Catalytic cracking of crude oil to light olefins and naphtha: Experimental and kinetic modeling

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Highlights

  • Cracking of Arab AXL crude oil over E-Cat yielded 13 wt.% C2–C4 light olefins.

  • Blending E-Cat with MFI-280 increased light olefins yield to 21 wt.% for AXL feed.

  • Up to MFI Si/Al ratio of 280, there was slight effect on coke formation.

  • Four-lump kinetic model predicted experimental yields between 500 °C and 550 °C.

Abstract

The direct catalytic cracking of three light crude oils have been evaluated over an equilibrated FCC catalyst (E-Cat) blended with MFI zeolite in a microactivity test unit at 550 °C and catalyst/oil ratio between 1 to 4. At 60% conversion, the Super Light (ASL) crude oil yielded about 10 wt.% C2–C4 olefins and 60 wt.% naphtha over E-Cat, Extra Light (AXL) crude oil yielded 13 wt.% light olefins and 52 wt.% naphtha, while for Arab Light (AL) crude oil, light olefins and naphtha produced were 12 and 51 wt.%, respectively. The addition of MFI with varying Si/Al molar ratio (Z30, Z280 and Z1500) to E-Cat increased the yield of light olefins with a maximum at 21.3 wt.% for AXL over E-Cat/Z280. PIONA analysis of co-produced naphtha showed an increase in aromatics content over all additives with a maximum obtained from the cracking of AL over Z30 (91 wt.%). Steam treatment of Z280 led to a slight change in the yield of light olefins and reduction of naphtha aromatics for the three types of crude oils. A four-lump kinetic model accurately predicted experimental yields of AL cracking over E-Cat and E-Cat/Z280 between 500 °C and 550 °C. From the kinetic model, the apparent activation energy for the conversion of naphtha to gases decreased from 21.2 kcal/mol over E-Cat to 16.2 kcal/mol over E-Cat/Z280 which indicates that Z280 facilitated the increased cracking of naphtha-range species to light olefins

Introduction

While thermal cracking of various hydrocarbons remains the main source of light olefins, intense research is ongoing to develop cost-effective catalytic routes that are less energy intensive and that emit less carbon emissions (Rahimi and Karimzadeh, 2011). The major source of ethylene and propylene is the traditional steam naphtha cracker that supplies about 57% of global propylene as a by-product to ethylene production. The fluid catalytic cracking (FCC) unit is also an important source of propylene producing about 35% of world propylene as a by-product to gasoline production (Chau et al., 2016, Sadrameli, 2016). The remaining 8% of world propylene is produced by ‘on-purpose’ processes such as propane dehydrogenation, olefin metathesis and methanol-to-propylene.

The FCC process is known for its flexibility to crack a wide range of low-value heavy hydrocarbons as well as new potential feedstocks such as straight-run shale oils and bio-oils (vegetable and pyrolysis) through modifications to catalyst and operating conditions (Bryden et al., 2014, Vogt and Weckhuysen, 2015). However, light paraffinic oils contain lower molecular weight components boiling below 343 °C that are more difficult to crack especially the components below 221 °C with low coke production (Letzsch and Ashton, 1993). Corma et al. (2017) reviewed the direct catalytic cracking of crude oil to produce basic chemicals, mainly light olefins, using technologies derived from FCC. Several oil and chemical companies have patented various catalytic methods for the conversion of whole crude oil to light olefins and naphtha by integrating FCC unit with other processes (Powers, 2006, Long et al., 2014). Chen et al. (2015) suggested a two-stage FCC riser for the cracking of a mixture of shale oil and 30% vacuum residue to enhance the production of propylene.

