RFCCU Design to Improve C3= Olefins, Especially Propylene Selectivity

Propylene through RFCCU is a subject that seem to be gaining interest among refiners worldwide when considering either new units or potential revamp of existing operating fuels producing RFCCU. The interest gains higher traction especially if the RFCCU objectives change from producing gasoline and a feed source to an onsite Alkylation unit to be an on-purpose Propylene unit to feed an onsite or nearby Polypropylene (PP) facility. As the future gasoline demand is projected to decline, the desire to maintain or improve margins becomes an overriding objective, especially if the regional and global demand for PP is expected to have a growth rate higher than other petrochemicals.

This article is only for educational sharing of my opinion and can have inaccuracies and omissions. This is not professional advice for any real situation of any kind. Readers assume all risks for any deduction or extrapolation. The Disclaimer at the end of this Article should be reviewed. It applies to the entire content and any comments or response

Once the above decision passes the management gate review, the project development and conceptual engineering starts. With the knowledge that technology for a Propylene Selective RFCCU is available, the focus shifts to the product quality and specifications that a PP manufacturing facility, which is a catalytic process that is sensitive to impurities and contaminants, may require. To add perspective, I think, PP needs polymer grade Propylene at around a minimum of 99.5% purity unless the facility has their own onsite feedstock preparation and treating facilities. In the way of comparison, the refinery grade Propylene, generally speaking, has a C3= content in the 50-70% range while the chemical grade is around 92-95% purity range.Thus, additional capital and operating cost is needed to go from refinery grade to polymer grade, mainly in the products recovery section, but some costs may also apply in the Rx/MC section as well as in designing the best possible available feed to maximize Propylene production

If the Propylene production objective is to meet the design feedrate for an on-plot PP unit rather than selling to third party producers, the design is a bit more complicated because C3= yield in the RFCCU, and recovery and purification, needs to be matched to the capacity and design of the of the attached world class PP facility. This may or may not include additional steps and processing to meet the polymerization reactions constraints and catalyst specifications. So, for the purpose of this Article, consider the case of polymer grade Propylene production in a refining environment meant to be sold over the fence (OTC) to third party customers

 From a conceptual design perspective there are four main considerations that need attention in the early stages of project development: Crude feed type; Technology; Catalyst selection and Additional equipment and design factors in the RFCCU products recovery (PRU/GCU) section. In addition, a general recognition that high Propylene production may lead to lower gasoline make and a plan to manage the associated C4 olefins and higher gas make. Let us take a quick look at each of these considerations:

Crude Feed Type. The chemistry favors higher Propylene make from non-aromatic feedstocks that have higher Hydrogen content because aromatics rich feed are generally lower in Hydrogen and paraffins and thus tend to produce lower amount of olefins precursors in the gasoline range during the catalytic cracking in the riser causing lower C3= yields. In my understanding, the aromatics in the RFCCU feed are difficult to crack and either pass through and end up in various draws from the main column or condense and polymerize and end up as coke on the catalyst, though side chains in the aromatic rings can find enough activation energy to break. The other consideration in the feedstock is the Conradson Carbon content. As most of these large resin and asphaltenes ends up as coke on the catalyst, these can block the pathways to the catalyst additive active sites in the micro pores to crack gasoline range olefins (e.g. C5-C10).

Technology: Propylene specific RFCCU equipment and configuration is same as for a typical gasoline unit, with the exception that product rates and density may be different due to large amount of LPG in the product vapor to the main fractionator. Unless the refiner has requested more novel designs such as HS-FCCU or DCC type, the technology providers, I think, usually propose same process flow & equipment. If the C3= specific unit is linked to a downstream PP unit, it is important that the onstream reliability of two should match. This means that the owner needs to evaluate carefully if potentially lower reliability optional equipment such as catalyst coolers, for higher CCR feed beyond what a two stage regeneration can handle, or power recovery turbine to maximize energy efficiency of the unit, stands justified when all factors taken into consideration.

