Catalytic Cracking of Light Crude Oil - American Chemical Society

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Catalytic Cracking of Light Crude Oil: Effect of Feed Mixing with Liquid Hydrocarbon Fractions Abdulhafiz Usman,†,‡ Abdullah Aitani,† and Sulaiman Al-Khattaf*,†,‡ †

Center of Research Excellence in Petroleum Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ‡ Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ABSTRACT: The direct catalytic cracking of Arab Extra Light (AXL) crude oil was conducted in a microactivity test (MAT) unit at 550 °C, where product recycling was simulated by mixing feed and liquid products in varying proportions. The effect of AXL mixing with total liquid product (LP) or a mixture of light cycle oil (LCO) and heavy cycle oil (HCO) was investigated at various ratios to determine the dependence of light olefins yield on feed composition. It was observed that mixing the least amount of LP with AXL yielded ∼20.3 wt % light olefins and 39.6 wt % naphtha. The cracking of AXL and LCO+HCO yielded similar results as the case of cracking LP. A configuration of 100% recycle of liquid product without blending with AXL feed showed an increase in the yield of light olefins from 20 wt % to 29 wt % associated with a decrease in naphtha yield from 40 wt % to 27 wt %. A second configuration of 100% recycle of LCO+HCO without blending with AXL feed showed that the yield of light olefins increased from 20 wt % to 26 wt % associated with an increase in naphtha yield from 40 wt % to 50 wt %. The study has shown that the recycling of low-value heavy fraction (LCO+HCO) is a cost-effective means to supplement the AXL feed without loss in the yield of desired light olefins and naphtha.

1. INTRODUCTION Refiners are looking for means to enhance their profit margins by integrating fuel production with the manufacture of selected basic chemicals.1 Intermediate refinery streams are utilized in steam cracking for light olefins production or in catalytic naphtha reforming for aromatics production. The changing properties of transportation fuels due to environmental regulations and the stagnant growth in the demand for transportation fuels have necessitated the coproduction of light olefins (mainly propylene) from the fluid catalytic cracking (FCC) unit.2 While the FCC process remains the main conversion unit for gasoline production, it is also the second important source for propylene after steam naphtha cracking.3−5 High severity operation (high reaction temperature and high catalyst/oil ratio) has increased propylene yield in FCC units from 5 wt % to more than 20 wt % in the cracking of vacuum gas oil (VGO) and other heavy fractions.6 Enhancing the production of propylene in the FCC unit requires the utilization of USY catalyst blended with stable zeolite additive.7−9 The main additive used in FCC catalyst to increase the yield of liquefied petroleum gas (LPG) is MFI (ZSM-5) zeolite, which allows the cracking of naphtha-range components to light olefins.10−13 MFI selectively cracks the C5+ olefins, thereby reducing the olefinicity of FCC gasoline and increasing propylene and butenes production. The flexibility of the FCC unit in handling a variety of conventional gas oils and vacuum residues has allowed refiners to utilize new feedstocks such as shale and tight crude oils. Various oil and catalyst companies have conducted research on the conversion of crude oil to naphtha and light olefins.14−17 A two-stage riser FCC unit was investigated for cracking a mixture vacuum residue and shale oil to increase propylene yield.15 © XXXX American Chemical Society

Corma et al. reviewed various strategies for the direct conversion of crude oil to chemicals utilizing the FCC unit and related catalysts.16 Our previous work17 has demonstrated that light crude oils can be cracked in a MAT unit under moderate conditions to yield light olefins and naphtha. Several studies have reported the recracking of various liquid hydrocarbon fractions in the FCC riser to increase the yields of naphtha and light olefins.18−22 While recycling part of the liquid products is done in the same riser reactor, the recracking of some of the products requires a costly separate reactor. Various naphtha schemes have been proposed and the optimum scheme was cofeeding cracked naphtha with VGO using the same feed injector to increase propylene yield.22 In another study,23 naphtha reactivity and propylene yield from the catalytic cracking of various types of naphtha increased in the following order: heavy-FCC cracked naphtha < light straightrun naphtha < heavy straight-run naphtha < light cracked naphtha. It was found that the cracking of liquid hydrocarbon fractions such as FCC naphtha or LCO increased the production of high-octane gasoline or light olefins, depending on the type of the feed, composition of the catalyst, and operation severity.24,25 The recycling of partially hydrogenated light cycle oil (LCO) and cracking over different Y zeolites with MFI additive suppressed hydrogen transfer reaction and yielded high propylene and butenes in LPG fraction.25 In this work, liquid products from the cracking of Arab Extra Light (AXL) crude oil were recracked over commercial equilibrium catalyst (E-Cat) and MFI additive by mixing the Received: August 7, 2017 Revised: September 22, 2017 Published: September 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b02324 Energy Fuels XXXX, XXX, XXX−XXX

