Alternative Routes To High Conversion
Most fluid catalytic cracking units (FCCUs) are operated to maximize conversion to gasoline and LPG. This is particularly true when building gasoline inventory for peak season demand or reducing clarified oil yield due to low market demand. Maximum conversion of a specific feedstock is usually limited by both FCCU design constraints (i.e., regenerator temperature, wet gas capacity, etc.) and the processing objectives. However, within these limitations, the FCCU operator has many operating and catalyst property variables to select from to achieve maximum conversion.
Conversion, as discussed in this report, refers to the percentage of fresh feedstock cracked to gasoline and lighter products and coke. It is calculated as follows:
Conversion = 100 -- (LCO + HCO + CO)
A low conversion operation for maximum LCO production is typically 40-60%, while a high conversion operation for maximum gasoline is 70-85%. The range depends on feedstock type. Every FCCU, which is operated for maximum conversion at constant fresh feed quality, has an optimum conversion point beyond which a further increase in conversion reduces gasoline yield and increases LPG yield as shown in Figure 1. This optimum conversion point is referred to as the overcracking point.
The primary variables available to the FCCU operator for maximum unit conversion for a given feedstock quality can be divided into two groups, catalytic and process. These include:
- Catalytic Activity
- Catalyst Design
- Availability of Cracking Sites
- Reaction Time
- Carbon on Regenerated Catalyst
- Reactor Temperature
The equilibrium catalyst activity, as measured by a microactivity test (MAT), is a measure of the availability of zeolitic and active matrix cracking sites for conversion. An increase in the unit activity can effect an increase in conversion. Activity is increased by one, or a combination of:
1. Increased fresh catalyst addition rate.
2. Increased fresh catalyst zeolite activity.
3. Increased fresh catalyst matrix activity.
4. Addition of catalyst additives to trap or passivate the deleterious effects of feed nitrogen, alkalis (i.e., calcium and sodium), vanadium and other feed metal contaminants.
5. Increased fresh catalyst matrix surface area to 'trap' feed contaminants.
In general, a two (2) number increase in MAT activity can give a 1% absolute increase in conversion.
Increased matrix surface area improves conversion by providing more amorphous sites for cracking high boiling range compounds in the feedstock which cannot be cracked by the zeolite. Increased zeolite, on the other hand, provides the necessary acid cracking sites for selectively cracking the amorphous cracked high boiling compounds and lighter boiling compounds.
In addition to zeolite and matrix activity, many of the catalyst's physical and chemical properties contribute to increased conversion through selectivity differences. These include zeolite type, pore size distribution, relative matrix to total surface area, and chemical composition.
Higher conversion and coke yield are thermodynamically favored by higher pressure. However, this variable is normally only varied over a very narrow range due to limited air blower horsepower. Conversion is not significantly affected by unit pressure since a substantial increase in pressure is required to significantly increase conversion.
Availability of Cracking Sites
Increasing the concentration of cataIyst in the reactor, often referred to as cat/oil ratio, will increase the availability of cracking for maximum conversion, assuming the unit is not already operating at a catalyst circulation limit. This can be achieved by increasing reactor heat load or switching to a lower coke selective (i.e., lower delta coke) catalyst. Reactor heat load can be raised by increased reactor temperature or lower feed preheat temperature. This, in turn, increases the cat/oil ratio to maintain the unit in heat balance.
An increase in reaction time available for cracking also increases conversion. Fresh feed rate, riser steam rate, recycle rate and pressure are the primary operating variables which affect reaction time for a given unit configuration. Conversion varies inversely with these stream rates due to limited reactor size available for cracking. Conversion has been observed in some units to increase 1% absolute for a 3-5% relative decrease in fresh feed rate. Under these circumstances, overcracking of gasoline to LPG and dry gas may occur due to the increase in reactor residence time. One approach to offset any potential gasoline overcracking is to add additional riser steam to lower hydrocarbon partial pressure for more selective cracking. Alternatively an operator may choose to lower reactor pressure or increase the recycle rate to decrease residence time. Gasoline overcracking may be controlled by reducing the availability of catalytic cracking sites by lowering cat / oil ratio.
