Base octane

Isomerization. Isomerization is a catalytic process which converts normal paraffins to isoparaffins. The feed is usually light virgin naphtha and the catalyst platinum on an alumina or zeoflte base. Octanes may be increased by over 30 numbers when normal pentane and normal hexane are isomerized. Another beneficial reaction that occurs is that any benzene in the feed is converted to cyclohexane. Although isomerization produces high quahty blendstocks, it is also used to produce feeds for alkylation and etherification processes. Normal butane, which is generally in excess in the refinery slate because of RVP concerns, can be isomerized and then converted to alkylate or to methyl tert-huty ether (MTBE) with a small increase in octane and a large decrease in RVP. 

SiHcone residue introduced to gasoline with toluene plugged catalysts on vehicles (83). Also a manganese-based octane improver known as MMT has been shown to clog catalyst surfaces (84). 

ZSM-5 s effectiveness depends on several variables. The cat crackers that process highly paraffinic feedstock and have lower base octane will receive the greatest benefits of using ZSM-5. ZSM-5 will have little effect on improving gasoline octane in units that process naphthenic feedstock or operate at a high conversion level. 

When using ZSM-5, there is almost an even trade-off between FCC gasoline volume and LPG yield. For a one-number increase in the research octane of FCC gasoline, there is a 1 vol% to 1.5 vol% decrease in the gasoline and almost a corresponding increase in the LPG. This again depends on feed quality, operating parameters, and base octane. 

Table I lists some of the available results from twenty of these commercial applications. In all cases, ZSM-5 increased both Research and Motor Octane. The ZSM-5 content in the unit inventory for these applications ranged from 0.2 to 3 wt %. The variations in the octane response from a given concentration of ZSM-5 are due to variations in the base octane (octane without ZSM-5), gasoline cut point, catalyst makeup rate and regenerator temperature.

Figure 1. Impact of base octane on ZSM-5 octane boost.

We now need to relate the activity of ZSM-5 to the octane boost achievable commercially. As discussed previously, the octane boost at any given ZSM-5 activity depends on the base octane, which is a characteristic of the concentration of low octane olefins and paraffins in the gasoline. Figure 1 illustrates the relationship between octane boost and activity at several different base octanes. As can be seen from the curves, as the gasoline base octane increases, more ZSM-5 activity is required to achieve a given octane increase. 

Table VIII shows the influence of base octane on the relative fractional ZSM-5 replacement rate required to achieve a 1 RON boost. Model estimates show that half as much ZSM-5 is required to achieve a +1 RON/+0.4 MON boost at a base octane of 88 as compared to an application with a base octane of 92.

The base octane for this application was 93.4 and the regenerator temperature was 1335 F. As the data show, the model matches the commercial response very well. The objective of the application at Refinery B, was to increase the Research Octane by 1.5 numbers within the first week. The base octane for this application was 92.7 and the regenerator temperature was 1275 F. Again, the model prediction fits the commercial data quite well. 

Table VIII. ZSM-5 Model - Parametric Sensitivity Effect of Base Octane...

The estimated average Western European pool RON without lead additive is about 92.5. The average lead based octane increase is 3 to 3.5. 

Each unit is unique in terms of its ability to retain catalyst due to its mechanical design and the operating conditions employed however, a general rule is that the additive should exhibit similar physical properties to the cracking catalyst. This philosophy is particularly true with CO promoters which are added at much less Aan 1 % of the cracking catalyst addition rate. From an economic standpoint, FCC additives are all significantly more expensive than FCC catalyst. CO oxidation promoters are twenty to fifty times more expensive while ZSM-5 based octane additives or SOx removal additives are about five to ten times more expensive than cracking catalyst. As a result, it is critical that the maximum amount of additive be retained in inventory for its useful (active) life. 

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