Oct-2020

We are aiming to boost our FCC propylene output. What recent developments would you recommend? (PTQ Q&A)

Responses to a question in the Q3 2020 issue of PTQ

Various from Axens, BASF, Shell Global Solutions and W. Aru.

Article Summary

Nicolas Lambert, Technology Team Manager - RDS, FCC & Sweetening Segment, Axens - Nicolas.LAMBERT@axens.net
Thibault Reffet, Technology Team Manager - Clean Fuels, Axens. Thibault.REFFET@axens.net

Beyond the obvious adjustments to the catalytic systems to include ZSM-5 and follow closely the latest catalytic developments, some upgrades on the technology side are possible. The principal hurdle will be the hydraulic limitations of the unit which could require reducing the unit throughput to achieve better performance.

Not so recent, but efficient in generating some margin in an existing unit, which can in turn be used to increase unit severity, is the FCC Alliance Stripper Packing. This largely proven equipment aims at thorough stripping of spent catalyst under all circumstances in the unit. Through an important reduction of hydrogen content in coke (versus disc and donuts, baffles, or other technologies), the air demand and regenerator(s) temperature are drastically reduced. Temperature drops of 10-15°C are routinely achieved, and possibly more depending on the starting point. The ‘room’ thus freed in the regenerator enables increasing the ROT in order to reach higher propylene yields.

Some other technical elements offered by the FCC Alliance and Axens have similar effects, albeit to a lesser extent:
- Feed injectors: FIT G-Series or FIT R-Series (depending on the unit feed diet) will reduce coke production through better, more intimate contacting of the feed. The liquid hydrocarbons are more efficiently and quickly vaporised to promote vapour phase cracking in order to limit coke formation by deposition on the catalyst. Following the same principle as above, reduced demand on the regenerator side allows increased severity
- The RS2 riser termination device will ensure minimum thermal degradation. The same load on the wet gas compressor can thus be achieved at the same unit throughput but operating at higher severity to promote propylene production
- MTC Technology: a riser quench that will force an increase in the catalyst circulation rate, bringing more energy to the riser and thus improving feed vaporisation and performances

In addition, if the LCN or external olefin-rich cuts can be disposed of, coupling the FCC unit with an external oligomerisation unit will prove a very efficient substitute to a secondary riser.

Indeed, the principal pathway for propylene from LCN cracking is through the intermediate oligomerisation of olefins subsequently followed by cracking. Cracking of paraffins requires much more energy for limited results. Externalising the oligomerisation step through Polynaphtha or PolyFuel processes will ensure a much higher extent of this reaction step, leaving only the cracking part to be performed by the cat cracker. In such a case, only oligomers are recycled to the FCC unit, requiring far less space than the full LCN cut, for instance, for much better results.

Moreover in addition to the LCN, the oligomerisation unit can convert the C4 olefins into oligomers that can be cracked to propylene. Hence, by using this type of technology, it is possible to tune the FCC product slate towards the desired products. This coupling of FCC with an oligomerisation process is called FlexEne (see  Figure 1).

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Hernando Salgado, Technical Service Manager ME, BASF - hernando.salgado@basf.com
Amid Shah, Technical Service Manager India, BASF - amit.shah@basf.com
Stefano Celestino Riva, Technical Service Manager EMEA, BASF - stefano.riva@basf.com

If all conventional and easy ways to increase propylene yield from the FCC unit have already been evaluated and implemented to the furthest extent possible — operating conditions, feed quality, naphtha recycle — then the market can be scouted for the latest technology in the development of ZSM-5 additives.

Recent R&D developments have enabled an increase in ZSM-5 additives’ crystal content from a traditional 25-40% level to higher than 50% crystals. BASF’s ZEAL is the latest ZSM-5 additive in that category. Using such a high crystal ZSM-5 additive, while maintaining the dosing rate the same as the incumbent additive, can significantly boost propylene yield by minimising the additive dilution effect, provided that sufficient olefin precursors are available in the light gasoline stream. The extent of incremental propylene production depends on the specific application. However, it is always a good idea to consult your FCC catalyst supplier to receive brainstorming support for new ideas or simply to evaluate the impact of conventional operational changes.

