(593d) Extraction High-Value Chemicals from Ethanol Co-Products: A Feasibility Assay on Phytate Extraction with Life Cycle and Techno Economic Assessment

Authors: 
Reis, C., University of Minnesota
Rajendran, A., University of Minnesota
Tiffany, D., University of Minnesota
Hu, B., University of Minnesota
Background

 The well-established dry-grind process today accounts for also over 80% of the operating ethanol facilities in the United States, producing not only ethanol, but also distiller’s grains. Distiller’s grains, commonly commercialized as dried distiller’s grains with solubles (DDGS) accounts for most of the heavy fraction from the ethanol distillation and rectification operations, and is processed through a complex downstream processing after the fermentation is unloaded. Despite being a nutritious animal feed, inclusion of DDGS as feed ration is still limited due to compositional properties, such as the high presence of non-starch polysaccharides, which are non-digestible to monogastric animals, the fatty acid profile, and the overdose of phosphorus, which is largely non-available to some animals. Phosphorus behavior in ethanol downstream processing is of particular interest, not only because it can lead to environmental issues, but also because it can potentially be an additional economic source to ethanol producers. Over 40% of the phosphorus present in ethanol co-products is present as phytate, the main form of organic phosphorus in cereals and grains, which is not completely hydrolyzed during the whole ethanol process. Phytate has a high market value, and today is a platform chemical utilized in a wide array of industries, and it is mostly exploited because of its high chelating potential (He et al. 2017). Phytate is also the raw material to produce myo-inositol, a cyclic polyol with a variety of uses by pharmaceutical and nutraceutical companies.

Phytate is found to be dissolved on the ethanol co-products(He et al. 2017). During the dry-grind downstream processing, thin stillage, i.e., the liquid-rich fraction derived from the centrifugation of whole stillage, accounts for the highest concentration of phytate among all the co-products. Recent research (He et al. 2017) has proposed an extraction method using ion exchange separation, which can efficiently separate phytate from thin stillage, with little to no effect on the residual nutritional profile of the final DDGS. Even though the concentration of phytate being relatively low on ethanol co-products, at around 1 g L-1 in thin stillage, due to the enormous volumetric production by dry-grind plants, the production leveraging costs in extracting phytate might become an industrial alternative.

The high volumes of DDGS produced by ethanol plants, alongside with its limited dietary inclusion, pose a challenge for higher revenues to ethanol plants. The second-most important co-product from dry-grind ethanol facilities, distiller’s corn oil, represent today a mature product suitable for biodiesel production and other uses of low-grade vegetable oil(Reis et al. 2017). Removal of distiller’s corn oil, on the other hand, has been criticized by animal nutritionists claiming a loss in energy value from DDGS. Despite still representing about 20% of the total revenue stream from dry-grind ethanol plants, DDGS are still ranking in low positions when compared to higher end animal feeds, such as soybean meal. The extraction of high-value chemicals through mature sound technologies may represent, thus, an alternative for higher economic security of ethanol plants. This current research describes a techno-economic analysis and a life-cycle assessment of a phytate extraction process on a large-scale dry-grind corn-to-ethanol plant in the United States, and its posterior in-loco conversion to myo-inositol.

Methods

The scope of this work analyzed the utilization of corn-to-ethanol thin stillage produced through the dry-grind process as raw material to extract phytate using an ion exchange process, followed by hydrolysis to generate myo-inositol. The process was modelled according to He et al. (2017) , and by additional empirical analysis, when required. Thin stillage was obtained from a dry-grind ethanol plant in the state of Iowa, USA. The process of converting thin stillage to a phytate brine solution is divided into the major steps: (1) filtration, (2) flow through ion exchange column, (3) desorption of phytate using brine solution, (4) reconditioning of column using sodium hydroxide, and (5) column washing using water. The conversion of phytate brine solution to calcium phytate is done by (6) addition of calcium chloride and sodium hydroxide, followed by product filtration, and production of myo-inositol is achieved by (7) hydrolysis of phytate brine solution, (8) precipitation of phosphate with calcium chloride and sodium hydroxide, and (9) evaporation and drying of myo-inositol.

