Views: 2 Author: Site Editor Publish Time: 2022-08-11 Origin: Site
The freeze-drying process is usually expressed by a function of variables such as freezing rate, product temperature, gas pressure, solution concentration, and product thickness (or filling volume). Variability in lyophilized bulk products.
In addition, variations in heat exchange during lyophilization can also introduce differences in product uniformity. For example, an experiment found that in the initial drying process, under the temperature of the plate layer of -25°C and the pressure in the chamber of 150mtorr, the vial at the front of the plate layer in a laboratory freeze dryer is more than that in the pilot freeze dryer. The vials at the front of the lamella receive 1.8 times more heat. This is mainly because the chamber doors of laboratory freeze dryers are usually constructed of plexiglass, while the doors of production equipment are almost always made of stainless steel, and the thermal emissivity of the two materials is significantly different.
Heat exchange is usually realized in three forms：
1 Conduction；2 Radiation；3 Convection；
For the freeze-drying process, taking laboratory freeze-dryers as an example, we can subdivide the following energy sources：
Qcontact. The heat exchange brought by the loading plate layer (the bottom plate of the vial), the direct contact surface between the adjacent vial and the target vial;
Qgas For the heat exchange caused by the ambient gas of the vial, the temperature effect caused by the gap air between the lower surface of the plate above the vial and the top of the vial, and between the bottom of the vial and the upper surface of the plate can be considered.；
Qrad Thermal radiation from freeze-drying cabinet walls, glass or stainless steel doors.
Qconv，There is thermal convection in the freeze-drying box under the condition of normal pressure or slight low pressure, and under the condition of low vacuum, the energy transfer of the convection is extremely small.
Among the above-mentioned heat transfer mechanisms, radiation Qrad and contact heat conduction Qcontact are relatively independent of pressure, that is, not affected by vacuum/pressure changes in the box; while convection Qconv and heat transfer through the gas in the vial and shelf gap Qgas depends on pressure.
Therefore, under vacuum conditions, radiation Qrad and conduction Qcond. are important ways of heat exchange, but due to the difference between the two heat sources and energy, it will be found that the vials placed in the middle and edge of the plate layer may exist after freeze-drying. The amount of residual water varies.
At the same time, related studies have shown that the convection Qconv contributes to the gas conduction Qgas when the pressure is greater than 100 mTor. Therefore,under typical laboratory freeze-drying conditions, convection Qconv can not be ignored as an important heat transfer route, and an in-depth understanding of it can also help to develop the design space of non-cilin vial freeze-drying process, such as pre-filled needles. See article Brief description of lyophilization of dual-lumen prefilled needles.