Simultaneous optimization of different strategies for co-production of sugar and ethanol
Systematic methods and tools for managing the complexity
Process Synthesis & Design - II (T4-1b)
Keywords: sugar, ethanol, co-production, optimization
Owing to the surplus of sugar on the World Market, the EU is reducing economic support for refined sugar by about a third. The aim is to reduce the mass of sugar produced, so that the excess is not exported to non-EU markets. Because of these new market conditions, it may be advantageous to assess alternative profitable usages for sugar beet which may assist in reducing environmental impact while improving the overall economy of sugar production. Due to reduced price support from the EU, it is obvious that only the most advanced sugar plants will survive these keen market conditions. Thus, a restructuring of sugar production by applying alternative production processes is inevitable for the sugar plants, in order to achieve sustainable development.
Efforts to improve production efficiency and economic viability in the sugar industry have traditionally focused on maximizing sugar yield per hectare of agricultural land and sugar produced per tone of sugar beet grown. Although some co-products of sugar production such as beet pulp and molasses are utilized in other industrial processes, priority is accorded to sugar production. The traditional focus on sugar has made the industry vulnerable to changing market prices and weather patterns, and prone to financial instability. There have been few attempts by sugar plants to consider all sugar beet resources as a bundle of potential products and services whose value could be maximized simultaneously.
Sugar plant that wishes to stay competitive must attain flexibility and diversity in adapting to changing markets. Co-production strategies present attractive options because key co-products of one process, such as molasses from sugar production, are available onsite for use as raw materials for the additional processes, for example bioethanol production. Sugar plants could be redesigned by annexing the distillery with the sugar plant, in order to gain a spatial advantage and flexibility in producing varied quantities of sugar, ethanol and electricity depending on prevailing market conditions.
The objective of our work was to economically evaluate investment needed for the co-production of sugar and ethanol. The aim was to optimize combined sugar–ethanol production and simultaneously improve the economic and environmental performances of the sugar and ethanol plant. A conceptual design for an integrated sugar and ethanol production was used. Both processes were simulated using computer simulators and results served as the basis for the optimization of the integrated sugar–ethanol process by the General Algebraic Modeling System (GAMS). Discrete variables were included into optimization problem, in order to find economically and environmentally optimal strategy for co-producing sugar and ethanol. Discrete variables were used to decide on the production in fixed or flexible quantities. Production in fixed quantity has reserved all of the economically extractable sugars for sugar production by using “C” molasses or “final” molasses for ethanol production. C molasses is not valuable for sugar production because sugar extraction has reached a point of diminishing returns. Such a strategy would be chosen when the market value of sugar was generally higher than that of ethanol in production-equivalent terms, and was expected to remain higher for the foreseeable future.
In the optimization problem, decisions with discrete variables were also made to enable sugar production with flexible quantities. Namely, sugar extraction can be halted after the first or second stages, resulting in “A” or “B” molasses, respectively. These molasses streams have fermentable sugars that can still be economically extracted. However, the presence of additional fermentable sugar increases the efficiency of ethanol conversion. Consequently, if ethanol is expected to have a market value close to or greater than that of sugar, then it makes economic sense to prioritize ethanol production over some sugar production, by using A or B molasses as the ethanol feedstock. In the case study, sensitivity analysis were used, in order to determine the impact of changing market conditions on the final optimization solution.
The study of a combined sugar–ethanol production showed that opportunities still exist for increased profitability in the sugar industry, while decreasing environmental impacts through a holistic approach. The optimization model used in our study is very valuable for the producers who are using (or will be using) combined sugar–ethanol production, since they have the flexibility to switch among different alternatives of molasses use when the market prices are fluctuating over time. Consequently the decision whether to emphasize sugar or ethanol production can be easily made.
Presented Tuesday 18, 15:40 to 16:00, in session Process Synthesis & Design - II (T4-1b).
