Increasing propylene for the petrochemical market

Increasing FCC propylene with shape-selective additives minimises the impact on unit process conditions and equipment

Solly Ismail, BASF Catalysts

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Article Summary

Global demand for propylene is projected to grow from 52 million tpy to 85 million tpy by 2010. This equates to a compounded annual growth rate of 4.6% over the next five years. Traditionally, steam crackers and FCC units have been the two major sources of propylene production. Steam crackers contribute 67%, while FCC units satisfy 30% of demand. However, recent supply and demand studies suggest a growing reliance on FCC units to meet expected shortfalls in supply, and it is believed that FCC-produced propylene will increase some 50% by 2010.

In the past, most FCC units have been swing producers of propylene, meeting short-term demands when the economic factors are favourable. At other times, when the economics are not as rewarding, refiners usually alkylate the propylene to produce higher-value gasoline products or downgrade the value of the propylene by blending it for sale as LPG. In some cases, producers will even use it as fuel gas in the refinery itself.

Contrary to the swing producer’s approach, some FCC units have become consistent suppliers of propylene, most notably in Asia. Several of these producers have made significant capital investments in new hardware, in the form of grassroot installations, such as deep catalytic cracking (DCC) units or hardware modifications to the reactor sections of existing FCC units. The latter enables them to over-crack the gasoline to create lighter components such as LPG and dry gas.

Advantages of shape-selective additives
To meet the growing demand for propylene without resorting to hardware changes, refiners have two non-capital options available to them: increase the reactor outlet temperature (ROT) or use additives. While it is relatively easy for some refiners to increase ROT, serious drawbacks minimise its practicality. Increasing ROT increases conversion, thus raising the production of all light products, dry gas, LPG and coke. The higher level of dry gas and coke impacts both the air blower on the regenerator side and the wet gas compressor on the reactor side.

Calculations based on a computer model simulation show that an 18°F (10°C) increase in ROT will raise propylene production by 0.45 wt%. Other expected increases are propane by 0.14 wt%, total butylene by 0.44 wt% and isobutane by 0.06 wt%. In addition, dry gas increases by 20 vol% and coke by 0.13 wt%. For some refineries, this increased load on the compressor and air blower may not be a problem. However, most refineries operate close to their practical limits for these two critical operating components of the FCC unit. Consequently, they may not be able to meet the additional air demands for the regenerator, either because of capacity limits or due to the pressure limitations of the regenerator.

For refiners confronting this reality, additives may be an economically attractive option. They produce a negligible amount of unwanted by-products, which are lighter than propylene. Products in the dry gas range include hydrogen, methane, ethylene and ethane.

This same increase in propylene production can be accomplished by adding shape-selective additives to the level of 0.6 wt% of the newly added catalyst. Refiners routinely dose upward to 5 wt% of the unit inventory without 
experiencing significant negative impacts. For refiners wishing to increase propylene output even further, loss of activity may become an issue, which will be discussed later.

With 0.6 wt% additives in the new FCC catalyst, the LPG produced is 0.45 wt% propylene, 0.31 wt% total butylenes and 0.09 wt% isobutane. These additives work equally well in gas oil or resid units. Experience shows that there is virtually no increase in coke or dry gas, and there are no reports of butadienes being an issue. Shape-selective additives have been used in FCC applications for over 20 years, with no increase in butadienes being observed. Also, the remaining gasoline’s RON increased by 0.5 numbers. In summary, shape-selective additives give FCC refiners both process flexibility and increased profitability.

When comparing the LPG products from an increased ROT operation with those achieved with the use of shape-selective additives, it quickly becomes apparent that the percentage of propylene in the LPG is much higher from the shape-selective additive: around 52% compared to 41%.

As seen in yield shifts generated by computer simulation models, increased ROT (or increased fresh FCC catalyst addition) is more suited to increasing butylenes than it is to making additional propylene. An additive approach to increase yields is distinctly more efficient. As previously mentioned, the other by-products from the additives are butylene and isobutane. The level of isobutane produced with additives is also higher.

Initially, additives will convert some gasoline to non-propylene LPG. However, this can be remedied by directing the butylene and isobutane to the alkylation unit, where it can be recovered as high-quality, high-gravity, high-octane product. Conversion of butylene in the alkylation unit will result in a small, but observable volume shrinkage of 0.22 vol% of the original butylene feed.2

As previously noted, the major advantage of using additives over changes in ROT is the minimal impact on the process conditions. Additives have virtually no effect on coke make and have a lower impact than increased ROT on the wet gas compressor. This means that refiners using additives do not require additional air. Many FCC units are already using ZSM-5 additives to further improve their propylene yield. Refiners now have a new generation of shape-selective additives to improve their profitability. For example, the proprietary Engelhard Maximum Olefins Additive utilises advances in new binding and stabilisation technology to meet the increased global propylene demand through better selectivity and unit retention.

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