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Light Cycle Oil Upgrading to High Quality Fuels and Petrochemicals: A Review. Georgina Cecilia Laredo, Pedro M. Vega-Merino, and Persi Schacht Hernández Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00248 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018
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Light Cycle Oil Upgrading to High Quality Fuels and Petrochemicals: A Review.
Georgina C. Laredo*, Pedro M. Vega Merino, Persi Schacht Hernández.
a
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, México 07730
CDMX, México.
*Corresponding author. Tel.: +52 55 9175 6615 E-mail addresses:
[email protected] (G.C. Laredo);
[email protected] (P.M. Vega-Merino),
[email protected] (P. Schacht Hernández).
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2 Abstract The processing of light cycle oil (LCO) for diesel fuel production by hydrotreating (HDT) process has been facing difficulties due to the currently stringent environmental regulations. The low-quality of this middle distillate with high sulfur, nitrogen and a high percentage of di-aromatic hydrocarbons, limits the possible upgrading alternatives. A HDT step, involving hydrodesulfurization, hydrodenitrogenation and partial hydrodearomatization, is combined with a hydrocracking (HYC) step for producing: 1) high-quality fuels (highoctane gasoline and ultra-low sulfur diesel) and 2) benzene, toluene and xylenes (BTX) enriched fraction. In this review studies regarding the HDT-HYC process for producing such products from real feedstocks are considered, trying to provide the state of the art developments for the LCO upgrading.
Keywords: LCO; BTX; High-octane gasoline; ULSD; Hydrocracking.
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Introduction Light cycle oil (LCO) is a middle distillate produced in the Fluid Catalytic Cracking unit. LCO is a very low-quality feedstock for diesel fuel production due to its high sulfur (up to 4.0 wt.%), nitrogen (up to 600 wppm), and aromatic contents (up to 90 wt.%)1,
2
which
make this stream difficult to be processed and comply with the worldwide stringent environmental regulations3, 4. The chemical nature of the LCO is highly aromatic (Figure 1). As an example, LCO from Mexican refineries contains up to 90 wt.% of aromatic compounds, and mostly of them, are di-aromatic hydrocarbons (i.e. naphthalene derivatives)2. By combining hydrotreating (HDT) and hydrocracking (HYC) steps, some researches have been able to upgrade LCO by producing high-octane gasoline and ultra-low sulfur diesel.5 Following a set of similar steps, other researches have obtained a benzene, toluene, and xylenes (BTX) enriched fraction5,6. In fact, some oil companies have commercial technologies for both processes5. In many cases, the production of both BTX and high-quality fuels from LCO firstly requires a selective HDT step to yield 1,2,3,4-tetrahydronaphthalene (tetralin) derivatives and also, to decrease the amount of sulfur and nitrogen contaminants. The product of the HDT step is then processed at the HYC section. A general scheme of the involved reactions should be kept on mind (Figure 2) in order to overcome the difficulties associated to the demanding operating conditions and the required high -active, -selective and -stable catalysts. The synergy of these factors can make the LCO upgrading economically feasible. Accordingly, upgrading LCO has been extensively studied using model mixtures. In this context, the catalysts required for both steps, play a key role in the partial hydrogenation of
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4 naphthalene followed by the selective hydrocracking of tetralin derivatives to one-ring aromatic hydrocarbons with alkyl chains6, 7. Bifunctional catalysts with acid (support) and metal (hydrogenation-dehydrogenation) functions are commonly used in research studies. It is comprehensible that a good balance between both functions is necessary for attaining the best catalytic performance7. The sulfides of group VI and VIII metals of the HYC catalysts comply the hydrocracking function; moreover, for the HYC of heavy oil feeds, metallicsulfide are considered more effective than metals.8 Also, when a catalyst or a process is almost ready for scaling-up or commercial application, experiments with real feedstocks are mandatory in process design and optimization studies9. This review intends to show results found in the available literature regarding the studies of real feedstocks like LCO for increasing its profitability either for producing high-quality fuels like high-octane gasoline and ultra-low sulfur diesel or as a source for valuable petrochemicals like BTX.
1. High-quality fuels from LCO A summary of the best experimental conditions for producing fuels from upgrading LCO is shown in Table S1 (supporting information). Fischer et al.
10,11
patented a two-bed catalytic process involving HDT followed by HYC
for producing high-octane gasoline fuel from a rich aromatic feedstock like LCO. The catalyst required in the HDT step is a nickel-molybdenum on alumina (NiMo/Al2O3) and in the HYC step, a noble metal like palladium on dealuminated Y zeolite. (Pd/Deal Y) is required. Preferred experimental conditions were 357 and 413 °C, respectively, 4.1 MPa, 1478 nL of H2/L of Hc, and a weight hourly space velocity (WHSV) of 0.6 h-1. The gasoline obtained attained an average value of research and motor octane numbers ((RON +
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5 MON)/2) up to 98. Conversions up to 20 wt.% were reached from a LCO having a final boiling point (FBP) of 385 °C, but increased up to 45 wt.% when the FBP of the LCO were 288 °C (42 wt.% of LCO) and up to 35 wt.% when the LCO had a FBP of 338 °C (70 wt.% of LCO).
A process for producing high-quality fuels was patented by Derr et al.12 Reaching a conversion as high as 55 wt.%, high-octane gasoline and high-quality distillate were coproduced by hydrocracking LCO at moderate hydrogen partial pressure (7 MPa). HYC conditions were: pressure 4.8-6.2 MPa, hydrogen to hydrocarbon ratio (H2/Hc ratio) of 270445 nL/L and temperature from 360 to 425 °C. The gasoline fraction may have a RON and MON as high as 100 and 88, respectively (average value of 94). The distillate fraction, boiling immediately after the gasoline fraction (>215 °C), was recycled to the hydrocracker to increase the paraffin content of this fraction by partial saturation and cracking of the aromatic hydrocarbons contained. In this way, a paraffinic distillate of’ low sulfur (
