Co-processing renewable and recyclable feedstocks in the FCC unit
As new regulations drive refiners to develop more sustainable processes to produce fuels and chemicals, they are exploring co-feeding renewable and recyclable feedstocks.
Lucas Dorazio, Jian Shi and James Fu BASF Corporation
Snehesh S Ail, Marco J Castaldi and Golam S Chowdhury The City College of New York
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While the refining industry currently plays a vital role supplying much of the world’s transportation fuels and commodity chemicals, new regulations are forcing them to produce fuels and chemicals more sustainably. As a result, refiners have begun exploring co-processing renewable and recyclable crude oils (RCOs) to replace conventional fossil based feedstocks. The new feeds present challenges such as additional metal contaminants and differences in feedstock chemistry. The ability of the FCC unit to manage metal contaminants and tolerate a wide variety of feedstock makes this process well suited to co-process these RCOs.
At BASF, we are exploring alternative feedstocks, their implications on the operation of the FCC unit, and how the catalyst can be designed to address the new challenges that will be created. Through collaborations and partnerships, we have access to a diverse collection of RCOs, as well as the resources needed to prepare custom RCOs. We have also adapted our existing catalyst testing laboratories to explore how these new feedstocks will behave in FCC units.
Challenge of RCOs
RCOs originate from three possible sources: pyrolysis of recyclable waste streams such as plastics and tyres, plant based oils such as soybean oil and corn oil, and pyrolysis of biomass such as wood residues and corn stover. Depending on the source and the conditions used to prepare the RCO, the properties will vary considerably. On one end of the spectrum, pyrolysis oils derived from polyethylene and polypropylene wastes contain high hydrogen to carbon and minimal oxygen content. This makes oil produced from these materials easily upgradable. In contrast, pyrolysis oil derived from biomass will present challenges due to its inherently lower hydrogen to carbon content, significantly higher oxygen content, and relatively high content of alkali metals. Between these two extremes will be plant based oils containing moderate content of oxygen and alkali metal contaminants.
One way to illustrate the diversity of the different feedstock options is to examine their differing carbon, hydrogen, and oxygen content, which is illustrated in a Van Krevelen diagram in Figure 1. The lower the hydrogen to carbon (H/C) content of the feedstock, the more challenging it will be to upgrade into transportation fuels and chemicals. For reference, H/C content of petroleum based crude oil is roughly 1.7. The relatively high H/C content of polyethylene and polypropylene (~2.0) makes these ideal starting points for deriving RCOs. While biomass appears to have a H/C content similar to petroleum based oil, the other factor that must be considered is the oxygen to carbon content (O/C). A lower O/C results in a feed that is easier for refiners to process. Figure 1 shows that biomass based and plant-derived oils have a higher O/C than crude oil. In plant-derived oils, oxygen is contained within the structure of the triglycerides comprising the oil. While sources of biomass wastes will vary, all are comprised of lignin, cellulose, and hemicellulose that are rich in oxygen. In an FCC process, the oxygen contained in biomass pyrolysis oils and plant oils will be removed through various deoxygenation pathways producing H2O (hydrodeoxygenation), CO (decarboxylation), or CO2 (decarbonylation), all low-value products for refiners. However, in addition to increasing gas yields, deoxygenation will influence yields of high-value products, depending on which pathway is dominant. Producing more CO/CO2 will consume carbon that otherwise could be used to make fuels and chemicals. Producing more H2O will consume hydrogen, reducing the effective H/C, and potentially increase the tendency to form coke.
New metal contaminants will also create challenges when processing RCOs. Plants are rich in alkali metals that will be carried over into biomass based pyrolysis oils and plant based oils. Similar to metals contained in conventional oils, the alkali metals will accumulate on the surface of the FCC catalyst, requiring new metal passivation strategies to be incorporated into the design of the catalyst. The extent of alkali metal contaminants will also vary with the source of the RCO. On one end of the spectrum will be plastic-derived oils with little or no contaminant metals. On the other end of the spectrum will be oils derived from biomass pyrolysis that will potentially have a significant content of alkali metals such as Ca, K, Mg, and Na.
Pyrolysis of renewable and recyclable materials
Understanding the process of pyrolysis is important in understanding the nature of different RCOs and how they will behave during co-processing in a refinery. Pyrolysis is a process involving the thermal decomposition of materials at elevated temperatures in an inert atmosphere. Pyrolysis results in the formation of a broad spectrum of hydrocarbon fragments that can be categorised by condensation temperature: waxes comprise products condensing at or above 25ᵒC, liquids comprise products condensing between -15ᵒC to 25ᵒC, and gases are products condensing below -15ᵒC. A fourth product category is char, which consists of a highly carbonaceous non-volatile residue forming in the pyrolysis reactor. The yields of gases, liquids, wax, and char will vary based on the design of the pyrolyser, pyrolysis conditions, and the material being pyrolysed. As one example of this variation, Figure 2 shows pyrolysis yields from a lab-scale pyrolysis unit for varying materials and pyrolysis temperatures. In general, the amount of upgradable products from plastics will be significantly more than that obtained from biomass.1 In the lab-scale results illustrated in Figure 2, plastic pyrolysis yielded 80-90% upgradable products (liquids and wax) compared to biomass pyrolysis yielding only 15-20% upgradable product (liquid).
Plastic pyrolysis oils
While any plastic can be pyrolysed to produce an oil, polypropylene and polyethylene are more commonly used as feedstock for commercial production of plastic- derived pyrolysis oil, as they produce oil well suited for upgrading. When these plastics undergo pyrolysis, the large hydrocarbon polymers are thermally cracked into a very broad distribution of smaller hydrocarbon fragments. Using the lab-scale pyrolysis oils shown in Figure 2 as an example, the pyrolysis of low density polypropylene (LDPE) yields a range of hydrocarbons from methane to C34 size hydrocarbons. In this experiment, the gas fraction contained hydrocarbons from methane to pentane (Figure 3). The liquid and wax fractions were comprised of hydrocarbons ranging from light naphtha to C34 size hydrocarbons, with the wax generally containing a larger portion of the heavier hydrocarbons (Figure 4).
Certainly, the heavier portions of the liquid and wax fractions can be easily co-processed with conventional gasoil in FCC. While significantly lighter than conventional gasoil, it is possible that the lighter portions of the liquid and wax could be co-processed as well, assuming the impact of the lighter feed on product yields can be tolerated. Both fractions can be readily mixed with gasoil without miscibility issues. When co-feeding plastic pyrolysis oil in a lab-scale ACE unit, the impact of the lighter feedstock results in higher yields of LCO, gasoline, and LPG yields consistent with the percentage being co-processed. Otherwise, the co-processing of plastic pyrolysis oil does not negatively impact yields.
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