Carbon on Regenerated Catalyst
The lower the carbon on regenerated catalyst (CRC), the higher the availability of cracking sites since less coke is blocking acid cracking sites. CRC is reduced by increasing regeneration efficiency through the use of carbon monoxide oxidation promoters such as COCATŪ and PROCAT, promoters supplied by BASF Corporation. CRC may also be reduced by more efficient air and spent catalyst contact. Increased regenerator bed levels also improve CRC through increased residence time but this must be traded off with reduced dilute phase disengager residence time and the possibility for increased catalyst losses.
Increased reactor temperature increases unit conversion, primarily through a higher rate of reaction for the endothermic cracking reaction and also through increased cat/oil ratio. A 10°F increase in reactor temperature can increase conversion by 1-2% absolute. Higher reactor temperature also increases gasoline octane and LPG olefinicity which is a very desirable side benefit of maximizing conversion through this route. The higher octane is due to the higher rate of primary cracking reactions relative to secondary hydrogen transfer reactions which saturate olefins in the gasoline boiling range and lowers gasoline octane. A 10°F increase in reactor temperature can give up to a 0.8 and 0.4 number increase in research and motor octane, respectively.
All of these variables are not always available for maximizing conversion. The reason is that most units are already operating at an optimum conversion level corresponding to a given feed rate, feed quality, set of processing objectives and catalyst at one or more unit constraints (e.g., wet gas compressor capacity, fractionation capacity. air blower capacity, reactor temperature, regenerator temperature, catalyst circulation). Therefore, the operator has only one or two operating variables to adjust. Once the optimum conversion level is found, the operator has no additional degrees of freedom for changing operating variables. However, the operator can work with the catalyst supplier to redesign the catalyst properties to remove operating constraints to shift the operation to a higher optimum conversion level.
Example 1 - LPG
For example, consider the operator who wants to increase conversion without sacrificing feed rate, but is operating at a wet gas capacity limit while using a USY octane catalyst with a low matrix surface area. Any increase in conversion through increased reactor temperature would cause the wet gas capacity limit to be exceeded. Since this alternative is not practical, the operator can increase conversion in the short term by increasing availability of catalytic cracking sites by higher catalyst circulation through lower feed preheat or additional reactor heat load while simultaneously reducing reactor temperature. This alternative will enable the operator to increase conversion but will result in lower gasoline octane due to the lower reactor temperature. A feasible alternative is to redesign the catalyst to optimize the catalyst matrix and zeolitic activities, other catalyst properties, and zeolite types. This would increase conversion to gasoline through greater bottoms upgrading without octane loss.
Example 2 - Regenerator
Consider the operator who wants to increase conversion but is operating at a regenerator temperature limit. Conversion can be increased through higher cat/oil ratio when it is obtained by lower feed preheat. Alternatively, the unit conversion can be increased within the regenerator temperature limit by redesigning the catalyst properties to achieve lower coke selectivity.
The following two examples of commercial data illustrate how operators with different constraints were able to raise conversion.
Example 3 - Dry
Gas & Coke Limit/ Maximum Reactor Temperature
In this example, the FCCU was operating on a rare earth zeolite Y (REY) gasoline catalyst with moderate matrix for maximum bottoms upgrading and high gasoline yield while processing a moderate level of resid. The unit conversion was limited by regenerator, reactor and feed preheat temperatures. Future processing objectives were the same, but without resid. See Figure 2.
To accomplish these new objectives, the catalyst was redesigned for lower matrix activity at constant total activity. This resulted in a catalyst with a higher zeolite surface area. The results shown in Figure 2 indicate the unit was re-optimized for nearly a 5 volume percent relative conversion increase.
Example 4 -
Maximize Octane Barrels
In this example, summarized in Figure 3, the refiner was operating the unit for maximum gasoline octane and bottoms upgrading using an Ultra-Stable Y Zeolite (USY) Octane Catalyst having a moderate matrix surface area. Unit conversion was limited by a maximum 975° F reactor temperature and 500° F minimum feed preheat when the processing objective changed to maximize octane-barrels, this required an increase in conversion. To achieve this, the catalyst was re-designed with less USY zeolite for higher hydrogen transfer and greater gasoline yield. The result was a 3 volume percent relative conversion gain with nearly a 5% relative increase in FCC gasoline octane-barrels.