We would like to explore oil to chemicals routes. Any process ideas for VGO/resid conversion?Apr-2021
Mel Larson, Becht, email@example.com
The world is moving away from pure residua conversion; RFCCs are out of favour considering the demand for low sulphur products. The shift has been to some version of hydrogen addition to bottom of the barrel. On the FCC front, east of Suez has focused on high conversion/propylene-making FCCs with recycling of a portion of FCC naphtha to recrack as diesel. This trend will continue by maximising the cracking of VGO to petrochemical feedstocks.
Bani Cipriano, W. R. Grace & Co., Bani.Cipriano@Grace.com
The fluid catalytic cracking (FCC) unit provides an excellent opportunity to increase the yield of petrochemical products from the refinery. Specifically, the FCC is the unit that generates the highest yields of propylene in the refinery. Propylene is a valuable petrochemical feedstock, the majority being converted to polypropylene while the remaining finds use in production of cumene and acrylonitrile among other end uses. Under suitable conditions of hardware, operating conditions, and catalytic system, FCCs can process VGO and/or resid feeds.
Conventional FCCs generate propylene yields in the range 4-5 wt%. FCCs can operate in max C3= mode and can reach upwards of 12 wt%. Commercially proven high olefin yield catalytic cracking processes exist that can achieve propylene yields of >20 wt%. One such example is PMcc process technology available through a joint effort of Technip Energies and Grace.1,2
The propylene yield depends in general on three key aspects1,2:
• Hardware configuration: as examples only, (i) regenerator design is different for resid vs VGO feeds, and (ii) addition of a secondary, external riser for cracking naphtha can increase the yield of propylene.
• Process conditions: in general, higher ROT and lower hydrocarbon partial pressures in the reactor will increase the yield of propylene.
• Catalyst for propylene maximisation: these catalysts in general (i) include the shape selective ZSM-5 zeolite to crack gasoline olefins into propylene, (ii) are designed to tolerate and retain their catalytic activity in the presence of harmful metals such as V, Ni, and Fe to name a few, and (iii) have engineered macro and mesoporosity to facilitate the upgrading of the largest feed molecules into valuable products.
It is also important to consider C3= and LPG olefin recovery constraints (wet gas compressor, splitting capacity). In Q2 2020, during the COVID-19 pandemic, many FCCs were processing lower than customary rates and this freed up C3= recovery capacity. Refiners turned to strategies to maximise propylene (ZSM-5 usage and higher ROT). If operating at full rates, refiners may need to invest in additional recovery and splitting capacity if operating in max C3= mode.
Finally, operating in max C3= mode can be very profitable for refineries. We recently estimated that the PMcc process can deliver approximately $2.60/bbl in uplift over an FCC processing a similar rate1, the uplift being the result of higher values for propylene over gasoline and fuels.
1 For information about high olefin catalytic cracking including hardware configuration, process conditions, and catalyst design for high olefin yields, the reader is referred to Singh et al, Conventional FCC to maximum propylene production, Hydrocarbon Processing, Sept 2020, and Dharia et al, Introducing PropyleneMax Catalytic Cracking (PMcc®), Grace Catalagram, Vol 125, 2020.
2 For more information about refinery strategies in the aftermath of COVID-19, the reader is referred to Roundtable Session: Turbulent Markets Getting the most out of the FCC during COVID-19, Hunt et al, AFPM 2020 Annual Meeting.