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
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.
Table 2. Experiment summary
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.