Additionally, If there is uncertainty on operating the unit in the Propylene mode on a permanent basis, then it is useful to request a rating case, on the same proposed equipment, for the usual gasoline or diesel mode operation. This can provide guidance on operating and capacity constraints in the reactor riser and cyclones and in the main fractionator overhead, draw-offs and bottom sections if the yields shifts significantly in favor of C5+ gasoline make. And, if the second design case for is for a higher CCR and metals feed, then adding some design margins in the pertinent areas of the Rx/Regen system can be helpful in the long run when running the operation in the alternate mode. Especially, important to ascertain the limitations of the LCO, HCO and Slurry circuits and heat integration of these streams in the Products Recovery Unit(PRU)

Catalyst: To achieve the objective of maximum Propylene make likely requires using the ZSM-5 additive to the base RFCCU equilibrium catalyst. The ZSM-5 is also a zeolite but with smaller pores and have shape selectivity, which, in my understanding, means that it has less diffusional and cracking limitations for the gasoline range straight chain olefins and less propensity to cause hydrogen transfer reactions (e.g. from naphthenes to olefins to form paraffins). The other reaction such as isomerization or cracking of isomerized molecules or aromatics production generally happens at base catalyst active sites and/or on an active matrix. The ZSM-5 is generally added as an additive on top of the base catalyst, which generically speaking, is the usual REY but with a lower level of rare earths to modulate hydrogen transfer reactions. However, formulations vary from one manufacture to the other so the best course is to discuss these points with catalyst vendors to arrive at the best possible combination for the feedstock in question and the yield objectives. In my opinion, ZSM-5 performance depends strongly on the base catalyst, and, in addition to C3= it also produces a higher amount of C4= as well as lighter gases and Hydrogen. Thus, when choosing a catalyst system, it is critical to evaluate both with respect to the acidity levels, matrix activity, rare earths loading and the compatible physical properties.

The details and the chemistry of ZSM-5 activity is well understood, in my opinion, but it is outside the scope of this brief article. Usually ZSM-5 is carefully manufactured and besides the original zeolite has various other things added to it to enhance the activity of the catalyst. Often it is impregnated with Phosphorus to improve the stability. From a cost standpoint, I think, it is more expensive than the usual REY fresh catalyst, so it can impact the operating cost.      

Additional Equipment and Design Factors: The core part of the Products Recovery Unit (PRU) is same as the regular RFCCU. But beyond the Debutanizer, additional equipment is needed. These may include C3/C4 splitter and a C3/C3= Splitter to produce the desired product grade. In addition, Propylene product treating equipment may be needed to meet the desired specifications on sulfur, water, unsaturates and other impurities. As for the C4 part of the product slate, the treatment or whether splitting into C4/C4= is needed, depends on the marketing strategy if the stream or a specific product is planned to be routed to an Alkylation unit or to a C4= based petrochemical manufacturing.  

It is worth mentioning that the C3/C3= splitter is a more complicated equipment than an ordinary fractionating splitter. Due to close relative volatility it can have many trays, (150+), and can be divided into two vessels. These trays are often proprietary high liquid capacity type with very close tray spacing. The reboiling, in many instances, uses overhead vapor compression heat pump to save on energy (needs large amount of low level reboiling heat) using in a specially designed exchanger with close temperature approach. All these factors mean that careful attention is needed on the details on this specific equipment since it is the key to the Propylene product specification for downstream processing.

The above are just a few highlights when considering a RFCCU for Propylene production. There are many other considerations, both in terms of the process and catalyst, since FCCU is historically a fuel producing unit and equipment, over the years, have been optimized for gasoline and diesel production while LPG and gases were mainly treated as peripheral to the main products. So, when roles are desired to be reversed, careful re-engineering is needed to fulfill the main objective in a cost efficient and reliable way to achieve Operational Excellence. The nameplate design capacity of a fuel producing RFCCU does not, in my opinion, translates directly into the same nameplate capacity for Propylene production 

Disclaimer: This is not professional advice, directly or indirectly, and can be edited or deleted at any time. Anyone accessing this Blog unconditionally agrees that the expressed views are only the personal opinions of the Author for educational sharing only, basis author’s knowledge only, and may contain omissions and inaccuracies. It must not be used for any actual new project or on an existing facility. Readers accept all risks and responsibility for any interpretation or extrapolation and any consequences stemming from such reliance