Maximizing propylene yield in FCC unit requires specific catalysts or additives and the operation at high severity and short contact time (Buchanan, 2000). MFI additives can increase propylene yield from about 5 wt.% in conventional FCC unit to about 15 wt.% depending on the type of feedstock (Aitani et al., 2000). MFI additives crack C7–C10 gasoline range components to light olefins, mainly propylene and butenes (Arandes et al., 2000). The amount of MFI crystal added to USY-based FCC catalysts has increased with values as high as 25–30 wt.% are reported in some FCC units. High propylene yields in FCC result from naphtha over-cracking and low hydrogen transfer reactions as a function of the strength of additive acid sites and fast diffusion of desired light olefins (Awayssa et al., 2014).

Various methods have been developed for testing FCC catalysts and diverse feeds ranging from light hydrotreated vacuum gas oil (VGO) to blends containing atmospheric and vacuum residues (Corma and Sauvanaud, 2013, Bryden et al., 2015, Vogt and Weckhuysen, 2015). The early method was the Micro Activity Test (MAT) which uses a fixed-bed reactor to measure conversion by changing the catalyst-to-oil (C/O) ratio. Despite its various drawbacks related to operation mode and reactant–catalyst contact, the MAT method is still used. Other laboratory fluidized-bed techniques have been developed to overcome the above drawbacks and provide more realistic laboratory testing taking into consideration FCC process short residence time and fast catalyst deactivation. These techniques include the CREC Riser Simulator reactor (Arandes et al., 2000, Passamonti et al., 2012), ACE Advanced Cracking Evaluation (Kayser, 2000), micro-downer (Corma and Sauvanaud, 2013) and circulating riser pilot plant unit (Lappas et al., 2015, Bryden et al., 2015).

In this work, we have studied the direct catalytic cracking of three light crude oils in a MAT unit at FCC operating conditions using commercial E-Cat blended with MFI additive. The effect of MFI Si/Al molar ratio (30, 280 and 1500) and post modification by steaming on light olefins yield were evaluated. Kinetic modeling of crude oil cracking was conducted using a 4-lump model to determine the kinetic constants and the activation energies between 500 °C to 550 °C and catalyst/oil ratios of 2 to 4.

Section snippets

Feedstocks and base catalyst

The crude oils used in this study, Arab Super Light (ASL), Arab Extra Light (AXL) and Arab Light (AL), were procured from a domestic oil company. The properties of ASL, AXL and AL are listed in Table 1. ASL feed is a low-sulfur crude oil with an API gravity of 51° while AXL has an API of 39° and higher sulfur content was about 1.2 wt.%. Arab Light crude oil with an API gravity of 34 has the highest sulfur content (1.8 wt.%) of the three feeds. The naphtha fraction (C5221 °C) in ASL, AXL and AL

Catalyst characterization

XRD patterns of the MFI additives are shown in Fig. 1. All additives showed characteristic peaks of MFI between 8–9° and 22–25° (Baerlocher et al., 2001, Treacy et al., 1996). There was no change in crystallinity for all the additives.

The nitrogen adsorption isotherms exhibited type I isotherm for all additives which is typical of microporous materials. The increase in the adsorbed amount between p/p0 of 0.9 and 1.0 is due to the adsorption in inter-particle voids. The textural parameters

Reaction scheme

A kinetic study was conducted on the catalytic cracking of one of the feeds; AL crude oil. Crude oil consists of a large number of reactants which belong to different distillation cuts and chemical groups. Since it would be complicated to represent all the equations in a kinetic scheme, lumping of the reactants and products based on their boiling points is used to simplify the kinetic model (Ancheyta-Juarez et al., 1997). For crude oil cracking, a four-lump model comprising (HCO + LCO), naphtha,

Conclusions

This study has demonstrated the ability for direct catalytic cracking of crude oils to light olefins and high-aromatics naphtha under FCC conditions. The cracking of AXL crude oil (°API = 39) over E-Cat/MFI yielded 16.5, 21.3 and 19.4 wt.% C2–C4 light olefins for MFI with Si/Al ratio of 30, 80 and 1500, respectively, compared with 13.0 wt.% over E-Cat alone (at 60% conversion). A similar trend was observed with another light paraffinic crude oil (ASL) with °API of 51, yielding 14.5 wt.% light

Acknowledgements

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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