Propylene production

[Hedewandro LucrediParticipant]

We are having propylene with high content of CO2 (> 2 ppm) besides COS is under control (< 10 ppb). The feed LPG is from our FCCU and we measure in the FCCU and the content is around 2-3 ppm of CO2 in the treated LPG. We checked the amine and caustic treatment and it seems ok. We increased the steam in the FCCU regenerator stripper. How can I reduce the CO2 in the LPG from FCCU ? And in the propylene unit ?

[dblewisParticipant]

We had a similar problem. The CO2 is probably coming from the Reactor as it gets entrained in the vapors. You can look at the absorber / stripper operation and see if you can minimize the CO2 by increasing lean oil and / or stripper bottoms temperature. You could ask your catalyst supplier if other refiners have had this issue when the catalyst was changed to increase LPG production. You mentioned that you increased the stripping steam to the Regen. I am not aware of any stripping steam to a Regen but to the Reactor stripper. I would try cutting the steam rather than increasing the steam.

[Hedewandro LucrediParticipant]

Thanks for your answer. In order to remove CO2 in the propylene unit we increased the steam and reduces the pressure in the deetanizer tower. It seems do not remove the CO2 enough. This tower remove the lighter streams on the top (ethane, CO2, etc) and the bottom have propane/propylene, but it seems do not remove enough the CO2. The propane/propylene stream in the tower bottom still have 2-3 ppm of CO2 and produces propylene with CO2. How can I improve this tower to remove more CO2 ?

[Paul R OrlowskiKeymaster]

Michael Edwards says:
The CO2 does not come from the reactor itself. The regen catalyst is bathed in flue gas components as it enters the regen standpipe, and even full combustion regen operation generates more CO2 (AND CO) in the few seconds the catalyst flows to the riser. Note, most regenerators fluidize the catalyst with air next to the standpipe, so flue gas and air enter the pipe with the regen catalyst. You will see both O2 and CO in reactor fuel gas, though there should be no CO in full regen, and O2 in fuel gas, though there should be no O2 in partial combustion. Some few units do fluidise the regen cat near the standpipe entry or in separate steam stripped withdrawal well next to regen. There, you would increase the stripping steam. More stripping steam in reactor does not affect amount of CO2 (essentially all flue gas components are already stripped), but can increase amount of LPG recovered, so reduce % flue gas in LPG and fuel gas.

[Paul R OrlowskiKeymaster]

Salvatore Mannello says
I totally agree with Michael about regen operation. I can add that if the RCSP is over fluidized, try to decrease fluidization medium; depending from the unit type, e.g., you can increase regenerator cat level, or if the combustion air is not proper distributed, if you can, try to even distribute. Moreover, once all the CO2 is stripped from the deetanizer, try to not absorb it again in the recontacting 2°stag high pressure accumulator and/or primary absorber. E.g., if the propane-propylene recovery from the fuel gas is well accepted try to increase the temperature around the 2° stg HP drum and primary absorber, or decrease pressure. In amine absorber system the grade of the CO2 and COS absorption from the LPG depends from the type of the solvent used (MEA, DEA, MDEA…); in any case can help: increasing the amine circulation and/or the concentration; decreasing the amine lean loading and the temperature; specialty amines can help COS absorption; the last option is molecular sieves.

[Christopher DeanKeymaster]

If your Refinery Propylene Unit is to send C3= to a PChem facility any remaining CO2 will be removed in that downstream P Chem facility. I agree with the previous comments to reduce CO2 in the FCC. As an additional comment , if your Propane /Propylene stream is separated from the FCC LPG stream producing chemical grade propylene may be beneficial.

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