For instance, one of the operating variables that is often neglected but affects propylene yield a lot is the hydrocarbon partial pressure, impacting both the olefinicity of the C3 fraction (C3=/C3 ratio) and the overall yield. A decrease of hydrocarbon partial pressure can be achieved by increasing injection steam or any other steam going to the riser, such as lift steam. Hydrogen transfer reactions are expected to be reduced under low hydrocarbon partial pressure conditions, hence preserving the olefins already made, and increasing cracking reactions (that are thermodynamically favoured by low pressure) towards products including light olefins such as propylene. Figure 1 shows the potential impact of increasing injection/lift steam in a typical high propylene unit with high ZSM-5 addition:

It must be noted that graph is specific for each unit since the actual response to light olefins yield depends on other process variables as well as unit design aspects. In addition, a substantial increase in injection steam or lift steam might require plant modifications to accommodate the higher steam rates, and a careful review of the capacity of some downstream systems such as the lines and pumps of the sour water system, including the main fractionator overhead drum collection boot, and the sour water stripping unit. Other design aspects such as maximum feed injector pressure drop and cyclone inlet velocities should also be monitored as they will directionally increase. As a side benefit, higher injection steam will also improve feed atomisation and vaporisation, thus reducing coke yields and coke deposition tendency.

Finally, producing maximum propylene from the FCC riser is only the start; it should also be recovered by reducing/minimising losses in the dry gas/fuel gas. In one patented application, a refinery added a chiller to cool the naphtha used in the primary absorber to minimise the loss of propylene to the fuel gas. The value of propylene in a petro-refinery can be high enough to pay back additional investment in a short time.

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Kevin Le, Senior Technologist - FCC, Shell Global Solutions (US) Inc - Kevin.Le@shell.com
On an existing unit, ZSM-5 additive has been used as the main handle for maximising propylene production. It provides the most flexibility for managing a unit’s constraints, as well as a quick response to the need for incremental propylene barrels. Optimising fresh catalyst formulation is also important to target for maximum LPG production, along with ZSM-5, while minimising the bottoms or slurry production. Generally, a fresh catalyst that works best in this case is formulated with a ‘lower’ rare earth and a ‘higher’ matrix surface area than the ‘typical’ catalyst formulation.

In most cases, an existing unit is typically constrained by the volumetric flow on one or more gas fractionation pieces of equipment — namely, the wet gas compressor (WGC), the absorber, the debutaniser, or the depropaniser. The constraint in these columns can often be relieved by upgrading the column internals. Shell has implemented a wide range of its own column internal designs to expand the capacities of these columns within existing vessels. To manage the constraints on an existing unit, optimal solutions are often found by balancing the riser outlet temperature (ROT), ZSM-5 concentration, fresh catalyst formulation, and in some cases feed rate and/or feed quality to the unit.

Maximising ROT could be a low cost option to maximise LPG production, by increasing conversion and over-cracking of gasoline into LPG; however, it also comes with a higher dry gas yield and a poorer propylene selectivity when compared to the effect of ZSM-5. Feed nozzle performance has a significant impact on the dry gas yield. Choosing the right feed nozzle technology is also important in the mix of options for maximising propylene production while managing constraints on an existing unit. Shell’s latest feed nozzle technology offers an excellent dry gas yield reduction that has been proven in many units.    
Other operating variables that would impact propylene production include feed quality, conversion, and reactor pressure. In general, an ‘easier to crack’ feed, typically with higher hydrogen content and lower aromatics content, would result in higher propylene yield. The FCC feed is typically made up of several feed streams. It is important to accurately predict yield from each feed stream in order to optimise the FCC feed slate for maximising propylene production. Using a well-tuned kinetic model, such as Shell Advanced and Rigorous Catalytic Cracking (SHARC) proprietary software, is recommended for quantifying yield from each feed stream.

