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Natural gas supplies ~23% of U.S. energy needs. For off-shore wells, treatment near the wellhead is critical to prevent clathrates from forming and plugging the pipeline as gas flows to the mainland. The raw gas is normally treated by adding chemicals or reducing its dew point, but standard processing equipment is often large and requires manned platform operation. An alternative approach is to use supersonic natural gas separators that (1) cool the gas in a supersonic expansion to induce droplet formation and growth, (2) separate the droplets from the gas, and, (3) recompress the dried gas using a diffuser to minimize pressure losses. These separators are smaller than traditional process equipment, have no moving parts, and require no chemicals. Thus, they are suited for both off-shore and sub-sea applications.

 As these devices are adopted, critical questions remain regarding droplet growth in these complex vapor mixtures. The goal of this work is to improve our fundamental understanding of water - hydrocarbon droplet growth at cooling rates comparable of those found in the novel separators. In particular, hydrocarbons can inhibit water condensation because they wet the water surface while water cannot wet the hydrocarbon surface. Since removing water is critical to preventing clathrate formation, understanding co-condensation in this highly non-ideal system is vital to the success of supersonic separator technology and efficient natural gas recovery.

Our experimental apparatuses include a series of supersonic nozzles with cooling rates that match the supersonic separators. The characterization methods include pressure trace measurements, infra-red spectroscopy measurements of the gas phase, and in situ small angle x-ray scattering measurements to directly follow the growing droplets. To date we have made extensive measurements using the pure components and limited experiments with the binary system. Figure 1 illustrates our preliminary position resolved droplet size measurements for pure nonane, pure D₂O and binary mixtures of these materials. In the experiments presented here, the flowrate of D₂O was fixed at 2 g min^-1 and the flow rate of nonane was increased from 12 to 22 g min^-1. In the first case, Fig. 1a, pure nonane nucleation occurs further downstream than D₂O nucleation and the nonane droplets are significantly larger than the D₂O droplets. When the materials co-condense, the droplets observed 5 cm downstream of the throat are significantly larger than either the pure nonane or pure D₂O droplets. By the end of the nozzle, however, the pure nonane droplets are larger. Since binary nucleation is not thought to occur, this behavior suggests that the nonane condenses on the D₂O droplets and that the final number density of the aerosol formed during co-condensation is higher than that of the pure nonane droplets. For the higher nonane flow rate, Fig. 1b, pure nonane nucleation occurs upstream of pure D₂O nucleation. In this case, the pure nonane droplets are always larger than the droplets formed in the presence of D₂O. This observation is consistent with the formation of additional particles via D₂O nucleation followed by growth via nonane condensation. Additional experiments, to better resolve the growth curves and to directly follow the condensation of both species via infrared spectroscopy, are currently underway.

Figure 1: The growth of pure and mixed droplets when nonane condenses (a) downstream of D₂O and (b) upstream of D₂O.