Octane losses

Saturation of olefins other than reactive olefins usually is not desired. The added hydrogen is often expensive or useful elsewhere, and it does not provide any real improvement in product quality. Acmally, product quality may be reduced in the case of gasolines. Research octane number losses may be correlated with increasing olefin saturation. So in many cases, hydrodesulfurization conditions are selected with an eye toward minimizing olefin saturation over and above that needed for product quality improvement. There is one exception saturation of certain olefins shows substantial improvements in Motor octane number. This is true for iso- and n-pentenes and to a lesser extent for higher boiling isoolefins. The higher n-olefins show octane losses upon saturation. 

The chemical objectives of naphtha and gasoline hydrotreatment are essentially the removal of S and N, and up to whether possible, to saturate the monoaromatic rings. One collateral requirement has to do with achieving the objective with no octane losses. The chemistry of the HDN of nitrogen compounds, commonly found in gasoline, has been the subject of studies, some of which are considered in this section. 

Baco, F., Debuisschert, Q. Marchal, N., et al., Prime G+ process, desulfurization of FCC gasoline with minimized octane loss, in Fifth International Conference on Refinery Processing, AICHE 2002 Spring National Meeting. 2002. New Orleans, LA, March 11-14. 180-188. 

Adding a downstream gasoline splitter can help when reducing gasoline sulfur by minimizing octane loss. Other towers may be expanded by using structured packing. 

Olefin Reductions—Alkylation capacity would increase with any reduction in olefin content because it would provide a way to use the C4 and some C5 olefins in gasoline. New isomerization capacity would be needed if substantial quantities of C5 and C6 olefins had to be removed. The C5/C6 olefins would have to be hydrogenated and isomerized to make up part of the octane loss. 

Lower the FCC unit riser temperature. This solution indeed lowers the S level (up to 7%) in the gasoline range, but many units would require large temperature drops (30 °C) leading to octane losses and operating difficulties (19)... 

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. 

CDHDS is used in combination with CDHyrdro to selectively desulfurize gasoline with minimum octane loss. Bottoms of CDHydro column, containing the reacted mercaptans, are fed to the CDHDS column where the MCN and HCN are catalytically desulfurized in two separate zones. HDS conditions are optimized for each fraction to achieve the desired sulfur reduction with minimal olefin saturation. Olefins are concentrated at the top of the column, where conditions are mild, while sulfur is concentrated at the bottom, where the conditions results in very high levels of HDS.76... 

In HE alkylation, the processing of 1-butene leads to a significantly lower RON. Therefore, to avoid this octane loss, HF units often employ upstream selective hydrogenation/isomerization to isomerize 1-butene to 2-butene. 

Reduces benzene in reformate streams by over 99.9% Minimal impact to hydrogen balance and octane loss... 

Besides the technologies on the way to commercialization, new catalysts have been proposed to achieve the effective desulfurization of gasoline without loss of octane number. Yin et al." reported that FCC naphtha could be desulfurized over Ni/HZSM-5 catalyst without octane loss due to aromatization activity of the catalyst. In their concept, an olefin is aromatized before the hydrogenation. Excess cracking as well as rapid saturation are a concern. The present authors are examining CoMoS catalysts supported on carbon materials in HDS of cracked gasoline. 

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