Jul-1998

Maximising distillate production from the FCC unit

In the heating oil season, many refiners switch from a gasoline to a distillate mode of operation of the FCC unit, to increase light cycle oil yield

John Black, Jon Petrunia and Robert T Powell, KBC Advanced Technologies

Article Summary

The most common and simplest operational change that refiners can use to increase distillate production from the FCC unit is to change the cut points of the products. The end point of gasoline can be decreased, with the heavy gasoline becoming part of the light cycle oil (LCO) product. The end point of the gasoline is reduced until flash temperature and other product specifications of the LCO product prevents any further decrease of the gasoline end point.

It should be possible to increase the LCO product by at least 4 to 6 vol% by this method. One refiner was able to increase LCO product liquid yield by almost 5 vol% by reducing the gasoline end point from 430°F to 380°F (The Catalyst Report – Maximising Light Cycle Oil Yield; Engelhard Corp). 

In addition, the end point of the LCO product can also be increased. The end point of the LCO can be increased until cloud, pour, and other LCO product specifications limitations are reached. Increasing the boiling range of the LCO product is a common industry practice; and, for many refiners, it is the only operational change necessary to satisfy any distillate production increases needed from the FCC unit.

Conversion reducing strategies
If additional quantities of distillate production are required, operational changes on the FCC unit can be employed that will generate higher yields of LCO. By reducing the conversion of fresh feed, refiners can increase the yield of LCO and heavier products. Conversion-reducing strategies include reducing riser temperature, decreasing catalyst circulation rate, reducing hydrocarbon contact time, and other methods.

Riser temperature
A common method to reduce unit conversion is by reducing the riser temperature. The riser temperature can be decreased by 10 to 30°F during the distillate mode of operation. The decrease in riser temperature can cause a significant loss of gasoline octane.

With many markets now requiring the use of oxygenates in the finished gasoline during the winter months, this octane loss from the FCC unit may not have a significant impact on gasoline blending. However, if octane loss is not acceptable, alternative strategies to increase LCO yield will have to be employed. In addition, the lower riser temperature will also increase the yield of less valuable heavy cycle and slurry oil products.

Catalyst circulation rate
Another conversion reducing strategy is to decrease the catalyst circulation rate. The reduction in catalyst-to-oil ratio (cat:oil) will result in a lower conversion and, thereby, increase LCO and heavier yields. The primary method to reduce catalyst circulation rate is to increase the feed preheat temperature. The hotter fresh feed will require less heat from the regenerator, so catalyst circulation rate is reduced.

Methods that cause an increase in regenerator bed temperature will also result in lower catalyst circulation rate. FCC units that have catalyst coolers can operate with minimum heat removal to increase the regenerator temperature.

Reduce hydrocarbon residence time
Conversion can also be reduced by decreasing the hydrocarbon residence time (contact time) in the riser. The oil residence time in the riser can be reduced by injecting or increasing the injection rate of inerts and/or light hydrocarbons with the feed. Some common feed diluents include steam, condensate, lift gas, and naphtha. However, the use of diluents will normally increase catalyst circulation rate, which can partially offset some of the effects of the reduced contact time.

If the unit has several feed injection locations, injecting a portion of the fresh feed in the upper location will decrease conversion by a reduction in hydrocarbon contact time. The amount of fresh feed injected in the upper feed location can be increased until the desired conversion level is obtained.

Other methods
An additional conversion reducing strategy is to operate with a higher carbon-on-regenerated catalyst (CRC) level. The higher CRC will reduce the effective activity of the catalyst, reducing conversion. However, too high a CRC can cause a significant conversion loss. The “snowballing” of the catalyst or a temperature runaway during a unit upset are also potential dangers with high levels of CRC.

Feed strategies
There are several feed strategies that can be employed to reduce conversion. If there is flexibility, the crude oil being processed can be changed to a higher aromatic and/or nitrogen content oil. The higher aromatic content gasoil  will not crack as easily as paraffinic feed and will result in a lower conversion. The higher nitrogen in the feed will act as a temporary catalyst poison and will cause a deactivation of the catalyst through neutralisation of the cracking sites.

The carbon residue content of the feed can be increased by increasing the gasoil  end point from the crude distillation unit. The higher carbon residue of the fresh feed will cause a higher regenerator bed temperature which will lead to a lower cat:oil ratio.  The carbon residue of the feed can also be increased by including atmospheric tower bottoms (ATB) or vacuum tower bottoms (VTB) with the gasoil. The quantity of coker gasoil or deasphalted oil in the fresh feed can also be increased.

These types of feed are more difficult to crack and will also lead to lower conversion. Increasing or introducing slurry and/or heavy cycle oil recycle streams to the riser is another means of reducing conversion. However, too high a recycle rate will result in too low a conversion and high production of heavy cycle oil. Increasing the feed to the FCC unit by including kerosene or virgin distillate with the gasoil will decrease oil contact time in the riser.