Supercritical ammonia physical properties

Liquid ammonia has been suggested as a solvent for the C4 separation(l). A drawback to its use in the liquid state, however, is the need for costly refrigeration. Its use as a supercritical solvent would also be acceptable were it not for its high critical temperature (405.45 K). High temperature favors the polymerization of the butadiene hence, its limitation in this role. In this study, a method was developed that seeks to circumvent this problem and yet achieve the desired separation of the C4 s. Prausnitz(2) discusses the use of a mixture of supercritical solvents whose properties provide the optimal physical conditions for efficient extraction. It is equally possible to prepare mixtures of solvents that not only modify those critical properties of the individual solvent component, but also introduce the chemical features needed to maximize the separation of the feed mixture. 

We have applied some of these principles to the extraction of 1-butene from a binary mixture of 1,3-butadiene/1-butene. Various mixtures of sc solvents (e.g., ethane, carbon dioxide, ethylene) are used in combination with a strongly polar solvent gas like ammonia. The physical properties of these components are shown in Table I. The experimental results were then compared with VLE predictions using a newly developed equation of state (18). The key feature of this equation is a new set of mixing rules based on statistical mechanical arguments. We have been able to demonstrate its agreement with a number of binary and ternary systems described in the literature, containing various hydrocarbon compounds, a number of selected polar compounds and a supercritical component. 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

A gas supply and pump or flow regulator usually make up the source when a GC-like set-up is being used. The most common mobile phase for SFC is carbon dioxide this is based on its low cost, low interference with chromatographic detectors and good physical properties. Other examples include nitrous oxide and ammonia. Supercritical fluids can... 

PbS is a semiconductor that exhibits extreme quantum confinement effects, with the absorption spectral properties being highly sensitive to the physical parameters of the nanoparticles. Thus, the effects of the experimental parameters in RESOLV on the PbS nanoparticles produced can be evaluated in a systematic fashion. For example, PbS nanoparticles were prepared by the rapid expansion of a supercritical ammonia/Pb(N03)2 solution at various temperatures. In the experiments all parameters [Pb(N03)2 concentration, expansion nozzle size, and concentration of Na2S and PVP polymer in the room-temperature ethanol solution] were held constant, except the temperature of the ammonia/Pb(N03)2 solution for rapid expansion. The absorption spectra of the two PbS-nanoparticle samples obtained by rapid expansions at 160°C and 130 C were quite similar. The similarity in absorption properties probably reflected the fact that the PbS nanoparticles obtained at the two different preexpansion temperatures had similar particle sizes, which was supported by the similar x-ray powder diffraction patterns of the nanoparticle samples. 

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