CATALYST FOR SUCCESS
With this preliminary work out of the way, the actual DOE
was relatively simple. Stites turned to Design-Expert software
(Stat-Ease, Inc.), which walks users through the process of
designing and running an experiment and evaluating the results.
The factors, runs and results for the DOE are shown in Table
1. The %<DP9 in the sample at the start of the experiment
is 25.00%. The response %<DP9 shows how much this value
increased during the experiment. The results show that pH is the
most important factor and that the other factors all had minimal effects, although incubation at 60 C made things slightly
worse by speeding up the reaction. The best results were provided by pH adjustment to 2.0 and storage at 3 C with the DP
increasing to only 25.04% after 48 hours. This is less than the
standard deviation of the HPLC method and this method can be
easily performed in the field.
Next, Stites addressed the question of whether or not these
results are significant or could have been achieved by chance.
Factorial design analysis uses the half-normal plot to identify significant effects. The orange and blue rectangles on the
half-normal plot in Figure 2 show the effects, positive and negative respectively, and the position of these rectangles reflect the
relative size. The further the factor effects are from the line near
zero, the more likely they are to be significant. In this case, the
factor effects of variable A, which is pH, are much greater than
the variation between the insignificant effects, demonstrating
the statistical significance of the experiment.
The method of run 6 was selected and worked very well from
the beginning for samples from the slurry and liquefaction steps.
Some strange results were seen with fermentation samples—most
of the carbohydrates were gone. It was discovered that the sulfuric
acid was interfering with the HPLC measurements. The method
Figure 1: HPLC results for DP measurements of maltodextrin Figure 2: Half-normal plot demonstrates that DOE results are
Table 1: DOE used to explore sample preservation methods
was modified to remove the sulfuric acid before analysis by treat-
ment with barium hydroxide and filtering. Since this modification
was made, the method has worked with samples from all three
Hundreds of samples were shipped from ethanol plants
around the Midwest and used to evaluate the performance of
the new product. Some plants found significantly better results
and others did not see significant improvements.
“However, the DOE was clearly successful in its ability to
identify and validate a measurement method that has enabled us
to accurately evaluate the performance of the new product in a
large number of plants under a wide range of operating conditions,” said Stites.