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cracking of crude oil alone or mixed with liquid fractions. The E-Cat was calcined at 500 °C for 3 h at a rate of 5 °C/min before use. The commercial MFI additive was procured from Zeolyst Intl. in ammonium form (CBV28014 with nominal Si/Al molar ratio of 280). It was calcined in air at 550 °C for 3 h (3 °C/min) to get the Hform. The additive was pelletized, crushed, and sieved using an 80−90 μm sieve. The properties of the E-Cat and MFI additive are presented in Table 3. Based on our previous work,17 E-Cat/MFI-280 additive

AXL feed with liquid MAT products and commercial FCC liquid products to simulate two recycle configurations. Total liquid product and a mixture of LCO and HCO were mixed at different ratios with AXL feed in a MAT unit to study their impact on the incremental yields of naphtha and light olefins.

2. EXPERIMENTAL SECTION 2.1. Feedstocks. The AXL crude oil, LCO, and heavy cycle oil (HCO) used in this work were procured from an industrial FCC unit at a local refinery. LCO and HCO fractions were used as representative products obtained from the fractional distillation of the FCC liquid product in industrial FCC unit. The physical and distillation properties of AXL are presented in Table 1. AXL is a light

Table 3. Physicochemical Properties of E-Cat and MFI-280

Table 1. Properties of Arab Extra Light (AXL) Crude Oil property

value

gravity, °API density at 15 °C (kg/m3) sulfur (wt %) vanadium (ppm) nickel (ppm) microcarbon residue (wt %) kinematic viscosity, @ 20 °C (cSt) elemental analysis (wt %) carbon hydrogen nitrogen simulated distillation (°C) initial boiling point 50% final boiling point PIONA naphtha fraction (wt %)

property

E-Cat

MFI-280

SiO2/Al2O3 ratioa BET surface area,b m2/g micropore volume, cm3/g mesopore volume, cm3/g total acidity (NH3-TPD),c mmol/g

3.3 158 0.07 0.14 0.09

295 370 0.05 0.15 0.07

a Measured by ICP analysis. bDetermined by t-plot. cCalculated using high-temperature NH3 desorption peak.

39.3 828 1.6 2.7 3.0, only marginal increases in light olefins yield were observed. As shown in Table 4, the cracking of AXL produced a total light olefins yield of 20.4 wt % (2.8 wt % ethylene, 9.5 wt % propylene, and 8.1 wt % butenes). The yield of naphtha was the highest among all products (40.3 wt %) which may be attributed to the unconverted naphtha in the AXL feed and the formation of naphtha from the cracking of LCO and HCO fractions.29 The PONA composition of naphtha in the AXL feed and product showed an increase in aromatics content from 26.8 wt % to 48.4 wt % and substantial formation of olefins (26.0 wt %). This was likely due to the strength of acid sites within the MFI additive. Naphtha paraffins content decreased from 64.7 wt % to 22.0

Product Yield, % parameter

0% recycle

Cat, AXL feed was cracked using different ratios of E-Cat/MFI mixture. Our previous studies showed that the highest yield of light olefins was obtained at MFI additive concentration of 25 wt % blended with E-Cat for the cracking of VGO.10,11,27,28 Adewuyi et al.27 reported that the highest yield of light olefins occurred at an MFI concentration of ∼25 wt % associated with an increase of ∼150% in propylene and isobutene yield. However, above 25 wt %, the conversion of gasoline-range olefins was essentially complete, and the major effect of MFI was dilution of E-Cat with a concomitant loss in overall conversion. D


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Figure 4. Product yields (naphtha, light olefins, and propylene) from the cracking of total liquid product (LP) and AXL mixture at different recycle ratios.

