Packing sublimation energy

The packing energy of an organic crystal can be easily calculated by a lattice sum over pairwise interactions. The potential parameters for these calculations are summarized in Table 15. The packing energy is usually a quite accurate estimate of the crystal sublimation energy. 

Similar small but significant deviations of the carbon atoms from a planar conformation have been found in chrysene (10) and in 20-methylcholanthrene (12) (Iball and MacDonald, 1960). The deformation energies for these molecules (0-025-0-040 and 0-155-0-260 kcal mole-1 respectively Ali and Coulson, 1959) are of the order of one-hundredth of the sublimation energy and serve to demonstrate that the small out-of-plane deviations found by Iball and his associates could easily arise in the process of packing the separate molecules into the crystal. 

The activation energies for surface self-diffusion are also much lower than sublimation energies, since here again only a limited number of bonds need to be broken to allow movement. Diffusion coefficients are not surprisingly much greater, and activation energies are lower, the more closely-packed the atoms in the plane thus for example values of the latter for rhodium atom migration vary... 

Based on molecular packing calculations with and without symmetry constraints the lattice dimensions for RDX crystal have been reproduced to within 0.9% deviation relative to experimental results and with a very small translation and rotation (less than 1.1°) of the molecules inside the unit cell. Additionally, the lattice energy was found to be practically identical to the static lattice energy estimated based on the experimental enthalpy of sublimation (E AH S0M + 2RT). 

In Table V. is also listed the lattice energy L (sublimation heat extrapolated to absolute zero after subtraction of the zero-point energy) for some molecule lattices, calculated on the basis of the same simplified formula (22). In all cases we have assumed closest packed structure, as this structure is at least approximately realised in the molecular lattices in question. The summation of (22) over the lattice gives... 

Kq is related to the overall hardness of the crystal the sublimation enthalpy A//s may be estimated from the calculated packing energy. 

Energy balances can be formulated for both fronts. At the defrost front, heat is required to heat up the packing material from the saturation temperature (Tj) to the inlet temperature (T2). Furthermore, heat is consumed due to the (endothermic) sublimation of component i. The required energy is provided by the feed gas, which is cooled down from T2 to Tj. The energy balance results in... 

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