What a Low Delta Coke Catalyst Means to the Refiner
Literature about low delta coke catalysts has increased considerably with the introduction of ultrastable Y zeolite (USY) octane catalysts. These USY catalysts comprise the most recognized form of low delta coke catalysts. However, the refiner does not necessarily have to look to a full USY octane cataIyst to enjoy the benefits of a low delta coke catalyst.
BASF's DYNAMICS series catalysts allow the refiner to choose from a full catalyst spectrum ranging from high delta coke (typically very gasoline and light cycle oil selective) catalysts to low delta coke (typically octane producing) catalysts. BASF can tailor the catalyst to the refiner's specific need, moving from a higher delta coke catalyst without going to a full USY octane catalyst.
The means by which the DYNAMICS concept achieves lower delta coke hinges on the catalyst properties that make up such a catalyst. Low delta coke catalysts may have an increased content of USY zeolite and/or a comparatively higher zeolite-to-matrix ratio. At constant conversion, catalysts with higher USY content increase LPG yield, reduce gasoline yields, and increase octane. When higher zeolite-to-matrix ratio is used to lower delta coke, the relatively lower matrix content results in less bottoms upgrading but no loss in gasoline yield. BASF's DYNAMICS concept allows the refiner to use either option to reduce delta coke. The choice will depend on unit constraints and refinery economics.
Benefits of a Low Delta Coke Catalyst
At first glance, the decrease in gasoline yield or the increase in bottoms make that are characteristic of constant conversion evaluations of low delta coke catalysts may negate the catalysts' benefits. In practice, these catalysts typically improve cat cracker yields by allowing an increase in conversion up to the FCCU limits. The largest increase can be obtained in units where regenerator temperature limits conversion.
In order to understand how low delta coke catalysts improve conversion, it is first necessary to understand the relationship between delta coke, the FCCU heat balance, and the unit operation. The definition of delta coke is given in Figure 1. According to the definition, a lower delta coke catalyst must either decrease the coke yield or increase the cat-to-oil ratio. Since overall heat balance states that the coke yield is essentially set by reactor heat requirements, a low delta coke catalyst will increase the cat-to-oil ratio in the FCCU through increased catalyst circulation.
The increased catalyst circulation that is characteristic of low delta coke catalysts also leads to higher conversion and/or lower regenerator temperatures. The regenerator heat balance states that the energy required to heat the circulating catalyst and the flue gas to the regenerator temperature is supplied by coke burning. Low delta coke catalysts increase circulation while the coke yield remains essentially constant. Therefore, less heat is generated per pound of circulating catalyst and the regenerator temperature cools. Meanwhile, the higher circulation supplies more cracking sites per weight of oil, so conversion increases. The result of higher circulation is thus higher conversion at equal or lower regenerator temperatures.
Use Lower Delta Coke Catalysts to Increase the Cat-To-Oil Ratio
If a commercial FCCU is not catalyst circulation limited, the increased cat-to-oil ratio combined with the other properties of low delta coke catalysts can provide numerous benefits to a refiner. Figure 2 shows the yield shifts that occur at constant coke yield with a change from a high delta coke to a low delta coke catalyst. The reduction in catalyst delta coke was achieved by both an increase in zeolite-to-matrix ratio and an increase in the USY zeolite content of the catalyst. The most noteworthy yield changes are:
- Higher conversion due to higher cat-to-oil ratio.
- Lower dry gas make due to higher zeolite-to-matrix ratio.
- Greater potential gasoline yield via increased LPG. This effect is due to the higher content of USY zeolite.
- Higher liquid volume yield due to the conversion increase and the lower dry gas yield.
- No significant increase in bottoms yield because the higher cat-to-oil ratio compensated for the reduction in matrix cracking.
Figure 2 represents the optimum yield shifts for the low delta coke cataIyst only if air blower capacity limits the FCC conversion. If catalyst circulation or LPG handling capacity were limits on the unit conversion, the refiner would have exceeded these limits at constant coke make. In units where these constraints apply, the advantages of a low delta coke catalyst are less than stated above. However, the lower regenerator temperature and the reduced dry gas make may allow certain refiners to take further advantage of this low delta coke catalyst, as described below.
Besides using a low delta coke cataIyst, a refiner has two other controls that allow him to increase cat-to-oil ratio: reactor temperature and feed temperature. Either raising reactor temperature or lowering feed temperature will increase the reactor heat requirement. Higher heat requirements require increased coke yields. Increased cataIyst circulation is needed if delta coke remains constant, and this again results in higher conversion.
Figure 3 exhibits the results of increasing reactor temperature by 10°F and of decreasing feed preheat by 25°F. Benefits include:
- Higher conversion, which reduces the bottoms yield to a lower level than that of the higher matrix catalyst.
- Greater potential gasoline due to increased LPG production.
- Higher octane (only for the increased reactor temperature case).
Changing to a low delta coke catalyst benefits the refiner by increasing the cat-to-oil ratio without causing a corresponding increase in coke yield. The increased cat-to-oil ratio shifts cycle oil yields into higher valued light products. This shift increases the total liquid volume gain in the FCCU.
The lower dry gas yield and lower regenerator temperature may further benefit the refiner. It may be possible to increase reactor temperature, lower feed temperature, or process more heavy feed. All of these may prove beneficial .
FCCU constraints may prevent the refiner from achieving the full benefit of the low delta coke catalyst. For example, a catalyst circulation limit would not benefit from a lower delta coke catalyst due to the increased catalyst circulation it causes. A wet gas compressor limit would reduce the benefit of a low delta coke catalyst, but some benefit would still be realized due to the lower dry gas make.
- Increased conversion at same coke yield.
- Lower dry gas yield.
- Lower regenerator temperature.
Possible Problems in FCCU
- May reach cat circulation limit.
- May reach wet gas compressor limit.