In experiment set C, the contents of the center cavity were replaced with 90 L of buffer. The freezing time was greater than
that of pure water by two to three hours (compared to experiments B and C, center cavity). This reflects the lower thermal
conductivity of the solute containing ice and the freezing point depression from the cryoconcentrate. Similar results are
observed for standard freeze–thaw vessels. Experiment C2 also included thermowell sleeves, constructed from the same material
as the bags. A configuration involving thermowell sleeves integral to the bag instead of the original flexible tubing would
allow the use of a rigid thermowell made from any convenient material without introducing another product contacting surface.
This is shown in Figure 5. There were no significant differences in the measured temperature resulting from the addition of
thermowell sleeves (data not shown).
Figure 7.
|
Figure 7 shows a sequence of images taken during freezing and thawing from experiment C1. It shows that during thaw, a piece
of ice is kept at the bottom of the cavity by the thermowell but eventually fractures and floats to the top. The ice motion
is unpredictable and ice in close proximity to the heated wall will melt faster, as expected. This is likely a major contributor
to the variability in thaw times.
Table 3. Prototype 1 issues and possible solutions
|
In Table 3, we have summarized several important technical issues that were identified during the testing of the prototype
large freeze bag holder and offer some possible solutions.
Conclusions
We demonstrated that it is possible to conduct a freeze–thaw operation in large rectangular disposable bags housed inside
a jacketed container. The overall concept and dimensions of the system are compatible with the infrastructure (freeze–thaw
skids, transportation methods, and storage facilities) typically used for current commercially available large-scale freeze–thaw
vessels (e.g., IBI 300 L CryoVessel). Having a compatible system provides internal flexibility and facilitates potential technology
transfer to external sites and contract manufacturers.
The exploratory studies revealed that the freezing step is likely to require a slightly longer cycle and show more variability
in terms of freezing time as compared with that of the traditional freeze tanks. The bag holder is expected to be significantly
cheaper than a freeze–thaw tank of comparable capacity because of the simpler design and use of standard stainless steel as
opposed to higher grade alloys. In addition to capital costs, there are advantages such as quicker turnaround time caused
by the elimination of the clean-in-place and steam-in-place steps, the lack of requirements for pressure vessel certification,
and lower shipping weight. There remain some process consistency issues to be addressed. In-depth studies to examine the long-term
stability, shipping, and durability of the bags will need to be performed to guarantee robustness of this technology before
it can be implemented in production.
Acknowledgements
The authors would like to thank C. Hsu, S. Martin-Moe, and B. Wolk for their support and leadership. We would also like to
thank M. Goodwin and B. Buchanan of Thermo Fisher and the prototype vendor for their help with the design and fabrication
part of this project.
Philippe Lam is senior engineer and Samir Sane is principal engineer, both of Process R&D at Genentech Inc.,South San Francisco, CA. 650.225.1000, lam.philipe@gene.com
|
