It is common practice to freeze protein bulks for long-term storage, allowing product life to be extended up to several years. For high-volume products such as antibodies, the batch size can be hundreds of liters. Freezing such large quantities in a timely manner has generally been handled using "freeze–thaw vessels", which are jacketed metal tanks with a cooling surface provided by internal coil and fin assemblies. With the increasing cost of high-grade steel alloys, it is desirable to explore disposable technology alternatives to costly freeze–thaw vessels. The following is a report on the design and testing of a prototype large-scale bag freezing system developed at Genentech.
The lack of a commercial large-scale disposable container system that could meet our needs prompted us to develop the technology in-house. The following reports our efforts at designing and testing the first prototype of a large scale freeze–thaw system based on disposable bags that would complement our current frozen bulk storage strategy. We discuss our rationale and design methodology, and present experimental results from the first at-scale prototype.
Design Constraints and Initial Concepts
Disposable bags used as liquid bulk storage containers are readily available in volumes of 1,000 L and above. The technology is proven and, because of its high structural flexibility, is relatively resilient against mild impact from blunt objects. Unfortunately, these soft-wall containers are much less forgiving when the contents are frozen. Light bumps may compromise the integrity of the bag, and the puncture may not be discovered until the contents are thawed. Such difficulties must be taken into account when designing a freezable bag system. Our aim was to develop a system that is compatible with facilities already geared toward handling commercially available freeze–thaw tanks. We had four main design constraints:
1 Each individual container must hold at least 100 L. There is potential for large bulk sizes, possibly in excess of 1,000 L per batch, and the use of many small containers would cause increased handling, tracking, and testing.
2 The system must be compatible with current commercially available freeze–thaw skids. The system also must be able to function under the same conditions with operational times, equivalent (within a few hours) to those of current commercial freeze–thaw vessels.
3 The system must be compatible with the bulk transportation and storage infrastructure used by commercial freeze–thaw vessels. The footprint and vertical dimensions must not be larger than those of existing 300-L freeze tanks in order to accommodate storage freezers, air shipping containers, and entrances to existing facilities.
4 The system must cost less than current commercial freeze–thaw tanks.