Maximizing Light Cycle Oil Yield
While gasoline is usually the most valuable product of the FCC unit, other products can seasonally increase in value to the point where it is advantageous to increase their yields. For example, in winter the value of light cycle oil, when used as a blending component in heating oil, can be greater than that of gasoline. Under such circumstances, many refineries will adjust their FCC unit operation to increase light cycle oil yield at the expense of gasoline. This article compares the advantages and disadvantages of several methods of increasing the FCC unit light cycle oil yield.
Increasing LCO Yields
One obvious method of increasing LCO yield is to reduce the FCC unit cracking severity so that conversion declines. At the lower conversion, yields of heavy products (light cycle oil, heavy cycle oil, and clarified oil) will increase while yields of light products (gasoline, LPG, and gas) and coke will decrease. The cracking severity can be reduced in several ways - reducing catalyst activity, lowering reactor temperature, and reducing the catalyst/oil ratio are three common ways. These three alternatives have some similarities and differences, which are illustrated in Figure 1.
In each case, the refiner's target is a 3 vol % increase in light cycle oil yield as compared to the base case. All three cases illustrated are similar in that the increased light cycle oil yield is accompanied by reduced yields of coke and gas. These latter benefits would allow the FCC unit feedrate to be increased if it had previously been limited by wet gas compressor capacity, air blower capacity, or regenerator temperature. Unfortunately, all the lower conversion cases are accompanied by reduced gasoline and LPG yields as well as increases in low-valued heavy cycle oil and clarified oil. The gasoline yield loss is the smallest in Case 2 in which the reactor temperature has been decreased by 30° F. At the lower cracking temperature the gasoline selectivity improves significantly, but this higher selectivity is offset by a decrease in octane of nearly 2 RON.
Maintaining Cracking Severity
It is often desirable to maintain the cracking severity as high as possible while maximizing LCO yield. The simplest way to do this is to reduce the gasoline endpoint by adjusting the operation of the main fractionator. This allows the heavy end of the gasoline fraction to be drawn off with the light cycle oil, resulting in a barrel-for-barrel trade of gasoline volume for light cycle oil. Since this method affects only the apparent conversion and not the true conversion, all yields other than gasoline and LCO remain the same. This is an advantage over simply reducing the cracking severity which, as we saw in Figure 1, is accompanied by increases in undesirable heavy products and losses in valuable lighter products. An additional advantage of reducing the gasoline endpoint is that such a change usually increases the octane of the remaining gasoline because low octane components, which are present in the higher end of the gasoline boiling range, are removed.
Figure 2 illustrates the changes that occurred at one refinery which reduced the gasoline endpoint from 430°F to 380° F. This fractionation change caused a shift in liquid yield of 4.8 vol % from gasoline to light cycle oil. The gasoline octane exhibited a typical response by increasing by 1.0 RON.
Another way to increase the light cycle oil yield is to recycle heavy cycle oil or clarified oil. These heavy products don't crack easily, but some incremental upgrading can be achieved by having them pass through the reactor a second time. Recycle streams crack non-selectively, so some compensating process changes may be required to balance the additional coke and gas loads.
Choosing the Proper Catalyst
The proper catalyst choice can have a strong influence on the FCC unit light cycle oil yield. Catalysts which contain an active matrix provide more cracking sites for the large hydrocarbon molecules typically found in heavy cycle oil and clarified oil. This greater matrix cracking activity, which is usually associated with a high alumina content and a high surface area, allows such catalysts to upgrade bottoms to light cycle oil.
In a comparison of 17 commercial FCC catalysts, it was found that there was a strong correlation between the catalysts' LCO selectivities and their surface areas. Figure 3 illustrates this relationship, which indicates that catalysts with high matrix surface areas can increase the LCO yield by 3-5 wt% at 70 wt% conversion as compared to catalysts with low matrix surface areas.
The catalytic route to maximizing LCO yield may be attractive. However, a catalyst change in a commercial FCC unit can take several weeks or months to complete. This tends to make the previously discussed process changes, whose effects on LCO yield are immediate, more appropriate when LCO demand changes suddenly.
FCC unit feed fractionation can also help a refiner increase his total LCO yield. If the current feedstock contains light ends in the LCO boiling range, it is better to remove them before they are cracked to lighter products in the FCC unit reactor.
The potential value of feedstock fractionation was demonstrated in a laboratory study in which a feedstock containing 8 wt% light ends (430°F-660° F) was found to give the yield structure shown in Figure 4. After the light ends were removed, the gas/oil gave a slightly different yield structure as shown. A unit using the fractionated feed would experience a loss of LCO yield due to both lower LCO selectivity and the 8% lower feedrate. Nevertheless, the total refinery yield of products in the LCO boiling range increases when the light ends that were removed from the feed are added to the FCC unit light cycle oil production.
The feed fractionation method of increasing light cycle oil will be expensive if existing equipment can't be used. Even so, it can still be an attractive, long-range option because it can increase the yield of a valuable product while at the same time debottlenecking an overloaded FCC unit.
In this article we examined how an FCC unit's light cycle oil yield could be increased by reducing cracking severity, recycling heavy products, decreasing the gasoline endpoint, choosing an appropriate cracking catalyst, or removing light ends from the FCC unit feedstock. Each of these alternatives has associated advantages and disadvantages, so the refinery must choose between them carefully to obtain the most profitable operation. BASF can help the refiner determine the optimum route to take.