For a given feed quality, maximising the cat-to-oil (C/O) ratio also plays an important role in increasing unit conversion and propylene production. Shell designs FCC units with a unique feature that allows sour water to be recycled to the riser, besides the more typical naphtha recycle. The company has successfully utilised this feature to maximise C/O ratio and lower the partial pressure of the hydrocarbon feed in the riser, and at the same time to maximise the unit’s conversion and thus propylene production. It is a great addition to the tool kit for maximising the C/O ratio which could include lowering the feed preheat temperature, increasing CO concentration in the flue gas on a partial burn regenerator unit, and raising the catalyst cooler duty. It is important to note that recycling sour water and/or naphtha to the riser will impact coke yield and riser residence time, as well as loading to the reactor cyclones and main fractionator (MF).

Lowering the reactor pressure to reduce hydrocarbon partial pressure is good for unit conversion and propylene production, yet it is typically not a practical solution for an existing unit because of various constraints, like pressure balance, loadings to the MF and WGC, and so on.

For an existing unit operating without constraints in the gas fractionation section, a propylene yield up to 7-8 wt% of fresh feed has been achieved by using a combination of the above mentioned variables. To further boost propylene yield, up to 15-20 wt% of fresh feed will typically require major revamps for an existing unit. Shell has developed MILOS (Middle Distillate and Lower Olefins Selective) technology for maximising light olefins while offering a flexible operating mode for distillate (LCO/LGO) production. The MILOS process features a dedicated riser for naphtha recycle that is operated with a high ROT and C/O ratio, utilising a high ZSM-5 concentration in the catalyst to maximise the overall olefin production.

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Tom Ventham, Sales & Technical, Europe and Africa Unicat BV/G. W. Aru, LLC - tom.ventham@gwaru.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

Propylene is a key product for many FCC operations. This area is receiving renewed focus as the current market sees reduced gasoline demand and over-supply. ZSM-5 based additives provide the means to increase valuable propylene production as well as to destroy low value gasoline blend components. The response given here by G. W. Aru, LLC and Unicat B.V. hopes to break a common myth in the FCC world — that producing propylene using ZSM-5 has a finite limit. Attempts to increase propylene with ZSM-5 above 10% fresh feed appear to reach an asymptote, but additional increases in propylene are possible.¹ A complex set of reactions takes place over ZSM-5 which provide additional feedstock for cracking reactions to produce LPG olefins, propylene, and butylene. ZSM-5 efficiency can be improved through optimisation: minimising FCC catalyst rare earth content, reducing FCC catalyst zeolite-to-matrix ratio, reducing hydrocarbon partial pressure in the FCC riser, increasing Ecat activity, and adjusting FCC feed quality, including the recycling of a naphtha or butylene stream. Further increases in propylene production can be achieved through incremental increases in ZSM-5 addition. The best ZSM-5 for this type of high propylene operation is one of high intrinsic activity, good hydrothermal stability, and excellent retention.

Numerous studies conclude, and we agree, that ZSM-5 is the most selective and cost-effective method to generate incremental propylene from the FCC unit, especially in units that have dry gas or wet gas compressor limitations. It is far more economical to use ZSM-5 rather than elevating riser temperature or adjusting other operating variables. There can be significant positive synergy from proper design of the base FCC catalyst formulation; this cannot be ignored in the unit optimisation effort. ZSM-5 use is a near-real time optimisation tool that allows the entire refinery a rapid response to changing market conditions. However, full value can only be realised if the ZSM-5 is separately added rather than pre-blended or incorporated with the FCC main catalyst.

With the extremes currently seen in how refinery operation needs to be continuously manipulated to maintain optimum profitability, reducing flexibility by not having independent control of ZSM-5 addition for precise control of LPG production greatly diminishes the value that can be achieved. Further to this, inefficient addition of either ZSM-5 or fresh catalyst by not decoupling this process means unnecessary added costs to the FCC budget at a time when all opex is under pressure and should be minimised.

1 How ZSM-5 works in FCC, Bart De Graaf, Johnson Matthey Process Technologies Inc., AFPM 113th Annual Meeting day two Tuesday | March 24, 2015.