Figure 5. Product yields (naphtha, light olefins, and propylene) from the cracking of LCO+HCO and AXL mixture at different recycle ratios.

easier than LCO and HCO molecules, because of the ease of diffusion in the zeolite pores. The naphtha yield was high because LCO and HCO fractions were also further cracked to naphtha and other light ends. The yields of ethylene, propylene, and butenes were 2.3, 6.3, and 4.2 wt %, respectively. These yields were lower than those obtained when AXL alone was cracked. This is because AXL has a higher content of heavier ends (37.5 wt %), which are light olefins precursors, than the LP fraction (14.3 wt %), as shown in Table 2. Since naphtha is converted to lighter ends upon the cracking of LP, it is desirable to separate the light ends and naphtha from the cycle oils (LCO and HCO) by fractional distillation in the refinery. The LCO and HCO fractions can then be further mixed and cracked with AXL feed at different ratios. The LCO and HCO fractions were cracked separately and as a 50:50 mixture to assess their contribution to the product stream. At a conversion of 31.2 wt %, the product yields obtained at a constant CTO ratio of 3.0 are shown in Table 4. For LCO feed,

wt % and naphthenes decreased from 8.5 wt % to 5.1 wt %, because of aromatization and cracking reactions.17 The yields of LCO and HCO from AXL cracking were 17.6 and 9.6 wt %, respectively. These results are quite similar to product yields obtained from the cracking of conventional FCC VGO feed.13 The yield of dry gas and coke formation in the spent catalyst were 4.4 and 1.6 wt %, respectively. 3.4. Cracking of Liquid Product (LP), LCO, and HCO. The liquid product (LP) fraction obtained from the cracking of AXL was cracked at the same experimental conditions in which the AXL feed was initially cracked. The conversion and product yields of LP cracking are presented in Table 4. The low conversion of LP (1.2 wt %) can be attributed to the dilution of the feed by its high naphtha content (59.6 wt %), which is highly aromatic and has a low reactivity.23 The highest product yield from LP cracking was naphtha at 40.0 wt %, followed by LCO (25.4 wt %) and HCO (14.5 wt %). It is worth mentioning that naphtha molecules in the LP fraction convert E


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heavy oil (HCO) and to the activity of the acidic matrix in the base USY catalyst.31 Generally, most commercial FCC catalysts have sufficient acidic matrix activity to provide some precracking of high-boiling-point materials. These precracking molecules are small enough to enter the zeolite pores for further cracking to LPG and naphtha. The product distribution from the cracking of LCO+HCO mixture is presented in Table 4. The mixture contains complicated chemical structures that can be cracked to form various gaseous and liquid products, which are more complicated in structure and composition. The results show that the yields of ethylene, propylene, and butenes were 2, 6, and 4 wt %, respectively, which are similar to those obtained from the HCO cracking. However, the naphtha yield increased from 17 wt % in HCO cracking to 27 wt % in the cracking of LCO+HCO. This indicates that the mixing of AXL with LCO +HCO fraction might lead to a higher overall product yields than if HCO alone is mixed. The conversion from the separate cracking of AXL and cycle oils decreased in the following order: AXL > LCO+HCO > HCO > LCO. This order may be consistent with the trends in the aromatics content and average molecular weight of the feeds.32 3.5. Recycle Configurations. A practical recycle approach which involves the mixing of LP with AXL feed and the use of this mixture as a feed into the MAT unit was tested.25 The mixing of AXL with LP at different ratios and recracking in the MAT unit results in different product distributions.21 The first recycle configuration was conducted to examine the effect of cracking total liquid product comprising naphtha, LCO, and HCO fractions obtained from the cracking of AXL, as shown schematically in Figure 2. The effect of various recycle ratios on the desired product yields was examined. A total LP of 67.5 wt % was obtained from the initial cracking of AXL in the MAT unit. The recycling of this entire amount and making it up to 100 wt % with 32.5 wt % of AXL feed represents 100% LP recycle. A blend of 33.8 wt % LP and 66.2 wt % AXL feed corresponds to 50% recycle while mixing only 16.9 wt % LP and 83.1 wt % AXL represents 25% LP recycle. The 0% recycle indicates that there is no liquid product (LP) in the feed stream: just AXL. The second configuration was conducted to examine the effect of AXL mixing with LCO+HCO fraction and cracking in the MAT unit as shown schematically in Figure 3. The effect of various recycle ratios on the desired product yields was investigated. A total yield of 27 wt % LCO+HCO product was obtained from the cracking of AXL in the MAT unit. Recycling this entire amount back into the MAT unit and making it up to 100 wt % with 73 wt % of AXL feed represents 100% LCO +HCO recycle. A blend of 13.5 wt % LCO+HCO and 86.5 wt % AXL feed corresponds to 50% recycle. The 0% recycle indicates that there is no LCO+HCO in the feed stream: just AXL. The results of the two recycle configurations are presented in Tables 5 and 6 and in Figures 4 and 5. These results indicate that there is no change in the yield of the light olefins (∼20 wt %), as a result of AXL mixing with LP or LCO+HO feeds, which have a lower content of light olefinic precursors. For instance, the yield of light olefins remained at 20 wt % for 0% recycle as well as 100% recycle (Figures 4 and 5). This suggests that the low reactivity cracked naphtha fraction in the LP and LCO+HCO feeds contributed very little to the total light olefins yield when the two feeds were mixed with AXL.23 Naphtha yield decreased slightly from 40.3 wt % to 37.5 wt %

