streams. From Table 1, it is evident that the isomerate RVP was around 13.5 psia vis-à-vis the design of 12.9 psia, leading to increased VLI of the MS blend. A detailed hydrocarbon analysis of the isomerate was carried out. It was observed that C₄s were getting slipped, resulting in the higher RVP.

Furthermore, it was observed that the C₄ content in the isomerate increased with the rise of C₄s in the feed. Upon further analysis of individual product streams from the isomerisation unit, it was found that the lighter components, along with isopentane, being removed from the deisopentaniser (DIP) column were the major contributors to the high C₄ content in the isomerate.

To mitigate this issue, the DIP was bypassed, and the total feed was routed to the isomerisation stabiliser section via reactors. The entrained C₄s were purged from the stabiliser, resulting in an isomerate RVP of 12 psia and restoring MS production to the targeted level, as shown in Table 2.

Additionally, by bypassing the DIP, 10 TPH of steam was saved, which resulted in a reduction of 2.15 TPH of CO₂ emissions.

Conclusion
The issue of elevated VLI in the MS product stream was primarily due to C₄ slippage from the isomerisation unit. This problem was addressed by bypassing the DIP section in the feed, which successfully reduced C₄ slippage.

However, this also led to a decrease in RON from 88 to 87. Despite the reduction in isomerate RON, this led to increased MS production. As the primary objective of the DIP section is to enhance the RON of the isomerate stream and thereby maximise the MS RON barrel, DIP operations can be optimised based on the MS blend requirements, with the additional benefit of steam savings from the DIP section.

This short article originally appeared in the 2024 Refining India Newspaper, which you can VIEW HERE