Jul-2013

Towards a zero gasoline production refinery: part 1

Integrating products from the steam cracker, aromatics complex and FCC unit to produce petrochemicals without gasoline may offer more attractive margins

BLASIS STAMATERIS
Foster Wheeler

Article Summary

In some regions, the traditional export markets for gasoline-
focused refineries are diminishing. In this environment, investments in new refining projects (or in significant refinery upgrades) need to be optimised to fulfil market requirements. An opportunity exists to integrate the value chain from crude oil processing in refineries to the production of petrochemicals in line with market requirements.

The objective of this article is to present alternative refinery configurations that are able to process relatively heavy crudes, producing middle distillates, petrochemicals and aromatics, without producing any gasoline at all. The article will demonstrate how, by relying on well-proven refining process technologies, configurations could be adapted in an existing FCC-based refinery or can be applied to a grassroots project. It will show how to integrate products from the steam cracker, aromatics complex and FCC unit while rationalising investments.

Part 1 of the article presents ways of upgrading streams produced in refineries that have traditionally been oriented towards the production of transportation fuels to produce petrochemicals, which may offer more attractive margins, allow diversification of product slates and reduce the impact of refined product market volatility. Streams from the FCC unit offer building blocks towards propylene and aromatics production. Part 2, to be published in a forthcoming issue of PTQ, will show the impact of these processes on the overall product slate of the refinery for different refinery configurations.

FCC unit: not just a gasoline-making machine
In addition to gasoline, the refinery FCC process also produces one-third of the global propylene supply. Propylene yields from the FCC unit when operating in petrochemical mode can be at least 
10 wt% of the feed.

In order to maximise propylene production in the FCC unit, the following technical aspects need to be considered:1
•  Feed quality: hydrogen content of the feedstock strongly correlates to the propylene yield
•  Increased riser outlet temperature (ROT) and high catalyst-to-oil ratio: increased severity yields higher conversion and a higher propylene yield
•  Catalyst system: use of ZSM-5 zeolite, which converts the C7+ olefins into light olefins with high catalyst Micro Activity Testing (MAT) activity
•  Hydrocarbon partial pressure in the reactor: shifts the reaction equilibrium to favour low molecular weight olefins. This is achieved though a low operating pressure and the addition of steam, and should be balanced with increased plant costs due to larger vessel requirements.

Propylene is produced in the FCC unit by cracking of olefinic naphtha to lighter olefins. The cracking reactions that take place in the initial reaction step on the lower section of the riser with the feed and hot regenerated catalyst are endothermic. The catalyst supplies the necessary heat to reaction temperature. The riser is no more than a straight pipe, the diameter of which is set to provide the feed with a certain residence time.

Other converting reactions occur in a later step in the middle or upper section of the riser. In the sequential reactions, olefins that are initially produced from cracking reactions are consumed by subsequent secondary reactions yielding iso-paraffins and/or aromatics. The reactions producing light olefins are controlled by an equilibrium mechanism and thermodynamics limit the propylene production from the FCC unit. Table 1 summarises the FCC unit’s operating conditions as compared with  steam cracking.

As Table 1 shows, cracking in the FCC/DCC units occurs at moderate temperatures compared with steam cracking, which makes the process very efficient from an energy consumption perspective. Coke that is deposited in the catalyst is burnt to regenerate the catalyst and provides the heat required by the cracking reaction. Also, since the feedstocks employed, such as gas oils and residues, are relatively cheap compared to steam cracking (traditionally naphtha fed in Europe), the process can be economically attractive. In addition, the DCC unit requires very clean (desulphurised, low metals content, low Conradson carbon) feedstocks with a high hydrogen content.

The increase in the ROT brings increased production of olefinic liquefied petroleum gas (LPG), dry gas and coke. Ethylene produced in the FCC unit (typical yield <2 wt% of the feed) could be recovered instead of using it in the fuel gas system.

The addition of ZSM-5 zeolite, with its characteristic pore size, which provides shape selectivity by limiting access to the interior of 
the catalyst to mostly linear non-branched paraffin and olefin molecules, gears the resulting equilibrium distribution of the C3 and lighter olefins towards propylene, the olefin product with higher yields.

Ethylene is also produced, but its yield is largely dependent on reaction conditions, as mentioned above. The reaction chemistry and the use of ZSM-5 catalyst favour conversion of the olefinic molecules in the C7-C10 range to olefinic LPG. This depletes the catalytic naphtha of olefins, which, along with the fractionation of the light catalytic naphtha (LCN) typically composed of C5-C6 molecules, contributes to the high aromatics content of the heavy catalytic naphtha — another synergy that can be exploited for aromatics production.3

Processing of light catalytic naphtha (LCN)
The direct cracking of C5-C6-type molecules contained in the LCN to form light olefins requires initial dehydrogenation to form olefins that can then be cracked through olefin reaction pathways and require high severity (temperatures of about 650°C) and a high catalyst-to-oil ratio. So, because of the selectivity of ZSM-5 zeolites to crack larger molecules, the cracking of the lighter molecules is regarded more as a thermal cracking process. Coke make when cracking LCN is low. Therefore, in order to achieve a significant increase in propylene production, large amounts of LCN need to be processed. 

One option is to process the LCN into the same riser as the main feedstock. In this case, the LCN would be injected slightly below the main feedstock for it to detect a high catalyst-to-oil ratio. This option, although relatively low in cost, is not recommended, as the relatively large amounts of naphtha required to significantly increase the propylene yield (say by about 2%) would cool down the catalyst and the lower temperature would not allow complete vaporisation of the main feedstock, leading to unnecessary coke formation and deposition in the feed zone.