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4. Vacuum freeze-drying
4.1 A primer on freeze-drying
4.1.1 Units of measurement
First, it may be helpful to review the terms used commonly to express the amount of pressure to which the vacuum chamber used for drying is subjected. As the reader will note further on, freeze-drying is one of the processes carried out in a vacuum; the conditions are below normal atmospheric pressure Normal (sea level) atmospheric pressure is 1.033 kilograms per square centimeter, or 14.7 pounds per square inch, 760 torr or 1,000 millibars. In scientific parlance, 1 atmosphere is equivalent to 760 torr (1 torr is equivalent to 1 millimeter of mercury whose symbol is Hg). For many yearn the unit of measurement for pressure was in mm Hg. Then, in recognition Torrecelli' s work - the invention in 1843 of the mercury barometer - the unit was renamed the torn However, the two terms, mm Hg and torr are current and will be used interchangeably in this study.
As a matter of information, the unit of measurement in the Systeme Internationale D'Unites (SI System) is the Pascal which is related to the bar. Another Unit used but considered obsolescent is the micron.
Table 1. Typical Units Used to Measure Vacuum Pressure.
For the purpose of this study, freeze-drying is defined as a method of drying wetted archival and library materials by freezing, then under vacuum conditions by converting the solid to the vapor phase; the liquid phase is by-passed.
When a liquid is converted to the vapor phase by heating, the action is called evaporation. As a solid (frozen), its Conversion to the vapor phase by the application of heat without going through the liquid phase is called sublimation. An example of sublimation: preservation in nigh mountainous regions where water in meat freezes then evaporates (sublimes) with no liquid phase to produce unwanted side effects.
The process of both sublimation and evaporation depends ultimately on the relationship of temperature and pressure to the kinetic energy (energy of constant motion) present in water as a liquid or solid. For example, in a kettle of water, as the temperature is increased the kinetic energy increases and permits molecules from the liquid to escape as vapor. Maximum vaporization occurs at the boiling point, 100°C (212 °F) at 1 atmosphere. Conversely, the vapor pressure of water, that is, the pressure of the vapor in equilibrium with the liquid, is 760 mm Hg at 100 °C. If the atmospheric pressure is reduced to say, 525.8 mm Hg, as would be the case if the same kettle is boiled on the top of a mountain, the boiling point will be reduced by several degrees to 90 °C (194 °F). The reason: at this temperature the vapor pressure of water is 525.8 mm Hg.
Water as a solid behaves in much the same way as water as a liquid except that the temperature at which the energy of motion of the solid (ice) permits the release of water vapor molecules begins at the opposite end of the thermometer scale: the freezing point and below. The vapor pressure scales become numerically very low. By using selected temperatures below the freezing point together with suitable pressures, the sublimation of the solid will take place. This will be explained later in the text.
Figure 1. Temperature-Pressure Equilibrium Curves of Pure Water.
4.1.3 Temperature-pressure values of water
A graphic method of explaining the temperature-pressure influence in water as a solid, a liquid, and a vapor is in the temperature-pressure equilibrium curves of pure water shown in Figure 1. At the triple point TP, at a pressure of 4.58 mm Hg and a temperature of 0 °C (32 °F), the three states of water - solid, liquid, vapor - can exist in a state of equilibrium. The solid exists in the area between A-TP and B-TP; the liquid between B-TP and C-TP; the vapor below the area A-IPC.
A point on any curve represents a condition of equilibrium between any two states at a definite temperature and pressure. If the temperature is raised and pressure is increased the solid enters the liquid zone and melts. And if the temperature is reduced and pressure held constant or reduced the solid can become vapor. In other words, sublimation becomes possible because there is no ''liquid" zone between the '"solid" and "vapor" zones. If the system varies in either temperature or pressure from the triple point value at least one of the three states disappears.
Table 2. Vapor Pressure/Temperature Relationship for Water/Ice.
4.1.4 Vapor pressure
Ice within a chamber evacuated of air and maintained at a given temperature will give off molecules in the form of water vapor. Thin escape (sublimation) is continuous. The molecules will bump into each other and the chamber wall; some will return to the surface of the ice. Eventually, the rate at which the molecules return to the ice will equal the rate at which they leave the ice. At this point, the water vapor and ice are in a state of equilibrium; the pressure at which this occurs is called the equilibrium vapor pressure. If the temperature in the chamber is, say, 20 °C (-4 °F) the pressure will reach 0.78mm Hg. This is the equilibrium vapor pressure and, consequently, the vapor pressure as seen in Table 2. The pressure exerted by vapor depends only on temperature as long as there is some liquid present.
4.2 Conditions required for freeze-drying
First, the object must be in a frozen state from which, as we have seen, water vapor molecules escape. Next, in order to induce continued sublimation a method must be found which will remove the water vapor molecules in the vicinity of the ice. The most effective method is to draw the molecules away by creating a colder surface (condenser) with a lower vapor pressure elsewhere, but near the vicinity of the ice. Under this condition water vapor molecules will diffuse to the colder surface where they will recondense and remain trapped as ice crystals.
