(418e) Numerical Simulation on the Feasibility of Bulk CO¬2 Separation in Natural Gas via Supersonic Nozzle Expansion System

The feasibility of separating CO₂ from natural gas through a convergence and divergence nozzle in supersonic condition has been studied. Due to the decrement of pressure and temperature at the throat, separation of denser phase i.e. liquid CO₂ is made possible at supersonic, compressible flow condition. A full hydrodynamic CFD simulation in Fluent was performed on an axisymmetric nozzle to assess the pressure, temperature, velocity and the turbulence profile. To evaluate the separation of CO₂ from the natural gas, a selection of thermodynamic and physical properties has been requisite and assessed numerically as well as through experimental means. The combinations of these physical properties; nucleation, growth, coalescence and breakage of CO₂ liquid are combined in the Population Balance Model (PBM) to attain the amount of CO₂ liquid formed in the nozzle.

The non-equilibrium condensation is modelled by combinations of Classical Nucleation Theory and other empirical formulas coupled with SRK equation of state. A computational code written in C language is linked with Fluent as User-defined Function (UDF) to generate the particle size distribution of CO₂ droplets as well as the number density of the droplets.

Results generated by the full coupling exercise shows clear formation of CO₂ liquid droplets in the divergence section of the nozzle. The amount of droplets produced increases after the throat indicating that the balancing of each model had been achieved successfully in determining the separation efficiency of CO₂ in natural gas containing CH₄. The vapour CO₂ mass fraction is found to be decreasing as expansion grows whereas the distribution of droplet number density is mainly concentrated at the wall of the nozzle after the throat. The results indicate that separation of CO₂ through convergence-divergence nozzle is possible with further efficiency of separation can be done via optimum geometrical design of the nozzle.