What is the best route for maximum propylene from a resid feed to the FCC?Jun-2021
Todd Foshee, Shell, email@example.com
The best route for maximum propylene from a resid FCCU starts with considering the quality of the resid feed. For the heaviest of resid feeds, contaminants in the form of metals, nitrogen and sulfur can be high in addition to the coke producing compounds. As such, a resid hydrotreater for the FCC feed is prudent in order to allow the FCC unit to go to the high conversions needed for maximum propylene production from the FCC unit. The resid hydrotreater can dramatically reduce feed metals and nitrogen for lower fresh catalyst consumption, reduce feed sulfur for lower H2S in the dry gas and lower SOx in the flue gas, and reduce coke producing compounds (CCR and polyaromatics) to gain regenerator capacity.
For the FCC unit, maximizing conversion combined with using a properly formulated catalyst (typically with a high zeolite content) and using higher quantities of ZSM-5 with a high crystal content is necessary to maximize propylene production. For the highest propylene productions, FCC naphtha recycle to the riser should be considered (light naphtha is typically preferred, but full range can be used). Shell typically prefers using a separate riser for naphtha recycle so that the severity of the riser for the feed and naphtha recycle can be controlled independently, providing the highest propylene yields. Shell Catalysts & Technologies has a proprietary maximum propylene FCC process technology called MILOS that utilizes Shell’s FCC hardware technology in a two riser FCC. The MILOS process is based on decades of Shell FCC pilot plant development and know how.
Sanjay Bhargava, KBC, Sanjay.Bhargava@kbc.global
Propylene production from resid feed in a FCC depends on several factors including feed characteristics, operating conditions, base catalyst, catalyst additives and equipment design. At a fixed feed, the variables that will have the maximum effect will depend on unit constraints and you will need a sophisticated calibrated simulation model that includes the reactor/regen, catalyst characterisation and gas plant to determine the best route for maximising C3=.
Feed characteristics: residue is heavy and aromatic and is H2 deficient. C3= production is favoured by paraffinic resids. So the more paraffinic the resid, the higher the tendency to make C3=. Low metals and low concarbon is also preferred when processing resid in FCCs. Hydrotreating resid also increases the H2 content and conversion.
Operating conditions: maximising the riser outlet temperature (ROT) will result in increasing C3=. Lowering the preheat will also increase C3= content. Unfortunately, maximising ROT makes more dry gas and the increase will be limited either by wet gas compressor limits or gas plant limits. Increasing injection steam rates also helps increase C3= yield but this needs to be optimised based on effects on atomisation, erosion of feed nozzles, main fractionator flooding and sour water handling constraints. Lift steam and catalyst make-up rate also affect C3= production. This is a complicated optimization and will need a sophisticated simulation model with calibrated catalyst factors.
Catalyst composition and additives: best way to increase C3= with catalyst additives is with the use of ZSM-5 as an additive (preferred) or impregnated into the base catalyst.
Equipment design: resid processing through the RFCC poses challenges mainly with high regenerator temperatures that has a major impact on conversion which in turn affects C3= production. RFCC designs now incorporate catalyst coolers and/or two-stage regenerators with partial combustion. Both designs result in lower regenerator temperatures resulting in higher conversion.
Tom Ventham, Unicat BV / G. W. Aru LLC, firstname.lastname@example.org
To consider the twin objectives of maximum propylene production with residue processing was once the antithesis of FCC operation. However, advances in FCC catalyst technology and unit design mean this is now a common strategy to maximise unit profitability and is no longer the insurmountable challenge it may once have seemed. For a long time, an outdated myth existed suggesting that increasing ZSM-5 addition to the FCC would at some point lead to no further increase in propylene production – a ceiling or plateau existed beyond which further increases in propylene were no longer possible. Laboratory analysis by visionary FCC experts and the challenging of these conventions by pioneering refiners showed increasing ZSM-5 use will continue to improve propylene yield, albeit at admittedly reducing effectiveness.1 Therefore, increased use of an effective ZSM-5 additive, in the absence of advanced FCC redesigns or retrofits, presents refiners with the opportunity to immediately increase propylene yield beyond the current level. Today, ZSM-5 additives, such as Ultra C3Booster from Unicat & G. W. Aru, LLC, remain flexible and cost-effective means to achieve propylene targets. But what else can be improved in the resid FCC to aid effectiveness of ZSM-5 to maximise propylene output? Once the most appropriate ZSM-5 additive has been selected, normally a factor of cost, activity, and technical support and expertise offered by the supplier, reviewing FCC operation with the view of boosting effectiveness and propylene maximisation is critical.
