The FCC unit as a propylene source
Hardware and catalyst design changes can increase the propylene yield, but ZSM-5 and other additives are essential to optimise and retain the flexibility to respond to today’s and tomorrow’s challenges
Intercat (Johnson Matthey)
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Propylene has often been considered as “the other olefin”, implying a relative unimportance compared to ethylene. However, this view is rapidly changing, with annual worldwide production now in excess of 65 million tons and worth more than $25 billion.1
The supply and demand balance is also tightening, as propylene demand growth is forecast by SRI Consulting to exceed 4.5–5.0% per year over the next five years, led by Asia with even stronger growth at nearly 6% per year. This exceeds the forecast growth rate of ethylene by about 0.5%. The consequence of these different growth rates is that by 2015 propylene will form 40% of the total olefin demand.1
On the supply side of the balance, propylene is a by-product of ethylene plants and refineries. Currently, about 61% is produced in ethylene plants, 34% in petroleum refineries and less than 3% in on-purpose propylene-only production plants.
Ethylene plants, however, have very limited flexibility for increasing the relative yield of propylene. Table 1 shows that feedstock selection essentially determines the yield ratio, and even â€¨the heaviest gas-oil feeds will only produce about 37.5% propylene. In 2005, the average was only 29% propylene, and most of the relative future growth will need to be met from refinery sources.
In refineries, while cokers and visbreakers produce some propylene, the majority comes from FCC units. Indeed, it is only in refineries with FCCUs that the scale of production supports the investment in recovery of propylene as a separate product. Propylene production in refineries is also increasing as refiners struggle to blend FCCU naphtha and propylene polymer gasoline into the gasoline pool due to increasingly restrictive product specifications. The major advantage the FCCU has over steam crackers is its high degree of flexibility in low-cost feedstock selection and product yields. The FCCU operator can use a combination of operating conditions, catalyst selection and additives that can be adjusted to suit the current economics. All these factors combine to make propylene ever more important to refinery and FCCU economics.
This increasing importance of propylene to FCCU economics can be demonstrated using a simple model based on the commercial yields shown in Table 2. These come from a high- propylene yield unit running resid feed.2 They illustrate what a unit designed for this type of operation can achieve, and how using incremental high-activity ZSM-5 additive can further enhance the propylene yields.
Economics of FCCU propylene production
Figure 1 gives the product upgrade value per kg of ZSM-5 for these yields for three regions using the appropriate Platts average prices. Superimposed is the cross-regional average propylene price. The ZSM-5 margin closely tracks the propylene price, showing how important this price is to ZSM-5 economics. The steep rise in this margin since 2003 is a major contribution to significantly improved overall FCCU margins.
Increasing FCCU â€¨propylene yield
What can the refiner do to maximise the propylene yield from the FCCU? Not surprisingly, high propylene yields follow from high conversion and selective cracking. Using a combination of unit and catalyst design plus additives, refiners can substantially increase the propylene yield from the process.
Unit design Unit design changes are only an option when designing a grassroots project or revamping an existing unit. For propylene maximisation, the reactor/regenerator design objectives should be a high cat-to-oil ratio and high riser temperature to give a high conversion, and short contact time for selective cracking. Good catalyst activity maintenance is also important, since it increases conversion and improves selectivity, while allowing greater flexibility in catalyst design.
Light ends handling capacity is crucial when designing for maximum propylene. Areas such as the wet gas compressor, amine treating and fractionation should be sized to ensure the economic maximum propylene recovery is achieved. For revamps, switching tower internals and possibly even overhead refrigeration may be warranted, particularly when ethylene recovery is also intended.
Catalyst design The constant replacement of the FCCU catalyst is a significant part of the inherent flexibility of the process compared to the relatively fixed yields from a steam cracker. However, in designing a catalyst, there are always diverse needs, which force compromises and limit this flexibility. Inevitably, there are trade-offs, but high activity with low coke selectivity is usually a good starting point on the way to high conversion. For residue operation, the right balance between bottoms upgrading and metals tolerance is critical, while for propylene selectivity low hydrogen transfer activity has to be traded against stability. Added to this is the need to retain flexibility to respond to short-term opportunities and constraints. It is in this area in particular that the separation of base catalyst and additive functions can have a major impact.
Additive design Additive systems allow the refiner to compensate for the compromises in catalyst design, while still offering synergistic benefits and enhanced flexibility. For example, bottoms-cracking additives can increase conversion without coke penalties, while metals-trapping additives aid catalyst activity maintenance.
Although ZSM-5-based additives were first used to boost the gasoline octane rating during the phase-out of lead in the US and Europe, they have now become the key lever used by refiners to increase olefins production. ZSM-5 is now regularly used in over 50% of FCCUs worldwide.
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