Ice sublimation

There are additional factors to be taken into account that influence and also the quality and uniformity of products freeze-dried in vials  

Cooling rate Nucleation rate Number of ice crystals Size of ice crystals Ice sublimation time 

The actual drying (sublimation) time 4 is thus seen to depend on the following quantities  

Latent heat of sublimation at the particular drying temperature (kJ kg ) 

Total heat transfer coefficient from shelf to sublimation front [kJ (m h deg) ] 

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While A

metal-water interactions are better probed by thermal desorption spectroscopy (TDS) in which heat is used to detach molecules from a surface. TDS data are in parallel with A (and AX) data. This is illustrated in Fig. 19.35 The spectrum of Ag(110) shows only one peak at 150 K, corresponding to ice sublimation. This means that Ag-H20 interactions are weaker than H20-H20 interactions (although they are still able to change the structure of the... 

Sublimation is the direct conversion of a solid into its vapor. Frost disappears on a cold, dry morning as the ice sublimes directly into water vapor. Solid carbon dioxide also sublimes, which is why it is called dry ice. Each winter on Mars, solid carbon dioxide is deposited as polar frost, which sublimes when the feeble summer arrives (Fig. 6.24). The enthalpy of sublimation, AHsub, is the molar enthalpy change when a solid sublimes ... 

The phase diagram for water, shown in Figure 11-39. illustrates these features for a familiar substance. The figure shows that liquid water and solid ice coexist at the normal freezing point, T = 273.15 K and P = 1.00 atm. Liquid water and water vapor coexist at the normal boiling point, P — 373.15 K and P — 1.00 atm. The triple point of water occurs at 7 = 273.16 K and P = 0.0060 atm. The figure shows that when P is lower than 0.0060 atm, there is no temperature at which water is stable as a liquid. At sufficiently low pressure, ice sublimes but does not melt. 

Fig. 2.16.2. Ice sublimation rate (kg/m2 h) for five different types of tray.

A plant having a shelf area of = 30 m2 has been loaded with 300 kg of water in trays and frozen on the shelves. Water vapor transport and condenser temperatures have been measured in this case between 0.4 and 0.6 mbar, which is approx, two to three times higher than the normally expected operation pressure of the plant (to get an measurable quantity of ice sublimed in a reasonable test time). The data of the test are shown in Fig. 2.19. Three Pt 100 (resistance thermometers) have been frozen in the ice. One CA each have been... 

In this operation some ice sublimes from the condenser to the LN2-cooled surface. However, the surfaces of the LN2 plate can be controlled between -80 °C and -100 °C, that corresponds to a water vapor pressure of approx. 5 10-4 to 2 1 () " mbar. 

By drawing a horizontal line across the figure at p = we see how the line cuts the solid-gas phase boundary at —78.2°C. Below this temperature, the stable form of CO2 is solid dry ice, and C02(g) is the stable form above it. Liquid CO2 is never the stable form at in fact, Figure 5.5 shows that CCfyi) will not form at pressures below 5.1 x In other words, liquid CO2 is never seen naturally on Earth which explains why dry ice sublimes rather than melts under s.t.p. conditions. 

For some purposes solid carbon dioxide ( dry-ice ), sublimation temperature —78.5 °C or mixtures of dry-ice and acetone (temperature —78 to —95 °C) are used as coolants. These are obviously not as efficient as liquid nitrogen and they should not be used with chemicals which have an appreciable vapour pressure at the appropriate temperatures. 

A 40-cu ft tank is filled with air at 14.7 psia and 80°F. The tank also contains a 15-lb block of dry ice (solid C02, specific gravity of 1.53 based on water). After the dry ice sublimes, the temperature is 30°F. What is the pressure Assume ideal gas behavior. 

Drying. The conditioned frozen material to be dried is placed in a vacuum chamber, where sublimation and desorption of water occur. As soon as the chamber has been evacuated and the optimum vacuum has been reached (0.5 to 0.05 millimeters of mercury), heating is applied so that the ice sublimes. For large-scale production of food products, the combination of conduction and radiation that results from circulating a hot fluid through coils or plates has proved quite satisfactory for heating. 

A triple point is a point where three phase boundaries meet. For water, it occurs at 4.6 Torr and 0.01°C (see Fig. 8.5). At the triple point, all three phases (ice, liquid, and vapor) coexist in dynamic equilibrium. Under these conditions, water molecules leave ice to become liquid and return to form ice at the same rate liquid vaporizes and vapor condenses at the same rate and ice sublimes and vapor condenses directly to ice again at the same rate. The location of the triple point of a substance is a fixed property of that substance and cannot be changed by changing the conditions. The triple point of water is used to define the size of the kelvin by definition, there are exactly 273.16 kelvins between absolute zero and the triple point of water. The normal freezing point of water is found to lie 0.01 K below the triple point, so 0°C corresponds to 273.15 K. 

Carbon dioxide is formed by the action of acids on carbonates and as a by-product of fermentation, the conversion of carbohydrates to ethanol by yeast. Because its triple point is at 5.1 atm, it cannot exist as a liquid at ordinary atmospheric pressure. Solid carbon dioxide, which is sold as dry ice, sublimes directly to the gas—a property that makes it convenient as a refrigerant and cold pack. 

Fig. 2.16.2. Ice sublimation rate (kg/m2 h) for five different types of tray. 1, Flat tray as in Figure 2.14(a), with 20 mm filling height 2, flat tray with one rib of 20 mm height 3, ribbed tray as in Fig. 2.15 4, ribbed tray as in Fig.

Fig. 2.57. Laboratory freeze-drying plant. Microprocessor controlled, air-cooled compressor, condenser end temperature-53 °C, 3 kg of ice sublimed in 24 h. (a) Unit with 8 valve connections for 8 containers, condenser in an...

The decisive data during main drying (MD) is the temperature of the ice at the sublimation front, Tice, which can only be measured by barometric temperature measurement (BTM). Figure 1.77 shows the principle the valve between the chamber and condenser is closed for less than 3 s, to fill the chamber with water vapor of saturation pressure p., corresponding to Tke as shown in Table 1.11. The two conditions for the use of BTM a leak rate of the chamber below a certain limit and enough ice subliming during the time the valve is closed, are described in detail in Section 1.2.1. 

The transfer of process data from the pilot plant to the production plant 1 is possible, if the remarks above are taken into account the transfer to production plant 2 is not possible the possible vapor flow is not sufficient at the necessary pressure for the amount of ice sublimed under the conditions operated in the pilot plant. Theoretically, a plant with two valves of 1.2 m diameter each could handle 60000 vials in the process developed by the pilot plant or a plant with one valve and double MD is feasible. 

The temperature of dry ice (sublimation temperature at normal pressure) is —109°F. Is this higher or lower than the temperature of boiling ethane (a component of bottled gas), which is —88°C ... 

Another example is that a focus on terran sea-level atmospheric pressure can influence researchers views of the range of temperatures at which water is a liquid. On the surface of Mars liquid water cannot exist, except possibly transiently, because the pressure is too low, and water ice sublimes directly to water vapor without going through an intermediate liquid phase. Further, over most ranges of pressure, formamide has a larger liquid temperature range (255 to 480 K) than water and is also an excellent solvent for polar materials. 

Never handle dry ice with your bare hands. At 1 atm, dry ice sublimes at -78°C. At these temperatures, severe tissue damage could result. You should always use heavy thermal gloves or tongs. 

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