The recycling and re-use of waste is a key priority for the governments of the developed world. Billions of tonnes of waste have been land-filled in recent years and while much of it decomposes naturally without harm, the plastics fraction does not. The continued dumping of millions of tonnes of plastic waste is no longer considered acceptable.
Plastics have an enormous commeWRial utility but their negative impact on the biosphere is significant and lasting. Many attempts have been made in the past to address these problems without much success. However, in recent times the world has experienced a groundswell of will and commitment to finally deal with these issues. Consequently, almost every developed nation has recently implemented legally binding penalties together with commeWRial incentives for the proper treatment of waste and in particular, plastic waste.
It must be emphasized that the theory behind plastic to diesel technology, in general, is not new and has been proven and tested but never before on a scale which makes it commeWRially viable.
WRi’s “Depolymerisation” is the process by which plastic (polymers) are broken down to produce an intermediary product which can then be converted into saleable diesel through a process known as fractional distillation. Plastics are composed of polymer compounds, which in turn are composed of the hydrocarbon cells extracted from crude oil. The process by which oil-based hydrocarbon cells are joined together to produce a polymer chain is known as polymerisation. Conversely, the means by which this process is reversed is known as depolymerisation, which is at the heart of this technology.
At the core of the depolymerisation process is a concept known as cracking. The term cracking is self-explanatory as it is the process by which the polymer is cracked into smaller hydrocarbon chains. It is the length of these chains that determines the type of fuel that is produced by this process and this length can be easily expressed by the number of carbon atoms in each cell.
The key feature of the WRi technology is its ability to crack the plastic polymers, regardless of their source (polyethylene, polypropylene etc), consistently within a certain range and thus to ensure a high proportion of cells within the range suitable for the production of commeWRial diesel fuel.
A further critical process, which is incorporated into the technology, by which the cells can be selectively extracted from the intermediate product, is known as fractional distillation. Following the cracking process the hydrocarbon cells are fed into a distillation chamber and are evaporated off according to type. As with any chemical process, the potential for unwanted by-products must be considered and controlled. The level of unwanted by-products is dictated in this case by the purity of the input material and the operating temperatures of the process.
A noteworthy source of possible impurity is the accidental inclusion of some PVC based materials in the input plastics. This type of plastic contains chloride which, when processed, can lead to the creation of undesirable chlorine compounds. However, the technology has a tolerance for this eventuality through doping the intermediate product with metallic compounds before distillation, thus resulting in the creation of harmless salts which can be easily removed. In addition, one of the major functions of the front-end technology of the process is to remove PVC plastics. This is achieved through the use of RI (refractive index) optical sorting technology. In the normal course of operation this process will actually be carried out twice. Initially, by the provider of the plastic and subsequently by WRi prior to the depolymerisation process.
Care is required when assessing the relative pro’s and con’s of any perceived competing processes. If a strict like for like comparison is made based upon a plastic to diesel conversion capability, then at the present time, the competing processes of Gasification and Pyrolysis can be disregarded as this is not, in general, the primary focus of the technologies. The technologies are developing and there are many variants and combinations which attempt to achieve different objectives. However, in general, the processes operate at significantly higher temperatures than fractional depolymerisation, and are therefore unsuitable for plastic to diesel conversion. On the other hand these competing processes or variants of them have had some success but there are fundamental and intrinsic difficulties. In most cases, the application of these processes attempt to deal with the treatment of a wide range of organic materials within the one system. The reasons for broadening the scope of the allowed inputs is that to operate commeWRially the input material must have little or no cost and must be widely available i.e. unprocessed mixed organic waste. There are operational difficulties involved in dealing with a diverse input stream, an increased likelihood of the inclusion of unwanted contaminants and finally a divergent range of output products. In summary, while the competing processes have enjoyed some success (waste to energy) they are reliant on an ability to handle a wide range of inputs which is difficult to achieve on a large commeWRial-scale.