Hydrothermal carbonisation of Cotton and Polyester Clothing Waste for Hydrochar and Terephthalic acid Production

In the UK around 93,000 tonnes of clothing waste, which typically contain 55% cotton and 23% polyester, are sent to landfill every year. Under the European Green Deal, polyester terephthalate (PET) manufacturers are under pressure to increase recycling rates to 30% by 2030. Therefore, this study focusses on using hydrothermal carbonisation (HTC) to produce terephthalic acid from both coloured PET and clothing waste containing dyes. The purity of terephthalic acid is important for recycling and typically needs to be over 99% without measurable colour for manufacturing recycled plastic. Therefore, the specific aim for this study is to purify terephthalic acid from coloured PET bottles and polyester-containing clothing waste and, for the latter, to produce a hydrochar co-product that can be used as a solid hydrochar.

Prior to studying HTC of cloth containing cotton and polyester, it is important to understand the behaviours of pure cotton cloth. There are several previous studies on the HTC of cellulose, but none on cotton cloth. It was established that the main difference between cellulose and cotton cloth in HTC is the temperature at which aromatic carbon is produced in the hydrochar. These are 200 C and 240 C for cellulose and cotton cloth respectively. This is because of the flame retardants chemicals added during the cotton cloth manufacturing process. The impact of recycling the aqueous liquor was also investigated and it was found that this was beneficial for increasing the hydrochar yield for cotton cloth, implying that the water-soluble organics contribute to hydrochar formation through combination reactions. However, the increase in hydrochar yield was accompanied by a decrease in calorific value due to the oxygen content increasing.

Although the hydrolysis of PET has been widely investigated, there are no previous reports on how dye can be removed effectively to produce colourless high purity terephthalic acid from coloured PET bottles. The study of hydrolysis of polyester cloth showed that unreacted polyester increased with the Run of recycling aqueous liquor, due to the reactant (water) reduction. The yield of recovered terephthalic acid also increased with recycling the aqueous liquor, due to less dissolving which could be related to the water-soluble organics and ethylene glycol presence. For the coloured PET bottles, hydrolysis took place with pure deionized water. After that, the filtered solid product was mixed with sodium hydroxide solution to produce disodium terephthalate solution. The activated carbon adsorption was successfully applied to treat the disodium terephthalate solution. The removal of dyes produced a whiter and brighter sample of terephthalic acid compared to a commercial standard sample and reached over 99 % purity.

For the hydrothermal carbonisation of mixed cloth, the hydrochar yield increased on recycling the aqueous liquor containing any unprecipitated terephthalic acid and ethylene glycol. It is likely that the acid served as a catalyst to increase the hydrochar yield. This also reduced the oxygen content of the hydrochar leading to a higher calorific value. The study about producing high purity terephthalic acid from mixed cloth, needed one extra step to the process of removing dye from coloured polyethylene terephthalate bottles. The filtered HTC solid product needed heating without oxygen before being mixed with sodium hydroxide solution. After that, the other impurities, notably dyes, could be removed from the disodium terephthalate solution by activated carbon absorption as for the coloured PET bottles

The strategic vision is to develop a world-leading Centre for Industrial Doctoral Training focussed on delivering research leaders and next generation innovators with broad economic, societal and contextual awareness, having strong technical skills and the capability of operating in multi-disciplinary teams covering a range of knowledge transfer, deployment and policy roles.
The immediate beneficiaries of our activities will be the students we train and their sponsoring companies. These students are expected to progress to research/development careers in industry or academia and be future leaders. They will be able to contribute to stimulating UK-based industry into developing the next generation of technologies to reduce CO2 emissions from burning fossil fuels and ultimately improve the UK's position in the global economy through increased jobs and exports.

Other beneficiaries include the industrial and academic partners of the CDT, the broader scientific and industrial carbon capture and storage and cleaner fossil energy communities, skills base and society in general. The key application areas addressed by the CDT will impact on the major technical challenges in the sector over the next 10-20 years as identified by our industrial partners:
(i) Implementing new, more flexible and efficient fossil fuel power plant to meet peak demand as recognised by electricity market reform incentives in the Energy Bill.
(ii) Deployment of CCS at commercial scale for near zero emission power plant and development of cost reduction technologies
(iii) Maximising the potential of unconventional gas, including shale gas and underground coal gasification.
(iv) Development of technologies for vastly reduced CO2 emissions in other industrial sectors: iron and steel making, cement, refineries, domestic fuels and small scale diesel power generators.
These areas also cover biomass firing in conventional plant defined in the Bioenergy Priority Area where specific issues concern erosion, corrosion, slagging, fouling and the overall supply chain economics.

Technically, the students we train will graduate with specialised knowledge in CCS and cleaner fossil energy. This will be underpinned by a broad technical knowledge of the sector and a wider appreciation of the role carbon capture and storage and cleaner fossil energy can play in the UK and internationally. We will also support development of their professional skills including developing their creative thinking skills providing them with a solid foundation to rapidly progress to become the future leaders of innovation and growth in UK industry and academia.

In the short-term, the trained reseachers will apply their knowledge and skills to underpin applications-led activities at the partnering industrial organisations and participate in further academic-industry collaborations. In the longer term, they will progress to lead in the integration of dramatically enhanced carbon capture and storage and cleaner fossil energy technologies that will be of direct benefit across the UK fossil fuel industry and supply chain, leading directly to wealth creation with job protection and growth.
A company sponsoring a student will help define the research they undertake and will be of direct interest to the company. Further, the company will have had long term access to a potential employee. Timely application of the technologies developed will enable and accelerate the development and adoption of CCS and cleaner fossil energy knowledge bringing environmental benefits to the UK and internationally.

The publicity generated by the project will raise public awareness of the role of CCS and cleaner fossil energy igenerally in society. Ultimately the broader benefits to society will include improvements to the quality of life derived from the improved efficiency, flexibility and reliability of the technologies.