• What are the optimal routes to valorisation of olefins-rich FCC unit and coker unit off-gas?



  • Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

    One of the most attractive routes to add value to off-gas from FCC and Delayed Coking units is the olefins recovery from this stream. This solution will demand capital spending in olefins recovery processing plant and relies on the integration level of the crude oil refinery with petrochemical assets capable to recovery the ethylene and propylene contained in the off-gas, in some cases, downstream players carried out capital investments to build pipelines to sent the off-gas to petrochemical plants. A detailed economic analysis needs to be carried out before taking the capital investment decision considering the availability and price of natural gas which will substitute the off-gas as fuel for fired heaters and boilers in the refinery.

    We published an article regarding this topic in the Q3 2020 issue of PTQ magazine, which you can view and Download HERE



  • Celso Pajaro, Sulzer Chemtech, chemtech@sulzer.com

    ‘Optimal’ routes depend on the off-gas stream flow rate, composition, and refinery configuration. FCC unit off-gas can be sent to a cryogenic unit to recover the residual C₃+ and the C₂/C₂=. The C₂/C₂= rich stream can be injected downstream of the ethane cracking furnace, where ethylene will be recovered, and the ethane will be returned to the furnace. This arrangement requires the removal of water and other residual impurities.

    If there is no steam cracker, recovering ethylene may not be profitable, and a conversion process is preferred. There are several processes in the market that convert olefins into aromatics or other processes that alkylate benzene with light olefins, producing ethyl benzene and cumene.

    Coker off-gas has a lower olefin content than FCC off-gas but still has a significant amount of C₂+ material that can be recovered. It is not uncommon that coker off-gas is combined with FCC off-gas and fed to a cryogenic unit (as previously described). For small refineries, the previously mentioned options may not be economical. In this case, they may focus on recovering the C₃+ material in the off-gas by improving the efficiency of their gas plant. Changes in unit configuration, adding chilling cooling to absorbent naphtha, and other changes can reduce C₃+ in the off-gas by more than 2-3 mol% while increasing C₃/C₄ stream flow rate by 5% or more.



  • Carel Pouwels, Ketjen, carel.pouwels@ketjen.com

    Valorisation routes greatly depend on refinery configurations. FCC units that typically operate in ‘conventional’ gasoline mode are distinctly different from FCC units that operate in max propylene mode or even in very high severity mode. Therefore, it depends on the level of observed olefins. Refiners that aim to maximise propylene, whereby propylene yields are achieved at 9 wt% or higher, commonly operate a C3 splitter. The amount of ethylene produced may be too low for economic recovery, especially in an environment where gasoline demand is relevant. However, not only does ethylene yield count but so do the scale and capacity of the FCC unit.

    As refiners pivot to serve the petrochemical market, greater severity can drive both propylene and ethylene yields. In such cases, ethylene recovery may be used to enhance FCC profitability. For those refiners who doubt the economics of their own assets and question whether there is more potential, it is worthwhile to investigate the status quo. Benchmarking against industry peers, as illustrated in the following chart (see Figure 1), provides a health check and valuable insight into a refiner’s current position.

    To benefit optimally from such configurations, we strongly recommend working closely with partners to maximise the units in operation. It is advantageous to use the expertise of the licensor of process equipment and supplier of FCC catalysts to tune the operation conditions and reformulate to the optimal FCC catalyst.