In this response we look at the best approaches under assumptions that feed quality remains fixed and mechanical design of the unit cannot be changed. The first area to review is to ensure feed molecules that can be converted to LPG olefins, such as propylene, using ZSM-5 are maximised. This centres around avoidance of hydrogen transfer reactions that saturate gasoline range olefins to retain these molecules for catalytic cracking with ZSM-5. Hydrogen transfer is a bi-molecular reaction that takes place between an olefin and naphthene with final products being paraffins and aromatics. Therefore, hydrogen transfer reactions should be minimised to avoid conversion of gasoline olefins in the C6-C9 range that could be cracked to propylene using ZSM-5. Hydrogen transfer increases with increasing FCC catalyst rare earth on zeolite, as measured by unit cell size (UCS). High rare earth content catalysts tend to be favoured in residue FCC applications due to the advantages of rare earth stabilisation on zeolite activity retention in high contaminant metal environments. Furthermore, in some technology platforms rare earth based vanadium traps are a common feature of catalysts offered for residue operations.
An experienced refiner looking to increase propylene production from a residue FCCU will consider suitable alternatives to high rare earth on catalyst. The challenges of high metals operation warrant an alternative and more advanced solution than a simple high rare earth catalyst. Advanced metals traps, including Ultra CokeBuster supplied by Unicat / G. W. Aru, LLC, offer protection from deleterious FCC contaminants. Separate particle metals traps have proven effective in a number of FCC applications by protecting the main FCC catalyst to retain or improve inventory catalytic activity or unit conversion. This alternative metals protection capability allows the catalyst to be reformulated to further increase unit propylene production without sacrificing other valued yields.
In parallel with metals trap option discussed, another approach to be considered by the advanced refiner includes an operational spin to fully optimise the whole system when faced with this challenge of operating in maximum propylene mode with resid feed. When further increasing light-ends production a common limitation that will be reached is wet gas compressor (WGC), or associated gas plant, hydraulic constraint. This is common with residue feed units due to relatively high dry gas production, even when using metals trapping or passivation technologies. Another common limitation in residue units is maximum regenerator temperature due to the high coking tendency of the heavy feed. In both of these cases, a reduction in riser outlet temperature acts to alleviate the constraint. Any collateral loss in propylene yield through a reduction in riser temperature, including if reducing below “overcracking” temperature, will be more than fully compensated by an increased addition of ZSM-5. ZSM-5 only generates LPG olefins whilst overcracking operation creates a mix of thermal cracked products including low molecular weight C2- species, low value light paraffins, and problematic di-olefins. Reducing the yield of low molecular weight species through riser temperature reduction alleviates WGC volumetric capacity limitations creating an opportunity to efficiently refill the space with a higher proportion and mass of LPG olefins generated from an increase in ZSM-5 addition. Furthermore, riser temperature reduction will lead to a decrease in cat-to-oil ratio (C:O) at constant other parameters. Lower C:O will tend to a reduction in hydrogen transfer reactions, the negative effect of which is described above. The complex dynamics and balance of this optimisation, coupled with expected variations in residue feed quality, necessitates separate ZSM-5 and other additive addition to allow for a careful online tuning that will ensure profitability is consistently maximised.
