Flash Joule Heating Equipment: Recovery of Polyolefin Plastic Scrap in Short Contact Times by Rapid Joule Heating

Background

Polyolefins, consisting of low- and high-density polyethylene (LDPE, HDPE) and polypropylene (PP), account for about 60% of plastic waste. While mechanical recycling plays a key role in treating part of the plastic waste, it falls short in producing high-quality materials, especially polyolefins and mixed plastics. As a result, chemical up-cycling and recycling technologies have developed rapidly. However, activating the C-C bond in polyolefins is a challenge, making these processes both energy-intensive and unselective for high-value products. Advances in hydrocracking, hydrolysis and catalytic pyrolysis have produced fuels, lubricants and waxes with relatively high selectivity. However, these processes are not directly recyclable because their products require additional steps to produce monomers. One way to synthesize monomers is to produce naphtha through hydrocracking followed by steam cracking at elevated temperatures (Pathway 1 in Figure 1). However, this two-step process is very energy intensive. Alternatively, catalytic pyrolysis and ethylene cracking have been used to produce monomers directly. Etherolysis uses ethylene (the monomer of polyethylene) as a reagent and expensive precious metal catalysts, making it less recyclable and economical. Catalytic pyrolysis, while promising, has a high catalyst-to-polymer ratio (“1”), lower selectivity for ethylene and propylene, and slower penetration of heat into plastics due to the elevated reaction temperature (especially compared to mechanical recycling) and higher energy intensity.

The main point of this paper

Importance of process electrification:

Process electrification is key to decarbonizing the chemical industry by transitioning from fossil fuel processes to more sustainable and energy efficient alternatives.

Application of renewable electricity:

Renewable electricity as a clean energy alternative to carbon-intensive methods of producing key chemicals.

Electrification of catalytic and non-catalytic processes:

Microwave, induction heating and Joule (or resistance) heating processes are receiving attention.

Space-time heating (STH) research:

Dong et al. showed that STH (a form of Joule heating) enables selective production of plastic monomers even in the absence of a catalyst.

Advantages of Nanofiber Membrane Technology:

Mastalski et al. noted that Nanofiber Membrane technology can eliminate heat and mass transfer limitations.

Limitations of catalytic deconstruction studies:

Research in catalytic deconstruction is still limited.

Methodology of this study:

A laboratory-scale method for the selective deconstruction of polyolefins to C2-C4 olefins using rapid pulsed Joule heating (RPH) over H-ZSM-5 catalysts is presented.

Efficient single-step electrification method: deconstruction of polyolefin plastic wastes to light olefins over H-ZSM-5 catalyst using fast Joule heating

The importance of chemical deconstruction of polyolefins:

Deconstructing polyolefins into fuels, lubricants and waxes reduces their accumulation in landfills and the environment.

Recyclability Challenges:

Achieving the goal of converting polyolefins to C2-C4 monomers with high yields, low energy consumption and low CO2 emissions remains elusive.

Single-step electrification method for fast Joule heating:

A method for efficiently deconstructing polyolefin plastic scrap into light olefins (C2-C4) utilizing fast Joule heating over H-ZSM-5 catalyst is demonstrated.

High yields were associated with high polymer-to-catalyst ratios:

Higher yields were achieved at higher polymer-to-catalyst ratios compared to previous studies.

Critical role of catalyst:

Catalyst is critical for the production of light olefins with narrow distribution.

Advantages of pulse operation and steam co-feeding:

Compared to continuous Joule heating, pulse operation and steam co-feeding achieved highly selective deconstruction with greater than 90% C2-C4 hydrocarbon product ratio and minimal catalyst deactivation.

Effectiveness of the lab-scale method:

Demonstrated effective deconstruction of real-life wastes, adaptability to additives and impurities, and versatility in the management of recycled polyolefin plastic wastes.

Summarize

The proposed Rapid Pulsed Joule Heating (RPH) technology for catalytic cracking of polyolefin wastes is promising for depolymerizing polyolefins into monomers. Ultra-fast reactor heating utilizing CFP enables plastic deconstruction within 500 ms (via 10 ms and 50 ms pulses), and the H-ZSM-5 catalyst efficiently breaks down polymers into light hydrocarbons, with a C2-C4 product of >75% at full conversion. In addition, high productivity is achieved at about 50-200 times lower catalyst usage. While continuous Joule heating (CJH) increases the yield of light olefins compared to RPH, catalyst deactivation is also significantly increased. Passing steam in RPH minimizes coke formation and thus increases monomer yields, outperforming CJH. Notably, we demonstrate that the product fraction of C2-C4 hydrocarbons is greater than 90% at full conversion, which highlights the potential for RPH to produce monomers. The high ratio of propylene to ethylene (~2.6-3.5) suggests that the technology is particularly suitable for propylene production. In contrast to conventional catalytic pyrolysis, the thin film structure of the polymer eliminates heat transfer limitations and associated bulky reactors, allowing for a modular system.