Gasoline benzene reduction
By adding a new unit that reacts a benzene-rich stream with light olefins, a refiner complied with current regulations for benzene and achieved a payback in 20 months
EL-MEKKI EL-MALKI and BENJAMIN UMANSKY, ExxonMobil Research and Engineering Company
ERIK MOY, GRANT DONAHOE and TIMOTHY THOM, Badger Licensing
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Under clean fuels regulation in the US, specifically Mobil Source Air Toxics II (MSAT II), refiners are required to reduce the benzene in gasoline to 0.62 vol% on an average annual basis. This has been in effect since January 2011 for large refiners, with a deferment for small refiners until 2015. In Europe and in many other regions, a regulation of 1.0 vol% maximum benzene in gasoline has been in effect and others are expected to adopt similar regulations. The challenge for refiners is to meet the tightening gasoline specifications for benzene at the lowest cost and without significant octane loss.
There are several approaches available to refiners to reduce the benzene in their finished gasoline. Since naphtha reforming is the predominant source of benzene in a typical refinery, the prevention of the formation of benzene in the reformer is accomplished by pre-fractionation of the naphtha feed to remove the benzene precursors. However, for many refiners, pre-fractionation of the reformer feed does not provide sufficient benzene reduction to achieve 0.62 vol% in the gasoline pool. Alternatively, the conversion of benzene produced in the reformer is implemented downstream of a reformate splitter. The benzene containing light reformate fraction from this splitter is sent to a hydrogenation reactor, where benzene is converted to cyclohexane. Both of these strategies result in a loss of octane barrels and an additional burden on the hydrogen balance in the refinery. The third approach is benzene extraction for the petrochemical market. While petrochemical benzene can be an attractive product, there is significant investment required to recover benzene. And unless the refinery has existing facilities or capacity for such a process, it is very difficult to justify this investment on a small scale.
An alternative technology, BenzOUT, has been developed by ExxonMobil Research and Engineering Company (EMRE) and is licensed by Badger Licensing. The technology provides a low-cost alternative solution for refiners to meet benzene regulation without the octane and hydrogen debits associated with alternative options. In this article, we will discuss a project where significant benefits were achieved from the implementation of this technology.
EMRE developed the BenzOUT technology and catalyst to convert benzene into high-octane alkylaromatic blending components by reacting a benzene-rich stream with light olefins, such as ethylene or propylene. In December 2009, EMRE and Badger agreed to jointly market the technology to third parties, and ever since licences and related engineering services are provided exclusively through Badger.
In a typical application, the technology reduces benzene in reformate by reacting benzene contained in a light-cut reformate with refinery-grade propylene from an FCC unit over a proprietary EMRE zeolite catalyst. The typical benzene concentration in a light-cut reformate, which is produced in a reformate splitter, ranges from 10-30 vol%. A simple flow diagram of the process is shown in Figure 1. The key features are:
• Fixed-bed catalyst technology The process uses a fixed-bed liquid-phase reactor, resulting in low utility requirements. The reactor may be a single bed (stage) or multiple beds, depending on the benzene content of the feed and the desired benzene conversion. In revamp projects, it is possible to retrofit existing tubular or fixed-bed reactors for this application
• Catalyst The process utilises an EMRE proprietary highly active zeolite catalyst with long cycle lengths. In addition, the catalyst is regenerated ex-situ to further extend catalyst life
• Stabilisation Propane fed to the unit with propylene is removed from the BenzOUT product in a product stabiliser. This can produce a propane product of HD-5 quality. The product from the technology is a light reformate with a reduced RVP.
Besides benzene reduction, the process provides several advantages that make it economically attractive. The reaction of benzene with light olefin results in a volume swell, which largely depends on the benzene content in the feed and the degree of benzene conversion. Also, an octane gain of 2 to 3 numbers of (R+M)/2 in the total reformate is typical. Moreover, the BenzOUT unit offers reformer flexibility, since it allows refineries to process the full-range naphtha feed in the reformer to achieve increased hydrogen production and significant octane gain.
The first grassroots BenzOUT unit was started in December 2011 at Calumet Superior refinery in Wisconsin. A schematic of the unit’s process flow is shown in Figure 2. The naphtha feed splitter at the refinery was changed to become the reformate product splitter for the unit. Two new benzene alkylation reactors and a new product stabiliser with all associated equipment were installed.
The refinery had an aggressive timeline for the project’s execution, with a target mechanical completion date of November 2011. Given the climatic conditions at the refinery’s location, the short seasonal construction window was given full consideration. The selection of modular design accelerated the construction of the unit and allowed for a minimal footprint in the refinery. Six separate structural modules containing equipment, piping and instrumentation were delivered to the refinery to create a three-level structure. The major vessels such as alkylation reactors, the stabiliser and pumps were placed on the perimeter of this three-level structure. Figure 3 shows a photograph of the installed unit.
Benzout unit operation
Initial plant commissioning activities were commenced in mid-November 2011 with the assistance of operations and technology experts from EMRE, Badger and the client’s contractor. Catalyst was loaded on 9 and 10 November and, after a subsequent two-week period to set up rotational equipment and process controls, the unit began operation on 1 December, reaching steady-state operation within a matter of days. Operating and process control strategies were fine-tuned in the following weeks, and at the end of December the start-up was officially completed.
The operation of the unit has been straightforward and easy to control. No additional staffing is required to operate the unit. It is run without any online analysis and one daily set of samples.
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