FCC Octane MON Versus RON
Significance of RON and MON
For clarity it is useful to provide a general overview of an octane number. An octane number is a quantitative, but imprecise measure of the maximum compression ratio at which a particular fuel can be utilized in an engine without some of the fuel /air mixture "knocking" or self igniting. This self ignition of the air/fuel mixture in the cylinder results in a loss of peak power. Directionally as the compression ratio of the engine increases so does the required octane number of the gasoline if engine knocking is to be avoided.
The performance of an engine is dependent upon many factors, one of which is the severity of operation. Accordingly the performance of a fuel is also dependent upon engine severity. To account for differences in the performance quality of a fuel two engine octane numbers are routinely used. Thc Research Octane Number (RON, or F1) simulates fuel performance under low severity engine operation. The Motor Octane Number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. In practice the octane of a gasoline is reported as the average of RON and MON or R+M/2.
Classically, both numbers are measured with a standardized single cylinder, variable compression ratio engine. For both RON and MON, the engine is operated at a constant speed (RPM's) and the compression ratio is increased until the onset of knocking. For RON engine speed is set at 600 rpm and MON is at 900 rpm.
MON and RON Depend on Gasoline Composition
The octane number measured is not an absolute number but rather a relative value based on accepted standards. By definition, n-heptane has an octane number (RON and MON) of 0, while iso-octane (2,2,4-trimethyl pentane) is 100. Linear combinations of these two components are used to measure the octane number of a particular fuel. A 90%/10% blend of iso-octane/n-heptane has an octane value of 90. Any fuel knocking at the same compression ratio as this mixture is said to have an octane number of 90.
In general, RON values are never less than MON, although exceptions to this rule exist. For pure compounds the differences between RON and MON range from 0 to more than 15 numbers. Typical values for gasoline range hydrocarbons having boiling points between 30° and 350° F go from less than 0 to greater than 100 with the extreme values being generated by extrapolation. Table 1 summarizes actual RON and MON values for a variety of pure hydrocarbons.
In practice octane numbers do not blend linearly. To accommodate this, complex blending calculations employing blending octane numbers as opposed to the values for pure hydrocarbons are routinely employed. There is no universal blending program used industry wide. In fact, for a given oil company, blending calculations that are refinery specific are not uncommon. As an improvement over octane numbers of pure compounds, there are tabulations of blending octane numbers for both RON and MON. Summarized in Table 1, these numbers are measured by blending 20 vol.% of the specific hydrocarbon in 80 vol.% of a 60/40 iso-octane/n-heptane mixture. Although still not exactly indicative of the actual blending octane number for a specific gasoline composition, the blending octane numbers are more representative. In general, the blending octane numbers are greater than the corresponding pure octane number.
Octane in FCC Gasoline
Discussions of the source of octane in FCC gasoline are numerous. For the most part these discussions focus on RON and avoid the discussion of MON. For RON it is well known that increasing the aromatic, iso-/normal ratio and olefin content of gasoline results in significant increases in octane. Current catalyst technology requires that the hydrogen transfer activity of the catalyst be low if RON is to be increased. Most FCC octane catalysts make RON by increasing the olefin content of the gasoline rather than making aromatics. The use of Z-100 catalyst for increasing RON relies on cracking of low octane N-paraffins to generate olefins and concentrate the aromatic content. In fact it can be said that Z-100 catalyst does not make gasoline octane, but rather it concentrates it. A source of increased aromatic content can come from selective bottoms cracking. Selective bottoms cracking implies that substituted aromatic hydrocarbons boiling outside the gasoline range (>400°F) have their boiling point reduced to below 400°F by selective cracking of large substituent side chains. These can be either long chain paraffins or naphthenic rings. The saturated substituent will crack while the aromatic core cannot. Bottoms cracking of this type may most likely be achieved through improved understanding of the role of the matrix.
Improving the RON of FCC gasoline as discussed is for the most part well understood. The same cannot be said for MON. Although it is true that increasing the RON of gasoline does increase its MON, the incremental increase in MON is typically only 33-50% of the RON. Referring to Table 1, MON values (either actual or blending) are always less than RON except for highly branched paraffins. Increasing the weight fraction of isoparaffins in the gasoline should result in an increased MON/RON ratio although overall R+M/2 could in fact decrease. This is due to the fact that although isoparaffins have higher MON values than RON, their absolute values are generally less than their olefin counterparts.
Another approach to increase the MON / RON ratio of gasoline may result from increasing the weight fraction of those hydrocarbons having blending MON's nearly equivalent to the blending RON's. Examples include propyl or isopropyl-benzene, C-5 and substituted C-5 naphthenes and highly branched olefins like 4-methyl 2 pentene. The ability to selectively increase the concentration of these hydrocarbons may be more a feedstock property than anything that current catalyst technology can do.
If motor octane is to be increased to a greater extent than research octane it appears that the specific hydrocarbon types must be selectively produced or concentrated. Directionally, the degree of highly branched isomers, either paraffinic or olefinic, must be increased with emphasis on the degree of internal branching. Although aromatics have a large effect on both RON and MON, alkylbenzenes such as propyl or isopropyl benzene may effect MON to a greater extent than RON. To fully understand the factors effecting MON, detailed chemical characterizations of a variety of gasolines should be studied.
Light Straight Run Processing in the Reformer
Due to the demand for increased octane, any possible octane improvements in refining streams are being investigated. The best processing option for light straight run is isomerization. However, if that option is not open, then reforming that stream can be the next best option.
Reforming light straight run was tested in a commercial reformer and the results were a net upgrade of the light straight run by 11 to 14 research octane numbers.
The data from this test run follows:
The octane benefits are a result of the conversion of low octane n-hexane ( RON=24.8) to isohexane (RON=80) and the generation of isopentane (RON=93) and isohexane (RON=80) from the bulk reformer feed.
Based on these results, it may be useful to economically evaluate the octane gain that can be obtained by processing light straight run in your reformer.
References for Table 1
1. Modern Petroleum Technology; 5th Edition Part II; Edited by G.D Hobson, Wiley 1984, page 786.
2. Heterogenous Catalysis in Practice: C N. Satterfield, Wiley 1980, page 241.