Catalysis 2025 Issue

The level of contamination that reaches the hydroprocess- ing reactors determines the rate of catalyst deactivation. The higher the contamination, the shorter the run length that is observed; this is similar to fossil fuel deactivation. The indi- vidual contaminants have individual deactivation rates, and they are also cumulative. The contaminants are at higher concentrations and lower boiling ranges than the equivalent fossil fuels, resulting in higher overall catalyst deactivation. The plastic liquefaction step does not seem to impact the contaminant concentration. However, a pretreatment unit such as that used for renewable feeds is not required. The method of producing the plastic oil also determines the level of contamination, with hydroliquefaction (HTL) removing more contaminants and pyrolysis retaining more contaminants in the liquid phase. A Woody Shiflett, President, Blue Ridge Consulting, blueridgeconsulting2020@outlook.com WPOs contain a myriad of contaminants that are highly variable depending on what waste plastics constitute the pyrolysis process feedstock and what type of pyrolysis process is employed (thermal and catalytic). Some of these contaminants are in the form of particulates. Many of these contaminants can be removed simply by depth filtration in pretreatment reactors or beds, as has been reported in a joint Ghent University/Pall Corporation study. Mixed poly- olefin pyrolysis oils tested have shown some 80% of met - als removed in this manner and exhibit 40-60% less coke formation downstream. Fossil fuel feed contaminants tend to be limited to Ni and V in the heaviest stocks (vacuum gasoil [VGO], deasphalted oil [DAO], and residue), Fe in many feeds from upstream corrosion products, or Si in lighter coker-derived feeds (naphtha and kero). WPO can introduce high levels of Na (as 10s-100s ppm), higher levels of Si and Fe (10s of ppm), some Pb (~ <10 ppm), and significantly high levels of chlo - rine (Cl) (100s of ppm). Clearly, in any case, some signifi - cant pretreatment is and will be required. Most technology providers and catalyst suppliers actively engage in guard catalyst and ‘hydrodemetallisation’ catalyst development to meet the needs of emerging feedstocks, with the renewables co-processing and hydroprocess- ing area being a somewhat recent example over the prior decade or two. WPO processing guard catalyst development is and will be following. Speciality guard material innovators and sup- pliers, such as Crystaphase (Houston, TX), are and will be tailoring specialised trapping guard systems to address these needs. As WPO processes enter full commercial- scale applications, more detailed physical and chemical characterisation of contaminants will be needed in order to design and optimise appropriate pretreatment and guard material processes and products. A Chris Ploetz, Process Technology Manager at Burns & McDonnell, cploetz@burnsmcd.com When used as a petrochemical feedstock, the composi- tion and physical properties of raw WPO, also called waste plastic pyrolysis oil (WPPO) or plastic pyrolysis oil (PPO),

can cause various challenges in downstream processes. The nature of these challenges varies depending on the disposition of the oil as steam cracker feed, FCC feed, or hydroprocessing unit feed. Usage of these oils in a steam cracker or FCC supports circularity in the polyolefins market (for example, high-density polyethylene [HDPE], low-den- sity polyethylene [LDPE], and polypropylene [PP]), whereas usage in a hydroprocessing unit (with subsequent process- ing through a reformer and aromatics complex) supports circularity in the aromatic derivatives market (for example, polyethylene terephthalate [PET], polystyrene, and nylon). From the perspective of downstream processing as a petrochemical feedstock, notable characteristics of WPO include the following: high vapour pressure, low flash point, wide boiling range with heavy tail, high pour point, high levels of unsaturation (including diolefins), chemically bound oxygen and nitrogen, chemically bound halogens (primarily chlorine due to PVC in waste plastic), metals and other heteroatoms (for example, silicon and phosphorus), and particulates (reactor solids consisting of carbonaceous char and calcium halides). All of these properties can be problematic in downstream processing, but the issues are magnified if the feed is to be 100% pyrolysis oil. In lieu of this, many refinery and petro - chemical operators are considering pyrolysis oil blending at relatively small fractions with traditional feedstocks in order to reduce the adverse impacts of raw pyrolysis oil while still gaining credit for recycled content via certification from International Sustainability and Carbon Certification (ISCC) Plus or other third parties. Specific pretreatment needs upstream of a hydropro - cessing unit should focus on addressing diolefins, metals, silicon, phosphorus, and particulates. Diolefins should be saturated in a selective hydrogenation unit (SHU) to avoid oligomerisation at high temperatures in the reactor feed preheat train. Metals, silicon, and phosphorus should be removed using guard beds to avoid poisoning and plug- gage/fouling of the main reactor catalyst bed. Primary particulate removal should be accomplished at the pyrolysis facility, but users of WPO should also install filtration systems to prevent plugging of exchangers, catalyst beds, and control valves. Within the hydroprocessing unit, metallurgy should be evaluated for the presence of chemi- cally bound chlorine and other halogens, which will react to form hydrochloric acid (HCl) and hydrofluoric acid (HF) within the reactor. These acidic compounds will ultimately be removed with the acidic sour water decant streams at the cold separator vessels downstream from the reactor. Additionally, any user of WPO needs to consider the high vapour pressure (if unstabilised), low flash point, and high pour point. The vapour pressure of unstabilised pyrolysis oil can preclude storage in atmospheric tanks. The low flash point (typically <<100˚F) requires pyrolysis oils to be treated as a flammable liquid despite being relatively heavy. The high pour point (substantially above summer ambient temperatures) requires heat tracing or other methods of maintaining adequate storage and process temperatures to avoid pluggage due to wax build-up. Although these pretreatment steps add cost to a project,

Catalysis 2025