Recycling Plastics & Circular Economy

Recently, Neste, the world’s leading producer of renewable diesel, UK-based chemical recycling company ReNew ELP, and Australian technology developer Licella (Cat-HTR™ process) joined forces in a development project to explore the potential of using mixed waste plastic as a raw material for fuels, chemicals, and new plastics. This brings me to the topic at hand for many entrepreneurs who wish to look into the process thoroughly.

The global plastic production has increased over the last 60 years due to the vast applications of plastics in many sectors. The constantly increasing demand for plastics have caused substantial plastic wastes accumulation in landfills, oceans and rivers that have aggravated environmental pollution problems. The rising plastics demand has contributed significantly to the depletion of petroleum as part of non-renewable fossil fuel since plastics are petroleum-based materials. First (1^st) Generation alternatives that have been developed to manage plastic wastes include incineration, crude re-purposing recycling and energy recovery (melting) methods. However, there were some considerable drawbacks of these recycling methods as it required high labor costs for the separation process and caused atmospheric and water contamination that not only reduced the process sustainability but posed grave health hazards while hindering the big cause – addressing circular economy in the sector.

Due to these drawbacks, the focus has now shifted to compensate the high energy demand by subsequently recycling the feedstocks. Through extensive research and technology development, the plastic waste conversion to precursor materials is being pursued globally. As petroleum was the main source of plastic manufacturing, the recovery of plastic to liquid oil has great potential since the oil produced had high calorific value comparable with commercial fuels. Most importantly, the valuable chemicals from these waste streams can now be extracted and diverted to the supply chain already existing within the plastics industry, thus contributing immensely to the circular economy.

The largest groups in total non-fiber plastics production are PE (36%), PP (21%), and PVC (12%), followed by PET, PUR, and PS (<10% each). Polyester, most of which is PET, accounts for 70% of all PP&A fiber production. Together, these six groups account for 92% of all plastics ever made. Approximately 42% of all non-fiber plastics have been used for packaging, which is predominantly composed of PE, PP, and PET. The building and construction sector, which has used 69% of all PVC, is the next largest consuming sector, using 19% of all non-fiber plastics. If we reviewed the process for each type of plastic and the main process parameters that influenced the final end product such as oil, gaseous and char. On the technical front, the key parameters that are scrutinized include temperature, type of reactors, residence time, pressure, catalysts, type of fluidizing gas and its flow rate.

Figure 1. Overall process schematics © Kapil Shyam Lokare 2018

Plastic Types

Fundamentally, different types of plastics have different compositions (leading to different applications stemming from different monomer precursors) that normally reported in terms of their proximate analysis. Proximate analysis can be defined as a technique to measure the chemical properties of the plastic compound based on four particular elements which are moisture content, fixed carbon, volatile matter and ash content. Volatile matter and ash content are the major factors that influence the liquid oil yield in our process. High volatile matter favored the liquid oil production while high ash content decreased the amount of liquid oil, consequently increased the gaseous yield and char formation. Table 1 summarizes the proximate analysis of different plastics.

Based on Table 1, it can be observed that the volatile matter for all plastics is very high while the ash content is considered low. These characteristics indicate that plastics have high potential to produce large amounts of liquid oil (and thus more recyclability to monomers) through chemical processes. Since the results of plastics’ proximate analysis are very convincing, the following discussion would focus more on the process parameters involved during the any proprietary processthat would have major influence in the liquid production and subsequently recovery of precursor compounds addressing the circular economy.

Table 1. Proximate Analysis of Plastics

Process Parameters

Process parameters play a major role in optimizing product yields and compositions in any process. With any proprietary process, the key process parameters influence the production of final end products such as liquid oil, gaseous and char. Those important parameters may be summarized as temperature, type of reactors, pressure, residence time, catalysts, type of fluidizing gas and its rate. The desired product can be achieved by controlling the parameters at different settings.

Various parameters influence the liquid oil yield and the most critical factor was the temperature. Different plastics have different degradation temperature depending on their chemical structures. Therefore, the effective temperatures for liquid optimization also varied for each plastic and are also strongly dependent on other process parameters. Such parameters include the type of catalyst used, the ratio of catalyst/polymer and also type of reactors operated. Table 2 summarizes the optimum temperature required to optimize liquid oil yield for different plastics at different conditions.

Table 2. Summary of studies on plastic recycling using processes developed worldwide summarized.

Product Distribution

Table 3 shows the main chemical compounds from the plastics using mostprocesses. For PET, half of the liquid oil (around 49.93%) contained benzoic acid compound. PET tended to lose more aliphatic compound than the aromatic compound and this resulted in higher liquid yield than PVC. PET yielded 23.1 wt% liquid oil while PVC only produced 12.3 wt% liquid oil. This shows that the liquid yield of PET almost doubled the PVC. Most of the PVC gave naphthalene and its derivative, around 33.55%. In PP and HDPE oil, the liquid oil contained primarily aliphatic, monoaromatic and polyaromatic compounds. This indicates the complexity of the HDPE structure to degrade during thermal degradation process. Besides that, the BTX aromatics in PP oil (53 wt%) were found higher than in the HDPE fraction (32 wt%) at the same temperature as mentioned previously. In terms of hydrocarbon product distribution, paraffins were the main product observed (66.55%) for PP derived liquid compared to HDPE (59.70%). Hence, PP oil was more value added than the HDPE derived liquid since paraffins release extra energy for combustion than other hydrocarbon groups such as olefins and naphthenes. For the LDPE derived liquid oil, the aliphatic compound which consisted of alkanes, alkenes and alkadienes were the main compositions found. For PS, benzene, toluene and ethyl benzene were three main components in the PS oil product that increased with the temperature.

Table 3. Main components of the oil from multipleprocesses developed worldwide summarized.

Conclusion

I provide a concise global summary of plastic recycling for each type developed globally and a discussion of the main affecting parameters to optimize liquid oil yield using standard processes and technologies available. Some of the best alternative for plastic waste conversion and economics in terms of operation are highlighted. The flexibility that it provides in terms of product preference (and market demand) can be achieved by adjusting the parameters accordingly. Most processes could be done in both thermal and catalytic process. However, the catalytic process provides for lower operating temperatures with greater yields of liquid oil for most plastics with the right catalyst selection. The sustainability of the process is unquestionable since the amount of plastic wastes available in every country has attained millions of tons. With the gasification, pyrolysis, and CAT-HTR process and many others, the waste management becomes more efficient, less capacity of landfill needed, less pollution and also cost effective.