Design and Testing of a Prototype Large-Scale Bag Freeze–Thaw System - The development of a large-scale bag freeze–thaw system will have many benefits for the biopharmaceutical industry. -


Design and Testing of a Prototype Large-Scale Bag Freeze–Thaw System
The development of a large-scale bag freeze–thaw system will have many benefits for the biopharmaceutical industry.

BioPharm International Supplements

Figure 5.
After the bags were filled with liquid, we discovered that the flexible thermowell tended to coil up and did not provide adequate placement accuracy, which could severely affect temperature monitoring. Furthermore, a flexible thermowell would not allow for the removal or re-insertion of the temperature probe after the bag contents have been frozen. To a lesser extent, a similar problem affected the dip tube. In that case, the dip tube failed to reach to the bottom of the bag, leaving a considerable amount of residual liquid after emptying of the contents. Consequently, we decided to use rigid polycarbonate (PC) tubes for the dip tube and thermowell. A custom split bar assembly was used as a clamping mechanism to hold the PC tubes vertically. The split bars were secured to the metal bag holder by adhesive-backed industrial Velcro, which allowed minor adjustments in the position and easy removal when necessary. Figure 5 shows the setup with the rigid dip tube and thermowell. Recirculation mixing can be accomplished by pumping the liquid from the bottom of the bag by the dip tube and returning it near the top at the opposite side of the cavity, approximately 10–12 cm above the liquid level. The outlet for the return port is aimed toward the nearest wall to minimize potential foaming.

The tops of the bags were removed and the sides were secured to the cavity walls using heavy-duty adhesive tape to monitor the progress of freezing in each cavity. A video camera connected to a computer was used to record images at one-minute intervals as a time-lapse movie during the freeze and thaw phases of each experiment. This allowed us to visually assess the completion of the freeze or thaw process accurately.

Results and Discussion

Table 2. Experiment summary
We conducted a series of freeze–thaw experiments in the prototype bag holder filled with water as well as a 25 mM citrate buffer containing 150 mM sodium chloride (NaCl). Sodium chloride solutions form a eutectic at approximately –21 C; as a result, the NaCl solution exhibited very different freezing dynamics compared with pure water. Table 2 summarizes the results for a select number of runs, which are discussed in a subsequent section. The completion time for freezing or thawing was determined from the time-lapsed video data. In some instances, it was not possible to clearly observe all three cavities simultaneously; in those cases, no time data is given. Freeze time is defined as the time required to freeze all of the liquid; thaw time is the time required to melt all of the ice as visually determined from the time lapse movies. The freeze–thaw skid permits the recording of only one external temperature probe. The location of the probe varies from experiment to experiment and is indicated in Table 2.

A flow rate of 1 L/minute was used in all cases involving recirculation mixing during thaw. Similar conditions are adequate for post thaw mixing in freeze–thaw vessels. Only the cavity containing the temperature probe was connected to the pump. It should be noted that the recirculation time of three hours was chosen arbitrarily; the optimal duration will be determined later but is expected to be between three and five hours.

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