ZSM 5 is a catalyst additive used in FCCs to improve yield of light  olefins,   increasing refinery grade propylene production and/or providing more propylene and butylene feed to alkylation.

This comes through greater cracking of heavier molecules, including heavier (gasoline-range) olefins.

Use of ZSM-5 is favored under a number of conditions:

• Increasing olefin feed to serve expanded alkylation capacity - ZSM-5 produces more olefins for the same level of FCC utilization.

• Increasing olefins for direct propylene sales - ZSM-5 allows you to increase olefins so you can put more butylenes into alkylation and free up propylene for direct sales, if you have a propylene splitter and a direct outlet for RGP or PGP.

• Maintaining akylation throughput while reducing FCC gasoline production - in a soft gasoline market, refiners may want to reduce FCC throughput or maximize diesel yield over gasoline. ZSM-5 helps them maintain olefin production for alkylation feed by increasing yield of olefin relative to FCC feed rate.