Prediction of FCCU Gasoline Octane and Light Cycle Oil Cetane Index
Octane is a measurement of the knocking characteristics of a gasoline as determined in a laboratory engine by a standard ASTM test method. Research Octane number and Motor Octane number are the two tests employed and both can be run on the same test engine. The differences are that the two tests are run under different engine operating variables, for example, the engine speed used for the research method is 600 rpm compared to 900 rpm, for the motor method.
The Octane Number of a particular gasoline sample is determined by blending n-heptane, which has 0 octane, and iso-octane, which has 100 octane, in the correct proportion to produce the same knock intensity, as the sample being run in the test engine. The percent of iso-octane in the blend is then assigned as the Octane Number of the test sample.
The cetane number is also an ASTM test, used to measure the ignition qualities of diesel fuels. The higher the number the better the quality. The cetane index is a calculated value using the ASTM mid boiling point and API gravity, and correlates well with cetane number. The cetane index is used if the volume of sample is too small or a test engine is not available.
A diesel fuel with a high cetane index has a low spontaneous ignition point, that is, combusts at lower temperature from the heat of compression of the diesel engine. A high cetane index of FCCU light cycle oil (LCO) is desirable since the higher the index, the more LCO can be blended into the more valuable road diesel fuel.
Many factors affect the gasoline octane and LCO cetane index. Feed quality, catalyst type and unit operating conditions all play major roles in affecting the quality of FCCU products. The purpose of this report is to show ways to predict the changes in FCC gasoline octane and FCC LCO cetane index from changes in feed quality, unit operating conditions and product qualities.
Factors Affecting Gasoline Octane
One of the major variables affecting gasoline octane, is reactor temperature. As a rule an increase of 10 deg. F in reactor temperature generally increases the Research Octane of FCC gasoline 0.6 RONC. While Motor Octane is not as dramatically affected by reactor temperature directionally some improvement in Motor Octane should occur when increasing reactor temperature. Motor Octane is affected more by hydrocarbon types. At higher reactor temperature, paraffins crack leaving a higher olefin and aromatic content gasoline. The increased olefins are the major improvers to Research Octane and although olefins generally lower the Motor Octane, there is some slight improvement from the increased aromatic content.
An indication of the change in hydrocarbon type in the gasoline can be seen by the change in the gasoline bromine number and aniline point. The bromine number is an indication of olefin content. A higher number indicates a higher olefin content. The aniline point indicates aromatic content with a lower aniline point indicating a higher aromatic content.
Feed Quality Can Have a Major
Impact on FCC Gasoline
The other major factor affecting FCC gasoline octane is feed quality. Generally, the less paraffinic feed, the higher the gasoline octane. Feed paraffinicity or lack of it can be indicated by a number of tests. Aniline point, 'K' factor and gravity are all rough indicators of feed type.
Aniline point is the temperature at which a 50/50 mix of sample and aniline are mutually soluble. Since aromatics are readily soluble in aniline, a lower temperature is needed to achieve solubility. Thus, aniline point is a rough measure of aromaticity, a lower number indicating a higher aromatic content.
The 'K' factor also measures the hydrocarbon chemical nature. Feeds with 'K' factors above 11.8 indicate a higher paraffin content. Feeds with 'K' factors below 11.8 are generally more aromatic. 'K' factor is a calculated number based on boiling point and gravity. Altamont crude which yields a gas oil with a high 'K' factor produces a gasoline with a relatively low octane number. On the other hand, a crude such as Alaskan North Slope, which produces a cat feed with a relatively low 'K' factor, yields a gasoline fairly high in octane.
Another indicator of feed quality, although a very general indicator but one easily obtained, is API gravity. API gravity is a measurement unique to the oil industry. The chemical nature of the feed determines the API gravity. Aromatics are much denser than paraffins with the same boiling point and therefore, will have a lower API gravity.
A higher aromatic content feed (or lower paraffin content feed) as discussed above should generally produce a higher octane gasoline. Figures 1 and 2 are commercial data plots showing the effect of changes in feed gravity on the FCC gasoline Research and Motor Octane, respectively. The lower API gravity feed (or higher aromatic content) will increase FCCU gasoline octane 0.3 RONC and 0.15 MONC for every 1.0 deg API change. Altamont feed has a gravity in the 35 deg API to 40 deg API range, whereas Alaskan North Slope cat feed is usually 20 deg API or lower.
Other factors which affect gasoline octane within the control of the refiner and independent of reactor temperature and feed quality are Reid vapor pressure (RVP) and gasoline cut points as well as FCC Catalyst choice. RVP of gasoline indicates butane content and generally increases gasoline octane 0.2 RONC for every 1.0 psi increase in RVP due to the amount of butanes dissolved in the gasoline. Changes in the cut point of gasoline also affect the octane of FCC gasoline. Various tests have shown that the front end (0-30% point) of FCC gasoline is higher in octane than the middle cut (40-60%) and tail end (90%+) in an FCC gasoline whose boiling range is C-5 to 430 deg F end point. A lowering of the FCC gasoline end point and increase in the RVP, as usually occurs during the winter heating oil season, will result in a higher FCC gasoline octane.
