As novel hydraulic fracturing technology with directional drilling continues to improve, shale oil will continue to be a game-changer for North American refiners. Although credited with many advantages, shale oil does not come without its challenges. Suppliers and processors alike are urgently working to adapt to the changing oil landscape. Just a few years ago, investments were focused on processing heavy crudes. Now, however, the industry is faced with lighter, sweeter crude streams from shale plays.
In varying degrees at each refinery, shale oil makes up only a percentage of the total feedstock. Present estimates put shale oil production at 5% of total US crude demand. The percentage could grow substantially as shale oil production increases and refiners invest in process modifications to handle this lighter feed.
The rapid introduction of shale oil to the refinery has come with interesting consequences for the fluid catalytic cracking unit (FCCU). To aid refiners in understanding the implications of shale oil, a detailed feed analysis and cracking studies of a representative Bakken shale oil and its fractions, compared to a typical Mid-Continent vacuum gasoil (VGO), are provided here. These results aid in understanding how best to optimize operations and maximize FCC value. A key element in this optimization is appropriate catalyst selection to overcome some of the challenges commonly associated with processing shale oil. In particular, the presence of iron and calcium in variable quantities can be addressed through the application of a catalyst with an optimized matrix and mesoporosity.
Properties of raw Bakken crude
Shale oil is highly variable. Density and other properties can show wide variation, even within the same field.14 For this study, a sample of raw Bakken crude was obtained from a refinery. The crude was light and sweet with an API gravity of 42° and a sulfur content of 0.19 wt%. Table 1 presents the properties of this sample, compared to published assays of Bakken, West Texas Intermediate (WTI) and Light Louisiana Sweet (LLS) crude oils.
The properties of the Bakken crude used in this study closely matched those in the published assay. Similar to other light crudes, raw Bakken crude has a low amount of FCC feed (< 28% at 680°F+). The straight-run (SR) Bakken sample was distilled into a 430°F− gasoline cut and a 430°F650°F light cycle oil (LCO) cut, and the properties of these cuts were measured. The gasoline composition and properties were analyzed via proprietary octane calculation software based on detailed gas chromatography analysis.6,7 The gasoline fraction from the straight-run Bakken sample was highly paraffinic and had low octane numbers [a research octane number (RON) of 61 and a motor octane number (MON) of 58]. The LCO fraction had an aniline point of 156°F and an API gravity of 37.6°, resulting in a diesel index of 59. While the Bakken crude sample was light and paraffinic, it also had a heavy end.
Light sweet crudes are generally easy to process, although challenges arise when these crudes are the predominant feedstock in refineries designed for heavier crudes. Shale oils, like other light sweet crudes, have a much higher ratio of 650°F− to 650°F+ material compared to conventional crudes. Bakken shale oil has a nearly 2:1 ratio, while typical crudes, such as Arabian Light, have ratios near 1:1.
A refinery running high percentages of Bakken oil could become overloaded with light cuts, including reformer feed and isomerization feed, while at the same time growing short on feed for the FCCU and the coker. Refiners running predominantly shale oil could shut down the vacuum distillation and coker units, and send the entire atmospheric tower bottoms (ATB) portion to the FCCU. Many refiners would still be short on FCC feed, and some have considered bypassing a portion of the whole shale oil around the crude distillation unit to fill up the FCCU capacity.
Also, while the highly paraffinic Bakken ATB would crack to high conversion, the expected low delta coke would result in low regenerator temperatures and possible difficulty in circulating sufficient catalyst to maintain reactor temperature.
Shale oil cracking yields
To examine the impact of shale oil on FCC yields, cracking was performed with whole Bakken crude, a 430°F+ distillation of Bakken, a 650°F+ distillation of Bakken and a reference sample of a typical Mid-Continent VGO. Feed properties are presented in Table 2. Cracking was done over an FCC catalyst in a fixed-fluidized bed advanced cracking evaluation (ACE) test unit8 at a constant reactor temperature of 980°F, using three catalyst-to-oil (C/O) ratios (4, 6 and 8) for each of the feeds. The catalyst used in the experiments was an FCC catalyst with an optimized matrix and mesoporosity, deactivated metals-free using a cyclic propylene steaming (CPS) protocol. Deactivated properties are given in Table 3.
Interpolated yields at a C/O of 6 are presented in Table 4. The whole Bakken crude resulted in low coke and a low-octane gasoline. While the whole Bakken crude yielded significant gasoline, much of the gasoline was from uncracked starting material in the feed. The yields of the 430°F+ and 650°F+ distillations of the Bakken crude were similar to those of the Mid-Continent VGO reference sample. The 650°F+ distillation of the whole Bakken crude had higher coke than the Mid-Continent VGO due to its heavier end and higher Conradson carbon number.
