Ammonia critical point

Figure 4.4 Comparison of the ratio measures Z and (p for pure ammonia at 100°C. The ammonia critical point occurs at = 405.6 K and = 111.5 atm. These curves are plots of data tabulated by Walas [9].

The Haber process for ammonia manufacture, which operates above the critical point of ammonia. 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

Since 1973, progress has been made in all the fields reviewed and a more complete review of Franck s work has appeared (29). For the present purposes it is necessary to concentrate on the two areas which are least well studied simple electrolytes between 300 C and the critical point and the chemistry of aqueous transition metal cations. A number of studies which do not fall into these categories must, however, be mentioned because of their direct relevance to boiler water chemistry. These are studies of sodium phosphate phase equilibria (30), ammonia dissociation (31) and isopiestic studies of calcium and magnesium chlorides (32). 

There have been many theories of metal-ammonia solutions that differ from that presented here. Cohen and Thomson (1968) and more recently Cohen and Jortner (1973) supposed that large fluctuations in concentration occur over large distances, sufficient to invalidate the pseudogap model and substitute a semiclassical percolation theory. For the reasons given above, we think that this is only likely to be so within a few degrees of the critical point. 

The problem in choosing a polar SF for solubilizing polar solutes is that both the boiling point and the critical point are elevated by the polarity since supercritical operation requires T>TC, high temperatures are mandated in such cases. Thus for ammonia, the critical temperature, Tc = 132°C, must be exceeded for practical operation. A widely used compromise between polar substances with high values of Tc and nonpolar substances with low values of 5liq is carbon dioxide, for which Tc = 31°C and 5Uq = 8.9. Other factors involved in choosing an SF phase are elaborated by Schoen-makers et al. [191. 

At its critical point, ammonia has a density of 0.235 g cm. You have a special thick-walled glass tube that has a 10.0-mm outside diameter, a wall thickness of 4.20 mm, and a length of 155 mm. How much ammonia must you seal into the tube if you wish to observe the disappearance of the meniscus as you heat the tube and its contents to a temperature higher than 132.23°C, the critical temperature ... 

On the other hand in the case of steam, alcohol, carbon bisulphide, as well as in the case of fluids commonly used in refrigerating machines, etc, carbon dioxide and ammonia, the behaviour is different In these latter cases the specific heat of the saturated vapour remains neg ative throughout the whole range of temperature up to the critical point, at which point its negative value becomes infinite... 

MCM-22 (Si/Al =12) and MCM-36 pillared with Si02 (Si/Al = 29) were modified by ion exchange of alkali cations. A critical point was the pH of the solution, which should be adjusted to about 9 and 4, respectively. At pH 9 the pillared mesoporous structure of MCM-36 was destroyed as indicated by nearly amorphous XRD patterns and severely reduced BET surface areas. The degree of exchange decreased from Na to Cs (except the lowest value for Li). In spite of this, the basic character in both series increased from Li to Cs as demonstrated by IR spectra of adsorbed pyridine and TPD of ammonia. 

Chemical recycling of polyurethanes by ammono-lytic cleavage of urethane and urea bonds under supercritical conditions has been described [48]. It is well known that a number of low-boiling materials give enhance solubility and reactivity under supercritical conditions. Ammonia has a critical point at 132.45°C and 112.8 bar (11.28 MPa) with a density of 0.235 g/cm. Being able to act as hydrogen-bond donor and acceptor, it provides good solubility for polyurethanes and... 

Figure 4.4 compares the two ratio measures, Z and (p, for deviations from ideal-gas behavior for pure ammonia along the subcritical isotherm at 100°C. The figure shows that Z(P) is discontinuous across the vapor-liquid phase transition, while liquid phases have different molar volumes. In contrast, cp(P) appears continuous and smooth, though in fact it is only piecewise continuous. That is, the (p(P) curves for vapor and liquid intersect at the saturation point, but they intersect with different slopes. Near the triple point that difference in slopes is marked, but near the critical point the difference is small the... 

Figure 9.24 Vapor-liquid critical lines computed from the Redlich-Kwong equation of state for binary mixtures of a methane-like component and an ammonia-Uke component Filled circles are pure-component critical points. Mixtures having compositions (0.43

Mad] Thermal analysis, light microscopy, chemical analysis of layers. Armko Fe and electrolitic Cu. The alloys were melted in a vertical molybdenum-wire furnace in an atmosphere of cracked ammonia. Alloys with 50 mass% Cu and Fe and fiom 0.05 to 2.0 mass% C. Stable C-Cu-Fe diagram, miscibiUty gap, the compositions of two Uquid layers at 1150°C (invariant monotectic equiUbrium), temperature and constitution of critical point on the miscibiUty curve. 

Fig. 3 Saturated densities of ammonia on the basis of different force fields by Impey and Klein (open diamonds) [108], Kristof et al. (open squares) [246], Eckl et al. (open circles) [97], as well as Zhang and Siepmann (open inverted triangles) [247]. The simulation results are compared with a reference equation of state (solid line) [249]. The calculated critical points (full symbols) are also shown...

---