Theory

Physical principles of vacuum freeze-drying.

The freeze-drying is inherently a dehydration of frozen material in resulting from the transition material (ice) from the solid to the gaseous state, bypassing the liquid phase. The term freeze-drying is most often used in our literature and practice, however, it is not the only one. This process also defined as “molecular drying” because of the nature of movement of the product vapor in the pores and in the drying chamber. In medicine and biotechnology  it is called ” lyophilization “, because in the end turn out lipophilic, that is readily soluble substances. In the foreign food industry often use the term freeze – drying (English).

Physical basis of the freeze drying process clearly illustrated diagram of phase equilibrium for water vapor in the pressure – temperature, Fig. 1. In the state diagram at the intersection of the boundary curves of the triple point at which water can exist simultaneously in all three phases: the ice – water – vapor. Corresponds to the triple point pressure of 4.58 mm. hg. art. (611 Pa) and the temperature of 0,01 ° C. If the heat to the frozen material with pressure below the triple point, there will be a process of sublimation, along the line A – B in Fig. 1.

Fig. 1. The diagram of phase equilibrium for water in the pressure - temperature.

Fig. 1. The diagram of phase equilibrium for water in the pressure – temperature.

Vacuum drying takes place in a hermetically closed apparatus, and heat transfer by convection is low. Therefore, to maintain a substantial rate of drying in a vacuum, the heat required for evaporation of the liquid is supplied to dry the material by conduction from the heated surface (the contact-drying), or by radiation from the heated screens (infrared drying). Thus, a method for supplying heat to the material vacuum drying is contact drying or infrared drying in vacuum.

Principal diagram of the freeze drying kinetics shown in Fig. 2.

Fig. 2. The kinetic curves of the molecular process of drying: tn - the final temperature of pre-freeze; t c1, t c2 - upper and lower permissible levels of temperature of the material at the stage of sublimation; t 2 - The maximum permitted temperature at the final drying stage; t, τ - respectively, the time and the final drying sublimation.

Fig. 2. The kinetic curves of the molecular process of drying: tn – the final temperature of pre-freeze; t c1, t c2 – upper and lower permissible levels of temperature of the material at the stage of sublimation; t 2 – The maximum permitted temperature at the final drying stage; t, τ – respectively, the time and the final drying sublimation.

Upon completion of the freezing step main part of moisture in material passes into ice, but with a portion of bound water – usually a few percent, is in a supercooled liquid state. Further, after the establishment of the space of a drying chamber of the vacuum level, starts the first period drying. The temperature of the material to be dried is almost constant in time during the first drying period, it corresponds numerically to a degree of vacuum of the equilibrium curve. Sublimation zone moves into the thickness of the material, the moisture content decreases. Upon completion sublimation of the entire mass of ice begins the second period, vacuum final drying stage. Material temperature quickly rises to ambient temperature, bound water is removed. This scheme is simple and serves to explain the basic laws of the process. In real processes recess zone sublimation speed unequally in different points of the material, and evaporation occurs throughout volume throughout entire cycle during dewatering.

Prime cost structure

Prime cost structure

Разработано компанией ООО "Русские системы"