Exergetic and Economic Assessment of Distillation Hybrid Configurations for Bioethanol Refining

Abstract

Thermo-economics analysis was used to identify the most economic distillation hybrid configuration to dehydrate bioethanol mash (12 wt%) to fuel grade (99.5 wt%) based on economic objective of minimization of operating cost in this work. Three different hybrids of THIDC with azeotropic and, extractive distillation units were assessed using similar feed and product specifications of 1200 kmol/h (12 wt% ethanol) and 55 kmol/h (99.5 wt% ethanol) respectively . The six hybrid configurations were simulated using Aspen Plus ®. The hybrid of THIDC with conventional extractive distillation (THEX1) was shown to have the lowest irreversibility rate (lost work) and highest exergetic efficiency followed by the hybrid containing thermally extractive sequence (THEX3). The latter also has the lowest energy consumption. However, economic evaluation showed that thermally coupled extractive distillation hybrid (with THIDC) is the most attractive hybrid configuration dehydrating bioethanol to fuel grade at commercial scale with the highest return on investment (ROI) and the least annual product cost. This indicates its economic attractiveness when compared with the other hybrids considered in this work. The trade-off existing between economic and exergy efficiency favors the selection of THEX3 as the preferred choice for bioethanol refining among all the six hybrids investigated.

Thermo-economic analysis of proton exchange membrane fuel cell fuelled with methanol and methane

“…The result shows that with exception of few streams (6, 12, 14, 24, 26, 27 and 28) all the streams energy flow and enthalpy were negative. The negativity of the stream enthalpies has to do with the reference condition at which the various streams properties were evaluated [65,36]. However, it was generally observed from the result that in both configurations streams containing air and compressed air were the only streams whose enthalpies are positive values.…”

“…Similarly, the methane fuelled system results in Table 7 showed that stream 9 has the highest total exergy of 948.72 kW. This is a steam reformer effluent stream and its higher exergy can be attributed to its very high flow rate (7.76926 mol/s) in addition to high standard chemical exergy of the stream components present [36]. This stream has the greatest potential to perform work and was used in the Pre-oxidation reactor where high energy in form of work is required for the reaction.…”

“…This is largely due to the fact that pressure and temperature of such streams are lower than the reference pressure and temperature respectively. The negative physical exergy in the streams exergy resulted from the incoming air being below the reference conditions of 25°C and 1 atmosphere [65,36]. Reference conditions (Temperature and pressure) are known to play a vital role on the numerical values of exergy analysis obtained [67].…”

“…Higher chemical exergy implies higher chemical composition of components with high standard exergy and higher streams flow rates. Negative physical exergy were obtained for stream 10,11,26,38 in Table 6 and stream 1, 5 and 25 in Table 7. This is largely due to the fact that pressure and temperature of such streams are lower than the reference pressure and temperature respectively.…”

“…The exergo-economics based analysis of thermodynamic processes considers the cost of exergy only not the entire cost of its elements (utility, equipment cost, labour cost, raw materials) for detailed economic analysis. The later approach has been reported to be used in the thermo-economic analysis of some thermodynamic systems which gives more realistic measure of performance such as in oil shale retorting processes with gas or solid heat carrier [34], lowgrade waste heat recovery in Yazd combined-cycle power plant by a CO 2 trans critical Rankine cycle [35], distillation based hybrid configurations for bioethanol refining [36], Organic Rankine cycle for exhaust waste heat recovery of a diesel engine [37], pressure swing adsorption process for bioethanol refining [38], combined supercritical CO 2 (carbon dioxide) recompression Brayton/organic Rankine cycle [39], ORCs (organic Rankine cycles) for low temperature waste heat recovery [40], milk spray dryer exhaust to inlet air heat recovery [41], Dual-purpose Power and Desalination Plants [42] and air energy storage (CAES) system integrated with a wind power plant in the framework of the IPEX market [43]. In addition, significant progress has been made in the design of hybrid system to convert the waste heat in PEMFC to electricity [44,45].…”