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Apr-2008

Maximise olefins through catalytic cracking Indmax FCC process

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

Dalip Soni, Philip J Angevine and Rama Rao, Lummus Technology
G Saidulu, D Bhattacharyya and V Krishnan, Indian Oil Corporation
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Article Summary
While other light olefins were always produced, the low volumes did not attract much attention because of the separation costs and modest margins.Production of propylene from FCC units has grown significantly in the last decade and is now approaching 20-30% of the total propylene produced worldwide.1 The price spread between propylene and gasoline and the increase in demand for propylene have been the key drivers. Significant improvements have been made in the FCC catalyst formulation and process technology to maximise propylene yield from the FCC units. This relatively new strategy for on-purpose production of FCC propylene will help meet approximately 5% of the annual growth rate in worldwide propylene demand.1

Early attempts to increase light olefins from FCC were based primarily on process variables, but poor selectivity of this approach resulted in excess dry gas and coke make. 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 below shows the chronology of catalyst and additive developments for light olefins enhancement in FCC units.

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

Approaches to higher olefins production
Process variables

Some of the FCC process variables employed for olefins production include: (a) higher severity operation, (b) recracking of the cracked material, and (c) partial catalyst recycle. Table 2 summarizes the benefits and drawback of such variables.

High-severity operation is achieved by a high reaction temperature with or without higher residence time. The general concept for higher severity operation is to overcrack (beyond maximum gasoline mode) and produce increased quantities of light olefins. The major drawback of this approach is the production of excess dry gas and subsequent overloading of the gas plant (compressor and separator). Also, higher severity results in a higher coke make2 and a higher diene content in gasoline, which can lead to gum formation. This whole approach is somewhat of a “brute force” method, with little control over the desired products.

Recracking of cracked material such as naphtha can yield incremental light olefins. This recracking can be done in the same or a separate riser/reactor. The concept of injecting in the same riser/reactor has been used for upgrading visbreaker or coker naphtha in FCC risers. Recycle of FCC naphtha to the same riser reactor has also been implemented; however, dry gas yield increases significantly.

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 average catalyst activity is low compared to the case when a higher catalyst-to-oil ratio is achieved with all regenerated catalyst. Therefore, the partial-catalyst-recycle approach results in lower conversion and selectivity to light olefins.

Catalyst variables
Since the 1960s, the refining industry has used various X and Y zeolites as its primary cracking catalyst in FCC.3,4 Stability and selectivity benefits quickly led to the use of zeolite Y exclusively. The phase-out of lead caused researchers to seek alternate routes to increased gasoline octane. Several versions of Y zeolites were used, and these are listed in Table 3.

Many methods were tried to dealuminate zeolite Y to balance activity, stability, gasoline yield and octane. The use of zeolite Y for increased olefins focused on ultrastable Y, achieved mainly by its decreased 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 olefins yield reached a maximum at a unit cell size a0 of 24.15-24.20 Å. Figure 1 shows 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.9 ZSM-5 was first explored as a standalone FCC catalyst, but found to be lacking in conversion activity — not surprising for a medium-pore-size zeolite. 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 ZSM-5 use 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 now is used in almost 20% of worldwide FCC capacity. 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. Using the integral, single particle approach, the Y zeolites has to have some extra activity (so-called activity giveaway), which may not be easily achieved. This approach also does not allow the flexibility to change the percentage of ZSM-5 in the total catalyst inventory in the event of feedstock changes or fluctuations in market product demand.

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