Catalytic solutions for processing shale oils in the FCC
Monetising tight oils requires suppression of unconventional metals contamination and gas formation, and adherence to flexible catalyst functionality
Kenneth Bryden, E Thomas Habib Jr, Olivia Topete and Rosann Schiller
Grace Catalyst Technologies
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As novel technology for hydraulic fracturing with directional drilling continues to develop, shale oil (also referred to as tight 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. Current estimates put shale oil production at ~10% of the 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. While drilling technology advances and the rapid growth of shale oil production have made forecasts difficult, the US Energy Information Agency (EIA) currently forecasts that US shale oil production will top 4.8 million bpd in 2021.1 Shale oil resources are not confined to the US. Recent analysis indicates that shale oil formations are located throughout the world and constitute a substantial share of overall global technically recoverable oil resources.2 The January 2014 BP Energy Outlook projects that by 2035 shale oils will constitute 7% of the total global oil supply, with more than one third of shale oil production coming from outside the US.3 While the North American refining industry undergoes a renaissance due to abundant tight oil, the new feeds present challenges as well as opportunities.
Shale oil properties
Shale oil is highly variable. Density and other properties can show wide variation, even within the same field.4-7 Shale oils are generally light, paraffinic and sweet. Table 1 presents properties of three shale-based FCC feeds compared to a typical Mid-Continent VGO. While most shale oils are low in nickel and vanadium, they have been found to be high in inorganic solids, iron and alkali metals.5,9 Table 2 presents metals analysis of several shale-derived feed streams. While metals levels in the samples vary (as would be expected for shale oil), iron and calcium levels are generally high. Reports from the field indicate that Bakken crude is typically low in nickel and vanadium, while crudes sourced from the Eagle Ford shale have higher nickel and vanadium levels that can vary significantly based on their source.
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 when 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 shale oil could become overloaded with light cuts, including reformer feed and isomerisation feed, while at the same time short on feed for the FCC unit and the coker. Many refiners report that while they are benefiting from favourable crude prices they often are struggling to keep downstream process units full. At low FCC utilisation rates, the alkylation unit is often unconstrained, leading to an octane shortage.
Unconstrained downstream units are just one of the challenges faced by North American refiners. Unconventional oils can vary wildly in composition from cargo to cargo. Receiving crude in batches via rail, truck or barge can result in FCC feed changing rapidly over the course of several weeks or several days. To increase utilisation rates, heavier crudes may be blended with lighter shale oils, resulting in a ‘barbell’ crude, which has a lot of material boiling at each end of the boiling point curve, but little in the middle, reducing VGO yield for the FCC. Some refiners have tried charging whole crude to the FCC unit in order to boost utilisations, to the detriment of other key yields such as FCC naphtha octane.10
At the FCC unit, the challenges range from difficulty maintaining heat balance when the feed is very light to unexpected coke make when contaminant metals rise rapidly. When operating with highly paraffinic light shale oil feeds that crack easily and produce little coke, the FCC may become circulation constrained due to low regenerator temperatures. Refiners report spikes of both conventional (sodium, nickel and vanadium) and unconventional metals (iron and calcium) when running shale-â€¨derived feeds. Sodium and vanadium deactivate zeolite and suppress activity; nickel promotes dehydrogenation reactions, leading to high gas make. Unconventional metals such as iron and calcium deposit on the catalyst surface and cause a loss of diffusivity, which leads to a loss in conversion and an increase in coke and bottoms. To maximise profitability with rapidly changing feed quality, catalyst flexibility is key.
Flexible functionality is absolutely critical for processing unconventional feeds. The recently developed FCC catalyst family, that of Achieve catalysts, is designed to provide refiners that flexibility while mitigating the challenges associated with processing these new hydrocarbon streams.
Achieve features an optimised matrix technology to provide coke-selective bottoms conversion without a gas penalty. The technology in the high diffusivity matrix of the catalyst is based on technology embodied in the Midas catalyst, which has been commercially proven to be more iron tolerant than competitive offerings. Achieve FCC catalyst contains best-in-industry metals traps for nickel and vanadium, which are highly effective to minimise coke and gas formation due to conventional metals such as nickel and vanadium. This FCC catalyst is also formulated with ultra-stable zeolite that retains activity in the face of contaminant metals spikes. The catalyst can be formulated over a range of activity, rare earth exchange and isomerisation activities, to deliver an optimal balance of gasoline yield to LPG while maintaining an optimum level of butylenes for the alkylation unit. Increasing catalyst activity, via zeolite or rare earth exchange, can alleviate a circulation constraint and restore the heat balance to a comfortable level. ZSM-5 based additives can be used to boost octane, but the associated yield of propylene is not always desirable. A better solution is to boost zeolite isomerisation activity within the catalyst to selectively increase the yield of FCC butylene and iso-â€¨butane, keeping the alky unit full and maintaining refinery pool octane. The following examples illustrate how the flexibility of the Achieve catalyst family can address the challenges posed by shale oil.
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