Analytical solutions for the isobaric evaporation of pure cryogens in storage tanks

New analytical solutions have been derived for the isobaric evaporation of a pure liquid cryogen. In particular, expressions have been provided for the liquid volume, evaporation rate, Boil-off-Gas (BOG) rate, vapour temperature and vapour to liquid heat transfer rate as a function of time. Both equilibrium and non-equilibrium scenarios have been considered. In the former, the vapour and liquid cryogen are assumed to be in thermal equilibrium, while in the latter the vapour is treated as superheated with respect to the liquid and acts as an additional heat source. The derived solutions for two scenarios were validated against the numerical results for the evaporation of liquid methane and of liquid nitrogen in small, medium sized and large storage tanks that are used in industry. For the equilibrium model, the analytical solutions are exact. For the non-equilibrium model, the analytical solutions are valid for the whole duration of evaporation, except for a short transient period at the beginning of the evaporation. For physical quantities of industrial interest, they provide accurate estimates of liquid volume, BOG rate and BOG temperature, with the maximum deviations not exceeding 1%, 2% and 4.5%, respectively. The vapour to liquid heat transfer rate is also well predicted to within a maximum deviation of 5%

Predicting the viscosity of liquid mixtures consisting of n-alkane, alkylbenzene and cycloalkane species based on molecular description

1-component Extended Hard-Sphere (1-cEHS) model has been developed recently to predict the viscosity of liquid, n-alkane mixtures. It represents a mixture by a single pseudo-component characterized by an appropriate molecular weight and calculates the viscosity by means of the modified, extended hard-sphere model (EHS) that makes use of a universal function relating reduced viscosity to reduced volume. In this work we have extended the model to also predict the viscosity of mixtures containing alkylbenzene and cycloalkane species. Furthermore, we have developed a new 3-component Extended Hard-Sphere (3-cEHS) model which requires only a knowledge of the overall composition of n-alkane, alkylbenzene and cycloalkane species. Extensive comparison with the available experimental data indicates that both models (1-cEHS and 3-cEHS) predict the viscosity of binary and multicomponent mixtures containing n-alkane, alkylbenzene and cycloalkane species with uncertainty of 5–10%. The proposed models are a precursor of a new family of models that do not require a knowledge of the detailed composition of the mixture, but still take advantage of the underlying molecular description

A realistic vapour phase heat transfer model for the weathering of LNG stored in large tanks

A new non-equilibrium model relevant to LNG weathering in large storage tanks under constant pressure has been developed. It treats the heat influx from the surroundings into the vapour and liquid phases separately and allows for heat transfer between the two phases. The main heat transfer mechanisms in the vapour phase are assumed to be advection, due to upward flow of evaporated LNG, and conduction. It has been observed that the vapour temperature increases monotonically as a function of the height, in agreement with recent experimental results. In all the simulations performed the vapour to liquid heat transfer was small, also in line with recent experimental findings, and is estimated to contribute less than 0.3% to boil-off gas rates. The results of this work indicate that the heat transfer by the advective upward flow dominates the energy transfer within the vapour, while the natural convection, in the body of the vapour, can be neglected. The initial liquid filling has a pronounced effect on all the relevant variables, leading to a decrease in vapour temperature and boil-off gas temperature and an increase in boil-off rates. A rule of thumb for estimating the boil-off gas temperature in industrial storage tanks is provided

Cross second virial coefficients and dilute gas transport properties of the (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) systems from accurate intermolecular potential energy surfaces

The cross second virial coefficient and the dilute gas shear viscosity, thermal conductivity, and binary diffusion coefficient have been calculated for (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) mixtures in the temperature range from (150 to 1200) K. The cross second virial coefficient was obtained using the Mayer-sampling Monte Carlo approach, while the transport properties were evaluated by means of the classical trajectory method. State-of-the-art intermolecular potential energy surfaces for the like and unlike species interactions were employed in the calculations. All potential energy surfaces are based on highly accurate quantum-chemical ab initio calculations, with the potentials for the unlike interactions reported in this work and those for the like interactions taken from our previous studies of the pure gases. The computed transport property values are in good agreement with the few available experimental data, which are limited to (CH4 + CO2) mixtures close to room temperature. The lack of reliable data makes the values of the thermophysical properties calculated in this work currently the most accurate estimates for low-density (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) mixtures. Tables of recommended values for all investigated thermophysical properties as a function of temperature and composition are provided

