Increasing FCC propylene

ZSM-5-based catalysts are being used in FCCUs to increase propylene yields by as much as 15%

M Lesemann, W C Cheng, James R D Nee, Sergio Sobrinos and Gordon H McElhiney, Grace Davison Refining Technologies

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Article Summary

The demand for propylene will continue to grow significantly over the next few years. However, the increase in steam cracker capacity, the principle source of by-product propylene, is forecast to lag behind propylene demand, since it is tied to lower ethylene demand growth. Furthermore, much of the added capacity is being built in the Middle East, where ethane will be the primary (and economical) feedstock. Unlike naphtha, which is the major steam cracker feedstock in Europe, accounting for about 70% of the total feed slate, ethane cracking produces little by-product propylene. This will lead to structural tightness in the global propylene market, so the petrochemicals industry is looking to operators of FCC processes to fill the propylene supply gap.

In Europe, where diesel demand continues to increase at the expense of gasoline, there is a strong economic incentive to convert gasoline-range hydrocarbons into light olefins, notably propylene. Many FCC operators are already using additive technologies based on ZSM-5 to produce incremental propylene for sale into the petrochemicals market. Moreover, a number of refiners are considering revamping their FCC units to operate in petrochemicals mode, targeting propylene yields of 15 wt% or more, while others are looking at investing in grass-roots on-purpose propylene production facilities that utilise ZSM-5-based FCC catalysts.

Supply and demand
Propylene is a co-product of two refinery processes: steam cracking and fluid catalytic cracking units (FCCUs). Steam crackers, where the primary product is ethylene, also supply two-thirds of the world’s demand for polymer-grade/ chemical-grade (PG/CG) propylene. FCCUs contribute about 30% of the total propylene supply as a by-product of transportation fuels production. So-called on-purpose processes (for example, metathesis and propane dehydrogenation) supply the balance.

In Western Europe, however, propylene supply is primarily derived from steam cracking (more specifically, naphtha cracking), which accounts for about 75% of the regional propylene supply, while only about 20% is supplied from FCC units. Demand for propylene (mainly driven by the demand for polypropylene) is expected to grow by 2% annually over the next few years. Propylene supply from FCCUs in Europe is set to increase by more than 5% annually. Consequently, FCCUs will be contributing a larger share of the propylene supply in Europe by the end of the decade than they are now, but some imports into the region will be required to meet demand.

North America is and will likely remain a net exporter of propylene and its derivatives due to its large FCC capacity base. However, refining capacity is tight in North America, and strong demand for transportation fuels has led to record prices for gasoline this year. The steep price spikes for gasoline in the aftermath of Hurricane Katrina were a clear indicator of how far the supply side is already stretched. The economic incentive for refiners to devote FCC capacity to propylene production in North America will diminish, and propylene exports from North America into other regions will decrease. Given the strong demand growth for propylene derivatives in Asia, this leaves the Middle East as a potential exporter of propylene derivatives into Europe. Figure 1 shows the expected average annual growth rates (AAGRs) for the different regions.^{1, 2}

Incentives
ZSM-5-based catalyst additives increase LPG yields, especially propylene, in the FCCU by cracking gasoline-range hydrocarbons. The decision to use these catalyst additives therefore depends on an assessment of the economic trade-off between the relative values of gasoline and LPG/propylene.^{3, 4}

To illustrate this point, Figure 2 shows how the markets value CG propylene and gasoline in Europe. Although spot prices are volatile, the figure shows that the value differential between propylene and gasoline has been increasing over the past few years and is expected to reach 300–400/MT in the foreseeable future. A differential of this magnitude provides a strong incentive for refiners to increase propylene production at the expense of gasoline, even accounting for purification costs.

Not surprisingly, refiners in Europe have developed a significant interest in shifting their product slate in favour of propylene. Several refiners in the region are already in the process of or are planning to revamp their FCCUs for increased propylene production.

Yields
As the economic incentive for increased propylene yields from FCCUs grows, FCC operators have the following options to consider:
• Use ZSM-5 additives
• Increase riser temperature
• Increase conversion
• Change feed composition.

ZSM-5 additives, although introduced more than 20 years ago as a way to increase FCC octane value, are now the most effective way to increase light olefins yields from the FCCU. Typically, only a few weight percent of additive in the inventory is required to significantly improve light olefins yields. Basically, ZSM-5 cracks gasoline-range olefinic molecules primarily into C3 and C4 olefins. While ZSM-5 additives are formulated with different levels of ZSM-5 crystal, it is important to note that the additive’s actual activity is what determines its effectiveness. Only proper additive stabilisation will ensure that the FCC operator sees the full benefit in terms of yield optimisation.

ZSM-5 additives are also the fastest way to achieve a significant boost in propylene yield if market conditions demand such an action. In fact, some refiners, particularly on the US Gulf Coast (PADD III), use ZSM-5 additives in a dynamic fashion to capture profit opportunities that arise in volatile propylene markets. This activity and the size of the profit opportunities it creates have been previously described in other papers.4
Increasing the reactor temperature will raise LPG and therefore propylene yields, but the effect is not as pronounced as that of ZSM-5. Increasing the reactor temperature also raises the yield of dry gas and coke, which is undesirable.