(739f) Densities and Viscosities of H2S at Elevated Pressures and Temperatures Using Molecular Dynamics Simulations

Gas
reservoirs often contain undesirable components which need to be properly
treated. Acid gas is one such component that contains mainly H₂S,
some CO₂ and further less than 2 mol% of other species, mainly
hydrocarbon ¾ and for that reason referred
to as hydrocarbon content (HC) of the stream. Small amounts of H₂S
can be flared, however for H₂S rich gas plants it is reinjected into
the reservoir and the reservoir is sealed. Typically, the reinjections are
carried out at elevated pressures. Physical properties, such as densities and
viscosities of acid gas (to be) produced in H₂S rich gas plants and
to be injected safely at high pressure is important to the whole production
cycle.

Predicting
the physical properties is therefore important for the process of reinjection.
Due to the lack of high pressure and temperature experimental data for H₂S,
currently used semi-empirical methods such as TRAPP are limited for viscosity
prediction at elevated pressures and can result in large non-systematic
relative errors. As CO₂ and CH₄ has been extensively
studied and can be reliably predicted with such semi-empirical methods, we
concentrate this study on H2S where reliable experimental data is limited and is
further complicated due to its polar nature.

we use
Molecular Dynamics (MD) as an alternative route to predict the viscosity of H₂S
at elevated temperatures and pressures. Nowadays MD has become an important
method to study transport properties of complex fluids and gases as it relies
only on the knowledge of intermolecular potential. In addition, it also provides
detailed and valuable characteristic pictures of a molecular structure at the
atomic level. We have previously successfully used the method to reliably
determine the viscosity of fuels and lubricants (n-alkanes and branched alkanes
up to C₃₀) at elevated temperatures and pressures [1]. For the study
of highly polar H₂S molecule, we readjusted the Lennard-Jones
parameters of the forcefield developed by Nath et. At [2] using quantum
chemical simulations. A simplified force-field was selected to ensure proper
mixing of force-fields for any future simulations for acid-gas compositions
including CO₂ and CH₄ for which reliable force-fields are
reported in the literature. The modified force-field was used for MD
simulations for the calculation of densities and viscosities of H₂S at
desired temperature and pressure.

We
report that the densities of the H₂S at temperatures ranging from
388-413K and pressures ranging from 100-500 bar are systematically predicted
within 10% relative errors to experiment (see Figure 1 and Table 1). All
predicted densities are lower than the experimentally measured densities.
Further we find that viscosities are also systematically underpredicted by
about 35% relative error. For very low viscosities, the errors are higher (over
50% relative errors). This could partly be attributed to the accuracy of
experimental data, variation of source and measurement techniques. The MD
method for viscosity and density calculation with a reliable force-field can be
used for systematically predict the desired properties.

Figure 1 Simulated H2S densities at various pressures and
comparison to expt

Table 1 Calculated and
Experimental Viscosities

[1]
Payal et. al. , Shear viscosity of linear alkanes through molecular
simulations: quantitative tests for n-decane and n-hexadecane, Mol. Sim., 38,
1234 (2012).

[2]
Nath, S. K. Molecular Simulation of Vapor−Liquid Phase Equilibria of
Hydrogen Sulfide and Its Mixture with Alkanes, J. Phys. Chem. B, 107, 9498
(2003)