Extended hard-sphere model for the viscosity of dense fluids

An extended hard-sphere model is reported that may be applied to correlate and predict the viscosity of gases, liquids and supercritical fluids. The method is based on the hard-sphere model of Dymond and Assael and uses their roughness factors and molar core volumes to relate reduced viscosity to a universal function of reduced volume. The extended model behaves correctly in the limit of low densities and offers improved accuracy at high densities. The new universal reference function was determined from a large database of experimental viscosities for alkanes extending up to reduced densities of 0.84. It has been tested by correlating the viscosity of two high-viscosity liquids not used in the development of the universal function and has shown to perform satisfactorily up to reduced densities of approximately 0.9

Dilute gas viscosity of n-alkanes represented by rigid Lennard-Jones chains

The shear viscosity in the dilute gas limit has been calculated by means of the classical trajectory method for a gas consisting of chain-like molecules. The molecules were modelled as rigid chains made up of spherical segments that interact through a combination of site-site Lennard-Jones 12-6 potentials. Results are reported for chains consisting of 2, 3, 4, 6, 8, 12 and 16 segments in the reduced temperature range of 0.3 – 50 for site-site separations of 0.25 , 0.333 , 0.40 , 0.60 and 0.80 , where is the Lennard-Jones length scaling parameter. The results were used to determine the shear viscosity of n-alkanes in the zerodensity limit by representing an n-alkane molecule as a rigid linear chain consisting of c − 1 spherical segments, where c is the number of carbon atoms. We show that for a given n-alkane molecule, the scaling parameters ε and σ are not unique and not transferable from one molecule to another. The commonly used site-site Lennard-Jones 12-6 potential in combination with a rigid-chain molecular representation can only accurately mimic the viscosity if the scaling parameters are fitted. If the scaling parameters are estimated from the scaling parameters of other n-alkanes, the predicted viscosity values have an unacceptably high uncertainty

Reference correlation of the viscosity of ortho-xylene from 273 K to 673 K and up to 110 MPa

A new correlation for the viscosity of ortho-xylene (o-xylene) is presented. The correlation is based upon a body of experimental data that has been critically assessed for internal consistency and for agreement with theory. It is applicable in the temperature range from 273 to 673 K at pressures up to 110 MPa. The overall uncertainty of the proposed correlation, estimated as the combined expanded uncertainty with a coverage factor of 2, varies from 1% for the viscosity at atmospheric pressure to 5% for the highest temperatures and pressures of interest. Tables of the viscosity generated by the relevant equations, at selected temperatures and pressures and along the saturation line, are provided

In-situ acoustic-based analysis system for physical and chemical properties of the lower Martian atmosphere

The Environmental Acoustic Reconnaissance and Sounding experiment (EARS), is composed of two parts: the Environmental Acoustic Reconnaissance (EAR) instrument and the Environmental Acoustic Sounding Experiment (EASE). They are distinct, but have the common objective of characterizing the acoustic environment of Mars. The principal goal of the EAR instrument is "listening" to Mars. This could be a most significant experiment if one thinks of everyday life experience where hearing is possibly the most important sense after sight. Not only will this contribute to opening up this important area of planetary exploration, which has been essentially ignored up until now, but will also bring the general public closer in contact with our most proximate planet. EASE is directed at characterizing acoustic propagation parameters, specifically sound velocity and absorption, and will provide information regarding important physical and chemical parameters of the lower Martian atmosphere; in particular, water vapor content, specific heat capacity, heat conductivity and shear viscosity, which will provide specific constraints in determining its composition. This would enable one to gain a deeper understanding of Mars and its analogues on Earth. Furthermore, the knowledge of the physical and chemical parameters of the Martian atmosphere, which influence its circulation, will improve the comprehension of its climate now and in the past, so as to gain insight on the possibility of the past presence of life on Mars. These aspect are considered strategic in the contest of its exploration, as is clearly indicated in NASA's four main objectives on "Long Term Mars Exploration Program" (http://marsweb.jpl.nasa.gov/mer/science).Comment: 16 pages including figure