Completed Research
Solubilities and Mass Transfer Coefficients of Gases in Silicone Oil
and Polyalphaolefin
Dinara Abdrakhimova, MS, 2015
(Thesis: University of Pittsburgh ETD)
The equilibrium solubilities (C*) and volumetric liquid-side mass transfer coefficients (kLa) for He and N2, as surrogates to H2 and CO, and their mixtures were measured in a Silicone Oil and a SpectraSyn Polyalphaolefin as startup liquids in Fischer-Tropsch Slurry Bubble Column Reactors (SBCRs). The data were obtained within wide ranges of pressures (4-30 bar), temperatures (298-398 K) and mixing speeds (1000-1400 rpm) in one-gallon agitated ZipperClave reactor operating in the gas-inducing mode. The effects of different operating variables as well as the gas and liquid natures on C* and kLa for He and N2 in the two liquids were discussed. The interpretation of the experimental results and calculations reveals the following:
At constant temperature, the C* values of He and N2 in the Silicone Oil and SpectraSyn polyalphaolefin increased non-linearly with the gas partial pressure and hence Henry’s Law was not applicable.
Under similar pressures and temperatures, the C* values of both gases in the Silicone Oil were greater than those in the SpectraSyn polyalphaolefin; and C* values of N2 were greater than those of He in both liquids. This behavior was attributed to the differences among the Hildebrand solubility parameters calculated for the gases and liquids used.
The kLa values of both gases in the two liquids increased with the mixing speed, gas partial pressure and system temperature which was related to the increase of both the gas-liquid interfacial area (a) and the liquid-side mass transfer coefficient (kL). Increasing mixing speed, gas partial pressure and system temperature increased the gas holdup and decreased the liquid-phase viscosity and surface tension, leading to the formation of small gas bubbles and hence an increase of (a). Also, increasing mixing speed and system temperature increased the turbulence and the gas diffusivity in the liquid, resulting in an increase of (kL).
Under similar pressure, temperature and mixing speed, the kLa values of He in the two liquids were greater than those of N2. This behavior was because at the same temperature the diffusivities of He in both liquids were greater than those of N2 and hence (kL)He and consequently (kLa)He values were greater than those of N2 in both liquids, knowing that kL ∝ (DAB)n, n =1 for the two-film theory and 0.5 for the penetration theory.
Unlike gas solubilities, kLa values of He were lower in Silicone Oil than those in the SpectraSyn Polyalphaolefin, whereas the opposite was true in the case of N2. This behavior was attributed to the lower He and N2 diffusivities in Silicone Oil than in the SpectraSyn Polyalphaolefin, resulting in small kL values for both gases. On the other hand, the higher viscosity of Silicone Oil than that of the SpectraSyn Polyalphaolefin was expected to enhance the formation of large bubbles with small gas-liquid interfacial area (a). This was true in the case of the light He where smaller kL and, a values in the more viscous liquid (Silicone Oil) resulted in lower kLa values. In the case of N2, however, it seems that the induced heavy N2 bubbles were broken by the impeller and scattered throughout the reactor, leading to the formation of small bubbles with large gas-liquid interfacial area (a). Thus, the small kL values of N2 in the more viscous liquid (Silicone Oil) were overcame by its larger a values, leading to the higher kLa than those in the less viscous (SpectraSyn polyalphaolefin).
Five liquids, namely Silicone oil, SpectraSyn polyalphaolefin, reactor wax, n-tetradecane and paraffins mixture, were explored as potential startup liquids for the F-T synthesis in SBCRs at 410 K and 15 bars at two α values, 0.85 and 0.92. This investigation indicated that the SpectraSyn polyalphaolefin cloud remain in the reactor for over 200 hr, whereas the paraffins mixture would remain only for about 100 hr. This behavior was related to the startup liquid molecular weight, which was the main factor controlling its residence time in the SBCRs.