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Is it possible that refiners could be overlooking some practical solutions to increasing FCC olefins yields, such as in the gas plant/recovery section?

Feb-2023

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Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

No doubt, there are some relatively easy solutions to improve the yield of petrochemical intermediates in FCC units which can be overlooked by some refiners.

Considering the current market demand, the FCC units can be optimized to produce higher added value derivatives like light olefins, refiners facing gasoline surplus markets can operate the processing unit in maximum olefins operation mode, to minimize the production of cracked naphtha.

In this operation mode the FCC unit operates under high severity translated to high operation temperature (TRX), high catalyst/oil ratio. The catalyst formulation considers higher catalyst activity through addition of ZSM-5 zeolite. There is the possibility of a reduction in the total processing capacity due to the limitations in blowers and cold area capacity.

It’s observed an improvement in the octane number of cracked naphtha despite a lower yield, due to the higher aromatics concentration in the cracked naphtha. In some cases, the refiner can use the cracked naphtha recycle to improve the LPG yield.

In the maximum LPG operation mode, the main restrictions are the cold area processing capacity, metallurgic limits in the hot section of the unit, treating section processing capacity as well as the top systems of the main fractionating column. In markets with falling demand by transportation fuels, this is the most common FCC operation mode.

Through changing the reaction severity, it is possible to maximize the production of petrochemical intermediates, mainly propylene in conventional FCC units.

The use of FCC catalyst additives such as ZSM-5 can increase unit propylene production by up to 9,0%. Despite the higher operating costs, the higher revenues from the higher added value of derivatives should lead to a positive financial result for the refiner, according to current market projections. A relatively common strategy also applied to improve the yield of LPG and propylene in FCC units is the recycling of cracked naphtha leading to an over cracking of the gasoline range molecules.

Nowadays, the falling demand by transportation fuels has made the refiners optimize the FCC units aiming to maximize the propylene yield following the trend of a closer integration with the petrochemical sector. Among the alternatives to maximize the propylene yield in FCC units is the use of ZSM-5 as additive to the FCC catalyst as well as the adjustment of the process variables to most severe conditions including higher temperatures and catalyst circulation rates. Another interesting alternative is to recycle the cracked naphtha to the process unit aiming to improve the LPG and consequently the propylene yield.

The installation of propylene separation units can present a significant capital investment to refiners but considering the last forecasts that reinforces the trend of reduction in the demand by transportation fuels, this investment can be a strategic decision to all players of the downstream industry in the middle term both to ensure higher added value to the processed crude oil and market share. Another possible capital investment aiming to improve the yield of light olefins recovery from FCC units is the use of cryogenic processes in the gas recovery section against the conventional configuration with separation columns, in this case the recovery of ethylene is highly improved.

Feb-2023
Mel Larson, Becht, mlarson@becht.com

This is a compound question of FCC yield and recovery. On the yield front, most units are bounded by the wet gas compressor and subsequent recovery capacity. The value and recovery of olefins are regionally specific. In the US, the difference depends on economical access to chemical infrastructure for propylene vs alkylation. The US Gulf Coast has been maximising recovery within existing hardware and cooling limits for decades. It is the opposite in the EU market, as LNG prices have been driving plants to fuel more LPG, be it saturates or olefins. One major in the EU stated it had reduced LNG demand by 40% by fuelling LPG components. Thus ultimately, the issue is the value of olefin processing, including petrochemicals, alkylation, and/ or cat poly plant (there are a few left), and it is not a one-size-fits-all solution.

Feb-2023
Bani Cipriano, W. R. Grace & Co, bani.cipriano@grace.com

The yield of light olefins from the FCC can be increased by optimising different FCC process variables, including riser operating temperature (ROT), injection of ZSM-5 additives, hydrocarbon partial pressure, and the recycle of light naphtha, C4 or oligomer streams into the FCC. Increasing the yield of light olefins increases the load on the wet gas compressor (WGC) and the gas plant.

The FCC unit can realise maximum value within unit constraints by optimising the appropriate variables, and it is important not to overlook catalytic solutions to these constraints. For example, by increasing the yield of light olefins via ZSM-5 additives versus ROT increase, it is possible to make more light olefins per unit of dry gas production and, therefore, for a given unit limitation on the WGC and the gas plant.

Furthermore, in high metals applications, using an appropriate coke-selective catalyst with metals trapping functionality can result in lower H2 make, which further offloads the WGC and gas plant. In both cases, an increase in recovery of light olefins is enabled. As the cost of propylene production from the FCC is lower than other propylene production routes, the use of catalytic solutions or minor revamps to overcome the gas plant handling capacity are effective ways of realising the maximum value from the FCC.

Feb-2023
Michael Allegro, BASF - Refining Catalysts, Michael.allegro@basf.com

For C3s, propylene splitters are well-established solutions that most units concerned with C3= recovery have likely investigated. For C4s, most units interested in olefin recovery have at least conducted a paper study to determine if a separations solution would be economical. An opportunity for optimisation could be for customers to pay closer attention to ‘lost’ olefins.

A potential opportunity for further recovery efforts could be that of ethylene. Many feel that FCCs do not produce enough C2= for it to be worth recovering. A future shift in product economics could lead to more attention being paid to fuel gas olefinicity.

Feb-2023