Specific heat of ammonia

Table 3. Specific heat of ammonia at constant pressure in kj kg 1 K 1... 

Concerning enthalpy and specific heat of ammonia, the extensive work by Haar, Gallagher [17] shows values from — 50 to 4- 350 °C and from 0 to 1000 bar. Ideal gas properties of ammonia are given by Haar [16]. 

Assume that all species are highly dilute in ammonia. For the purpose of this problem, consider the following transport properties as constants n = 2.7 x 10-4 g/cm-s, k = 1.1 x 104 erg/cni S-K, and Dtmg-nh3 = 3 cm2/s. The specific heat for ammonia may be taken as cp = 2600 J/kg-K. Assume that the density may be determined from a perfect-gas equation of state for ammonia alone. 

Reference TDI contains an enthalpy table for ammonia at different pressures. Reference TD2 contains a series of tables in an appendix from which the specific heats of the reaction-gas mixture were calculated. Humidity charts were also useful. Reference TD3 is valuable for its steam tables, while Ref. TD4 contains both thermodynamic and chemical equilibria data for nitric acid. The final reference, Robertson and Crowe (Ref. TD5), contains formulae and tables for the sizing and choice of an air-feed compressor. 

The specific heat of water is abnormally high, and this fact has an enormous influence upon climate and geological phenomena. Liquid ammonia is the only liquid possessing a higher specific heat. 

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... 

As shown by studies on various zeolites (ZSM-5, mordenite, H-Y) [44,55], it is difficult to compare adsorptions of ammonia and pyridine, because of their difference in basicity and the specific interactions of each of these molecules with the host zeolite. The adsorption heats should be correlated, not to the basicity of the probe molecules in liquid phase or in aqueous solutions (where NH3 is more basic dian pyridine by about 20 kJ mol", because the pKa values are 9.3 and 5.2, respectively), but to their basicity in the gas phase, which can be expressed in terms of the proton affinities (PA). In the gas phase, isopropylamine is a stronger base than pyridine and tiian ammonia the PA value of isopropylamine (pKa = 10.7) is about 924 U mol, and tiiat of pyridine is about 922.2 kJ mol", while that of ammonia is only about 857.7 kJ mol". This feature is illustrated in Figure 3, which represents the differential heats of ammonia, pyridine and isopropylamine vs uptake on a H-mordenite zeolite [54]. Other parameters that have to be taken into account are size (the kinetic diameters of pyridine and ammonia are 0.533 and 0.375 nm, respectively) and the secondary interactions between these adsorbates and the zeolite structure (hydrogen bonds may lead to the formation of a monodentate bond with pyridine and a bidentate bond with ammonia), as shown by Parillo et al. [55-57], These differences are reflected in the heats of adsorption, which are usually 20-30 kJ mof higher in the case of pyridine, than in the case of ammonia. A similar difference is observed for the chemisorption limit [44,54]. 

---