(262o) Designing an Extraterrestrial Submarine for Titan. III. Estimating Titan Sea Mixture Properties

An extraterrestrial submarine is currently being designed to explore the depths of the hydrocarbon rich seas on the moon Titan under the NASA Innovative Advanced Concepts Phase 2 study. The submarine must be designed to cycle and operate for one year in the cold, cryogenic hydrocarbon seas of Titan. Critical to the design and operation of the submarine is knowledge of the amount of dissolved nitrogen gas within the cryogenic liquid ethane and methane seas. Solubility is defined as a property of a solute to dissolve in a particular solvent. Based on available binary and ternary solubility data available, gaseous nitrogen (GN₂) solubility in the Titan seas is relatively high. Dissolved GN₂ that comes out of solution will hinder operation of the submarine during quiescent, horizontal cruising, and vertical ascent and descent stages of the mission. Therefore, robust analytical solubility models for predicting the amount of dissolved nitrogen as a function of temperature, pressure (depth), and mole fraction of ethane and methane (location within the seas) are required to mature the design of the submarine. Aside from needing precise knowledge of the amount of dissolved nitrogen, accurate estimations of Titan sea mixture properties are required. Mixture properties can be readily computed with fluid property software such as REFPROP, but the software requires as initial conditions the mole fractions of all mixture components, which is determined through solubility modeling.

This presentation presents an analytical solubility model fit to over 2,000 methane/ethane/nitrogen vapor/liquid equilibrium (VLE) data points. Data is first fit to the following proposed functional form:

x = p*exp((a₀ + a₁T + a₂T²) + (b₀ + b₁T + b₂T²)p*), p* = (P â?? P₀)/(1 MPa).

where x is the mole fraction solubility of nitrogen in either methane or ethane in units of moles/moles, is the vapor pressure of the pure liquid solvent at the given temperature , is the pressure, and and are the coefficients to be fitted. For b_i = 0, the solubility equation reduces to Henryâ??s Law. Next, parity plots are shown to illustrate the goodness of the fit of the solubility models. Then, solubility models are used to estimate dissolved GN₂ in two mixtures which bound the Titan sea liquid ethane/liquid methane concentrations. Since the solubility model only specifies a value at the sea/atmosphere interface, two models are used to estimate the amount of dissolved GN₂ as a function of depth. One method assumes the surface solubility value is constant throughout, while the second method assumes a new solubility value at each depth, corresponding to an increased pressure. Finally, the solubility values at each depth are fed into REFPROP, using a detailed equation of state, to estimate relevant thermodynamic properties, such as viscosity, speed of sound, and dielectric constant. While a wide variety of effects could occur in the Titan seas, complexities such as freezing are for the time being ignored, and properties are estimated by assuming constant enthalpy at each depth. Mixture properties can then be used as inputs into much more complex modeling later, such as the ballast tank system and effervescence models.

This work was funded through the NASA Innovative Advanced Concepts (NIAC) Phase 2 Titan Submarine Project.