(681f) Optimal Integrated Plant for Waste to Biodiesel Production

Current developed societies generate large volumes of waste. The origin can vary from sludge from water treatment, urban waste, forest residues and manure mostly. Apart from the volume, their composition is complex and dangerous. Anaerobic digestion presents a technological option to stabilize the waste while generating added value in the form of methane and digestate that can be used as fertilizer. For decades, the aim of processing the waste has focused on power production [1]. However, biogas is an interesting carbon source, not only because of its content in methane, but also due to the amount of CO2 available. Dry reforming can be used to process this biogas into methanol [2]. A step further allows the production of biodiesel from waste. The production of biodiesel from algae requires nutrients for algae growing [3], that can be provided by the digestate. Next, the methanol produced from the biogas is used for the transesterification of the oil.

In this work we have optimized an integrated facility for the production of fully renewable biodiesel from waste via anaerobic digestion. On the one hand, biogas is produced from cattle and pig manure. After its purification, it is dry reformed to obtain syngas. Methanol is later synthesized from the syngas. On the other hand, the digestate remaining is used as nutrient to grow algae. The effect of the nutrient composition on the algae growth is considered. The algae are harvested and the oil is extracted using mechanical press and a solvent, that it is later recovered. The oil is transesterified using the methanol produced from the biogas. The problem is formulated as a large NLP for the optimal operating conditions, biogas composition and digestate nutrient content. Dry reforming, methanol and biodiesel synthesis reactors are modelled to account for the effect of the operating conditions on the yield. Distillation columns are modelled using short cut methods but including constraints on the reflux ratio [2,4], the operating pressures and temperatures from the literature and the rigorous simulation of the column in CHEMCAD [4].Optimal HEN and water networks are designed afterwards.

3110kt/y of manure are processed for the production of almost 90Mgal/y of biodiesel and 46.7 kt/y of methanol. The process uses part of the biogas to fuel the reformer but still needs 1.5 MJ/gal of FAME and 1.22 gal of water per galon of biodiesel. The production cost of biodiesel is competitive with previous work [2], 0.31€/gal, but the investment is higher due to the need to produce the biogas, 193 M€.

References

1.-León E., Martín, M. (2016) Optimal production of power in a combined cycle from manure based biogas Energ. Conv. Manag. 114 89–99

2.-Hernández, B, Martín, M. Optimal composition of the biogas for dry reforming in the Production of Methanol. IECR. 10.1021/acs.iecr.6b01044

3. -Xin, L., Hong_ying, H., Ke, G., Ying-xue, S. (2010) Effects of different nitrogen and phosphorus concentrations on the growth, nutrient uptake, and lipid accumulation of a freshwater microalga Scenedesmus sp Bioresource Technology 101 5494–5500

4.-Martín, M., Grossmann, I.E. (2012) Simultaneous optimization and heat integration for biodiesel production from cooking oil and algae. Ind. Eng. Chem Res. 51 (23), 7998–8014