Catalyst Choice Can Improve Octane
Catalyst choice is also within the control of the refiner, however, catalyst inventories cannot be changed as quickly as RVP or cut points. The amount of rare earth or hydrogen exchanged zeolite as well as the degree of ultrastability of the catalyst have different effects on the ultimate octane of the FCCU gasoline. A non-rare earth ultrastable catalyst will produce a higher octane gasoline than a non-ultrastable rare earth exchanged catalyst.
Compared to a rare earth exchanged gasoline type catalyst, a partial rare earth exchanged, ultrastable catalyst can improve gasoline octane by 1.0 to 2.0 research numbers . A completely non-rare earth, ultrastable catalyst can improve the octane 2.0 to 3.0 research numbers clear or higher.
Of course, certain known yield changes occur with octane catalysts such as a slight decrease in gasoline yield and an increase in C3 and C4 olefins. The refiners own product requirements determine whether a gasoline or octane catalyst or a catalyst in between is used.
The table below gives a listing of the octane improvement a refiner may expect to see using various BASF Octane Catalysts when compared at constant conditions to ULTRASIV 260 which is a more traditional rare earth exchanged gasoline catalyst.
Factors Affecting LCO Cetane Index Directly Relate to Paraffin Content
Cetane index is different from cetane number only in that the former is a calculation. The cetane index correlates well with the cetane number in diesel fuels which have not had "improvers" added. Paraffins exhibit the highest cetane index, aromatics the lowest. Therefore, high cetane index LCO has a high 'K' factor as well.
The cetane index of LCO is calculated using the 50% boiling point and API gravity. The higher the 50% point and / or API gravity, the higher the cetane index. Figure 3 is a graphical representation of the cetane index calculation. The graph shows that the cetane index changes 1.2 numbers for every 1.0 deg API change in gravity (at a constant 50% point) or 1.0 numbers for every 10 deg F change in 50% point (at a constant API gravity). Figure 4 shows the effect of feed gravity on LCO gravity. An increase of 1.0 deg API in feed gravity increased the LCO gravity approximately 0.75 deg API.
Combining the data in Figures 3 & 4, means that a 1.0 deg API increase in the feed gravity should increase the LCO cetane index 0.9 numbers as shown in Figure 5.
High paraffin content crudes, such as Altamont, yield gas oil feeds which produce an FCC LCO with very high cetane indices. LCO produced from this feed normally have cetane indices in the mid 40's. Conversely, crudes with low paraffin content, such as Alaskan North Slope, yield gas oils which produce an FCC LCO with low cetane indices. An LCO produced from Alaskan North Slope feed will have a cetane index in the low 20's.
Octane Catalysts Will also Improve Cetane Index
Feeds which produce a high octane gasoline generally produce a low cetane index LCO. The reason is that the hydrocarbon types which affect gasoline octane and LCO cetane index are inversely related. Steps taken to improve gasoline octane, on one particular feed, will generally improve LCO cetane index as well. For instance, increasing reactor temperature improves octane by cracking the paraffins in the gasoline boiling range leaving a higher percentage of olefins and aromatics which are higher in octane. Long chain paraffins in the slurry boiling range will crack as well to shorter chain paraffins in the LCO boiling range. This improves the LCO cetane index. Reducing the rare earth content of a cracking catalyst will improve the LCO cetane index as well as the octane of the gasoline fraction. A reduced rare earth level means a reduction in hydrogen transfer thus leaving the hydrogen in the LCO fraction. This improves the paraffinicity and hence the cetane index of the LCO. At the same time hydrogenation of olefins in the gasoline fraction is reduced and octane in increased. These reactions are illustrated by the reaction mechanism shown below:
Comparing the performance of two widely used FCC catalysts, this effect of catalyst rare earth concentration on octane and cetane index is clearly demonstrated in the commercial data comparison that follows:
The above data was taken from a commercial unit operating at constant reactor temperature unit conversion and FCC feed stock.
In conclusion it should be noted that the relationship of feed gravity to gasoline octane and LCO cetane index naturally can vary from the data presented here. However, as a general rule, a 1.0 deg API increase in feed gravity should decrease the gasoline Research Octane 0.3 RONC, decrease Motor Octane 0.15 RONC and increase the LCO cetane index 0.9 numbers.
[Technical information and data regarding the composition, properties, or use of the products described herein is believed to be reliable. However, no representation or warranty is made with respect thereto except as made by BASF in writing at the time of sale. BASF Corporation cannot assume responsibility for any patent liability which may arise from the use of any product in a process, manner or formula not designed by BASF.]
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