Once riser temperatures have been stepped down to a new propylene-led optimum, another challenge will be faced, that of a reduction in conversion and consequent increase in bottoms product yield. This again can be resolved by considering the additive approach. Refiners who have reduced riser severity of their FCC or RFCC operation, whether because of rate limitations or shift to diesel-mode type operation, use bottoms upgrading type additives to maintain minimal bottoms yield despite a deliberate reduction in conversion or riser severity. These matrix based additives, of which Ultra MCBuster can be supplied by Unicat & G. W. Aru, LLC, crack paraffinic slurry range molecules to more useful products to maintain maximum profitability. A further benefit of using a high matrix additive such as Ultra MCBuster is that the shift in the zeolite-to-matrix ratio (Z:M) of the catalyst inventory is in the direction that favours a reduction in hydrogen transfer reactions.
Further operational improvements that can be made if targeting maximum propylene with a resid feed would be to increase riser steam and nozzle steam rates. Not only does the increase in nozzle steam give the possibility of improving feed dispersion that would reduce WGC limiting dry gas production, but the presence of more steam in the FCC riser reduces hydrocarbon partial pressure which is unfavourable for the progression of the bi-molecular hydrogen transfer reaction.
The recommendations discussed, focused around short-term catalyst, additive and operational improvements, give a robust methodology for how to optimise a unit for maximum propylene while processing residue feed. These recommendations are based on the vast FCC experience within the Unicat and G. W. Aru, LLC group and gives operators the tools and knowledge to proceed with confidence to achieve these goals.
1 How ZSM-5 works in FCC, Bart De Graaf, Johnson Matthey Process Technologies Inc., AFPM 113th Annual Meeting, March 24, 2015
Charlie Chou, Sulzer Chemtech, Charlie Chou@Sulzer.com
The FCC heavy naphtha/gasoline portion actually has a lot of olefins that can be re-cracked in the FCC for propylene. However, the FCC heavy naphtha/gasoline also has significant aromatics which is not preferred in the FCC. GTC offers a GT-BTX PluS technology that utilises a simple extraction process to extract aromatics from FCC heavy naphtha/gasoline to produce high-purity BTX (extract) as petrochemical products; and the remaining FCC heavy naphtha/gasoline (raffinate) would be non-aromatics and rich in olefins, which would be an ideal stream to recycle back to the FCC to crack for additional propylene. By doing this, the propylene yield can increase by 2-6% if there is a second riser to be recycled to (or to add a second riser), or increase by absolute 1-3% if recycling to the only/first riser together with resid feed.
Carel Pouwels, Albemarle, Carel.Pouwels@Albemarle.com
Producing propylene from a resid feed is not as easy as processing the VGO fraction. Propylene make is governed by the ability to create and preserve gasoline olefins, the potential of which is reduced when processing higher boiling fractions. Nevertheless, it is certainly a profitable route given the lower price of resid feed. The choice of the residue, however, is important. The less refractory the feed, the more conversion can be obtained, thereby more gasoline olefins. Gasoline olefins are the precursors for the generation of propylene. It is thus key to maximise these by maximising conversion. While the crackability of the feed is of paramount importance, unit limitations ultimately determine how severely an operation can be pushed. In resid applications, this often comes down to regenerator temperature, air rate, and the maximum duty of a cat cooler, if present.
Several options are available to maximise propylene from an FCC unit, all related to the requirements the client has and the starting point. Considering a completely new FCC unit begins with the process technology. Process licensors are the right partners to work with to determine the best choices for strategic and economic long-term solutions. One can choose from conventional process designs that yield between 10 wt% and 12+ wt% of propylene. There are also several high severity applications that target propylene yields in the 15-20 wt% range. These applications commonly operate at 550-600 oC with cat-to-oil ratios about double those of conventional FCC operations.
In the case of an existing FCC unit where debottlenecking downstream units is considered, or where hardware changes to the FCC unit are feasible, process licensors are again good partners. They can suggest various scenarios and help customers make the right choices for their goals.
