Refinery gasoline pool

Depending upon the refinery needs, the raw C5 plus steam cracked naphtha may be sent to isoprene extraction, treated to remove gum forming diolefins and sent to the refinery gasoline pool, or else completely hydrogenated and then fed to an aromatics extraction unit. 

The debutanized gasoline is cooled, first by supplying heat to the stripper reboiler or preheating the debutanizer feed. This is followed by a set of air or water coolers. A portion of the debutanizer bottoms is pumped back to the presaturator or to the primary absorber as lean oil. The balance is treated for sulfur and blended into the refinery gasoline pool. 

A number of refiners split the debutanized gasoline into light and heavy gasoline. This optimizes the refinery gasoline pool when blending is constrained by sulfur and aromatics. In a few gasoline splitters, a third heart cut is withdrawn. This intermediate cut is low in octane and it is processed in another unit for further upgrading. 

Heavy aromatics formed in the octafiner also have excellent octane number blending values and are generally returned to the refinery gasoline pool. The stream consists entirely of Co and Cio alkylbenzenes. Blending octane numbers (I) of these compounds range from 118 to 170. The Co fraction contains approximately 50 vol % pseudocumene and 20 vol % mesitylene. 

FCC gasoline is a major contributor of aromatics, olefins and sulfur to the refinery gasoline pool (Table 7 based on NPRA survey results). As FCC severity increases to generate more light olefins, aromatics will become even more concentrated in the gasoline. 

Environmental limits on the aromatic content of gasoline and diesel fuel have led to a further application of supported nickel hydrogenation catalysts. Benzene can be completely removed from light Ce reformate or other similar streams by liquid phase hydrogenation, before blending into the refinery gasoline pool. 

Altering the refinery process to put more aromatics into the gasoline pool. This would increase the crude oil requirement per litre of fuel it would also increase exposure of the general public to higher levels of toxic benzene. This was not viewed as a significant problem. 

In addition to its octane enhancement ability described above, ZSM-5 also increases the feed to alkylation, methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) units. Since the products from all these processes contain high Research and Motor Octane components, ZSM-5 provides the refiner additional flexibility in his downstream processing whenever the need exists to increase overall gasoline pool octane. In addition, the overall refinery can be rebalanced to take... 

In Western Europe it is expected that new isomerization capacity may exceed alkylation installations since naphtha availability generally exceeds demand. By selecting isomerization over alkylation the octane number of the gasoline pool may be increased without increasing the volume. Moreover, olefinic charge stock avails for alkylation are considerably smaller in Europe since there are fewer catalytic cracking units per refinery than in the United States and Canada. It is predicted that C5, and to a lesser extent C5/C6 isomerization, will prevail over alkylation in Western Europe until more catalytic cracking units are installed and/or a shift in the demand for naphtha over fuel oil is experienced. 

Isomerization of C5 and Q paraffins is a refinery process that generates high octane for the gasoline pool, while contributing no olefins or aromatics (11). Branched C5 and C6 paraffins have much higher octanes than normal pentane and normal hexane, as shown in Table 4.9 (5). Furthermore, C5 and C6 isomers can be separated by distillation, and flow schemes can be employed to recycle lower-octane isomers to the isomerization reactor. 

The need for benzene reduction is one of the determining factors in the way refiners will have to modify their process portfolio to meet future specifications. Apart from lowering the reformer severity, pre-fractionation and post-fractionation provide viable tools to reduce benzene in the gasoline pool. Pre-fractionation and subsequent hydrogenation of benzene is a typical solution. However, the products (cyclohexane and alkyl-cyclohexanes) are low in octane. Therefore, this option is only feasible if the refinery is not short in octane. The octane loss can be compensated for by the addition of oxygenates or preferably by the addition of alkylates. If more octane is needed, post-fractionation is one of the solutions. 

A problem present in the refinery is that, due to its fast transport in water and low biodegradabUity, MTBE addition to gasoline pool has been banned in some countries (from 2003 in Cahfomia). MTBE is formed by add-catalyzed reaction of isobutene with methanol. Other alcohols could be used to form different oxygenated additives, as discussed below, but the alternative is to use isobutene for conversion into another high octane number component such as isooctane, which could substitute in part the need of the alkylation process and related environmental/safety problems. 

From the inspection of the gasoline pool of a conversion type refinery, it is clear that major contributions with respect to octane optimization, may be expected from the fluid catalytic cracker and the downstream upgrading of its products. The development of zeolites contributes very substantially to these goals, both by their introduction into FCC catalysts and their use in the upgrading of some of the side streams. 

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