Question
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What is the best route for maximum propylene from a resid feed to the FCC?
Jun-2021
Answers
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Gary Martin, Sulzer Chemtech, Gary Martin@Sulzer.com
Whether you are producing propylene from a resid or VGO FCC, the economic limit to maximising propylene often is constrained by the gas concentration section (gascon). Modern FCC units provide an important link between a fuel refinery and the production of light olefins and aromatics for petrochemical use. FCC reactor licensors are designing units to produce higher propylene production. Economic factors influence the optimum propylene production with one factor being the capex associated with recovering the propylene. Revamps of older FCC units can provide for increased propylene production at the expense of gasoline, but a constraint to increasing production is the reuse of existing equipment to minimise capex. Increasing FCC unit severity, addition of ZSM-5 and varying the reactor partial pressure, total pressure, and catalyst-to-oil ratio enable increasing propylene production. However, this can have a significant effect on the gascon. The shift in additional propylene production produces more dry gas, making it more difficult to recover the propylene. To maintain high propylene recovery along with the shift in product slate can lead to much higher loadings in the sponge absorber, primary absorber, deethaniser, debutaniser, and the C3/C4 splitter. These columns are often shell limited, and it is usually not economical to replace or parallel these five columns by conventional means. While shifting product slate to petrochemicals makes sense for the forward-looking refiner, it must also make economic sense for today’s business.
For existing processing units, achieving the shift in yields requires an economical means of retooling existing assets. Dividing wall column (DWC) technology is a technique providing a much lower capex revamp to overcome the limitations of a light ends recovery section. A DWC design using a single vessel that combines the functions of a sponge absorber, primary absorber, deethaniser, and C3/C4 splitter can be utilised. This design can unload the existing gascon to enable improved recoveries in the existing unit along with handling additional load from shifting to increased propylene production. With the addition of only one vessel, this design unloads the existing sponge absorber, primary absorber, deethaniser, debutaniser, and C3/C4 splitter. The DWC design can produce equal or higher propylene recovery and product purities than that of a conventional design configuration and add any additional capacity needed. The only potential limitation is the bottom of the debutaniser and bottom of the C3/C4 splitter but there are other solutions if this is the case. Typically, this will not be a problem as changes in the product slate result in these areas not being a limitation or at least allow for changes to handle the new design conditions. The shift in product slate decreases gasoline yield which helps unload the bottom of the debutaniser. The DWC can also be used to overcome limitations in plot space and the construction can often be completed prior to a shutdown with only tie-ins required.
Jun-2021
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