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Aug-2018

Converting LCO to gasoline with high RON (TIA)

Light cycle oil (LCO) is an important byproduct of fluidised catalytic cracking (FCC). With the increasing severity of FCC operations, LCO with higher aromatics and lower cetane number is no longer appropriate as a blending component for clean diesel even after being hydrogenated.

Sinopec Research Institute of Petroleum Processing (RIPP)
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Article Summary
Converting LCO with high aromatics to gasoline becomes attractive to refiners. Accordingly, a new FCC technology, LCO to aromatics and gasoline (LTAG), was developed, which is capable of converting inferior LCO into gasoline with a high research octane number (RON).

LTAG technology combines selective hydrosaturation of LCO with selective catalytic cracking. The reaction mechanism is shown schematically in Figure 1. Firstly, diaromatics enriched with LCO are hydrogenated moderately into monoaromatics with tetralin structure. In the following FCC operation, monoaromatics crack readily into alkylbenzene which is a typical gasoline component with a higher RON. By contrast, the deep hydrogenation of diaromatics into bicyclic naphthenes is undesired in LCO hydrotreating since naphthenes will be converted to monocyclic naphthenes with a lower RON in FCC. By controlling the operating parameters of LCO hydrotreating, the selective formation of monoaromatics is guaranteed, together with low hydrogen consumption. With further optimisation on the FCC processing of HLCO, ring opening rather than hydrogen transfer of monoaromatics with tetralin structure occurs preferentially to produce alkylbenzene with a higher octane number.

LTAG can be operated in flexible modes, currently Mode I or Mode II (see Figure 2). Commercial trials with LTAG in both Mode I and Mode II have been successfully applied at a Sinopec refinery. In 2013, LTAG in Mode I was realised by the integration of No. 2 FCC unit and a typical diesel hydrogenation unit operated at a hydrogen partial pressure of 6.4 MPa. After the revamp, a HLCO conversion zone in the riser of No. 1 FCC unit, the conversion of LCO to gasoline was achieved by LTAG in Mode II in 2014.

For LTAG Mode I, LCO from a FCC unit and extraneous LCO are processed in a hydrotreating unit. The HLCO produced then returns to the original FCC unit as a sole feedstock. The preliminary calibration data collected have revealed a good result. When HLCO with a density of 0.89 g/cm3, hydrogen content of 11.3% and total aromatic content of 55.9% is used as the feedstock for FCC, a gasoline yield of 48.18% can be obtained at an HLCO conversion of 70.85%. The content of olefins in gasoline produced is only 5.9 vol%, while that of aromatics is 40.2 vol%. Furthermore, the RON of gasoline may reach up to 94.8.

For LTAG Mode II, both heavy oil and HLCO serve as the feedstock. To process HLCO and heavy oil separately in a single riser, an additional conversion zone at the lower section of the FCC riser is introduced. According to the scheme shown schematically in Figure 3, one may identify two conversion zones of the FCC riser in Mode II; that is, the upper part for heavy oil cracking and the lower part for HLCO cracking. The operating conditions and the performance of LTAG in Mode II during commercial trials is as follows: HLCO with a density of 0.91 g/cm3, hydrogen content of 10.95% and total aromatic content of 76.8% is used as one of the feedstocks of FCC. Compared with a traditional FCC operation, the yield of gasoline is increased from 42.95% to 59.03% and the yield of LCO is reduced from 21.25% to 0.94%. What is more, the content of olefin in gasoline produced is reduced by 20.2 vol% and that of aromatics is increased by 18.11 vol%. At the same time, the RON of gasoline is increased by 0.6 units. In the conversion zone of HLCO, the selectivity of gasoline can be as high as 78.83% at an apparent conversion of HLCO of 70.2%.

LTAG is characterised by the 
following technical features:
• Once through conversion of HLCO up to 70%
• Gasoline selectivity up to almost 80%
• Hydrogen consumption of about 2-2.5%
• For gasoline produced in Mode I, RON is over 94 and olefin content is about 5 vol%
• For gasoline produced in Mode II, olefin content could be reduced by 4-5% and RON increased by 0.5-1.0 units.

Up to now, there have been 19 LTAG applications in China and more units are under modification or construction. According to statistical data from the LTAG units available, the average hydrogen consumption in hydrotreating units is merely 2.0-2.5% since a moderate LCO hydrogenation condition is vital to the reaction mechanism involved in LTAG. For LTAG in Mode I, the once through conversion of HLCO and the yield of gasoline are about 70% and 48.5%, respectively. For LTAG in Mode II, the once through apparent conversion of HLCO and the selectivity of gasoline in the HLCO conversion zone are about 68% and 75%, respectively. In the meantime, the content of olefins in gasoline produced is reduced by about 20 vol% and that of aromatics is increased by about 15%. Also, the RON of gasoline is increased on the whole by 0.5-1.0 units. Based on a comprehensive evaluation of product slates, product quality, energy consumption and hydrogen consumption, the incremental profit margin of LTAG in Mode II is $23.55 per tonne of recycled LCO.

Efforts are now focused on further reducing the hydrogen consumption of LTAG. In addition, the maximisation of BTX production from LTAG is under development. It was found that the light fraction of LCO contains a high percentage of mono-aromatics with long alkyl branches. In FCC processing, these aromatics can be converted to monoaromatics with short alkyl branches, resulting in an increasing octane number for gasoline, higher than 100. In the next generation of LTAG, which has completed its work in the laboratory and is now being commercialised, LCO will be separated into light and heavy fractions first. The light fraction is fed directly back to FCC without hydrotreating, while the heavy fraction is subjected successively to hydrogenation and FCC.

For more information: gongjh.ripp@sinopec.com

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