Molecular Physics seminar

Nucleation and Droplet Growth during Co-condensation of Nonane and D2O in a Supersonic Nozzle

by Harshad Pathak (Stockholm University)

FA 32 ()

FA 32

Raw natural gas consists mainly of methane and has impurities like water vapor, higher alkanes, H2S etc. Dehydration of natural gas is important to prevent hydrate formation in pipelines carrying natural gas over long distances. A novel method of dehydration is by using a mechanical process of supersonic separation where raw natural gas is cooled down by adiabatic expansion resulting in condensation of water vapor and higher alkanes. The goal of this work is to understand the nucleation and droplet growth when droplet sizes are of the order of nm and timescales are of the order of microseconds when water and alkanes, two substances which are immiscible, condense together. We use supersonic nozzles in this work where cooling rates are of the order of 105-106 K/s. The supersonic velocities of the flow enable measurements on a resolution of the order of microseconds. Pressure trace measurement (PTM) is our basic experimental technique and it characterizes the flow by measuring the pressure profile inside the supersonic nozzle as the vapor-gas mixture expands and vapor condenses inside the nozzle. We use Fourier transform infrared spectroscopy (FTIR) to get the composition of the condensed liquid/vapor. The size and number of droplets is characterized using small angle x-ray scattering (SAXS) that are performed in Argonne National Laboratory. The nucleation rates for pure D2O and nonane agree with previous measurements done by other researchers. The subsequent process of growth of the droplets can be sensitive to droplet temperatures Td. For pure nonane droplets, we observe that Td is not important enough to alter the growth rates unlike pure D2O. The growth of D2O droplets is further affected by coagulation once condensation has slowed down. We also observe that when nonane and D2O both are condensing, the presence of nonane inhibits D2O condensation even when D2O dominates the nucleation process. Prediction of the droplet structure of composite nonane-D2O droplets is challenging because the SAXS spectra of these droplets does not fit to standard shapes like spheres or core-shell structures and the results from the fit violate mass balance. Although the ‘lens-on-sphere’ structures derived from MD simulations performed by our collaborators fits the scattering spectra better, the overall composition from this structure predicts that the amount of D2O condensed is 30-40% less than that measured from FTIR.