Oct-2014

'Snakes and ladders' for maximising propylene

Changes in process conditions in tandem with ZSM-5 additives widen the potential for petrochemicals production from the FCC unit

BART DE GRAAF, MEHDI ALLAHVERDI, MARTIN EVANS and PAUL DIDDAMS
Johnson Matthey Process Technologies

Article Summary

The modern day equivalent of turning lead into gold is upgrading oil into petrochemicals. Whereas in some parts of the world fluid catalytic cracking (FCC) units are being shut down because of poor economics, new units are still being built in growth regions like Asia and the Middle East. These new FCC units are typically designed as petrochemical FCC units, in particular to produce propylene, often from heavy residual feedstocks. Nowadays, margins for standard FCC units producing mainly transportation fuels are small or even negative, however not so for petrochemical FCC units.

Demand for petrochemicals is increasing, especially in Asia where there is a shortfall in mixed xylenes and propylene production. Mixed xylenes exports from the US account for 800 000 t/y or 25% of the Asian market’s consumption.1 Propylene is a key chemical for plastics such as polypropylene, acrylonitrile, propylene oxide derivatives, and so on. Global demand for polypropylene in 2000 was 25.1 million t/y, increasing to 42.3 million t/y in 2011 and is expected to grow to an estimated 62.4 million t/y by 2020. This growth is largely driven by increasing demand in Asia Pacific (to 62% of global demand), the Middle East and Africa.2 Growth in these regions outpaces that in North America. Future growth in the European market is expected to be very limited because of the Eurozone crisis.

There is a clear growing demand for petrochemicals, and the FCC unit is well positioned to meet it. Worldwide, 60% of the propylene is supplied by steam crackers, 30% by FCC and 10% by propylene on-demand units. To boost propylene yield in the FCC unit, typically ZSM-5 additives are used. The widely assumed model of how ZSM-5 additives work, that is commonly used, is simple: ZSM-5 cracks olefins out of the gasoline. This article shows this model to be incorrect. ZSM-5 does not just crack gasoline range olefins, it produces these too via oligomerisation. ZSM-5 acts both as a ‘snake’ and ‘ladder’, as in the board game, for olefins in the FCC unit. The interaction between hydrogen transfer (see Figure 1) and ‘snakes and ladders’ for olefins determines gasoline composition in the FCC unit. With this new model, gasoline compositions when using ZSM-5 can be explained and the ‘gasoline cracking’ model is left wanting.

Maximum propylene chemistry
Many reaction pathways play an important role when cracking a vacuum gas oil (VGO) or residue feedstocks into products over an FCC catalyst. The primary step is ‘cracking’, that is catalytically breaking carbon-carbon bonds (also known as beta scission). In this reaction, individual molecules react with acid sites on the catalyst forming carbenium ion intermediates that are readily able to crack. Cracking results in olefin formation via a) cracking large paraffins, b) ring opening of naphthenes, c) dealkylation of aromatic side-chains, and d) the subsequent cracking of the olefins formed in a) to c):

a) Paraffin cracking:     large paraffin → Olefin + Smaller paraffin
b) Ring opening: Naphthene (ring) → Olefin
c) Dealkylation: Alkylaromatic → Olefin + Aromatic
d) Olefin cracking: Large olefin → Smaller olefin + Smaller olefin

Concurring reactions include thermal cracking, hydrogen transfer, dehydrogenation, cyclisation, trans-alkylation, oligomerisation and polymerisation.

Thermal cracking and dehydrogenation are the primary sources of dry gas (C1-C2 and H2), and polymerisation produces coke. These reactions do not significantly directly contribute to the creation of propylene (or other light olefins), but may have an indirect contribution via their impact on FCC unit constraints.

Propylene yields from the FCC unit are improved by increasing operation severity, conversion, and through the use of ZSM-5 zeolite containing additives. The higher the conversion the higher the propylene yield will be, especially when in the naphtha overcracking region (typically overcracking begins at 70-75 wt% conversion depending on feed properties). Operating parameters are modified in such a way that beta scission is maximised relative to thermal cracking, dehydrogenation, hydrogen transfer and polymerisation. Various process licensors have developed technologies for this purpose. Typically, the following process parameters are optimised to maximise conversion within unit constraints:
• Increase cracking temperature
• Increase catalyst circulation/catalyst to oil ratio
• Increase Ecat activity
• Improve feed quality – hydrotreating for example.

Other parameters that significantly help to increase propylene selectivity are:
• Minimise hydrocarbon partial pressure. Reducing hydrocarbon partial pressure via dilution with riser steam shifts the reaction equilibrium towards light olefins by reducing light olefin oligomerisation (recombination) and hydrogen transfer reactions
• Reduce hydrogen transfer reactions. These have the undesired effect of saturating light olefins to paraffins. This can be done by:
ν Reducing contact time: advanced riser termination systems provide rapid catalyst/oil disengagement which decreases hydrogen transfer
ν Reducing backmixing: minimising catalyst slip in the riser reduces hydrogen transfer
ν Reducing fresh catalyst rare earth (RE) on ultrastable-Y (USY) zeolite is of particular importance in reducing hydrogen transfer. By reducing hydrogen transfer reactions, the gasoline becomes more olefinic and more readily crackable. Gasoline olefins are the prime source for LPG olefins. A catalyst with minimum rare earth helps to maximise propylene selectivity
• Naphtha recycle - to crack every potential propylene precursor out of the gasoline
• Use a ZSM-5 additive.

All of these parameters help to maximise the propylene yield, but by far the greatest variable for increasing propylene yield is the use of ZSM-5 additives. ZSM-5 additives selectively crack gasoline range molecules into LPG olefins (C3 and C4), with highest selectivity for propylene – typically 50-60 wt% of the incremental LPG is propylene.

Maximum propylene FCC units employ high levels of ZSM-5 additives, often 10% or more in the circulating inventory. Some concern has been expressed that at extremely high inventory levels the ZSM-5 additive can cause dilution of base catalyst activity. However, dilution effects are rarely observed in practice because: target Ecat activity is maintained by adjusting the fresh catalyst make-up rate – if slightly higher addition rates are required they effectively lower the ‘age’ of the inventory; and ZSM-5 additives do not significantly contribute to delta-coke (about 20-30% of the delta coke of the base catalyst), so as long as the FCC unit has some capacity to increase catalyst circulation rate ZSM-5 additives will not negatively affect conversion.