(282a) Optimal Integrated Plant for Biodegradable Polymers Production

The use of plastics and polymers has improved society´s lifestyle. Their properties have provided advantages in many fields from the automobile industry to the electronics that, together with a lower unit cost and improvements in their performance specifications, results in an increasing use of plastics. Over one third of the total consumption of plastics is employed in packing applications¹ making the recycling a need. However a secondary effect of employing large amounts of this material is the impact of microplastics in water bodies (e.g. oceans, seas) since it compromises the health and integrity of ecosystems ². For this reason, a substitution of the conventional polymers by biodegradable ones can provide an immediate positive impact.

In order to achieve this goal, an integrated process is conceptually designed for the production of high value products as biopolymers from renewable and waste sources as sawdust, manure and CO₂ , obtaining the intermediates required (e.g. methanol, glycerol, starch) for the synthesis of the biopolymer, a polyol (plasticising agent), as well as useful by-products such as biodiesel, promoting a circular economy. The process consists of six sections including biogas synthesis from manure wastes, syngas reforming and clean-up, methanol synthesis, algae growing to produce oil and starch, biodiesel synthesis and biopolymer production from the by-product glycerol, sawdust and the algae starch. Starting from organic wastes, the manure is digested anaerobically for the production of biogas and digestate. Last one is employed as source of nutrients for algae growth, where CO₂ , partially from the methanol synthesis section, is injected as carbon source. Once the algae is harvested, the oil and starch extraction is carried out. The biogas obtained from anaerobic digestion is reformed (dry and steam) to obtain syngas whose H₂ to CO ratio is adjusted towards the optimal methanol synthesis. Methanol is mixed with algae-oil in the proper ratio for the production of biodiesel and glycerol. In parallel, the lignocellulosic waste, sawdust, is dried and added to the liquefaction reactor together glycerol as solvent and sulfuric acid as catalyst for the production of the polyol. Once the polyol is purified by mechanical centrifugation, it is employed as plasticising agent within the polymeric matrix of starch to produce the biodegradable plastic. Then, the biopolymer is processed in a extruder to obtain pellets, used as plastic based pieces or to produce agriculture plastic films. The process is modelled based on mass and energy balances, including the thermodynamics equilibria involved in the biogas reforming and methanol synthesis together with surrogate models for the transesterification to biodiesel and polyol synthesis, formulating an NLP problem with a 5680 equations and 7870 variables solved in GAMS^®. Heat integration and cost estimation are performed next.

Optimal solution suggests full use of the sawdust in the production of biodegrable polymers. In this way, the facility process a 20 kt/yr of sawdust and 4723 kt/yr of waste (50% cattle manure and 50% pig manure). The plant produces 354 kt/yr of biodegradable polymer, 85 Mgal/yr of FAME and 35 kt/yr of methanol, a production size according to algae-based biodiesel and sawdust-based plastics production plants ^3,4. Once the heat integration is perfomed and the heat exchanger network is designed, 0.0035 kg of steam per kg of biopolymer produced are required where a 27% of the total biogas produced is employed to satisfy the energy requirements of the reformer. Regarding the economic assessment, a total investment cost of 1039 M$ are required with a biopolymer cost of 0.98 $/kg.

References

1. Andrady AL, Neal MA. Applications and societal benefits of plastics. Philos Trans R Soc B Biol Sci. 2009;364(1526):1977-1984.
2. Isobe A, Azuma T, Cordova MR, et al. A multilevel dataset of microplastic abundance in the world’s upper ocean and the Laurentian Great Lakes. Microplastics and Nanoplastics. 2021;1(1):1-14.
3. Martín M, Grossmann IE. Optimal engineered algae composition for the integrated simultaneous production of bioethanol and biodiesel. AIChE J. 2013;59(8):2872-2883.
4. Roldán-San Antonio JE, Martín-Hernández E, Briones R, Martín M. Process design and scale-up study for the production of polyol-based biopolymers from sawdust. Sustain Prod Consum. 2021;27:462-470. doi:10.1016/J.SPC.2021.01.015