G ASIFICATION AND S YNTHESIS
4 Flow Sheet Analysis
Previous studies[6,8,89] have demonstrated the importance of process optimisation for second generation fuel production through flow sheet analysis, a technique primarily based on process simulations in Aspen Plus®[90] to which economic analysis are coupled. To arrive at the most economically viable process design for stand-alone FT fuels production for example, Tijmensen et al[6] developed various flow sheets, by combining alternative gasification designs with alternative FT reactor configurations. It was shown that pressurised gasification combined with advanced FT reactor technology was most viable, while atmospheric-air gasification combined with conventional FT reactor configurations are least viable, since conventional FT reactor technology and atmospheric-air gasification had excess capital and operational costs. A similar analysis for biomethanol production[55] had shown that regardless of the method of gasification and reactor technology, recycle streams are essential to improve the yield of liquid product, so the specific production costs can be reduced.
In the study [59] that determined the most viable option for integrating FT fuels production with a pulp mill, a scenario where a portion of the synthesis was diverted for heat and power generation against a scenario where all syngas went to the FT reactor was compared. It was shown that the scenario where syngas was diverted offered a slight advantage in investment potential since it exported more electricity, which was favoured in that particular economic context where the price
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of electricity relative to the price of oil was 2.0 $.MJ-1 of electricity per $.MJ-1 of oil. In a South
African context, this ratio of the electricity price to the oil price is 1.6 $.MJ-1 (based on prices from
Eskom[91] and Department of Energy[92]), which shows that the same result might not be obtainable.
Flow-sheet analysis has also been used to demonstrate the effects of expected technological improvements on the overall performance of a biofuel process. In a study pertaining to bioethanol production by integrating the second generation bioethanol route into autonomous ethanol distilleries, Dias et al[17] developed processes based on the states of technology for hydrolysis and fermentation that are to be expected with 2010 and 2015 time frames. It was shown that yeasts capable of efficiently fermenting pentose sugars was the most significant factor in assuring low production costs. Those authors also showed a similar outcome in a separate study[82], although increasing process complexity by alkaline delignification in order to improve hydrolysis yields had resulted in unnecessary costs. Furthermore, those authors in a separate study[93] also showed that vacuum induced “multi-effect” distillation is advantageous over the conventional distillation in terms of utility consumption, especially when considering power generation techniques such as BIGCC, that have high electricity generating efficiencies. In another study pertaining to integrating second generation bioethanol processes into ethanol distilleries, Macrelli et al[81] demonstrated that heat integration to lower the process energy demands and integrating process streams for identical unit operations are important steps to lowering production costs. For stand-alone second generation ethanol production, quantifying the technological progress of the various steps in the bioethanol processes on the overall process performances were demonstrated by flow sheet analysis[94,95].
Flow sheet analysis has also been used to demonstrate the effect of Pinch Point Analysis (PPA) on the utility usage of biofuel processes, and hence, the overall efficiency. Dias et al[96]. had shown
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that the application of PPA on first generation ethanol production had reduced the steam consumption by about 32%, while Petersen et al[4] had shown that the application of PPA on second generation biofuels improves the efficiency by about 5 percentage points. PPA is an algorithm that integrates the thermal energy of streams and units within a process in order to reduce the reliance on heating and cooling utilities[97,98]. For both the endothermic and exothermic energies, the magnitude and temperatures are qualified as respective cold and hot profiles called “composite curves” on a Cartesian plane, with the heat magnitude on the X-axis and temperature on the Y-axis. A heat magnitude is then calculated to shift the cold composite curve, so that the closest distance between the profiles (pinch region) corresponds to the minimum approach temperature. This magnitude then represents the cooling requirement, while the portion of the cold composite that is not overlapped by the hot composite curve above the pinch region is the heating requirement.
Thus, it is seen that flow sheet analysis had been used to assess the impact of process scenarios from various perspectives. In some studies, optimised flow sheet configurations have been found through assessing the outcomes of the various combinations of process unit alternatives in the various process stages[6,13,55,82]. Others have sought to determine the effect of the improvements that are to expected for individual process units on the entire process, thus providing a quantified motivation as to why research efforts should be focused on those improvements[17,94,95,99]. Furthermore, others have sought to determine the benefits that process intensification measures offers, such as heat integration and process stream combination[81].