Yingtai: Basic Principles of Freeze-Drying Technology

Yingtai: Basic Principles of Freeze-Drying Technology

Freeze-drying involves first freezing a substance with a high moisture content into a solid and then sublimating the solid water molecules under a vacuum environment. Once all the water molecules have sublimated, the substance achieves complete dryness.

Freeze-drying is fundamentally a process of heat and mass transfer, based on the phase changes of water at its triple point.

Since water is the primary component of the substances to be freeze-dried, our starting point of study is water.

1. Phase Diagram

At the triple point, water exists in three states: solid, liquid, and gas. These states can transform into each other. Generally, the process of transforming from a solid to a liquid, from a liquid to a gas, or directly from a solid to a gas involves a change from a state with closely packed molecules and strong intermolecular forces to a state with more loosely arranged molecules and weaker forces. This process requires heat input from the surroundings. Conversely, the reverse process releases heat to the surroundings.

2. Phase Diagram Curve

The evaporation rate of liquid water increases with temperature, reaching its maximum at 100°C, where boiling occurs. Under normal atmospheric pressure at sea level, water will not exceed 100°C and will continue boiling at this temperature until it is completely converted to steam. Thus, 100°C is known as the boiling point of water, which is the boiling point of water at standard atmospheric pressure. In an open environment, this refers to standard atmospheric pressure. As altitude increases, the atmospheric pressure decreases, and the boiling point of water decreases accordingly. In a sealed environment, once water turns into steam, it cannot escape, leading to increasing pressure within the space, which also alters the boiling point. This principle is used in pressure cookers.

Since the boiling point varies with pressure, plotting the boiling points at different pressures yields a curve, which represents the critical curve between the liquid and gas phases of water. Similarly, the melting (freezing) point and sublimation point (deposition point) are also influenced by pressure, and similar curves can be drawn. By combining the boiling point curve, melting point curve, and sublimation point curve on a single graph, we obtain the phase diagram of a substance.

To remove water, the solution is first frozen, and then vacuum is applied to lower the pressure, allowing the frozen water in the solution to sublime. This process of first freezing and then removing moisture through sublimation is known as freeze-drying, or "lyophilization." This is the fundamental principle behind freeze-drying, also known as vacuum freeze-drying technology.