Table 7. Product Yields from AXL Cracking Mixed with Recycled Liquid Product (LP) at 550 °C and CTO = 3.0 parameter

AXL feed

recycled LP feed

60.0

1.2

4.4 0.06 0.7 2.8 0.8 25.6 9.5 2.9 8.1 1.6 3.6 20.4 40.3 17.6 9.6 1.6

2.2 0.0 0.3 1.5 0.3 10.3 4.3 1.3 2.8 0.5 1.4 8.6 27.0 17.2 9.8 0.4

conversion, wt % product yield, wt % dry gas H2 C1 C2= C2 LPG C3= C3 C4= n-C4 i-C4 C2=−C4= naphtha LCO HCO coke

total yield

6.6 0.1 1.0 4.3 1.2 35.9 13.8 4.2 10.9 2.1 5.0 29.0 27.0 17.2 9.8 2.0

Table 8. Product Yields from AXL Cracking Mixed with Recycled LCO+HCO at 550 °C and CTO = 3.0 product conversion, wt % product yield, wt % dry gas H2 C1 C2= C2 LPG C3= C3 C4= n-C4 i-C4 C2=−C4= naphtha LCO HCO coke

AXL

recycled LCO+HCO

60.0

31.4

4.4 0.1 0.70 2.8 0.8 25.6 9.5 2.9 8.1 1.6 3.6 20.4 40.3 17.6 9.6 1.6

1.1 0.0 0.2 0.7 0.2 6.6 2.6 0.7 2.1 0.4 0.9 5.4 9.8 6.6 2.8 0.3

total yield, wt %

5.5 0.1 0.9 3.5 1.0 32.2 12.1 3.6 10.2 2.0 4.5 25.8 50.1 6.6 2.8 1.9

a naphtha yield of 31.8 wt % was obtained compared with a naphtha content of 28.6 wt % in the feed. The yields of ethylene, propylene, and butenes were 1.6, 5.4, and 3.8 wt %, respectively. At a conversion of 27.9 wt %, ∼46.8 wt % of LCO product was produced compared with 67.7 wt % in LCO feed. The high yield of LCO may be attributed to the composition of LCO feed which contains multiring aromatics that are difficult to crack.25,30 On the other hand, the cracking of HCO feed produced 43.2 wt % LCO which was the major product compared with 61.8 wt % in the HCO feed. At a conversion of 31.2 wt %, ∼21 wt % of HCO product was produced. The other yields from HCO cracking were naphtha (17.3 wt %), ethylene (2.1 wt %), propylene (6.1 wt %), and butenes (4.0 wt %). The cracking of HCO feed resulted in a higher yield of light olefins than cracking LCO (12.3 wt % vs 10.8 wt %). This may be attributed to the presence of larger amount of light olefins precursors in F


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Figure 6. Schematic representation of LCO+HCO recycle configuration for catalytic cracking of AXL crude oil with product yields (RP = recycled product; GP = gas product).

