The analyses obtained above imply that the actual optimum was
outside these operating conditions previously investigated. Thus, the
steepest ascent method was preferentially called up to find out the optimal
operation conditions. The as-fitted multilinear model (BE-90),
shown as Eq. (5), was used in this study. The path of the steepest ascent
is the line through the center of FFD moving the fastest to the fitted surface.
Usually, it will go until no further increment in response is observed
[28]. In the upper part of Table 4, the moving steps are
presented, while the observed and predicted conversion efficiency are