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Meeting RVP with Revised Operating Targets

One refiner changed the operating targets for their Light Straight Run (LSR) Stabilizer to help meet mandated lower gasoline RVP's. The Stabilizer removed butane and lighter material from the C5+ gasoline blendstock, LSR

The LSR Stabilizer overhead went to a Depropanizer. The Depropanizer produced an overhead propane product for sales, and a normal butane stream which was routed to a Butamer Unit for isomerization. The Butamer Unit isomerizes normal butane into iso-butane which is fed to the Alkylation Unit. The Butamer can handle only limited amounts of C5's to prolong the catalyst life.

To meet gasoline blending requirements, the LSR specification was lowered to the range of 8.5-9.0. The overhead specification for the LSR Stabilizer remained minimum C5+. Case I shows typical plant operating data following the specification change. Operations tried various operating parameters to meet both the overhead and bottoms specifications. Unfortunately, it appeared that the tower could not meet both specifications simultaneously. An engineering study of the tower begun.

An existing simulation of the tower was revised to match the plant RVP. Case II shows the simulation results. The simulation matched the tower temperatures, reflux rate, and LSR RVP. The overhead and bottom product rates deviated somewhat because the old feed composition was not revised for the quick study. Overall, the simulation matched the tower operation well allowing accurate case comparisons.

The Case II simulation predicted 0.87 LV% butane in the bottom LSR to match the observed plant RVP. Case III increased the reflux and reboiler rates to reduce the LSR butane concentration and hence RVP. Case III indicates a bottoms’ RVP of 9.0 which just meets the upper limit of the new RVP specification. The calculated butane content was 0.19 LV%. Case IV indicates that further increases in reflux and reboiler rates show negligible gains. Case IV used reflux and reboiler rates which may not be achievable without equipment modifications. The bottom LSR was virtually butane free, yet the bottom RVP dropped to only 8.9. All of the extra reboiler duty and reflux reduced the bottoms RVP by only 0.1 psi as compared to Case III.

Case V shows what would have been required to meet the lower RVP target of 8.5. The C5+ content in the LSR Overhead had to be increased to 6.24 LV% to reduce the bottoms RVP to 8.5. The RVP reduction came not from eliminating butane from the LSR, but by rejected C5+ material to the overhead. The additional overhead C5+ not only violated the Butamer feed limit, but did nothing to reduce the overall gasoline pool RVP. The C5+ material would go through the Butamer, and assuming minimal reaction, would enter the Alkylation Unit. The non-olefinic C5+ material would pass through the Alkylation Unit and end up in the alkylate product. Since the refinery did not have a C5 conversion unit, the C5’s contained in the refinery’s crude ended up in the gasoline blending pool no matter what processing route they took.

The LSR RVP operating target was adjusted slightly allowing good separation in the LSR Stabilizer. Additional gasoline RVP reduction had to come from other units’ operating adjustments.

 

Equipment Fouling

Minor Equipment Debottlenecking