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Oct-2016

HS-FCC for propylene: concept to commercial operation

A FCC process provides a high light olefin yield from a wide variety of feedstocks utilising high severity reaction conditions and a novel down flow reaction system.

Nicolas Lambert, Axens
IWAO Ogasawara, JX Nippon Oil & Energy
Ibrahim Abba, Saudi Aramco
Halim Redhwi, King Fahd University of Petroleum & Minerals
Chris Santner, Technip Stone & Webster Process Technology
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Article Summary
The fluid catalytic cracking (FCC) process has undergone a long evolution of hardware and catalyst changes, from bed cracking with amorphous catalyst to short contact time riser cracking with sophisticated zeolite catalyst systems. Improvements to the process have provided a wide degree of flexibility to selectively target the production of distillates or gasoline, or propylene from VGO and residue feeds, thereby making FCC the most widely used conversion process.

More generally, the objective of the process is to produce high valued products, and increasingly this includes fuels and petrochemicals, such as light olefins and aromatics. At present, over 30% of the worldwide propylene supply comes from FCC-related processes (FCC, RFCC, DCC). Fluctuating product demand and price have caused most new project developers to demand product flexibility for long-term profitability and process integration with petrochemical facilities for added synergy and cost savings.

In order to respond to these market demands, a new high severity down flow FCC (HS-FCC) process has been developed by an alliance of Saudi Aramco, JX Nippon Oil & Energy (JX) and King Fahd University of Petroleum and Minerals (KFUPM), culminating in a 3000 b/d semi-commercial unit in operation since 2011 in Japan (see Figure 1). The process provides a high light olefin yield from a wide variety of feedstocks utilising high severity reaction conditions, a novel down flow reaction system and proprietary catalyst. HS-FCC is now available for licence from a Global Alliance by Axens and Technip Stone & Webster Process Technology.

Features of HS-FCC
FCC utilises acidic zeolite catalysts to crack heavy hydrocarbons into lighter fuels such as gasoline and distillate and, under more severe conditions, into lighter olefins such as propylene and butylene (and, to a lesser extent, ethylene). Complex secondary reactions that can degrade the primary products to less valuable components should be limited to retain product selectivity and refinery profitability. For HS-FCC, the objective is to not only improve the selectivity for normal fuels production, but also to maximise the potential of light olefin and petrochemical production at high severity. HS-FCC provides a total system to maximise product selectivity and, in particular, propylene yield. Three key elements are required to attain this objective:
•    Highly selective catalyst and additive system
•    Optimised reaction conditions
•    Down flow, short contact time reaction system with rapid catalyst separation.

The balance of these elements and realisation at commercial scale is the key to success.

Catalyst system
The catalytic cracking reaction pathways are complex, with the primary formation of olefinic products and parallel bi-molecular hydrogen transfer reactions leading to paraffin formation and aromisation of naphthenes. Managing the acid site density of the catalyst can suppress hydrogen transfer and isomerisation reactions to maximise olefins production. When coupled with ZSM-5 pentasil cracking catalyst additives, the increased olefins in the gasoline cut can be selectively cracked to further increase the propylene yield.

The HS-FCC catalyst uses a high USY zeolite content system with very low acid site density, formulated to minimise hydrogen transfer reactions for high olefin selectivity, and low coke and gas selectivity. This catalyst has been shown to be more effective for propylene production when coupled with ZSM-5 additives (see Figure 2). Commercial catalysts and HS-FCC catalyst exhibited a similar trend in gasoline and propylene yield as a function of conversion (severity), but the customised HS-FCC catalyst was much more effective in ‘feeding’ the ZSM-5 additive with more olefins, and more accessible linear olefins, to produce more propylene.1 

Optimised reaction conditions
When targeting maximum petrochemicals production, HS-FCC operates under more severe conditions than conventional FCC. The main reaction conditions applied and the advantages and challenges presented are shown in Table 1.

High reaction temperature coupled with short contact time increases the primary reactions towards olefins, while limiting the unwanted secondary reactions of hydrogen transfer and thermal degradation. A consequence of the increased severity and short time is the need for higher catalyst circulation (catalyst-to-oil mass ratio, or C/O) to provide the required heat to the reactor and sufficient catalyst activity to achieve high conversion at short contact time. The range of operating conditions for a conventional FCC and HS-FCC are summarised in Table 2.

Down flow reaction (DFR) system
The specific reaction conditions with very high C/O result in certain challenges in a conventional up flow FCC riser reactor system, where the catalyst required for the reaction is lifted up the reactor pipe or riser by the vaporised and cracked hydrocarbon feed. In up flow fluid-solid systems, the solids or catalyst are conveyed upwards against the force of gravity by drag forces from the rising gases (hydrocarbons). As a result, all riser reactor systems have varying degrees of catalyst back-mixing and reflux along the walls, particularly in the feed injection or catalyst pick-up zone at the bottom of the riser reactor. At very high C/O, significant back-mixing is unavoidable. This problem is overcome in a down flow reactor (DFR), where both the catalyst and feed flow downwards together (see Figure 3).

Down flow fluid-solid reaction systems have been of increasing interest in recent years to achieve plug flow reaction conditions, as summarised by Cheng.2 When plug flow conditions are achieved, more selective primary cracking results in greater selectivity. FCC pilot work demonstrating the effects of short contact time and down flow have been reported by Del Poso3 and Abul-Hamayel4 (see Figures 4a and 4b). The general trend is that of greater gasoline selectivity at short contact time down flow, with a maximum yield achieved at a higher conversion level. This effect is seen in Figure 4a, where the maximum gasoline yield is about 5 wt% higher in the down flow system. When olefins are of interest, the more selective down flow reaction environment can produce substantially more light olefins (C2-C4) at the same gasoline yield compared to a conventional up flow system (see Figure 4b).
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