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Oct-2009

A novel process to reduce aromatics and benzene in reformates

A catalytic scheme for reducing the aromatics and benzene content in reformates without octane barrel loss uses existing reforming units at Moscow Oil Refinery

Mikhail Levinbuk, A Meling, V Zuber and A Lebedev
Moscow Oil Refinery

V Khavkin
All-Union Scientific Research Institute of Petroleum Processing

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Article Summary

A reduction in aromatics, by 
25–30 vol%, and benzene in finished gasolines, without any octane barrel loss, is essential to further improve the quality of reformulated gasoline. As Table 1 shows, the majority of the quantitative aromatics (around 60 vol%) and benzene content (around 80 vol%) in finished gasolines is found in reformates, which in turn represent 35–60 vol% of the composition of all gasoline components.

This article looks at new ways of dealing with these issues and focuses on the reconfiguration of existing semi-regenerative reforming units at Moscow Oil Refinery. The aim of the research was to change the rates of major reactions to obtain novel routes, by loading various catalysts into the conventional reactors to obtain reformates with a reduced aromatics content and without octane barrel loss. The normal routes for removing benzene from reformates are either pre-fractionation of the reforming feedstock (to separate sources of benzene formation from the hydrocarbon feedstock) or processes that upgrade reforming post-fractionation cuts (to remove benzene through conversion to other hydrocarbons or through extraction for application to petrochemicals production).

Reducing aromatics content in reformates
Figure 1 shows the components of finished gasolines in US reformulated gasoline (34 wt% of the total gasoline pool), which are major contributors to the aromatics content in reforming gasoline, catalytic cracking gasoline, alkylate gasoline, methyl tert-butyl ether (MTBE) and ethanol. The largest contributors to the total aromatics content are reforming gasolines, and limits on the solution of aromatics by high-octane components are connected mainly to the total oxygen content of the finished gasoline (not more than 2.0 vol%). Therefore, following the ban on using MTBE (with an oxygen content of 18 wt%) in US reformulated gasolines, the total ethanol content (oxygen content 32 wt%) can substitute only 50% of MTBE’s share in the structure of the entire gasoline pool (see Figure 2).

To preserve octane barrel and overcome these limitations, the alkylate component would need to increase considerably. This would require the construction of alkylation units with a combined annual capacity of about 8–10 million tonnes of alkylate. To reduce the capital investment, it would be more acceptable to take a number of innovative steps involving semi-regenerative reforming units. The combined capacities of such units worldwide is about 60% of the total required capacity. To define the focus of scientific research needed for these developments, certain thermodynamic parameters of the main reactions in the conventional reforming process need to be first considered.

The two main reactions dominating the reforming process are naphthene dehydrogenation and dehydro-cyclisation of n-paraffin hydrocarbons. The latter reaction, achieved using a process with a fixed catalyst bed, can be dealt with using a new process with a moving catalyst bed at a reduced pressure and higher temperature. This provides an increase in reformate yields and octane number valued by an increase in aromatics content in reformates. One of the main tasks when creating a new reforming process is the replacement of n-paraffins dehydro-cyclisation reactions from the conventional reforming process with a reaction that supports high octane numbers in reformate at the expense of non-aromatic hydrocarbons.

Figure 3 shows the dependence of octane number values on the fractionation composition of reforming feedstock. An “octane number pit” in the graph is determined by the 20–25 vol% of low-octane C7-C9 n-paraffins in the reforming feedstock that must be isomerised (rather than cracked) to increase octane number values and support acceptable reforming yields. The general concept of developing catalysts for isomerising n-paraffins with differing hydrocarbon chain lengths is presented in Figure 4. Depending on the length of the paraffin chain (C4-C6, C7-C18 and C25-C55), it is possible to develop three types of catalyst with different matrix acidities and the same noble metals content (for dehydrogenation) reaction.

This development requires the creation of a new catalyst for isomerising medium-chain paraffins (n-C7-C9) and the reloading of semi-regenerative reforming units with both new and established catalysts (see Figure 5). With the arrangement shown in Figure 5, isomerisation of C7-C9 n-paraffins and hydrogenation reactions to convert aromatics into naphthenes (with an increase in octane number of 71–72 RON) occur in the first reactor. Naphthene dehydro-genation reactions to form aromatics (with an increase in octane number of 95–96 RON) take place in the second reactor, which contains conventional catalysts. This new process is called hydroisoforming.

Figures 6 and 7 compare the results of upgrading narrow fractions from straight-run naphtha IBP-180ºC using traditional processes and hydro-isoforming: IBP-70ºC fraction using isomerisation; 70–100ºC and 105–180ºC fractions using traditional benzene and gasoline reforming (Figure 6); and 70–100ºC and 110–180ºC fractions using hydroisoforming (Figure 7).

The best results for reforming straight-run gasolines are seen in the hydroisoforming of the 110–180ºC fraction. In this case, the aromatics content is reduced by 10 vol%, compared with the traditional process, and the benzene content falls from 
4–5 vol% to 1.2–1.5 vol%. This enables the application of the resulting reformates as reformulated gasoline. During hydroisoforming of the 70–100ºC fraction at low octane number


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