Browsing by Author "Dogbe, Eunice Sefakor"

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    Exergy, techno-economic and exergoeconomic analyses for improving energy efficiency of a typical sugar mill and designing integrated biorefineries
    (Stellenbosch : Stellenbosch University., 2020-03) Dogbe, Eunice Sefakor; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The sugar industry is energy-intensive, consuming about 350–600 kg of steam and 25–32 kWh electricity per ton of sugarcane processed into raw crystalline sugar. Though mostly energy-self-sufficient, improving its energy efficiency is necessary to produce sugar more cost-effectively. Besides, the decreasing trend and fluctuations in the world sugar prices necessitate product diversity to ensure the economic sustainability of the industry. Therefore, improving the current energy status of the sugar industry will make sugarcane resources available for further valorisation. This study aimed to improve the energy efficiency of a typical South African sugar mill towards its economic sustainability and competitiveness. The first objective (objective 1) towards achieving this aim was to identify the locations, magnitudes, and causes of inefficiencies in a typical South African sugar mill through exergy analysis. This analysis was based on rigorous mass and energy balances calculated from an Aspen Plus® simulation of a typical 250 ton per hour sugar mill. The cogeneration system had the highest exergy destruction (90 582 kW) representing 86% of the total sugar mill irreversibility. However, with the lowest exergetic efficiency of 9.6%, the crystallization unit recorded the most inefficient use of energy due to the process complexity. Following the exergy results, objective 2 was to select energy-efficient technologies to improve the sugar mill efficiency while objective 3 involved assessing the economic feasibility of integrating them into the mill. Organic Rankine cycle (ORC) and absorption heat pump (AHP) technologies were selected, which improved the cogeneration exergetic efficiency by 1.7% and minimized overall system irreversibility by 0.14%, saving 0.83% on total bagasse for valorisation, respectively. Though only marginal improvements were achieved, both ORC and AHP integrations were economically feasible and could be optimized to achieve better energy improvements. Furthermore, promising biorefinery products; succinic acid (SA) and short-chain fructooligosaccharides (scFOS) were integrated for the economic competitiveness of the industry in objective 4. Based on the exergy results, the biorefineries were developed to utilise A-molasses for the production of SA and scFOS in seven different scenarios, which were allhighly profitable with internal rates of return (IRRs) between 24 and 62% compared to theminimum required IRR of 9.7%, due to the integration benefits. Moreover, co-utilization of C-molasses and lignocellulose residue as first- and second-generation (1G-2G) feedstocks for the production of SA was also considered to fully valorise the sugarcane plant considering the current crystallization scheme. Objective 5 applied an aggregated system exergoeconomic methodology to assess the holistic performance of the biorefineries and to identify the most cost-effective one. With the lowest cost rate of 1 029 US$/h and exergoeconomic factor of 0.56, the scFOS powder scenario (S-FP) showed a good balance between the irreversibility- and investment-related costs and was considered the most cost-effective biorefinery for integration into the sugar mill. Overall, this study presented a broad spectrum of solutions to the energy and economic challenges of the sugar industry to be explored further for implementation, using exergy/exergoeconomic methodology as better design tools than conventional energy and economic analysis.