Hydrogen solubility limit

When a fluid is compressed and heated above the critical conditions (or to supercritical conditions, sc), the differences between gas and liquid disappear. For carbon dioxide, this occurs for temperatures above 31 °C and pressures above 7.3 MPa. For reactions (such as alkylations, aminations, hydroformylations, hydrogenations and Fischer Tropsch synthesis) occurring in supercritical fluids, the reaction rate is often increased dramatically because of improved desorption of heavy molecules minimizing the oxygen and hydrogen solubility limitations, improved heat transfer, and improved selectivity by a catalyst by minimizing pore diffusion limitations. 

In the case of a-Si H, it turns out that Qab is positive [439]. This can be inferred from the bond energies of Si—Si (2.35 eV). Si—H (3.3 eV), and H—H (4.5 eV). A Si—H network is thus unstable compared to pure Si and H2. A mixture of Si—Si and Si—H bonds will be driven towards their two stable mixture phases, which leads to the solubility limit of H in Si. Acco et al. [69,70] have determined this limit to be about 4%. Excess hydrogen in -Si H forms SiH2 or SiH around microvoids. 

Successive burial of hydrogen-rich surface layers leads to the formation of the a-Si H material. The large amount of hydrogen at the surface is to saturate the surface dangling bonds. The much lower hydrogen content in the bulk is due to the solubility limit of hydrogen in silicon. 

Fig. 25. Limits of the miscibility gap in (O) homogeneous, ( ) two-phase Pd-Rh alloys calculated from lattice constants 151), solid line hydrogen solubilities at 760 Torr 152), dotted line 60).

Chloroform in aqueous solutions at concentrations ranging from 1 to 10% of the solubility limit were subjected to y rays. At a given radiation dose, as the concentration of the solution decreased, the rate of decomposition increased. As the radiation dose and solute concentration were increased, the concentrations of the following degradation products also increased methane, ethane, carbon dioxide, hydrogen, and chloride ions. Conversely, the concentration of oxygen decreased with increased radiation dose and solute concentration (Wu et al, 2002). 

Alcohols from esters. The major problem is reaction selectivity. Paraffin by-product in alcohol results if the catalyst activity is too high. Yet the reduction of esters to alcohols is a difficult reaction. Copper chromite catalyst, 3000-5000 psig hydrogen, and a temperature of 270-300°C are required for the reduction. An alternate catalyst is CuO/ZnO, which is used for methyl ester reduction only. Hydrogen solubility in alcohol is limiting. 

Hydrogen concentration in carbon steei is solubility limited... 

Results obtained with the theoretical mod-el described above appear to be quite reasonable. An important assumption contained in the model is Assumption 4 hydrogen concentration in carbon steel is solubility limited. A necessary result of this assumption is a discontinuity in concentration gradient at the SS/CS interlace (illustrated in the figure at the beginning of this Appendix). 

It is likely that at least some of the hydrogen that is transported to the catalyst is dissolved in the substrate rather than in the ionic liquid. Therefore it is difficult to decide whether mass-transfer of the gas into the ionic liquid is the limiting factor. From the data available it appears, however, that substrate solubility in the ionic liquid is of greater importance with respect to the reaction rate than hydrogen solubility. 

Experience in air separation plant operations and other cryogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubility limit. For this reason, the carbon dioxide content of the feed gas subject to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

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