refining india 2024
Maximising MS production from the isomerisation unit within VLI constraints
Akriti Garg, Udayakumar V Unnikrishnan, Sumedh S Shirsat and K Sri Ganesh HPCL Mumbai Refinery
Introduction 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). Each of these streams has distinct spec- ifications. The blending process aims to maximise profit while avoiding quality giveaways and adhering to BS-VI specifi- cations, 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 gaso- line engines. It is influenced by the fuel’s volatility, temperature, and pressure characteristics. Key factors affecting VLI include: • Reid Vapour Pressure (RVP): This meas- ures 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 con- sists of hydrocarbons with different boiling points. The presence of lighter compo- nents that vapourise at lower tempera- tures can contribute to a higher VLI. The distillation curve of the fuel indicates how quickly different components evapo- rate. 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:
The vapour lock index 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
16
4
Product RVP
C in product
3.5
14
12
3
10
2.5
2
8
1.5
6
1
4
0.5
2
0
0
1 2 3
4 5 6 7 8
9 10 11
12 13 14
15 16 17
18
Days
Figure 1 RVP and C₄ in product on various days
6.5
7
5.75
C in feed wt% C in product wt%
6
5.05
4.49
5
4.45
3.59
4
3.1
2.99
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 isomer- ate RVP of 12 psia and restoring MS pro- duction 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 prod- uct stream was primarily due to C₄ slip- page 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 sec- tion is to enhance the RON of the isomer- ate stream and thereby maximise the MS RON barrel, DIP operations can be opti- mised based on the MS blend require- ments, with the additional benefit of steam savings from the DIP section.
2.53
2 3
2.38
2.06
1.61
1
0
1
2
3
4
5
6
Figure 2 C₄ in feed and product on various days
0 0.5 1 1.5 2 2.5 3.5 3 4
3.68
3.3
C in Feed wt% C in Product wt%
3.08
2.84
1.99
1.65
1.53
VLI=10(VP)+7(E70)
0.36
0.26
0.27
0.21
0.19
0.2
0.14
The VLI for MS varies between sum- mer and winter formulations to ensure optimal fuel performance under differ- ent temperature conditions. From April to July, the VLI limit is a maximum of 750, while from August to March, it is a maxi- mum 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 refin- ery restricts additional straight naphtha blending to MS; this would, in turn, reduce MS production.
1
2
3
4
5
6
7
Figure 3 C₄ in feed and product after DIP bypassing
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 fur- ther analysis of individual product streams from the isomerisation unit, it was found that the lighter components, along with isopentane, being removed from the deiso- pentaniser (DIP) column were the major contributors to the high C₄ content in the isomerate.
Contact: corphqo@hpcl.in
Stream
Quantity, tons/day
RON
RVP, psia
Benzene,
Arom.,
Olefin,
Stream
Quantity, tons/day
RON
RVP, psia
Benzene,
Arom.,
Olefin,
vol%
vol%
vol%
vol%
vol%
vol%
Reformate Isomerate
2,076
100.9
4.8 13.5 16.1
0.7
74
2
Reformate Isomerate
2,076
100.9
4.8
0.7 0.0 0.2 0.8
74
2
914 693
88
0
0 0
0
914 693
87
12
0.0
0.0
Light cracked naphtha Heavy cracked naphtha Straight-run naphtha
95.2 83.6 68.4 63.5 92.4
0.2 0.8
60.7
Light cracked naphtha Heavy cracked naphtha Straight-run naphtha
95.2 83.6 68.4 63.5 91.5
16.1
0
60.7
1,263
3.1
30
31
1,263
3.1
30
31
82
8
6
7
278 143
8
6
7
0 0
DHT naphtha MS production
143
3.5 8.3
1
13
DHT naphtha MS production
3.5 7.9
1
13
5,172
0.6
34.5
17.4
5,368
0.81
33.4
16.7
Table 1 MS blend with DIP in line ensuring summer VLI
Table 2 Typical properties of the streams with DIP bypassing operation
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