Octane Catalyst Effects on Heat of Cracking and Coke Make
Some zeolitic fluidized catalytic cracking catalysts have a high rare earth content, while others have low or even zero content. The advantages that rare earth content offers were discussed in CATALYST REPORT #789. This report deals with the effects and benefits of zeolitic catalysts without rare earth. It summarizes the results of a study of existing knowledge and laboratory and computer tests, and is based on a paper by BASF scientists Ernest Leunberger and Linda Wilbert that was presented at the 1987 annual meeting of the National Petroleum Refiners Association in San Antonio. An article based on the paper was published in the Oil & Gas Journal(Ref. 1) on May 25, 1987.
Low-content or zero-content rare earth catalysts are made by various manufacturers. BASF products that contain 100% non-rare-earth-exchanged sieve include OCTISIV® PLUS 580 and OCTIDYNE 1150 catalysts.
FCC octane catalysts of the ultra-stable Y (USY) type with little or no rare earth content can be expected to increase the endothermic heat of cracking by 25 BTU per pound of feed and reduce the catalytic coke-making tendency by 5 relative percent for each 1 number increase in research octane.
An analysis of commercial data, laboratory results, and simulation on an FCCU computer program indicate that the heat-of-cracking increase is due to two factors: the "recracking" of gasoline to "wet gas," principally C4, and the reduction of hydrogen transfer reactions when octane catalysts replace rare-earth-exchanged catalysts.
Besides the octane gain, which is typically on the order of 2 research numbers, the benefits of using octane catalysts include lower regenerator temperatures- as much as 30° F in a commercial FCCand higher cat-to-oil ratios. These benefits are desirable in most FCC units, especially when a refiner wishes to process more of a high carbon residue feedstock such as resid.
The main disadvantage of octane catalysts is the recracking of gasoline to wet gas. This occurs because there is not significant hydrogen exchange to promote the hydrogen-transfer reactions that compete with recracking. The higher heat of cracking that is characteristic of octane catalysts can also be a disadvantage when a unit without the ability to increase preheating of feedstock is at a coke-burning limit.
Endothermic and Exothermic Reactions
Catalytic cracking reactions are endothermic: they create products with higher heat contents than the reactants, and they absorb heat from the environment. In the catalytic cracking of paraffins a high-molecular-weight-paraffin is cracked to form a lower-molecular-weight olefin and a paraffin. Table 1 uses the cracking of normal decane and normal heptane as examples .
Approximately the same BTU's of heat are required to crack a carbon-carbon bond whether the hydrocarbon has low or high molecular weight. On a per-pound basis, however, the energy increases as the molecular weight of the feed decreases. Thus Table 1 shows a higher endothermic heat of cracking (366 BTU's) for heptane than for decane.
In competition with cracking reactions are hydrogen transfer reactions which are exothermic and thus give off heat. Hydrogen transfer reactions are promoted when rare earth is present in a zeolite catalyst. As indicated above, one effect of hydrogen transfer is to limit the production of C4 gas, as well as C3, reducing the endothermic heat of reaction.
The hydrogen transfer reactions promoted by rare earth exchange also tend to reduce the endothermic heat of cracking because they are exothermic. In the first example in Table 2, a naphthene and lower-molecular-weight olefins react to form an aromatic and light paraffins. When the olefins saturated by hydrogen transfer are in the gasoline boiling range, this reaction reduces gasoline octane. The second reaction shows how hydrogen can form a carbonaceous deposit (6C) transfer on the catalyst from a heavy aromatic. When this type of hydrogen transfer is eliminated, as in the reactions shown in Table 1, coke-make is reduced.
FCC octane catalysts maximize the octane of the cracked gasoline by minimizing the transfer reactions that saturate gasoline olefins. That the reduction of hydrogen transfer must also increase the endothermic heat of cracking has been observed by J. L. Mauleon et al. The results of his work, presented in Table 3, show that heat of cracking can be correlated with rare earth content. Lines 3 and 4 show that ultra-stable Y catalysts, with smaller unit cell size, which promotes less hydrogen transfer, have the effect of further increasing reaction heat.
Table 4 shows the correlation between rare earth exchange and coke-make observed by K. Rajagopalan and A. W. Peters(Ref. 2).
Ascertaining the Heat of Cracking
Two different approaches to ascertaining the heat of cracking were used in this study.
