Resid to propylene: the two-step approach
For maximum propylene from the FCC unit, refiners should customise their catalyst for properties best suited to deep conversion that can handle residue feeds.
MELISSA CLOUGH MASTRY, HERNANDO SALGADO, BAHA EREN and WEI SHEUN GAN
BASF Corporation, Catalysts Division
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The fluid catalytic cracker (FCC) remains one of the most important conversion units within a refinery for the generation of transportation fuels (i.e. gasoline and diesel precursors).1 However, in certain markets, especially where integration between refineries and petrochemical plants is high, there is great incentive to maximise propylene over other products, including gasoline. In these regions, mainly in Asia and the Middle East, the financial incentive of propylene over gasoline and other liquid products is clear and is driving many of the new project builds. Furthermore, new hardware technologies are advancing, including new unit designs. For these reasons, catalyst technologies must also contribute to advancements in this field.
Catalyst technologies and performance are monitored on a weekly basis from most refineries in the world. The performance can be compiled over many years to demonstrate trends in FCC.2 Historical equilibrium catalyst (Ecat) yields demonstrate an upward global trend in propylene, particularly in the Asia, EMEA, and Latin American regions (see Figure 1). This trend supports further utilisation of the FCC that will produce propylene from resid reliant regions and how important it is to design an optimal catalyst.
The FCC utilises a fluidised solid catalyst to crack oils into valuable products. The main contributor of catalyst activity is the zeolite Y, an important ingredient in all FCC catalysts. The zeolite Y is critical for generating a mix of products, including LPG olefins, naphtha (a gasoline precursor), and light cycle oil (a diesel precursor). Most FCC catalyst manufacturers employ ultra- stable zeolite Y, or USY, an outcome of careful calcination in the manufacturing process.3
A second and important zeolite in the maximum propylene FCC is ZSM-5. ZSM-5 is a very shape selective zeolite, capable of cracking near-linear naphtha range olefins.4 For this reason, ZSM-5 is very selective and cracks mainly C5-C9 linear or near-linear olefins. Without these temporary precursors in the FCC reaction, the ZSM-5 would contribute nothing to the FCC process. Therefore, the balance of high activity USY zeolite and ZSM-5 zeolite is critical. For example, with too much ZSM-5 in the FCC system, one might dilute the USY, penalising catalyst activity to an extent that there no longer remain C5-C9 linear or near-linear olefins for the ZSM-5 to crack.
Furthermore, for resid FCC applications there are additional challenges. For instance, resid feeds bring contaminant metals that can disrupt the FCC process, either catalytically or through the generation of unwanted byproducts. The most critical contaminants that come in with resid feeds include vanadium (V), nickel (Ni), sodium (Na), and iron (Fe).
Vanadium is well known to destroy USY within an FCC catalyst, attacking the Si-O bonds and resulting in zeolite collapse. Ultimately, this leads to a loss in catalyst activity and thus will result in lower conversion, with all things constant.5 Lower conversion means lower naphtha, lower feed for the ZSM-5 zeolite, and ultimately lower propylene. Nickel, on the other hand, operates with a very different mechanism.
Instead of destroying catalyst activity, nickel promotes dehydrogenation side reactions. Nickel is a very strong dehydrogenation agent, and thus will result in the generation of H2 and coke, two byproducts that are unwanted in resid applications. Higher H2 means higher loading on the wet gas compressor, which can often be a constraint for FCC units. Higher coke generation means lower generation of other valuable products, including naphtha and LPG.
Sodium is a very detrimental contaminant and much like other alkali and alkaline earth metals will work to neutralise the acid sites on USY with the positive charge. This immediately and irreversibly results in a loss in catalyst activity, with results similar to those of V poisoning. However, it takes very little Na contamination to see an effect on activity, so keeping Na out of the FCC unit is critical.
The last contaminant on the list is iron. Iron deposition occurs on the outside surface of the catalyst and in some cases can lead to pore blockage, acting as a physical barrier between feed and the important USY cracking sites.6 This too will lead to a loss in conversion on the FCC unit, resulting in lower naphtha and ultimately lower propylene.
In most cases, the deposition of contaminant metals occurs on the active catalyst itself (containing USY), and not on the ZSM-5 material. For this reason, the focus of contaminant metals technology should be on the base catalyst.
As a result of the very well understood mechanisms of USY cracking, ZSM-5 cracking, and metal contamination pathways, the science behind maximising propylene from an FCC unit, when confronted methodically, can identify an optimum solution for each maximum propylene unit. Since every FCC unit is different in terms of feed, contaminant metals, downstream processing constraints and product values, the answer to each maximum propylene resid unit will be unique, delivering in some cases up to $2.5/bbl or more in comparison with no optimised units.
The following sections are dedicated to explanation of the cracking mechanisms from resid feed to propylene product, the main contaminants and their effects in the USY catalyst system, the catalytic solutions to mitigate contaminants, and strategies to maximise propylene by ZSM-5 cracking. Thereafter, case studies are offered to demonstrate a two-step approach for propylene maximisation. These cases will further highlight the fact that, for each refinery, there exists a different solution for propylene maximisation, which highlights the need for a highly customised approach at every refinery.
Catalytic cracking mechanisms
First step: primary cracking using Y-zeolite
Catalytic cracking on USY zeolite is a complex process with multiple reactions occurring on the zeolitic acid sites, under the well accepted mechanism of carbenium and carbonium ion formation plus β-scission, leading to smaller molecules compared to molecules in the feed. A simplistic approximation of these primary reactions is shown in Figure 2.
Other secondary reactions such as hydrogen transfer and isomerisation also take place, affecting the quality of the products in terms of paraffins, olefins, and aromatics content. Since the catalytic cracking process was originally developed to convert VGO into gasoline through reactions 1, 2, and 3, most olefins are in the range 6 to 12 carbon atoms. Reaction 4 yields aromatic compounds containing two, three, or more rings in addition to small olefins and coke. Although β-scission can yield LPG molecules by itself, including propylene, due to the unit cell size of the Y-zeolite and contact time, most products from reactions 1, 2, and 3 are within the range of gasoline.
In this first step, the matrix also plays an important role, providing cracking of the biggest feed molecules and accessibility to the acid sites of the zeolite to maximise bottoms cracking and naphtha yield. Matrix, in addition to other important parameters such as metals passivation, imparts important physical characteristics to the catalyst architecture by defining the pore network connectivity.
Second step: secondary cracking of naphtha to smaller molecules
The lighter range gasoline molecules, from 6 to 9 carbons, coming from cracking on Y-zeolite or matrix can further crack in a secondary set of reactions to be converted into smaller molecules, including propylene. However, a different active zeolite with smaller pore openings is required: the ZSM-5 zeolite. An added advantage of the secondary reactions comes from the fact that linear olefins are more reactive, leaving highly branched olefins and paraffins in the gasoline fraction, thus improving its octane number.
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