To illustrate this frozen object-condenser relationship: if the chamber where the frozen object rests has a temperature of, for example, -14°C (6.80 °F) its vapor pressure is 1.36mm hg according to Table 2. If the temperature of the nearby condenser is -40 °C (-40 °F), the vapor pressure on the cold surface is 0.097mm Hg. This is equivalent to a vapor pressure ratio of 14 to 1, a difference that will go far in drawing water vapor away from the frozen object.
The next step is to find a method of providing an easier path for the water vapor molecules to travel on the way to the condenser trap. This requires the reduction of the air and other non-condensable molecules with which the water vapor molecules will collide. An effective method is the use of a vacuum pump system.
The rate at which freeze-drying proceed" will depend largely on the rate at which the frozen object gains heat. As ice sublimes heat is absorbed; this energy is required to speed up the release of molecules in the form of water vapor. This energy is called the latent heat of sublimation. If it is not replaced the temperature of the frozen object will suffer a progressive decrease in the rate of sublimation.
For small chambers sufficient heat energy from surrounding outside air and the room temperature will suffice. The frozen object is supplied with heat input through the walls and door of the chamber through conduction or radiation.
Resistance heating, or similar device, is normally used to supply heat energy when there are large qualities of frozen materials to undergo freeze-drying: In any case, the amount of heat energy supplied to the frozen object must not exceed the rate at which water vapor leaves the object, otherwise there can be a change from the solid to the liquid state. In such a case sublimation does not take place.
Figure 2. Simple schematic of a vacuum chamber for freeze-drying. A, chamber. B. refrigerated condenser. C, refrigeration units for chamber and condenser. D, vacuum pump.
4.3 The basic components of a freeze-dry system
Figure 2 illustrates a simplified freeze-dry system whose basic components, usually placed in series as in the diagram, comprise a chamber, a condenser, and a vacuum pump. The chamber can be of any cylindrical size as long as it can withstand an exterior pressure of 1.033 kilograms per square centimeter (14.7 pounds per square inch). It must be vacuum tight, refrigerated, and have an opening that provides easy accessibility.
The important feature of a refrigerated condenser is that it should be located in the direct path of moving water vapor molecules where they can be trapped. When contact with the condenser surface is made, the water vapors give up their heat energy, turn to ice crystals, and are removed from the system and prevented from traveling to the vacuum pump.
Condensers are primarily of two types, internal or remote. The choice depends on the application desired. In the remote type the condenser is housed in a vacuum chamber which is separate from the chamber that houses the frozen objects. This is shown in Figure 2. This type of condenser can be isolated by a valve that will permit defrosting. Smaller freeze-dryers can be defrosted with warm water or natural air. Some mid-size freeze-dryers have internal condensers designed to form an ice "plug" that can be pulled out after Using hot gas to that it loose.
Apart from the technical design features required of a vacuum system, the pump should have the capacity to reduce the chamber pressure to levels below 4mm Hg. At pressures above this level the ice DO longer sublimes to water vapor but turns to liquid. A look at Figure 1 will bear this observation out.
There are two refrigeration compressors in the simplified freeze-dry system shown in Figure 2. One serves the frozen object; it should have the capacity of producing controlled temperatures that go below -5 °C (23 °F). The other serves the refrigerated condenser and should have the capacity to produce temperatures of 40 °C (-40 °F) or less. Keep in mind that the force that drives the water vapor from an ice surface is the difference in vapor pressure produced by the difference in the temperature between the frozen object and the condenser.
As already pointed out, heat energy is required for the sublimation process. Where it appears that conducted or radiated heat will not suffice for the quantity of materials to be freeze-dried, heating devices can be installed. In certain types of proprietary vacuum chambers warming devices are incorporated with the design.
4.4 Degree of vacuum required for freeze-drying
For freeze-drying, Rowe and Snowman (12) state the degree of vacuum required is usually between 0.3 and 0.03 torn These values are made up of the partial pressures of air and water vapor in the chamber, the sum of which equals the total measured pressure. For example, 0.3 torr could have an air component of 0.03 torr and a water vapor of 0.27 torn
RHM Research Ltd (13) place frozen documents in a freeze-dry chamber (the shelves are cooled to -30 °C or -22 °F before loading) and apply the vacuum to a level of 0.1 torr (condenser temperature is-50 °C or -58 °F). About one hour later heat is applied to the chamber shelves at 40 °C (-40 °F). Depending on the thickness of documents the time for drying is about five days. At this stage the vacuum is about 0.06 torn
In freeze-dry operations, the Lockheed Missiles and Space Company (14) places the frozen books on shelves and the chamber is quickly evacuated to less than 4 torr; any free water in the books is frozen. For the first 56 hours the chamber is at 10 3 torr (the condenser is at less than -101 °C or -149°F); the shelf temperature not over 54.4 °C (130 °F), book temperature not over 37.8 °C (100 °F). When the weight of ice on the condenser is stabilized, the chamber is brought to ambient pressure, about 760 torr, it is opened, the ice removed, and if any of the books are dampish, or nearly dried, they are placed on non-heated shelves. The chamber is re-evacuated and held at 10 torr for 26 hours; shelf temperature is not aver 54.4°C (130°F). When book temperatures read above 12.8 °C (55 °F) the cycle is completed, the chamber is brought down to ambient pressure.
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