Study On Gas Phase Mass Transfer Using Laser Absorption Spectroscopy

Mass transfer is the transport process of materials due to the chemical potential difference. Mass transfer is not only ubiquitous in nature, such as global water circulation, transpiration and climate changes, but also in engineering, such as petro-chemical, atomic energy and microelectronic and optoelectronic processes. The fundamental process has been investigated for a few decades. However, the mass transfer mechanism is not fully understood. Part of the reason is the lack of advanced experimental methods. In this thesis, mass transfer is deeply studied using tunable diode laser absorption spectroscopy (TDLAS), which is a high resolution measurement technique for gas media. The absorption of the spectrum can be figured out totally in the scanned laser signal due to the narrow line width of the diode laser. The concentration and temperature of the absorbing gas species are obtained by Beer-lambert law. Two typical mass transfer phenomena are investigated:the first is the transports of acidic species in natural gas without phase change; the second is the phase change of water in the process of evaporation and condensation.The specific research contents include:(1) The transports of acidic species (H2S and CO2) in natural gas were investigated. A sensor system for acidic gas in natural gas was developed and tested in stability for a long time in laboratory and in situ. The test shows the maximum error of H2S and CO2 sensor system is-1.3% and-0.5% of full measurement scales, respectively. The linearity is 0.159% and 0.12%, respectively. The test on transient response in situ shows the maximum error is less than ±1% caused by disturbance. Furthermore, when the gas well is closed, the concentration of H2S and CO2 in acidic gas pipeline rises up. On the other hand, when the gas well is opened, the concentration of H2S and CO2 falls down suddenly. The reason that the concentration of H2S and CO2 in acidic gas pipeline varies is the difference of molecular characterization based on the Maxwell-Stefan equation. The concentration of acid gas in the natural gas pipeline depends on time due to the difference of mass diffusion coefficients. The concentration of acid gas in gas well varies with depth due to the difference of molecular weight. The model predicted that the concentration of H2S in the natural gas pipeline increased from 0.9% to 24%.(2) A tunable diode laser emitting at around 1370 nm is used to in situ detect the water evaporation from a free water surface subject to crossing wind at various velocities. Results show that the observed profiles of water vapor partial pressure agree with the prediction of the similarity principle, with overall deviation less than 4%. Based on the investigation on evaporation from the macro free water surface, the evaporation from nanopores was also investigated under saturated humidity. The water partial pressure near the nanopores was also detected by the diode laser emitting at around 1370 nm. The results of comparative experiment show the water vapor pressure near nanopores is up to 1.29 times higher than that near macro free water surface. Then the nanopores evaporation model was improved by considering the diffusive constriction resistance. The prediction of the theory agrees well with the experimental results, with deviation less than 8.5%.(3) The vapor condensation with multicomponent non-condensable gases was investigated. A new non-isothermal model considering the accompanied thermal diffusion across the diffusion layer is proposed in this paper. Component variations in the diffusion layer in the presence of N2 and H2 are predicted based on the proposed model. Results show that N2, the heavier component, accumulates close to the condensing surface and H2, the lighter component, is relatively uniformly distributed. Therefore, the heavier component may have a bigger impact on vapor condensation. The total heat transfer coefficients of condensation with air are calculated using the proposed model and compared with experimental data, improved agreement is obtained over the traditional diffusion layer model. Based on multi-field synergy principle, the enhancement of mass transfer is caused by the synergetic effects between mass diffusion field and other fields including velocity field, temperature field, gravity field or other external fields. The Comsol simulation results show that there exists mass exchange obviously in the boundary layer that is able to enhance mass transfer after the droplet departure. The experimental results show that the boundary layer is disturbed, which agree well with the simulation results.In summary, gas species transfer was investigated on-line using tunable diode laser absorption spectroscopy technique. The detail of mass transfer was observed by laser absorption spectroscopy method, which provides an effective experimental method to study the fundamentals of gas species transfer.