Results of processing SR shale oil
While FCC is typically used to reduce the molecular weight of the crude oil heavy fractions (such as VGO and ATB), in some cases refiners are charging whole shale oil as a fraction of the plants FCC feed. As a model case to understand the cracking of whole crude oil in the FCC and the effect of process conditions on yields, the whole Bakken crude described in Table 2 was processed in a circulating-riser FCC pilot plant at three riser outlet temperatures: 970°F, 935°F and 900°F.
As a reference case, the Mid-Continent VGO described in Table 2 was cracked at a riser outlet temperature of 970°F.9 The catalyst used in the experiments was a high-matrix FCC catalyst, deactivated metals-free using a CPS-type protocol. Deactivated properties are shown in Table 3.
Fig. 1 presents the yield structure of the starting feeds and the cracked products for a riser outlet temperature of 970°F. The Mid-Continent VGO is a typical VGO feed with a large portion of 650°F+ material and a small fraction of LCO-range material. When cracked, the LCO-range material cracks to liquefied petroleum gas (LPG) and gasoline, and the 650°F+ material cracks to the typical distribution of LPG, gasoline and LCO, resulting in a net increase in LCO.
| Fig. 1. Yield structure of starting feeds |
and cracked products for SR Bakken and
The whole Bakken crude starts with large fractions of gasoline and LCO-range material and a low amount of 650°F+ material. The amount of gasoline produced after cracking is high since the LCO-range material cracks to predominantly gasoline, and much of the starting gasoline is unconverted. LCO yields are low since there is little starting 650°F+ material to crack to LCO.
For the three different reactor outlet temperatures, plots of C/O ratio, gasoline, LCO and coke yields vs. conversion are shown in Fig. 2. As expected, lowering the reactor temperature increases the amount of LCO produced. Cracking SR shale oil produces little coke and bottoms. At the same conversion level, lowering the reactor temperature results in slightly more gasoline yield (due to increased C/O), which is consistent with prior research.10
| Fig. 2. Product yields as a function of riser outlet temperature and feed.|
At a riser outlet temperature of 970°F, the whole Bakken feed produces more gasoline, less LCO and less coke than the reference Mid-Continent VGO. Compared to the VGO, which produced gasoline with a RON of 93 and a MON of 80, the SR Bakken oil produced a paraffinic low-quality gasoline (at all three reactor outlet temperatures) with a RON of less than 80 and a MON of less than 70.
Synthetic crude from the pilot plant runs was distilled to recover the 430°F650°F LCO fraction. The aniline point and API gravity of the LCO were measured to calculate the diesel index, which is a measure of LCO quality. Fig. 3 presents data for LCO yield and quality as a function of conversion. Increasing conversion lowers LCO quality as a result of increased cracking of the LCO-range paraffins to lighter hydrocarbons. As seen in prior research,11 LCO quality follows LCO yield and did not appear to be influenced by reactor temperature at constant conversion. Diesel index values of the LCO produced by cracking whole shale oil were significantly higher than those obtained when cracking the reference Mid-Continent VGO. At a conversion of 78 wt%, the whole Bakken sample gave an LCO with a diesel index of 40, compared to a diesel index of 10 obtained for the LCO produced from the Mid-Continent VGO.
| Fig. 3. Effect of conversion level and |
feed type on LCO yield and quality.
This study of the effect of operating variables shows that whole shale oil responds to FCC operating conditions in a similar way to conventional oils. However, the product yield slate is substantially different in that good-quality (high-diesel-index) LCO is produced in the FCC along with large amounts of low-octane gasoline.
Metals in shale oil
While most shale oils are low in nickel (Ni) and vanadium (V), they have been found to be high in inorganic solids and in iron (Fe) and alkali metals.2,12 Table 5 presents metals analyses of whole Bakken crude, a Bakken 650°F+ distillation, and a sample of Mid-Continent VGO.
Also included in the table are other published metals analyses of shale oils. While metals levels in the samples vary, Fe and calcium (Ca) levels are generally high. Reports from the field indicate that Bakken crude is typically low in Ni and V, while crudes sourced from the Eagle Ford shale have higher Ni and V levels that can vary significantly based on their source.
To better understand the possible sources of metals in shale oil, the sample of whole Bakken crude was filtered through an 0.8-micron filter, and the solids were recovered. Scanning electron microscopy of the solids identified irregular micron- and submicron-sized particles. Energy dispersive spectroscopy maps of Fe, sulfur and Ca are shown in Fig. 4. The Fe in the sediments is associated with sulfur.
| Fig. 4. Energy dispersive spectroscopy |
maps of sediment in Bakken crude.