The first was to measure the heat absorbed by the cracking reaction through heat-balance methods. This was useful in analyzing commercial data.
The second, applied by Dart and Oblad(Ref. 3) in their classic study, was to analyze the reaction products and assign each one a heat of combustion, then add up the heats of combustion of the various products. For constant feedstock, the highest product heat of combustion correlates with the highest endothermic heat of reaction. This method was found to be applicable for both laboratory and commercial analysis.
An example of the changes in product distribution that are responsible for the heat-of-reaction gain with octane catalysts is shown in Table 5. The hydrogen-exchanged octane catalyst shows a lower gasoline yield and higher yields of C3 and C4. The shift is due to the "recracking" of gasoline which also increases the heat of reaction because it is highly endothermic.
The laboratory test results in Table 5 also show the reduction in delta coke typical of hydrogen exchanged octane catalysts. The data shows a small decrease in coke make and a significant increase in catalytic circulation which results in a 20 relative percent reduction in delta coke for the octane catalyst. Lower total coke make for an octane catalyst is possible in a laboratory unit because the unit is not heat balanced, so the coke make need not increase to supply the higher reaction heat reactions that produce octane require.
Laboratory test results comparing the rare-earth-exchanged catalysts and three hydrogen-exchanged octane catalysts showed an average heat-of-reaction increase of 30 BTU per pound and a reduction of delta coke by 6 relative percent for each 1 RON gain.
Four commercial octane catalyst trials showed agreement between the two methods of calculating heat-cracking differences to within 20 BTU per pound of feed. Although the catalysts used in the commercial trials were made by different manufacturers, with consequent variations in zeolite contents and matrix surface areas, changes in heat of cracking and delta coke made could be estimated from the octane difference. A 24 BTU average increase in heat of cracking and a 6 relative percent decline in delta coke per 1 RON gain for three octane catalysts two of them partially hydrogen-exchanged agree closely with the results from laboratory testing.
Effect of Heat-Cracking and Delta Coke Differences on Commercial FCCU Operation
When a catalyst increases the heat of cracking in a commercial FCCU, the unit must either increase its heat generation or reduce its heat requirements to stay in heat balance. Since the catalyst changeout usually occurs over a period of several weeks, the changes will be gradual, and the unit will have sufficient time to respond without a crisis. A unit with normal slide valve controls will respond to the increase in heat requirements by gradually increasing catalyst circulation. The higher circulation will increase the coke burned in the regenerator and bring the unit back into heat balance. If the unit does not have the air blower capacity to burn more coke, changes must he made in the unit operation to reduce the heat requirements. When no more coke can be burned, it is most economical to reduce the feed sensible heat requirement.
The heat balance can also be affected by the reduction in coke-make that results from introducing an octane catalyst. Again the standard FCCU control system would raise catalyst circulation to increase the coke-make back to its original level. This control strategy for handling lower delta coke will fail only if the catalyst circulation is at its maximum. Alternate strategies to increase coke-make include introducing a feed to the FCCU with a higher carbon residue, such as vacuum resid or slurry recycle. Spraying torch oil into the regenerator can be used as a last resort.
The increases in catalyst circulation caused by lowering the delta coke and raising the heat of cracking tend to reduce the regenerator temperature. According to one FCCU computer simulation, if enough octane catalyst is added to the unit to maintain catalyst activity, then a 20° F loss in regenerator temperature, a 15 percent relative increase in cat-to-oil, a 7 percent relative increase in coke make, and 2 volume percent increase in conversion would be expected. Correlations show this catalyst would also increase gasoline octane by 2 RON.
In a second projection where only enough octane catalyst is used to hold conversion constant the more usual case for commercial octane catalyst trials a 35°F drop in regenerator temperature and a 20 relative increase in cat-to-oil ratio are predicted.
These are the beneficial effects on unit operation lower regenerator temperatures and higher cat-to-oil ratios that an octane catalyst brings in particular to a refiner wishing to process more of a high carbon residue feedstock.
1. J.L. Mauleon and J.C. Courcelle. Oil and Gas Journal, October 21, 1985, page 64.
2. K. Rajagopalan and A. W. Peters, Preprint of the ACS Division of Petroleum Chemistry, Vol. 30, No.3, page 538 (1985).
3. J.C. Dart and A. G. Oblad, Chemistry Engineering Progress, Vol.45, page 110 to 118,(1949).
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