Nov-2024

Biofeed FCC co-processing and maximising low-carbon propylene yield (ERTC 2024)

With the refining industry striving towards decarbonisation of operations and pivoting towards the production of lower carbon-intensity products, new challenges and opportunities are arising. Close collaborations between partners are vital for adapting the existing assets of the refining industry to assure a rapid progression in the world’s journey towards lower CO₂ emissions.

R. González and S. Brandt
W. R. Grace

Article Summary

Propylene is one of the main petrochemical products from crude oil refinery operations. While current market conditions for propylene are somewhat suppressed globally, there is a projected increase of 45 MM tpy in global demand for C₃= by 2030, which will drive the demand for fluid catalytic cracking (FCC) propylene accordingly.² Propylene produced by the FCC already has a favourable carbon intensity compared to other on-purpose processes.³ The application of ZSM-5-containing technology to improve FCC propylene yields is favoured due its neutral impact on the heat balance of FCC units and, therefore, Scope 1 emissions.

In addition to its favourable carbon intensity, the carbon impact of FCC C₃= can be further reduced by using ZSM-5-based technology and/or co-processing biogenic feedstocks to the FCC unit. Grace has been partnering with a number of refineries globally to contribute to assessing the opportunities and risks of co-processing bio-derived feeds, as well as closely monitoring commercial trials and servicing continuous operation.1, 2, 4, 5

FCC proceeds via a β-scission mechanism on the active sites of the catalyst (Figure 1).⁶ The end product of β-scission is C₃=. To further reduce the carbon intensity of the FCC C₃= co-processing, some bio-derived feed streams to the FCC unit can be considered.³

The higher the co-processing rate, the greater the impact on the carbon intensity of the related FCC products. Assuming an equal distribution of the renewable carbon among the FCC products, the co- processing rate (mass-based) can be directly related to a reduction in carbon intensity. Consideration of the oxygen content of the renewable feed source is required, as this oxygen content is mostly converted to water, carbon monoxide (CO), and CO₂ yields. It can be estimated that considering a co-processing rate of 10 wt% renewable feed with an oxygen content of about 10 wt% (in the range of many seed oils), the carbon intensity of the resulting C₃= would reduce by 9%.

The ultimate impact of co-processing renewable feed components on the yield structure is likely to be different to this theoretical mass balance approach. However, this must be determined in FCC unit pilot plant testing and commercial applications, as they depend on the fossil feed type, unit conditions, and FCC catalyst properties. To assess the amount of C₃= stemming from the renewable feed component, highly sophisticated analytical methods for modern carbon determination might be required.⁷

Data testing for some renewable feed types and test conditions within Grace showed that the renewable carbon- containing feed might be preferentially converted to C₃= compared to fossil feed components. Figure 2 shows bench-scale pilot plant testing results, which indicate that the C₃= yield in this case increased by about 0.3 wt% FF by blending 9 wt% palm oil with the vacuum gasoil (VGO). Considering the incremental yield concept,⁸ it is estimated that palm oil yields 6-7 wt% FF C₃=, nearly double the yield of the fossil-based VGO in this particular case.

While Figure 2 illustrates the potential C₃= increase by renewable co-processing, challenges with co-processing should be considered. These potential challenges are often associated with the significantly higher oxygen content of the renewable feed component relative to traditional feedstocks. Despite the absence of added hydrogen (H₂), the FCC process offers a high degree of deoxygenation of renewable feed streams. Most oxygen species are converted to hydrocarbons and water, CO₂ and CO, which will leave the FCC unit on the reactor side and could pose challenges downstream. In pilot plant testing of renewable feed co-processing, the effects of trace oxygenates are often not considered. Nevertheless, these are likely to occur with oxygen-containing feed streams. Trace amounts of oxygenates are commonly found in fossil feed-based FCC product streams like liquefied petroleum gas (LPG) or cracked naphtha. Increasing the combined FCC feed oxygen content by the co-processing of renewable feed streams like vegetable oils will increase the amount of these oxygenate species. This might negatively influence the downstream processing of the FCC unit products while also causing products to exceed specification limits. Pilot plant testing will help understand the magnitude of changes in oxygenates and water, CO, and CO₂ yields. In addition, close cooperation with the catalyst supplier will help discover areas of concern and monitoring requirements.

Conclusions
The drive towards decarbonisation and the energy transition inspire the refining industry to a new way of thinking, reconsidering value chains and associated process schemes. The importance of the FCC process in refining and its high flexibility makes it one of the main processes to be considered for adaptation to new opportunities arising.

Besides lower carbon intensity transportation fuels, one of the target products of the FCC process is propylene. The demand for low carbon intensity and bio-derived polyolefins is increasing, and the adaptability and sophistication of the FCC process are ideal conditions to contribute to meeting the demand for bio-derived polymers.
Grace is supporting several refining customers on their paths to decarbonise the FCC unit’s operation and products. In addition, Grace’s expertise in product purification by adsorbents or hydrogenation and downstream processing to polyolefins is providing solutions for the new challenges that can arise with the co-processing of bio-derived feed streams.

This short article originally appeared in the 2024 ERTC Newspaper, which you can VIEW HERE