Feb-2021

How can we raise butanes production from our FCC unit?

Responses to a question in the Q1 2021 issues Q&A Feature

Various from Sulzer, Unicat and G. W. Aru

Article Summary

Mark Pilling, Head Refinery System Business Group, Sulzer - mark.pilling@sulzer.com and Celso Pajaro, Head Refinery System Business AME, Sulzer - celso.pujaro@sulzer.com

Net butanes production in an FCC unit is a function of the reactor and the gas plant recovery section. Reactor conditions enable higher butanes production while the gas plant enables recovery of the butanes produced in the reactor.
- Some reactor adjustment/changes that increase butanes production are:
 Î½  Increasing conversion increases C4 production
 Î½  Changing catalyst formulation targeting higher C3/C4 production
 Î½  Adding ZSM-5 increases C3/C4
 Î½  Modifying the reactor by adding state of the art feed injection nozzles and riser termination devices. This reduces coke and dry gas, allowing the use of higher activity catalyst and cat/oil ratio, improving the unit conversion.
- Gas plant adjustments
 Î½  There are several improvements that can increase butanes recovery in the gas plant; some involve small equipment changes (like replacing trays in the absorbers with a fourth generation random packing like NeXRing). Another option is to improve the process configuration (like adding a saturator drum downstream of the primary absorber). The following is a list of potential changes per column:
  ν  Primary absorber
      Ï… Lowering the temperature of the naphtha absorbent increases recovery
      Ï… Adding an intercooler reduces the absorbent temperature and improves C4s recovery
      Ï… Increasing the amount of debutanised naphtha to the top of the absorber
      Ï… Increasing the operating pressure of the absorber
   ν  Sponge absorber
       Ï… Increasing absorbent rate
       Ï… Increasing operating pressure
   ν  Debutaniser
       Ï… Adjust debutaniser conditions to produce lower RVP FCC naphtha

--------------------

Tom Ventham, Sales & Technical Europe and Africa, Unicat - tom.ventham@unicatcatalyst.com, CJ Farley, Senior Technical Services Engineer, G. W. Aru LLC - cj.farley@gwaru.com, Natalie Herring, Director of Technology and Business Development, G. W. Aru LLC - natalie.herring@gwaru.com, Kate Hovey, Senior Technical Services Engineer, G. W. Aru LLC - kate.hovey@gwaru.com

Normal butane is generally the lowest value FCC LPG component, along with propane. Propane and normal butane are sold as liquefied petroleum gas to be used as portable fuel for domestic heating and cooking or for light industrial use. C4s can be blended into gasoline up to a vapour pressure limit, used as a refinery fuel gas, or as incremental fuel in a refinery hydrogen plant or steam cracker. This balance shifts, sometimes seasonally, depending upon local economics and final product specification.

Iso-butane on the other hand is of high value due to its use in the alkylation process. Most refineries that operate an alkylation unit typically enjoy an excess of butylene produced by the FCC and are required to import incremental iso-butane to achieve the required stoichiometric balance. The Butamer process isomerises low value normal butane into iso-butane, making it a highly valued unit.

Options to increase iso-butane (iC4) production from the FCC will involve the use of an effective ZSM-5-based additive and balancing the main FCC catalyst rare earth content and other catalytic properties. Selective FCC cracking reactions, whether taking place on the main catalyst or ZSM-5, produce olefins via β-scission mechanisms. Butane can be directly produced from unselective thermal cracking reactions, or will result from secondary reactions whereby an olefin is saturated to a paraffin via hydrogen transfer. The degree to which hydrogen transfer reactions occur depends on many variables, with FCC catalyst unit cell size (UCS) being a key factor. Production of LPG olefins, including iso-butene and normal-butene, can be performed using a ZSM-5-type additive. For this purpose, speciality C4 selective additives, such as Ultra C4Booster from G. W. Aru and Unicat, may be selected to give particular focus on the production of C4 species. Iso-butylene is the most readily saturated olefin in the LPG range via hydrogen transfer, hence a balancing approach needs to be taken to account for the relative values of all isomers of both butane and butylene.