Oxygen Supply Characterization
To estimate the k
L
a of the pilot scale reactor at processing conditions, the dynamic gassing method was explored. A procedure was drafted to
measure the k
L
a while accounting for the DO sensor lag. This method development is on-going.
Media Steaming Characterization
Table 2. Sterilize-in-place (SIP) gain or loss results. Pilot scale test results are presented. For large scale, historical
SIP data are presented.
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Table 2 summarizes the gain or loss observed during SIP cycles at pilot and large scale. For the large scale, no experiments
were performed because historical SIP data showed minimal and consistent gain and losses. Testing performed at pilot scale
demonstrated that the weight change during SIP also was consistent and minimal. Therefore, it was concluded that the water
amount used to batch medium did not need to be adjusted to account for any considerable gain or loss during the SIP operation.
From the evaluation of data from SIP cycles done on actual media at the pilot scale, it can be concluded that media and water
behave similarly and that water is a good surrogate for testing purposes before starting actual production runs.
Evaporative Losses During Media Hold Characterization
Figure 2. Pilot-scale evaporative losses. Tests 1 and 2 were performed with the condenser on the fermenter enabled, and test
3 was performed with the condenser disabled. The condenser reduces the evaporative loss by almost a factor of two.
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Figure 2 shows the evaporation rate as observed at the pilot scale. The test was performed at process conditions. Tests 1
and 2 show very consistent results with an average evaporative loss of approximately 0.12% of starting fermenter weight/h.
Media hold conditions or the amount of water used to prepare the medium should be further evaluated to ensure the target starting
media concentration is met.
Figure 3. Large-scale evaporative losses.
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Large-scale data demonstrated that the need for any water adjustments to achieve the target media concentration is eliminated
when the airflow rate is decreased during the media hold. With the airflow rate at 10% of the processing set point, the evaporative
loss was negligible. Over approximately 40 h, the reactor weight dropped by approximately 0.02% of starting weight (Figure
3), which is within the calibration tolerance of the fermenter load cells. At the end of the hold, the air flow rate was increased
to the processing set point. The data indicated that no additional equilibration time was needed to achieve DO saturation.
Conclusions
The wet testing presented in this article represents a platform approach to characterizing microbial fermenters. These tests
can be used to gain an understanding of the equipment capabilities before starting actual process runs and further ensure
project timelines and success criteria are met. Measuring the heat removal and oxygen transfer capability of a fermenter helps
ensure a reactor meets process needs. Water-based testing can be performed to help develop appropriate media SIP and hold
conditions, thus ensuring the target starting media concentration is met.
These equipment characterization studies are not required to be performed for every new product that is introduced into a
facility as long as major equipment changes have not occurred (e.g., SIP cycles). However, it is useful to confirm that there
have been no shifts in equipment capability by periodically repeating these tests.
Acknowledgements
The author would like to thank Greg Naugle and Sushil Abraham for reviewing the manuscript, and Arun Tholudur for troubleshooting
support during testing and useful technical insights, Justin Bingham for providing data from large scale testing, and Colter
Davidson, Jaime Foster, and Tom Folger for testing at the pilot scale.
KIRSTEN HAYDA is a process development engineer, MARIA WIK is principal process development engineer, and VALERIE PFERDEORT is senior process development engineer, all at Amgen, Inc., Longmont, CO, 303.401.5156, khayda@amgen.com
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