Bottom of the barrel upgrading
The predicted and demonstrated role of additives in raising bottoms product yields and properties from the FCC unit
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One thing is certain in the ever-evolving world of refining: refiners need to capitalise upon opportunities that allow them to profitably crack increasingly difficult feedstocks in the fluid catalytic cracking (FCC) unit. This is apparent in the sheer quantity of new technologies appearing in the industry within the past few years. Two decades ago, Albemarle pioneered the technology platform from which the original Bottoms Cracking Additives (BCA) were developed. Subsequent improvement of these first BCAs, with enhanced zeolites and metals trapping components, has resulted in today’s BCMT series of additives.
Although market demands vary globally, refinery economics always present a considerable driving force towards reducing bottoms. Traditionally in the European marketplace, distillate yield is optimised on the FCC unit. The demand for diesel fuels has been driven by aggressive automobile efficiency regulations and policies favouring diesel-powered vehicles. Conversely, North American refiners have typically operated the FCC unit to generate the optimal amount of gasoline barrels. Recent regulations within the US, such as the Renewable Fuels Standard, have upset the traditional paradigm. Now, North American refiners are finding themselves in long positions in both gasoline and octane. This has resulted in the oversupply of high-octane gasoline and depressed gasoline margins. Coupled with increased global demand for diesel, many refiners have shifted focus away from optimising the gasoline yield towards increasing diesel production instead.
Figure 1 shows the concept of the yield space of the FCC unit. The yield space defines, in a three-dimensional co-ordinate system, the primary factors to be considered when optimising the FCC unit profitability. Economics always drive bottoms minimisation (vertical axis), market factors determine the predominant yield (horizontal axis) and heat balance considerations determine the optimal delta coke target (orthogonal axis).
When utilising a BCMT additive, additive choice depends not only on the primary product slate, but also on the configuration of the refinery, especially the existence of a feed hydrotreater. Refiners charging resid to the FCC unit without the capability of feed hydrotreating may face a delta coke limitation. This limitation often manifests itself as a metallurgical limit within the FCC unit regenerator or associated downstream piping and hardware; or as a reduction in the catalyst-to-oil ratio, resulting in unacceptable yields from the FCC unit. Resid processors also suffer from increased metals contamination on the equilibrated catalyst, leading to wet gas compressor constraints resulting from increased hydrogen and light gas production.
At the other end of the feed spectrum, some refiners choose to severely hydrotreat the feed sent to the FCC unit. This improves conversion and provides the opportunity for the FCC unit to generate products with specific properties, such as 10 ppm ultra-low sulphur gasoline. Refiners processing deeply hydrotreated feeds can find themselves in the situation where the regenerator temperature is too cool to maintain stable operation, but the wet gas compressor is operating at its design limit. In this situation, a coke-selective increase in delta coke is required, but this change must occur in a manner by which the gas volume is not increased.
Regardless of whether or not a feed hydrotreater is present in a refinery, the minimum bottoms yield will ultimately be defined by the concentration of non-crackable, multi-ringed aromatic cores present in the FCC unit feed that boil above 340ºC. Besides feed, there are two other main variables strongly influencing bottoms cracking.
Accessibility, defined as the ability of the large feed molecule to diffuse into the catalyst particle (without forming coke along the way) and subsequently crack upon the appropriate active site, is absolutely critical for profitable bottoms cracking. Just as critical is metals tolerance. Nickel, vanadium and iron increase coke, hydrogen and dry gas yields. When not properly passivated, these metals can lead to reductions in the catalyst-to-oil ratio and increased bottoms yield. Vanadium and sodium both destroy the zeolite crystal and neutralise acid sites, reducing conversion. Iron is known to glaze the outer surface of catalyst particles lacking sufficient metals tolerance, significantly reducing accessibility. The presence of calcium, vanadium and sodium further exacerbates accessibility loss.
Returning to the concept of the yield space (see Figure 1), refiners agree that minimising bottoms (moving downward in the yield space) is a fundamental driver behind profitable FCC unit optimisation. How-ever, feed, accessibility and contaminant metals commonly prevent them from moving towards greater profitability in their yield space.
Solving the bottoms cracking problem
While improved profitability is found by moving downwards in the yield space, maximised profitability is realised by simultaneously moving in the other two yield space dimensions as well. The primary product yield (the horizontal axis in the yield space) is defined by the local or regional product market. Delta coke considerations (the orthogonal axis in the yield space) will vary from refiner to refiner, depending on hardware and unit constraints. Given the myriad of possibilities present in these two dimensions of the yield space, it is illogical for one single additive solution to meet every refiner’s needs. Simply stated, the same additive cannot be expected to provide maximum gasoline barrels and a cooler regenerator temperature in one FCC unit, and increase LCO yield and delta coke in another unit.
Necessity therefore dictates three distinct solutions for refiners employing BCMT additives to maximise profitability within their yield space. Figure 2 overlays the names of these different additives, each of which is especially formulated to target specific regions of the yield space. Selection of the correct formulation depends on each refiner’s requirements for primary product yield and heat balance.
The BCMT solutions available to refiners are as follows:
• BCMT-500 minimises bottoms yields through a combination of Albemarle Developed Matrices (ADM), specifically ADM-20 and ADM-60, which in combination provide a robust bottoms cracking matrix with tailored mesoporosity and acid site distribution. The ADM-60 alumina also provides metals trapping capabilities, allowing the additive to increase the metals tolerance of the unit inventory.1 A rare earth stabilised zeolite is also incorporated into BCMT-500, bestowing excellent hydrothermal stability, increased gasoline yield and lower gas make
• BCMT-MD utilises the raw bottoms cracking strength of ADM-20 matrix, the metals trapping of ADM-60 and a combination of speciality zeolites designed to minimise the overcracking of light cycle oils into gasoline, while at the same time ensuring a sufficient wet gas yield to maintain volume expansion. Together, the combination of ADM-20 and the speciality zeolites result in a low delta coke and gas make, which can be leveraged to increase the catalyst-to-oil ratio or increase the fraction of resid charged to the FCC unit
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