When the refiner is not planning an overhaul of the FCC unit or its downstream units, the options to maximise propylene are restricted to process conditions, the choice of feed, and catalyst design. The catalyst supplier can be consulted to provide good insight into the optimal operation combined with the best catalyst technology and design for customer goals.
Albemarle provides such expertise and offers maximum propylene catalysts AFX and DENALI AFX. Both are proven in all current process technologies offered by licensors today. While a good catalyst technology is a prerequisite, more important is catalyst application knowledge. At Albemarle, using collected data from all our industrial applications, our Technical Services, Applications, Modelling Department, and R&D teams work together to build deeper understanding of catalysis in relation to process conditions and catalyst technology.
Gary Martin, Sulzer Chemtech, Gary Martin@Sulzer.com
Whether you are producing propylene from a resid or VGO FCC, the economic limit to maximising propylene often is constrained by the gas concentration section (gascon). Modern FCC units provide an important link between a fuel refinery and the production of light olefins and aromatics for petrochemical use. FCC reactor licensors are designing units to produce higher propylene production. Economic factors influence the optimum propylene production with one factor being the capex associated with recovering the propylene. Revamps of older FCC units can provide for increased propylene production at the expense of gasoline, but a constraint to increasing production is the reuse of existing equipment to minimise capex. Increasing FCC unit severity, addition of ZSM-5 and varying the reactor partial pressure, total pressure, and catalyst-to-oil ratio enable increasing propylene production. However, this can have a significant effect on the gascon. The shift in additional propylene production produces more dry gas, making it more difficult to recover the propylene. To maintain high propylene recovery along with the shift in product slate can lead to much higher loadings in the sponge absorber, primary absorber, deethaniser, debutaniser, and the C3/C4 splitter. These columns are often shell limited, and it is usually not economical to replace or parallel these five columns by conventional means. While shifting product slate to petrochemicals makes sense for the forward-looking refiner, it must also make economic sense for today’s business.
For existing processing units, achieving the shift in yields requires an economical means of retooling existing assets. Dividing wall column (DWC) technology is a technique providing a much lower capex revamp to overcome the limitations of a light ends recovery section. A DWC design using a single vessel that combines the functions of a sponge absorber, primary absorber, deethaniser, and C3/C4 splitter can be utilised. This design can unload the existing gascon to enable improved recoveries in the existing unit along with handling additional load from shifting to increased propylene production. With the addition of only one vessel, this design unloads the existing sponge absorber, primary absorber, deethaniser, debutaniser, and C3/C4 splitter. The DWC design can produce equal or higher propylene recovery and product purities than that of a conventional design configuration and add any additional capacity needed. The only potential limitation is the bottom of the debutaniser and bottom of the C3/C4 splitter but there are other solutions if this is the case. Typically, this will not be a problem as changes in the product slate result in these areas not being a limitation or at least allow for changes to handle the new design conditions. The shift in product slate decreases gasoline yield which helps unload the bottom of the debutaniser. The DWC can also be used to overcome limitations in plot space and the construction can often be completed prior to a shutdown with only tie-ins required.
Lynne Tan, BASF Refining Catalysts Asia, email@example.com
There are various routes for maximum propylene production and the best route is what is suitable for the respective refinery’s target and scope. Many licensors provide technological solutions to revamp existing FCC units for more propylene production but a significant investment and a long lead time are expected. A suitable catalyst design is a quick and easy alternative for FCC units to maximise propylene under existing hardware configurations.
To accomplish maximum propylene from a resid feed, a catalyst needs to have high metals tolerance to prevent activity and conversion loss due to contaminant metals, and good coke selectivity to better manage heat balance which is often a key challenge. An ideal maximum propylene catalyst will leverage on these unique characteristics to deliver optimal bottoms conversion to produce light naphtha, the precursor for propylene, and accentuates the effects of ZSM-5 to produce high propylene yield and selectivity.