Capturing maximum value with tight oil feeds in the FCC
FCCs processing tight oil feeds require catalyst choices to deal with higher conversion, heat balance concerns, and higher sodium, calcium and iron.
Viewed : 5047
Tight oil production has changed the refining landscape for major portions of the US and market forecasts show dramatic changes in the international refining community as tight oil production rates continue to ramp up. According to Hart Energy estimates, tight oil production will represent 46% of domestic crude oil production in 2020.1 While tight oil is of higher quality, many US refineries have been configured to process the increasingly heavy sour crudes previously projected to be available. Fluidised catalytic crackers (FCC) at refineries processing tight oil have seen serious operating changes; choice of catalyst and overall catalyst management strategy is a critical factor in achieving successful optimisation. BASF is the market leader in supplying FCC catalysts to refineries processing tight oil. In general, tight oil FCC feeds are light, paraffinic, have different contaminants, and are low in vacuum gasoil (VGO) and residuum content. Typical challenges for FCCs processing these crudes include high amounts of liquefied petroleum gas (LPG), maintaining stable heat balance, higher alkali metals, increased iron loadings and reduced feed rates. This article will illustrate how BASF is helping refiners to capture maximum value with tight oil feeds through operating strategies, innovative catalysts and technical service. BASF has a diverse catalyst portfolio providing the flexibility required to help solve these problems including high activity, optimum delta coke and high iron tolerance.
Tight oil quality varies between oil fields and has been shown to be highly variable even from the same field. Batch shipping these crudes by truck and rail increases this variability. Given this, there are many general properties which tight oil crudes exhibit. Tight oil formations are relatively young, light, low boiling point, low in Concarbon, have high naphtha and distillate yields, lower vacuum gasoil cuts, contain almost no vacuum resid, have low contaminants of sulphur, nitrogen, nickel and vanadium but have higher sodium, calcium, potassium and iron. The high naphtha and distillate yields can choke the crude column, limiting crude rates. Low sulphur will reduce the sulphur load across the refinery, thus lowering the sulphur plant loads. Due to the high naphtha yield, which is more paraffinic, maintaining the octane balance can become difficult, requiring the need to maximise alkylation and reforming, and placing higher emphasis on FCC gasoline octane. With the low resid content of the crude, refiners may consider shutting down the resid processing unit and feeding the resid directly to the FCC. Crude compatibility also needs to be addressed when blending the light/sweet tight oil crudes with heavy/sour crudes, creating a ‘dumbbell’ feed that does not act as a homogeneous mixture, resulting in asphaltene precipitation.
Processing tight oil in the FCC brings benefits and challenges to refiners. The VGO cut of tight oil is typically light, low carbon producing, and low in contaminant sulphur, nitrogen, nickel and vanadium. Table 1 shows the VGO cut of two tight oils compared to West Texas Intermediate (WTI) and Maya. The resid cut of tight oils shows the same trend in properties as the VGO cut, including being light and low carbon producing. The light, low boiling point feed is easily converted to lighter products. The lower sulphur and nitrogen in the feed reduces gasoline sulphur and flue gas NOx and SOx, making it easier to meet regulations. With less nickel and vanadium, hydrogen and coke are lower. However, high conversion and high LPG yield may limit the gas plant’s throughput and thus the FCC rate. The lower coke making tendency of the feed can constrain the unit on heat balance and catalyst circulation.
While traditional contaminants are low, tight oil crudes can contain high iron, sodium and calcium, requiring higher catalyst addition rates. Iron in FCC feed in particular is a concern to many refiners.2 Iron deposits on the outside of the catalyst forming iron nodules which are spike-like protrusions on the surface of the catalyst. Figure 1 shows a BASF equilibrium catalyst (e-cat) having 1.5 wt% iron from processing tight oil. Iron acts as a mild dehydrogenation catalyst, increasing hydrogen and coke, and can act as a CO promoter (which can be a problem in partial burn units). The iron nodules reduce the e-cat’s apparent bulk density and, at very high levels, can cause pore mouth plugging. BASF catalysts such as the DMS and Prox-SMZ platforms3,4 have high porosity, giving excellent tolerance to pore mouth plugging due to iron. Figure 2 shows a histogram of all e-cats BASF analyses across the globe. The BASF units are highlighted in red, showing how BASF supplies the majority of high iron (over 0.9 wt% iron on e-cat) FCC units.
Sodium is an alkali metal which neutralises the catalyst’s acid cracking sites. BASF has some of the lowest fresh sodium catalyst in the industry, giving them excellent sodium tolerance. Figure 3 is a histogram showing how BASF has the majority of the low sodium e-cats. Abnormal contaminants seen in tight oil feeds include phosphorus, lead and barium but these are at low levels and have no discernible effects on FCC yields or catalyst selectivity.
Heat balance is normally the main challenge for refineries processing tight oil. Low coke producing feeds cause low regenerator temperatures and result in catalyst circulation constraints. With lower coke yield, ensure the air blower is rated for the lower air requirement and the pressure drop across the air grid is maintained. Minimum regenerator temperature is set by maintaining efficient coke burn, typically 1250-1260°F (677-682°C). Operating moves to increase bed temperature include using a CO promoter to reduce afterburn and heat up the bed, reducing partial burn or going into full burn, increasing feed pre-heat and using oxygen injection. Increasing delta coke through increasing HCO/slurry recycle (nozzle erosion may be a concern), lowering the FCC feed hydrotreater severity (if an option), and feeding more resid to the FCC will increase bed temperatures as well. Catalyst solutions to increase delta coke include higher e-cat activity through higher additions, higher rare earth (REO) content, or changing to a less coke selective catalyst. If the unit cannot maintain heat balance, some less desirable options to investigate are turning on the air pre-heater, adding torch oil and reducing dispersion or stripping steam. The air pre-heater and air grid designs need to be reviewed to avoid damage when operating outside of start-up. Adding torch oil and reducing dispersion or stripping steam cause the negative impacts of higher catalyst deactivation rates and burning higher value products over coke. Another concern with low regenerator bed temperatures is the unit may become circulation limited due to slide valve pressure drop. To reduce catalyst circulation, increase the feed pre-heat temperature which has the added benefit of increasing the liquid yield due to less coke production. If the unit has a fuel gas fired heater, operating at higher pre-heat temperatures also has the economic benefit of burning lower cost natural gas over burning coke. Changes in catalyst bed heights may also provide small benefits. A long term option is to change out the slide valve port size to remove the slide valve pressure drop constraint.
Due to the paraffinic nature of tight oils, they produce low octane straight run naphtha and low octane naphtha from the FCC. Catalysis options include reformulating to a lower REO catalyst which increases the gasoline octane but offsets the heat balance. Also, consider using ZSM-5 to increase the gasoline octane up to the wet gas compressor limit.
Add your rating:
Current Rating: 3