The reference output (\(y_{\text{ref}}\)) was also determined online
during the experiment after process startup. The previously calculated
PCC cycle time was applied during the first few cycles until steady
state had been reached. The area of the eluate peak from the polishing
step was then automatically calculated and was used to calculate the
product output, which was set as the reference signal.
Pooling control
The cutoff times of the pooling of the product in the polishing step
were obtained based on the UV absorbance at a wavelength of 280 nm,
measured online with a U9-M UV monitor, i.e., pooling started and ended
at a certain value of absorbance, as described previously (Löfgren et
al., 2018). In addition, in the present study, the second absorbance
cutoff (i.e., when pooling ends) was automatically adjusted to
compensate for the variation in the incoming concentration. The height
of the eluate peak was obtained online, and the absorbance cutoff was
calculated based on the height. Previous runs of the process were used
to determine the linear relationship between the absorbance cutoff and
the peak height.