X-ray diffraction of the sediment identified the following crystalline phases: anhydrite (Ca2SO4), magnetite (Fe3O4) and pyrrhotite (substoichiometric FeS). Anhydrite and pyrrhotite have been mentioned in previous studies as being present in the Bakken formation.13,14 Based on this analysis, it appears that much of the iron in the Bakken crude comes from very small particles of iron oxide and pyrrhotite.
Iron and calcium effects
Fe and Ca have negative effects on catalyst performance. While particulate tramp Fe from rusting refinery equipment does not have a significant detrimental effect on catalyst, finely dispersed Fe particles in feed (either as organic compounds or as colloidal inorganic particles) can deposit on the catalyst surface, reducing its effectiveness.15,16
The Fe deposits combine with silica (Si), Ca, sodium (Na) and other contaminants to form low melting temperature phases, which collapse the pore structure of the exterior surface, blocking feed molecules from entering the catalyst particle and reducing conversion.17 Fe, in combination with Ca and/or Na, has a greater negative effect on catalyst performance than does Fe alone. The symptoms of Fe and Ca poisoning include a loss of bottoms cracking as feed particles are blocked from entering the catalyst particle, along with a drop in conversion.
Catalytic solutions for FCC processing of shale oil
The variability in shale oil properties requires a catalyst capable of process flexibility and metals tolerance. Several catalyst design factors are important. A catalyst that cracks deep into the bottom of the barrel increases FCC flexibility and maximizes total distillate yield. A moderate zeolite-to-matrix catalyst ensures that activity is not compromised while maintaining optimal bottoms cracking. The appropriate level of rare-earth exchange on zeolite is also a crucial aspect in maintaining optimal coke selectivity. A catalyst with optimized matrix and mesoporosity is a highly effective system for the effective processing of a variety of feedstocks.
Catalyst design can be optimized to resist the effects of contaminant Fe and Ca. High alumina catalysts, especially catalysts with alumina-based binders and matrices, are best suited to process Fe- and Ca-containing feeds because they are more resistant to the formation of low-melting-point phases that destroy the surface pore structure. Optimum distribution of mesoporosity also plays a role in maintaining performance because diffusion to active sites remains unhindered, despite high-contaminant metals.
The paraffin, aromatic and porphyrin molecules in the 700°F1,000°F boiling-point fraction of FCC feed have dynamic molecular sizes between 10 Angstroms (Å) and 30 Å.18 These molecules are too large to fit into zeolite pores (which are typically smaller than 7.5 Å) and must first be cracked by the matrix activity of the catalyst. For free diffusion of the 700°F1,000°F boiling-range molecules to occur, the catalyst pore diameter needs to be 10 to 20 times the size of the molecule, or 100 Å600 Å.18
Based on these considerations, a catalyst with an optimized alumina matrix and mesoporosity in the 100 Å600 Å range was designed.19 While two catalysts may have similar total pore volume, their mesoporosity can vary greatly. Table 6 compares the mesoporosity of this optimized matrix and mesoporosity catalyst to a competing catalyst. Note: The mercury intrusion method measures pore sizes greater than 36 Å, so the values represent porosity associated with the catalyst matrix only. Micropores smaller than 100 Å are undesirable and lead to poor coke and gas selectivity as the result of poor diffusivity and over-cracking.15 As seen in the table, the 100 Å600 Å mesopore volume of the optimized matrix and mesoporosity catalyst was twice that of the competing sample.
The resistance of the optimized matrix and mesoporosity catalyst to Fe and Ca poisoning was demonstrated in a commercial application. A refinery was processing resid feedstock high in Fe and Ca. Over time, the unit exhibited the symptoms of Fe poisoning. As Fe nodules built up on the catalyst surface, equilibrium catalyst activity, unit conversion and bottoms cracking all began to suffer. The refiner switched from a competing catalyst to one with optimized matrix and mesoporosity.a Upon switching, activity, bottoms cracking and coke selectivity improved, even at higher contaminant metals levels (Fig. 5).
| Fig. 5. Despite higher contaminant metals, |
the catalyst with optimized matrix and
mesoporosity improved bottoms cracking
and coke selectivity.
The shale oil boom has resulted in a renaissance in the North American refining industry. While shale oils are generally light, sweet and easy to crack, quality can vary greatly, and shale-derived feeds can contain sediments with high levels of iron and alkali metals. Catalyst formulations with optimized matrix and mesoporosity provide the best resistance to iron and calcium poisoning. Proper catalyst choice allows refiners to fully exploit the opportunities of shale oil while minimizing the detrimental impacts of processing.
The authors thank their colleagues at Grace for their assistance with the testing for this article. Special thanks go to Larry Langan for his analysis of the Bakken sediment.
a Graces MIDAS FCC catalyst contains a specialty alumina matrix and is designed to maximize the 100 Å600 Å mesoporosity optimal for coke-selective bottoms conversion.
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