Sep-2022

Refiner’s role in upgrading plastic waste

Plastic consumption is projected to skyrocket in the coming decades for packaging, automobiles and construction, increasing to 1, 231 million tonnes in 2060, according to recent estimates.

Rene Gonzalez
PTQ

Article Summary

Against this backdrop, chemical recycling of plastics-rich waste streams could become a preferred alternative. In select operating environments, hydrocracking of these streams can deliver a monomer that can be used directly without subsequent processing.

Single-use plastic consumption and the pollution resulting from discarding on land or in rivers and oceans has reached unprecedented levels the world over and is projected to rise significantly in the next decades.

According to the OECD’s Global Plastic Outlook Policy Scenarios to 2060 report, without urgent global action, the volume of plastics in rivers and lakes could reach 348 million tonnes in 2060, while plastics leaking into the world’s oceans could rise to 145 million tonnes during the same time period.

A hydrocracker can be upgraded to processes different types of plastic waste (including PVC and PS). The processing of plastics as a raw material through a hydrocracker reactor is highly dependent on a suitable catalyst and optimum operating conditions. A recent study by Ratnasari et al. focused on the catalytic pyrolysis of waste plastic in the form of high- density polyethylene (HDPE) in a two-stage pyrolysis-catalysis system.

Pyrolysis of the plastic took place in the first reactor and catalysis using solid acid catalysts in the second stage. The influence of using Mobil composition of matter No. 41 (MCM-41) catalyst and zeolite ZSM-5 catalysts on the yield and composition of the derived gases and oils was investigated. In addition, the influence of staging a layered mixture of the MCM-41 with its mesoporous structure followed by zeolite ZSM-5 on the composition of the oils and gases was also investigated.

The staged catalysis catalytically cracks the higher molecular weight hydrocarbons produced from pyrolysis of the HDPE to lower molecular weight hydrocarbons over the MCM-41 catalyst, allowing them to enter the microporous structure of the zeolite ZSM-5 catalyst more easily. Additional investigation by Ratnasari, et al. noted that too many types of hydrocarbon molecule-based plastics result in too many combinations and sizes of plastic-derived molecules.

These molecules need to be upgraded through microporous catalysts like MCM-41 or even the larger ZSM-5 catalyst. In addition, the effect on catalyst activity is rapidly affected by all the different types of co-contaminants in the plastic-derived molecules like HDPE and LDPE, even more so than catalyst deactivation seen in refinery FCC units, hydrotreaters and hydrocrackers when processing heavy residual hydrocarbon feedstocks high in contaminants.

Further investigations reported in the trade press point out that the use of microporous zeolite catalysts limits the range of higher molecular weight hydrocarbons derived from plastics pyrolysis that can enter the pores of the catalyst for reaction. For example, the MCM-41 is an ordered solid acid catalyst with a mesoporous pore structure (1.5—30.0 nm pore size range) whereas zeolite catalysts such as zeolite ZSM-5 and Y- zeolite are microporous (0.4—1.0 nm pore size range) solid acid catalyst.

A holistic approach to control fouling of the catalyst and process equipment is critical to the successful co-processing of pyrolysis oils with contaminants such as Ca, Na, Fe, Mg, etc., that are known to increase the catalyst deactivation rate. Lack of accessibility leads to reduced profitability through loss of activity and bottoms upgrading ability, while also increasing the coke and dry gas make in the unit.

Hence, catalysts with enhanced metals tolerance and high accessibility are required to maintain catalyst activity to profitably process these plastics derived unconventional feedstocks. Understanding these challenges may eventually allow for integration of stand-alone or integrated plastic waste pyrolysis processing within refining and petrochemicals operations. It is with these challenges that further elaboration on plastic waste upgrading will be discussed at the October 10 — 11 Refining India 2022 Conference in New Delhi - https://refiningindia.com/refiningindia2022/en/page/home