Oct-2009

Catalytic cracking process enhances production of olefins

A FCC technology and catalyst formulation selectively crack a wide range of feeds, including residue, to yield high quantities of light olefins

Dalip Soni and M Rama Rao, Lummus Technology
G Saidulu, D Bhattacharyya and V K Satheesh, Indian Oil Corporation

Article Summary

The primary purpose of the fluid catalytic cracking (FCC) process historically has been to convert low-valued, heavier petroleum streams into gasoline, alkylation feed (eg, isobutane and butene) and, to a lesser extent, other distillate products. While other light olefins were always produced, the low volumes did not attract much attention because of separation costs and modest product margins.

In the last decade, production of propylene from FCC units (FCCUs) has grown significantly and is now approaching 20–30% of all propylene produced worldwide.1  Key drivers of this surge in production include the price spread between propylene and gasoline, and increased demand for propylene. To maximise the propylene yield from FCCUs, significant improvements have been made in catalyst formulation and process technology. This relatively new strategy for the on-purpose 
production of FCC propylene will help meet approximately 5% of the annual growth rate in worldwide demand for propylene.¹

Early attempts to increase production of light olefins from the FCCU were based primarily on process variables, but the poor selectivity of this approach resulted in excess production of dry gas and coke. By the 1970s, researchers found that non-Y zeolites could also co-produce light olefins (C2= to C5=), often at the expense of gasoline. Table 1 shows the chronology of development for catalysts and additives to enhance the production of light olefins in FCCUs.

This article discusses the growth of  on-purpose production of light olefins from the FCCU and the approaches employed. Specifically, the Indmax FCC (I-FCC) process will be addressed, including its benefits, the underlying concepts, the chronology of its development and commercialisation.

Approaches to production of increased olefins
Process variables
Some of the variables of the FCC process employed in the production of olefins include a higher severity operation, recracking of cracked material and partial catalyst recycle. A high severity operation is achieved by a high reaction temperature, with or without a higher residence time. The general concept for a higher severity operation is to overcrack (beyond the maximum gasoline mode) and produce increased quantities of light olefins.² The length of contact time depends on the type of catalyst employed. A longer contact time results in excessive production of dry gas and coke, and may also result in reduced selectivity for light olefins due to hydrogen transfer reactions.

A high severity operation with a short contact time overcomes the drawbacks of a longer contact time. Short contact time produces less dry gas and coke, and provides a higher selectivity to light olefins. However, it requires a catalyst formulation that can deliver the desired levels of conversion and yield for light olefins.

Recracking cracked material such as naphtha can yield incremental light olefins. This recracking can be done in the same or a separate riser reactor. Injecting in the same riser reactor has been used for upgrading visbreaker or coker naphtha in FCC risers. Recycling FCC naphtha to the same riser reactor has also been applied, but the yield of dry gas increases.

In the partial catalyst recycle approach, part of the spent catalyst is recycled back to the reaction section to increase the catalyst-to-oil ratio. The drawback is that the catalyst’s dynamic activity is lower than when a higher catalyst-to-oil ratio is achieved with all-regenerated catalyst. This may result in lower levels of conversion and selectivity to light olefins.

Catalyst variables
Since the 1960s, the refining industry has used various X and Y zeolites as the primary cracking catalyst in FCC.^3,4 Benefits of greater stability and better selectivity quickly led to the exclusive use of Y zeolites. The phase-out of lead in fuels convinced researchers to seek alternative routes to increase gasoline octane. Several versions of Y zeolites were used, and these are listed in Table 2.

Many methods were applied to dealuminate Y zeolite in order to balance activity, stability, gasoline yield and octane. The use of Y zeolite for increased production of olefins focused on Ultrastable Y (USY), achieved mainly through its reduced zeolite unit cell size and decreased framework Al content.^5,6 At constant conversion, USY catalysts yield more C3-C4 olefins, higher octane gasoline, and less gasoline and coke. One patent⁷ showed that the olefin yield reached a maximum at a unit cell size of 24.15–24.20 Å. Figure 1 shows the olefin yield as a function of unit cell size. Unfortunately, USY is less active and stable than its REY counterpart. RE-USY can achieve benefits similar to USY, but with about 35% higher activity than USY.⁸

Currently, the popular approach to olefin production is the use of ZSM-5 additive.s ZSM-5 was first explored 
as a stand-alone FCC catalyst, but 
was found to be lacking in conversion activity — not surprising for a zeolite of medium pore size. It was later proved that ZSM-5 enhances gasoline octane when used as an additive to the main FCC catalyst, due to the ability of ZSM-5 to produce more olefins.

While early use of ZSM-5 focused on octane enhancement, the use of larger quantities of ZSM-5 for on-purpose propylene production started in the 1990s. Refiners have since increased their reliance upon ZSM-5, to the point where it is now used in almost 20% of FCC capacity worldwide. This approach has proven to be a relatively inexpensive route 
to propylene.

ZSM-5 can be used as a separate additive or as an integral part of the FCC catalyst. Each approach has its benefits and drawbacks. Since the two zeolites (Y and ZSM-5) age at different rates, the separate additive method can adjust better for this effect. However, the additive can build up in the FCC inventory to the point where this dilution can reduce the overall conversion performance of the catalyst mix.