The slope method: a tool for analyzing semi-continuous data.
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abstract
The MixAlco process is a biorefinery that converts lignocellulose into useful chemicals and hydrocarbon fuels via mixed-acid fermentation. For a semi-continuous-staged fermentation train, during each transfer, discrete amounts of material are moved between fermentors and data are tabulated. Because of natural day-to-day variations, the data are inherently noisy. To calculate performance parameters (e.g., yield, conversion, selectivity, productivity), the average flowrate of each stream component must be determined. To minimize error associated with noise, three data analysis methods were compared: Average, Accumulation, and Slope. The Average method determines the flowrate by averaging the amounts moved each transfer. The Accumulation method stores the solids and liquids that exit the fermentation train in separate vessels. After an extended time period, the mass in each storage vessel is measured so the average flowrate can be calculated. The Slope method calculates the flowrate of material in each stream from the slope of the moving cumulative sum with respect to time. For all three methods, the measured rates were virtually identical; thus accuracy was not affected by the method. However, for the examples presented, the Average method had >40% error and the Slope method <4% error; thus, precision was significantly affected by the method. The Accumulation method calculated the flowrate with a single data point so it is not possible to determine the statistical error.
