Techno-economic analysis of textile recycling integration with a pulp mill

Abstract (Swedish)

Under de senaste åren har efterfrågan på textilier ökat och andelen textilavfall som återvinns är i dagsläget liten vilket har lett till en ökad miljöpåverkan. Det finns därmed ett behov av nya återvinningsprocesser för textilier, där en möjlighet är att de-polymerisera cellulosa-baserade textilier till glukos genom en alkalisk förbehandling, följt av enzymatisk hydrolys. Att integrera textilåtervinningen med ett massabruk kan ge positiva effekter genom att överbliven energi från massabruket kan användas för att täcka det stora energibehovet i förbehandlingen.

Syftet med denna studie var att genomföra en tekno-ekonomisk undersökning över integrering av textilåtervinning med ett massabruk, baserat på processimulering i Aspen Plus® och känslighetsanalyser i Microsoft Excel®. Det totala utbytet i simuleringen var 84,7%, vilket var högre än utbytet på 60,3% rapporterat i studier för validering. Det högre utbytet kan förklaras av den stora recirkulationsströmmen som introducerades i simuleringsmodellen, som dessutom potentiellt kan bidra till att minska kostnader för råmaterial, energi och processutrustning.

Den slutgiltiga processen skalades till en kapacitet att behandla cirka 17 000 ton textilier per år, baserat på ett tillgängligt överskott av elektricitet på 30 GWh/år från integrering med Södras massabruk i Mörrum, södra Sverige.

Den största bidragande faktorn till den totala kostnaden är råvaror, varav textilierna står för 70% av råvarukostnaden, följt av citratbuffert och enzymer. För att täcka kostnaderna för råvaror skulle försäljningspriset för glukos behöva vara minst 4 gånger högre än priset rapporterat i litteratur. En delstudie genomfördes där massflödet av citratbuffert minskades genom en ökad cellulosakoncentration i reaktorn för enzymatiska hydrolys, samt att priset för textilier och enzym antogs vara lägre, vilket validerades genom priser funna i litteratur. Med dessa förändringar hittades ett fall som kan resultera i en lönsam process baserat på det aktuella referenspriset för glukos.

Abstract

Over the past years there has been an increased environmental impact due to an increased textile demand. Only a small fraction of waste textiles is recycled today which indicates a requirement for new textile-recycling processes, where one possibility would be to depolymerize cellulose-based textiles into glucose, with an alkaline pretreatment followed by enzymatic hydrolysis. Due to a high energy requirement in the pretreatment step, it could be favourable to integrate this process with a kraft-pulp mill, taking advantage of excess energy and alkaline solution from the recovery boiler.

The purpose of this thesis was to investigate the techno-economics of textile recycling integration with a kraft pulp mill, based on process simulation in Aspen Plus® and sensitivity analysis in Microsoft Excel®. A simulation model was implemented, scaled, and validated by literature data and used to investigate the process mass flows, energy flows and reaction yields. Literature reported an overall glucose yield of 60.3%, while a higher overall yield of 84.7% was obtained in the simulation, thanks to the implementation of a large recirculation stream in the pretreatment section. The recirculation stream was also found to have a potential to lower the costs of raw materials, utilities, and process equipment.

The energy requirement for heating in the pretreatment section was considered as the limiting factor in the scalability calculations. The textile recycling process was scaled to a capacity of treating almost 17,000 tons of textiles per year, based on integration with Södra’s pulp mill in Mörrum, Sweden, producing an available excess electricity generation of 30 GWh per year.

The largest contributor to the total cost was the raw materials, where the textiles accounted for 70% of the raw material cost, followed by citrate buffer and enzyme. The selling price of glucose would have to be around 4 times higher than the market price to compensate for the high raw-material costs in the initial case study. In a further development of the model, the cellulose concentration in the enzymatic hydrolysis step was increased, yielding a lower mass flow of citrate buffer, and the purchase price of textiles and enzymes was lowered, based on the large variation in price estimations found in the literature. After implementing these changes, a case was found where the process could be feasible based on the benchmark price of glucose.

Popular Abstract

Could a process for textile recycling benefit from integration with a pulp mill?

Just like the human body, a textile recycling process is dependent on a constant energy flow to function. Some of this energy demand could be covered through integration with a pulp mill. The following study investigates the techno-economics for such an integration, where recirculation streams and raw material cost were found central for the process feasibility.

Before digging into the economics, a fundamental question needs to be answered. Is textile recycling a problem that needs to be discussed? Yes, it is! The world is facing today some big environmental challenges, to which the textile industry is a contributor. As an example, the fashion industry is estimated to stand for 10% of the global carbon emissions. In addition, there is an increased demand of textiles, while only 20% of all waste textiles are recycled or reused today. Can´t we do better? One way to increase recycling of textiles is to implement new methods, where one solution is to chemically break down the building blocks of textiles into smaller units, which can be used to manufacture new products!

In this master thesis, a process for textile recycling was studied, in collaboration with ShareTex, a Swedish company. A simulation model for the process was successfully implemented in the software Aspen Plus®, based on available energy information from a pulp mill. The textile recycling process consisted of two main parts, a pretreatment with an alkaline solution and an enzymatic hydrolysis. The pretreatment can be seen as the step that makes the textile fibers more accessible, while the enzymatic hydrolysis can be seen as the biochemical scissor that makes the final cutting once the material has been exposed. Process simulation can be seen as a tool to get a better understanding for the process behavior and to catch a glimpse of the implementation at commercial scale. For example, a recirculation stream was explored in the simulation, which identified benefits that could not be seen in the laboratory experiments.

So, back to the economics, could this process be feasible? As a true businesswoman would say, it depends! The report enlightens several cases, where the main finding can be summarized as the need to lower the raw material costs. Specially, the cost of waste textiles was found to be very large, partly due to a high uncertainty in the purchase price. However, a case was found where it could be feasible to integrate textile recycling with a pulp mill.

The report sheds light on which process parameters to focus on to further optimize the textile recycling process. In addition, it can help to enlighten us on how to solve the environmental challenges the world is facing, as this work could be the first stone in the path to further implement textile recycling processes.

• author: Andersson, Gabriella
• supervisor: Ola Wallberg; Miguel Sanchis-Sebastiá
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KETM05 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Chemical Engineering
• language: English
• id: 9089688
• date added to LUP: 2022-06-20 10:14:27
• date last changed: 2022-06-20 10:14:27