The life cycle assessment of calcium phytate extracted from corn-to-ethanol thin stillage was estimated using GREET model. Calcium phytate extraction, and posterior conversion to myo-inositol were implemented on two stationary models on GREET – Dry mill ethanol production w/ corn oil extraction, and Dry mill ethanol production w/o corn oil extraction. Techno-economic assessment for ethanol plants was considered using the BuGal model, with added market and technical data for the modified phytate extraction and conversion units. Since the basis of the BuGal model for techno-economic analysis has been modified to analyze both conditions (with and without distiller’s corn oil extraction), the two stationary models on GREET to describe corn-to-ethanol process were utilized.

Results and discussion

LCA results indicate that the proposed phytate extraction does not impact significantly most of the environmental impact factors of ethanol. As a gallon of ethanol was considered as functional unit, the results indicate that for a system with phytate extraction and oil extraction, CO2 emissions will increase from 3.21 kg to 3.28 kg per gallon of ethanol, whilst for a system without distiller’s corn oil extraction, 3.23 kg of CO2 will be increased to 3.30 kg of CO2 per gallon of ethanol produced. NOx, SOx, N2O, and other gases also increased with phytate extraction by a factor of about 1%. Analyses were carried out for techno economic assessment in 4 different cases, (1) with distiller’s corn oil extraction and phytate extraction, (2) with distiller’s corn oil extraction and without phytate extraction, (3) without oil extraction and without phytate extraction extraction, (4) without oil extraction and with phytate extraction, and were consistently all across different cases.

Unlike major recent upgrades to the dry-grind ethanol industry, such as distiller’s corn oil removal, extraction of phytate and production of myo-inositol are related to products found in dilute concentrations. The low concentration of phytate in ethanol co-products requires low energy input on an ethanol basis, while still providing a high-value co-product. BuGal model calculates the operating expenses per gallon of denatured ethanol sold as USD 0.2673 for the control conditions, i.e., without phytate extraction, and USD 0.2686 for a system considering 20% of thin stillage being redirected to phytate extraction, due to increase in energy consumption for additional processing. Regarding chemical costs, phytate extraction and conversion to inositol likely increased the value from USD 0.1201 to USD 0.1226 per gallon of denatured ethanol, mainly due to the requirement of NaCl for phytate desorption from the ion exchange process, additional process water, and calcium chloride addition. These costs are balanced by the calcium phytate, myo-inositol, and calcium phosphate as fertilizer (derived from the phytate hydrolysis) sales, at prices estimated at USD 12, USD 10, and USD 0.20 per kg, respectively. BuGal estimates an average large-scale dry-grind operation, with factor of equity and debt being 0.7 and 0.3, respectively, with an interest rate charged on debt as 0.07, and the rate of return required by investors on the plant’s equity as 0.12. Even though these numbers are variable, the economic considerations on such assumptions are beyond the scope of this work, and remain constant through all assumptions. Following similar numbers as Kwiatkowski et al. (2006), 2.795 gallons of ethanol are estimated to be produced from each bushel of corn, and 17.5 lb of DDGS, and CO2 are produced on a bushel basis. Denatured ethanol sales are estimated to be at USD 1.70 per gallon, an estimate within current market prices , and that phytate extraction would not affect the price of DDGS, thus, being marketed at a price of USD 161 per ton, also within current market estimates.

Conclusion

The removal of phytate from thin stillage may represent a novel process in the dry-grind ethanol industry. The removal of phytate via ion exchange does not increase significantly (around 1% of increase) the emissions involved on the production of one gallon of ethanol, and that it can be a source of profit if operated within a certain range of thin stillage processed within a plant. The results also presented herein describe the potential of converting the extracted phytate to myo-inositol. Myo-inositol has a more flexible market when compared to phytate, and could also represent another source of revenue for ethanol plants.

References

Kwiatkowski JR, McAloon AJ, Taylor F, Johnston DB (2006) Modeling the process and costs of fuel ethanol production by the corn dry-grind process Industrial crops and products 23:288-296

He Q, Rodrigues Reis, CE, Wang F, Hu B (2017) Phytate extraction from coproducts of the dry-grind corn ethanol process. RSC Advances. doi:10.1039/C6RA27409A

Reis CER, Rajendran A, Hu B (2017) New technologies in value addition to the thin stillage from corn-to-ethanol process Reviews in Environmental Science and Bio/Technology 16:175-206 doi:10.1007/s11157-017-9421-6

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