Oct-2009

Opportunities for increasing on-demand light cycle oil

Swift response to demand for higher LCO yields from the FCCU may involve optimising variable parameters and employing a bottoms cracking additive

Ray Fletcher
Intercat (Johnson Matthey)

Article Summary

Fuel markets are observing an increase in demand for diesel, primarily in response to favourable regulations. This trend is also observed in markets where gasoline has long been dominant, such as the US. Where gasoline is still favoured over diesel, the differential growth of diesel is outpacing that of gasoline.

A balancing force that will dampen the shift of world fuel markets to diesel is the overwhelming number of gasoline-fuelled vehicles on the road. Furthermore, refiners have fixed assets in the form of FCCUs, which are largely for gasoline production. These factors will ensure that careful balancing of gasoline and diesel yields will be required within the FCCU for years to come.

There will be regions where light cycle oil (LCO) will remain in demand, either seasonally or intermittently. There will be a continuing need for refiners facing variable demand for diesel to switch their units swiftly and economically from one service to another. This article reviews the independent variables available to achieve a swift change-over from gasoline to LCO mode. In addition, it will introduce an additive approach to minimise the inevitable increase in slurry yield that accompanies an increased LCO yield when dropping conversion.

LCO upgrading potential
It is advisable first of all to define the slurry upgrading potential prior to initiating a potentially lengthy, step-wise procedure to optimise each independent variable. The most efficient means of evaluating upgrading potential is via a 
histogram-based analysis. As a first step, it is recommended that the as-produced slurry density be analysed for frequency over an extended period of time. The second recommended step is to plot slurry yield versus slurry density for the same time period used for the frequency analysis.

An example of this approach is shown in Figures 1 and 2, which represent actual operating data received from a Western European refinery. This unit consistently produces a slurry of 0.99–1.04 specific gravity. A fully optimised FCCU should be able to achieve a slurry of 1.10–1.12 specific gravity. A slurry yield reduction of about 4 vol% is feasible for this refinery, assuming it began optimising its unit in the stepwise manner described below.

Independent variable review
There are three primary means available to the refiner to enable maximum production of LCO: operational, catalytic and design. The focus of this article will be on the changes immediately available to the refiner to shift operations from gasoline to diesel. Therefore, it does not include a discussion related to changes in FCC design (hardware). The primary independent variables for the control of LCO yield are:
Operational
• Reduce riser outlet temperature
• Increase preheat temperature
• Recycle heavy cycle oil (HCO) or slurry oil
• Reduce gasoline and increase LCO end points
• Increase carbon on regenerated catalyst, if equipped with a CO boiler
• Reduce circulating catalyst inventory activity via reduced catalyst additions

Catalytic
• Reduce activity:
    ν Reduce active component (zeolite and matrix)
    ν Reduce rare earth on zeolite
• Enhance LCO selectivity:
    ν Reduce zeolite-to-matrix ratio
    ν Increase particle meso porosity (intra particle effect)
    ν Increase catalyst particle accessibility (surface morphology effect).

Operational variable overview
An overview of each independent variable with its influence on LCO yield, including any negative effects, follows. The actual yield observed on a specific unit will be a function of several variables, including feed quality, operating parameters, base catalyst selectivities and equilibrium catalyst activity, among many others. This comment applies to the subsequent selectivities quoted in this article.

Reduce riser outlet temperature
The effect of a reduced riser outlet temperature on LCO yield is immediate and significant. A refiner wishing to increase their LCO yield will observe approximately a 0.75 wt% increase in LCO for every 10°F (5.5°C) decrease in riser outlet temperature.1,2 However, offsetting this LCO increase will be a similar increase in slurry yield (see Figures 3 and 4). A side benefit will be a reduction in the demand for combustion air, enabling increased charge in certain units.

Increase preheat temperature
The effect of an increased preheat temperature is less than that of a reduced riser outlet temperature, but the two are directionally consistent. The LCO yield will increase by approximately 0.15 wt% for every 10°F (5.5°C) increase in preheat temperature. The slurry yield increases by approximately the same amount as the LCO (see Figures 5 and 6).

Reduce gasoline cutpoint
The impact of a 10°F (5.5°C) reduction in gasoline cutpoint is approximately 1.7 wt% (see Figure 7). This is the first and most obvious operational change to be made in the operating unit. The constraints limiting the magnitude of the cutpoint change are the product qualities of the LCO, such as the cold flow properties and flash. An additional advantage of this shift is a net reduction of sulphur in the LCO. The total sulphur of the LCO cut reduces by about 0.05 wt% for each 10°F (5.5°C) shift.