Sustainability analysis of soybean refinery: soybean oil extraction process
Soybeans are one of the main sources of oil crops around the world. Soybean oil is the most common product of soybean refinery. It is a resource of edible oil and has other food and industrial applications.
Techno-economic analysis (TEA) is applied to estimate the economic feasibility of the soybean oil extraction process. The mechanical extruding-expelling process, hexane extraction, and enzyme assisted aqueous extraction process (EAEP) are analyzed. Total capital investment, operating costs, and revenues are the three basic indices for evaluating gross profit and net profit; which are general indicators of economic feasibility of a manufacturing venture. Additionally, cash flow analysis and sensitivity analysis are used to evaluate profitability while considering the net present value (NPV) and the driving force of manufacturing individually.
According to the analysis, the extruding-expelling process is a profitable and product-leading process as the scale of oil production is over 12 million kg of annual soybean production. In addition to soybean oil, soybean meal provides over 70% of total revenues, due to its proper nutrient content for livestock feed applications. Hexane extraction is also a profitable process when the scale is over 173 million kg of annual soybean oil production. Before EAEP can start to earn profits, the capacity must be scaled up to over 40 million kg of annual soybean oil production. Additionally, the enzyme recycling and moderate strategy of co-product handling are required. The co-product handling includes selling aqueous fraction and insoluble fibers derived from EAEP, as these materials can be used for further corn-soybean based bioethanol production.
Besides, environmental impact analysis is used to evaluate the potential environmental impact and greenhouse gas (GHG) emissions resulted from these three oil extraction processes. The environmental impacts are evaluated based on mass balance of the processing; and is used to calculate environmental indices. The potential GHG emissions are estimated according to the energy consumption of the process. Results show, the EAEP and extruding-expelling have similar general environmental impacts, while hexane extraction has the highest environmental impact because the organic solvent, hexane is used as medium to extract oil. GHG emission results show, the extruding-expelling process has the highest GHG emissions due to its lower oil recovery and high-energy requirements needed to squeeze oil out from soybeans. By contrast, hexane extraction has the lowest GHG emissions because of its high oil recovery. Though the pretreatment of EAEP requires high-energy consumption, higher oil recovery than extruding-expelling process results in lower GHG emissions than the mechanical process.
In addition to the oil extraction process, co-product, distiller’s dried grain with solubles (DDGS), from the corn-soybean integrated bioethanol production is another main revenue source to increase profits for the whole refinery system. The combination of sieving and aspiration is used to fractionate DDGS based on the physical properties of nutrients, especially the density profile. Particle size of DDGS and the flowrate used in aspiration are the main variables for fractionation. The proper combination and interaction of variables for protein and oil separation are higher flowrate, smaller particle sizes, and heavy fraction. The best efficiency for protein and oil separation reaches about 29.7% and 68.15% respectively. For fiber separation, a mild condition results in higher fiber content approximately 7%.