May-2022

Maximising crude-to-olefins pathways

Considering that high-purity olefins are necessary for maximum conversion of polymerisation reactions, selective hydrogenation is crucial for purifying olefinic streams into highly differentiated products. 

Rene Gonzalez
Editor, PTQ

Article Summary

Considering that high-purity olefins are necessary for maximum conversion of polymerisation reactions, selective hydrogenation is crucial for purifying olefinic streams into highly differentiated products. 
When cracking Câ‚‚, C₃ and Câ‚„ alkane molecules to their equivalent monomers (ethylene, propylene, butylene), hydrogenation for separation and purification eliminates byproducts and other contaminants (e.g., vinyl acetylene [VAC]) from steam cracker-produced olefins. 

According to information available from Axens, their technology for hydrogenation of steam cracker-produced olefins also benefits upgradation of pygas and the C4, Câ‚… and naphtha cuts from the FCC unit. Two-stage hydrogenation of pygas prepares the stream for the C₈-C₉ aromatics complex, while installation of a fractionator between the 1st and 2nd stage of the pygas hydrogenation unit can provide marketable gasoline pool quality volumes from naphtha-based steam crackers.      

The aim of Câ‚‚ cut (ethylene) selective hydrogenation is to reduce acetylene content as low as possible while maximising ethylene yield and minimising oligomerisation (i.e., green oil formation) to extend operating cycles. Selective hydrogenation technology benefits from promoted palladium (Pd) and nickel (Ni) based catalysts. 

For high propylene (C₃=) yields, Pd-based catalysts have been developed specifically for maximum methyl acetylene and propadiene (MAPD) conversion and C₃= yields, suppressing green oil formation and minimising over-hydrogenation. Against this backdrop, Pd and Ni based catalysts have been formulated for either liquid or vapour phase selective hydrogenation. 

In addition, short alphaolefins (i.e., 1-butene, 1-hexene) are important building blocks for several routes to plastics grades (polyethylenes), making Axens technology and catalysts for dimerisation of ethylene to 1-butene and 1-hexene part of the olefins valorisation block of olefins optimisation technologies. Selectivity and purity are crucial. 

In conventional refinery operations wet gas compressor constraints and ZSM-5 catalyst limits only allow about 11 wt% propylene production from resid FCCs. Further efforts at unit optimisation to increase C₃= yields become problematic, such as with an inability to shift the FCC’s thermodynamic equilibrium for producing more olefins. Technology has been developed to overcome these barriers to boosting olefins production and will be discussed in more detail in the Q3 issue of PTQ. 

Revamp projects to increase olefins production from existing assets benefit from:
-    Higher FCC reactor operator temperature (ROT)
-    Advanced internals (e.g., riser termination devices)
-    Improved stripper packing and new feed injector designs. 

These capabilities, whether for grassroots projects for large-scale production capacities, or for mature markets with limited investment capabilities, enables much deeper penetration into the olefins market.     
Polymer demand has accelerated development of FCC technology able to produce over 25 wt% propylene by converting heavy residue under severe FCC conditions using VGO and residue feedstock. The most effective processes for producing such high wt% of propylene could also produce a considerable number of valuable by-products, including butenes feedstock for the lubricants and resins market. 

Due to the rigours of operating high complexity facilities, engineering staff at catalytic-crude-to-chemicals  and crude-to-olefins facilities stand to benefit from the technical and pilot plant support that accompanies licensed olefins processes available from global licensors of refining and petrochemical technology.