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46737-AC9
Multicomponent Droplet Growth in Supersonic Natural Gas Separators

Barbara E. Wyslouzil, Ohio State University

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 summarizes the pressures and temperatures that correspond to the maximum nucleation rates for a series of n-alkanes condensing in the nozzle. Figure 2 illustrates some of the position resolved growth measurements for pure nonane. In these experiments, the partial pressure of the nonane at the inlet of the nozzle is fixed while the total pressure of the Ar is increased by a factor of two. The average particle size is larger in the low pressure case because the heat released by the growing droplets quenches nucleation faster than when there is more carrier gas available, fewer particles are formed, and these can then grow to a larger size.

Figure 1. The conditions corresponding to the maximum nucleation rates for the condensation of a series on n-alkanes follow systematic trends: for a given carrier gas, N₂ or Ar, the lines are parallel; at fixed temperature, the pressure increases as the number of carbons in the alkane decreases; for a fixed hydrocarbon partial pressure, expansions conducted in Ar condense at a lower temperature. The latter observation is consistent with the fact that the cooling rate of the expansion  is higher when Ar is the carrier gas and, thus, we can probe the metastable vapor region more deeply.

Figure 2. The average size of nonane droplets increases rapidly as a function of position in the nozzle. We are able to observe scattering from smaller droplets for the high pressure Ar experiments because the aerosol number density is higher. The aerosol size distribution parameters are derived from fits of the small angle x-ray scattering spectra to a Schultz distribution of polydisperse spheres.

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