Optimum FCC Conditions Give Maximum Gasoline and Octane
The strong demand for higher octane gasoline dictates that most refineries operate their fluid catalytic crackers (FCCUs) at as high a reactor temperature as possible to produce maximum octane
Catalyst activity should also be targeted at a high level, but activity should not be high enough to overcrack the FCC gasoline to lighter C3's and C4's.
After high reactor temperature and targeted catalyst activity have been established, catalyst circulation should be increased by reducing feed preheat until the FCCU reaches an air blower or LPG handling constraint.
Results of laboratory studies indicate that maximum gasoline yield is achieved by maintaining an appropriate target catalyst activity at all catalyst-circulation rates.
The purpose of an FCCU is to convert gas oils to high-octane gasoline. Refinery profits are usually improved when the cat cracker is operated to achieve maximum yield and octane of the cracked gasoline.
There are three main controls on an FCCU that can be increased to convert more gas oil to gasoline and lighter products: catalyst activity, catalyst circulation rate, and reactor temperature. Maximum gasoline octane barrels are produced by increasing catalyst circulation rate and reactor temperature until conversion is limited by two or more unit operating constraints.
Catalyst activity is maintained at the highest target level consistent with good gasoline selectivity, and is not further increased unless the economics favor recracking gasoline to C3's and C4's.
Catalyst is maintained at a target level by replacing a portion of the catalyst inventory with fresh catalyst each day. In order to increase activity, more of this fresh catalyst makeup is added.
It is essential to adjust fresh catalyst additions to maintain catalyst activity within a target range where maximum gasoline selectivity is required. When activity is too low, the refiner is losing an opportunity to increase gasoline yield while realizing a minimum increase in gas yields.
When activity is too high, the cataIyst will overcrack gasoline to C3's and C4's, and gasoline yield will decline despite an increase in conversion.
Gasoline overcracking, caused by excessive catalyst activity, is illustrated by the laboratory data in Figure 1. In this experiment, catalyst was steam deactivated to various activity levels and then used to crack a Midcontinent gas oil at constant cat-to-oil ratio and reactor temperature.
Conversion increased about 1 wt % for each number increase in catalyst activity over the 64 to 77 activity range of the experiment. Gasoline yield increased 0.8 wt % for every number in catalyst activity from 64 to 71 activity.
At higher activities, additional conversion did not increase gasoline yield. Instead, the LPG yield increased by 0.8 wt % for each number of activity. The target activity range that achieves maximum gasoline selectivity for this feedstock is 70 to 72. Above this target range, no increase in gasoline yield can be expected for higher activity.
Before the activity becomes high enough to overcrack the gasoline to LPG, increasing catalyst activity is the most effective way to obtain maximum gasoline yield. The 0.8 wt % gasoline increase/ 1 wt % conversion represents a 1.0 vol % gasoline increase for each volume percent conversion increase.
The other methods of increasing conversion, higher cat-to-oil ratio and higher reactor temperature, give a lower gasoline increase per unit conversion. It is therefore important to establish a target activity for an FCCU and maintain the activity at the target level. Figure 2 shows gasoline and LPG selectivity in laboratory circulating pilot plant yields at various catalyst-circulation rates and selectivities.
At every conversion level, gasoline yield was highest at the target activity of 70 to 72. These results indicate that operating too far above or below the optimum activity reduces gasoline yield.
Although operators of many units find that 68 to 72 is the best target activity, this is not the case for all units and all feedstocks. Units with long catalyst-to-oil contact times, such as bed crackers, require lower target activities to avoid overcracking.
Units that process high-nitrogen feedstocks require higher target activities because the nitrogen poisons a portion of the catalyst zeolitic activity. If the target activity for a commercial unit has not been established, the potential gain in gasoline yield is well worth the catalyst cost of raising activity to see if the unit has yet reached the point of overcracking.
Maximum Reactor Temperature
Although increasing reactor temperature is the least effective way to increase gasoline yield, it is the most effective way to increase gasoline octane. Because octane is in tight supply in most refineries due to the declining demand for leaded gasoline, it is usually best to operate the FCCU at maximum reactor temperature.
There are a number of ways to increase the temperature, but circulating more catalyst to increase the coke burned in the regenerator is the most common. Increased reactor temperature often occurs simultaneously with an increase in catalyst circulation.
Our survey of commercial fluid crackers indicates that a 10° F increase in reactor temperature increases gasoline research octane 0.4 to 0.8 research octane number (RON). This increase in octane is accompanied by a 1 to 2 vol % increase in conversion caused by the higher reactor temperature.
The increase in cat-to-oil ratio required to increase reactor temperature will further increase conversion. Most of the converted products resulting from increased reactor temperature contribute to higher wet gas make.
Figure 3, a statistical analysis of one refinery's yield and operating data, shows a 0.1% increase in gasoline volume for each 1% of conversion. It also shows that liquid yields decreased as reactor temperature increased because dry gas production was higher.
The combination of wet and dry gas production from higher reactor temperature will often place the unit at a gas-handling limit before the reactor temperature is at a metallurgical limit.
Maximum Catalyst Circulation
After the reactor temperature has been increased to a gas-handling limit, it is often possible to further increase conversion by increasing catalyst circulation. The commercial data in Figure 3 show that higher circulation does not increase dry gas make, and may even decrease it.
Gasoline yield increases 0.8 vol %/ volume percent by conversion increase, while 0.5 vol % additional liquid volume can be recovered from increases in C3's and C4's.
Figure 4 shows a survey of test results from three additional commercial units that increased catalyst circulation by reducing feed preheat. All three refineries achieved significant increases in gasoline yield without increased dry gas make.
All of the refineries also experienced an increase in light olefins and isobutane that expanded total liquid yield. Catalyst circulation increases are usually limited by the refinery's ability to handle the increased C3 and C4 make, or by the air required to burn the additional coke. All four surveys agree that coke make increases 0.1 wt % for a 1 vol % increase in conversion.
This added coke must be burned in the regenerator, so it is possible to run out of regeneration air at higher cat-to-oil ratios.
The additional 0.5 to 1.0% LPG yield for each volume of added conversion can exceed gas compressor capacity, LPG recovery capacity, or alkylation and polymerization capacity.
Increasing catalyst circulation by lowering feed preheat is not likely to exceed any regenerator metallurgical limits because the higher circulation almost always cools the regenerator. Until the unit reaches an air blower or an LPG limit, our data show that increasing the catalyst circulation will improve gasoline yield.
[This article appeared in the March 21st issue of the Oil & Gas Journal.]