Supercooling process liquid water

To calculate the entropy changes, it is necessary to consider a series of reversible steps leading from liquid water at —10°C to sohd ice at —10°C. One such series might be (1) Heat supercooled water at —10°C very slowly (reversibly) to 0°C, (2) convert the water at 0°C very slowly (reversibly) to ice at 0°C, and (3) cool the ice very slowly (reversibly) from 0°C to —10°C. As each of these steps is reversible, the entropy changes can be calculated by the methods discussed previously. As S is a thermodynamic property, the sum of these entropy changes is equal to AS for the process indicated by Equation (6.97). The necessary calculations are summarized in Table 6.2, in which T2 represents 0°C and Ti represents 10°C. 

However, one finds that, in cooling a liquid below its freezing point, the liquid may not always turn into solid phase at the freezing point. In fact, in some cases, such as water, even at around -40°C, liquid water does not turn into a solid phase. It stays in what is called a supercooled state. A major phenomena is the freezing of supercooled clouds. However, if certain so-called nucleating agents are used, then the clouds would turn into liquid droplets (and form rain). The nucleation process is a surface phenomena and is observed in transitions from... 

Very pure liquid water can be supercooled at atmospheric pressure to temperatures well below 0°C. Assume that 1 kg has been cooled as a liquid to -6 C. A small ice crystal (of negligible mass) is added to seed" the supercooled liquid. If the subsequent change occurs adiabatically at atmospheric pressure, what fraction of the system freezes and what is the final temperature What is 5,ou] for the process, and what is its irreversible feature The latent heat of fusion of water at 0°C = 333.4 J g l, and the specific heat of supercooled liquid water = 4.226 J g-1 °C I. 

In Figure 11-1 the lines below and to the left of the hydrate/ice formation line represent a meta-stable equilibrium between water vapor in the gas phase and supercooled liquid water. The actual equilibrium with solid ice or hydrate is at a lower water content. The effect is depicted in Figure 11-3, which also extends the water content scale of Figure 11-1 down to 0.1 lb water/MMscf. The data on equilibrium water contents in the 0.1 to 1.0 lb water/MMscf range are necessary for the design of the recently developed superdehydration processes. Water content data down to as low as 0.001 Ib/MMscf are plotted by Buck-lin et al. (1985). Such extremely low values are of interest in the design of natural gas turboexpander plants. 

Weak detonations are believed to represent the condensation shocks observed in supersonic wind tunnels [12], [51]. Supercooled water vapor in a supersonic stream has been observed to condense rapidly through a narrow wave. The amount of liquid formed is so small that the equations for purely gaseous waves are expected to apply approximately. Since a normal shock wave would raise the temperature above the saturation point (thus ruling out the ZND structure, for example), and the flow is observed to be supersonic downstream from the condensation wave, it appears reasonable to assume that condensation shocks are weak detonations. This hypothesis may be supported by the fact that unlike chemical reaction rates, the rate of condensation increases as the temperature decreases. Proposals that weak detonations also represent various processes occurring in geological transformations have been presented [52]. 

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