FCC product fractionation for maximum LCO
For increased production of diesel over gasoline, maximising recovery of LCO from the FCC reactor can be part of the scheme
PHILLIP NICCUM, WILLIAM McDANIEL and HOWARD POLLICOFF
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The economics and technology of maximising LCO production in the FCC reactor is a complex topic.1 Once the FCC reactor has been adjusted to maximise LCO production, the remaining challenge is how to best capture the LCO being produced in the FCC reactor.
Lowering the FCC naphtha end point is a common strategy employed when LCO demand is high, as it shifts heavy FCC gasoline into the LCO product.
Minimising loss of LCO in slurry oil product is also commonly employed and goes hand-in-hand with maximising LCO product end point.
Minimising the LCO content of recycled oils is desirable because it preserves LCO already made in the FCC reactor.
Typical FCC main fractionator characteristics and operating strategies
The fractionation between LCO, HCO and slurry oil in the bottom of a traditional FCC main fractionator (see Figure 1) is very coarse. As Figure 2 shows, coarse fractionation is the result of reactor products feeding the fractionator through the bottom of the tower where the slurry oil product is withdrawn and because there are few fractionation trays between the slurry product, HCO and LCO product draws.
Naphtha/LCO cut point adjustment
Adjusting the cut point between FCC naphtha and LCO without changing the true FCC conversion is considered standard practice in many refineries for making seasonal adjustments for swings in gasoline vs distillate demand. This strategy works to preserve the LPG production, octane and total liquid volume associated with a high conversion FCC operation. Reducing the end point of the FCC naphtha product simply shifts heavy naphtha into the LCO product without changing the true FCC conversion level. The limitation to the adjustment is most often the flash point specification of the LCO product or fractionator top temperature minimum limits.
Figure 3 shows an example of how changes in gasoline/LCO cut point impact the naphtha and LCO yields.
In low severity FCC operations where maintaining adequate regenerator temperature is not an issue, such as may be the case when processing residue, HCO may be preferred over slurry oil as a recycle stream due to its very low carbon residue content and higher hydrogen content (see Figure 4).2 Ideally, the HCO would also have its LCO boiling range material distilled from it before recycling it, but the economic practicality of redistilling the HCO can be questioned if this requires yet another cycle oil fractionator.
On the other hand, if higher carbon residue is needed in the recycle to keep the regenerator temperature elevated, slurry oil would be the preferred recycle oil.
Basis of study
Data from a low conversion commercial FCC unit, shown in Table 1, are used as a basis for contrasting traditional and non- traditional FCC product fractionation. The objective of this work is to identify the best fractionation schemes for maximising LCO production.
Pro-II Version 9.3.2 was used for the modelling work, utilising the Soave-Redlich-Kwong (SRK) technical data package. Pseudo-components were generated using the FCC unit test run product yields, distillation and gravity data to generate average boiling point distillation and density curves for defining psuedocomponents consistent with traditional FCC technology practices. Finer psuedocomponent resolution in the highest boiling ranges were used relative to those normally used for FCC main fractionator design work to better define the impact of small distillation changes.
Fractionation design options
In this article, seven different design options are evaluated for fractionating the reactor products and recycle streams. In all cases, the LCO ASTM 90% boiling point is held at 640°F (338°C). Significant recycle of HCO and/or slurry oil is included in all cases to provide a further point of consistency between the fractionation options. For the purposes of this fractionation study, the cracking of recycle streams is not modelled. The recycle streams are treated as inert streams that simply pass from the reactor to the fractionator and back again to the reactor.
Base case: maximum gasoline
This case represents an FCC unit main fractionator being operated to maximise the production of gasoline product. However, 5000 b/d each of HCO and slurry oil are recycled to the reactor to provide consistency with the following cases.
Case 1: lower gasoline cut point
The cut point temperature between the gasoline and LCO products is reduced to shift heavy FCC naphtha into the LCO product. In this case and in all subsequent cases, the gasoline/LCO cut point is controlled to provide a 130°F (54°C) LCO flash point. This has a large impact on both gasoline and LCO production. Recycle rates are maintained the same as those of the base case.
Case 2: add bottoms quench
Referring to Figure 5, this case serves to demonstrate the value of quenching, or sub-cooling, the fractionator bottoms liquid. Cooled slurry oil is introduced into the main fractionator bottom to sub-cool liquid, allowing higher temperatures in the quench zone without coking the fractionator bottoms circuit, driving more vapours up the tower. The practice of routing cooled slurry into the fractionator bottom is commonly, but not always, practised in the industry during both maximum gasoline and maximum LCO operations. This practice is preserved in subsequent cases. Recycle rates are maintained the same as those of the base case.
Case 3: add slurry stripper
A steam stripper is added to the FCC main fractionator to produce a slurry oil product with reduced gasoline content, increased flash point and less LCO boiling range material. While this option recovers a modest amount of LCO from the slurry oil, it does not impact the LCO content of HCO recycle or slurry oil recycle streams.
Case 4: replace stripper with vacuum column
Referring to Figure 6, a slurry oil vacuum distillation column and associated HCO side stripper are added in place of the slurry steam stripper to recover more LCO from the slurry product and recycle streams. The vacuum HCO and vacuum slurry recycle rates are set equal to the main fractionator HCO and slurry recycle rates of the previous cases.
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