and 35 wt % upon increasing the recycle ratio from 0% to 100% for LP and LCO+HCO, respectively. While the yields of other products did not change, only coke content decreased from 1.6 wt % to ∼1.0 wt % for all AXL mixed feeds. 3.6. Strategy for Increasing Light Olefins Yield. A quantitative examination of the absolute product distributions obtained on cracking AXL feed followed by separate cracking its liquid products (naphtha, LCO, and HCO) was carried out. This was done to gain insight into the relative increases in light olefins yield and loss of naphtha fraction that may be achieved on recycling liquid product from crude oil cracking. This approach is not a practical recycle scheme, since it involves a separate FCC riser for the cracking of liquid product streams. Two separate cases were considered for this scheme. The first case considered was the separate cracking of total LP in the MAT unit. In Table 7, the results obtained from cracking the total LP yield of 67.5 wt % are shown. This fraction was cracked separately and added to the initial AXL product yields, as shown in Table 7. Some important conclusions can be drawn from these results. It can be observed that the overall yield of light olefins on recycling the entire liquid product was 29 wt %, which represents a 42% increase, compared to cracking AXL without recycle (20.4 wt %). The naphtha yield, decreased from 40 wt % to 27 wt % while the LCO+HCO yield remained almost unchanged at 27 wt %. While these results suggest that when LP is recycled into the MAT unit, only the naphtha fraction gets converted to light olefins, the LCO and HCO fractions are also recracked to lighter ends. The total yield of LCO and HCO (27 wt %) is the net yield from the conversion of all fractions in the LP feed (naphtha, LCO, and HCO). The recracking of the three fractions in the MAT unit at 550 °C and CTO = 3.0 has made these liquid fractions less reactive, because of the formation of heavier compounds.32

The second case considered for enhancing the yield of light olefins was the cracking of LCO+HCO fraction in the MAT unit without mixing with AXL feed. Table 8 shows that the overall yield of light olefins obtained upon recycling the LCO +HCO fraction is 25.8 wt %, which represents a 26.5% increase, compared to the cracking of AXL alone (20.4 wt %). The naphtha yield increased from 40.3 wt % to 50.1 wt %. This scheme, shown in Figure 6, provides a more attractive option than the cracking of LP because the amount of naphtha produced was significantly higher when LCO+HCO was cracked (27 wt % vs 50.1 wt %). Besides, the relatively lowvalue LCO+HCO fraction gets reduced significantly in the product stream from 27 wt % to 9 wt % when LCO+HCO fraction was recycled. However, in the case of LP recycling, the LCO+HCO content remained the same at 27 wt %. The two quantitative cases demonstrated that, in order to maintain a balance between high naphtha and light olefins yield, it is better to recycle LCO+HCO only after fractionation of the products, rather than the entire liquid product stream.

4. CONCLUSIONS It has been shown that the recycling of liquid hydrocarbon fractions obtained from the catalytic cracking of AXL crude oil is a viable means for increasing the yield of light olefins. Total light olefins yield increased from 20 wt % to 29 wt % on the separate cracking of total liquid product obtained from AXL cracking over E-Cat/MFI-280. A practical recycle configuration, which involves the mixing of AXL with its liquid product at different ratios, resulted in different product yields, depending on the composition of the mixed feed. The separation of LCO +HCO fraction from the liquid product stream and its recycling for further reprocessing with AXL in the same riser reactor seems to be a more viable recycle configuration than the cracking of the total liquid product. It is concluded that the G


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recycle of low-value heavy fraction (LCO+HCO) is a costeffective means to supplement the AXL feed without loss in the yield of desired light olefins and naphtha. This route is attractive to petrochemicals industries that are interested in the direct conversion of crude oil to petrochemicals. The study has shown that, at the laboratory level, it was possible to blend crude oil feed with its liquid products, crack them further, and obtain light olefins comparable to that obtained by the cracking of crude oil alone.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +966-13-860-2029. Fax: +966-13-860-4509. E-mail: [email protected]. ORCID

Sulaiman Al-Khattaf: 0000-0001-5071-4034 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support from the Ministry of Education, Saudi Arabia in the establishment of the Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP) at KFUPM.

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REFERENCES

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