Page 146 - Fluid Catalytic Cracking Handbook

FCC Catalysts 121

preferential cracking of long-chain, low-octane normal paraffins, as
well as some olefins, in the gasoline fraction.
ZSM-5 additive is added to the unit to boost gasoline octane and
to increase light olefin yields. ZSM-5 accomplishes this by upgrading
low-octane components in the gasoline boiling range (C 7 to C 1O) into
light olefins (C 3, C 4, C 5), as well as isomerizing low-octane linear
olefins to high-octane branched olefins, ZSM-5 inhibits paraffin
hydrogenation by cracking the C 7+ olefins.
ZSM-5's effectiveness depends on several variables. The cat crackers
that process highly paraffinic feedstock and have lower base octane
will receive the greatest benefits of using ZSM-5. ZSM-5 will have
little effect on improving gasoline octane in units that process naphthenic
feedstock or operate at a high conversion level.
When using ZSM-5, there is almost an even trade-off between FCC
gasoline volume and LPG yield. For a one-number increase in the
research octane of FCC gasoline, there is a 1 vol% to 1.5 vol%
decrease in the gasoline and almost a corresponding increase in the
LPG, This again depends on feed quality, operating parameters, and
base octane.
The decision to add ZSM-5 depends on the objectives and con-
straints of the unit. ZSM-5 application will increase load on the wet
gas compressor, FCC gas plant, and other downstream units. Most
refiners who add ZSM-5 do it on a seasonal basis, again depending
on their octane need and unit limitations.
The concentration of the ZSM-5 additive should be greater than 1 %
of the catalyst inventory to see a noticeable increase in the octane.
An octane boost of one research octane number (RON) will typically
require a 2% to 5% ZSM-5 additive in the inventory. It should be
noted that the proper way of quoting percentage should be by
ZSM-5 concentration rather than the total additive because the activity
and attrition rate can vary from one supplier to another. There are new
generations of ZSM-5 additives that have nearly twice the activity of
the earlier additives.
In summary, ZSM-5 provides the refiner the flexibility to increase
gasoline octane and light olefins. With the introduction of reformulated
gasoline, ZSM-5 could play an important role in producing iso-
butylene, used as the feedstock for production of methyl tertiary butyl
ether (MTBE).