(225b) Vapor-Liquid Equilibrium Data for the Systems H2S-MDEA-H2O and CH4-H2S-MDEA-H2O at High Solvent Concentrations and High Pressures

Vapor-Liquid
Equilibrium Data for the Systems H₂S-MDEA-H₂O and CH₄-H₂S-MDEA-H₂O
at High Solvent Concentrations and High Pressures

Eirini Skylogianni¹,
Diego D. D. Pinto¹, Hanna K. Knuutila¹, Christophe
Coquelet²

¹ Norwegian
University of Science and Technology (NTNU), Trondheim, Norway

² Mines
ParisTech, Fontainebleau, France

INTRODUCTION

Trouble-free
operations and stringent environmental requirements in the oil and gas industry
demand the removal of, among other, acidic constituents from natural gas. The
presence of carbon dioxide (CO₂) and hydrogen sulfide (H₂S)
increases corrosion, creates safety hazards for operations and results in an
export gas of lower value. The most commonly used technology for acid gas
removal is chemical absorption into aqueous amine-based solvents. When the
selective removal of H₂S over CO₂ is desired, tertiary
amines are preferred since hydrogen sulfide reacts instantaneously through a
proton transfer while carbon dioxide cannot react directly with the amine (Versteeg et al., 1996). In our work,
aqueous solutions of N-methyldiethanolamine (MDEA) are studied. This amine
demonstrates high selectivity for H₂S over CO₂,
degradation resistance, lower losses by vaporization because of its low vapor
pressure and lower energy requirements during regeneration thanks to its low
enthalpy of reaction (Kohl and Nielsen, 1997).

OBJECTIVE

Vapor-liquid equilibrium (VLE) data
over a wide range of pressures, temperatures and concentrations of the various
solvents used for acid gas removal are essential for the development of the
thermodynamic models employed during the design and operation of the absorption
and regeneration units. Plenty of equilibrium data are already available in the
literature for the system CO₂-MDEA-H₂O, at various MDEA
concentrations, pressure and temperature conditions since the use of
amine-based solvents is also the dominant technology for CO₂ capture
from the flue gases emitted by fossil fuel power plants. Moreover, the effect
of the co-existence of carbon dioxide and hydrogen sulfide to the acid gas
solubility into an MDEA solution has also been studied. However, few
experimental measurements of the solubility of H₂S into aqueous
amine solutions have been performed. In addition to this, to the authors’ best
knowledge, there are not any available data for high MDEA concentrations (MDEA
> 50 wt. %). The objective of this work is, therefore, to provide new experimental
data for the system H₂S-MDEA-H₂O at pressures,
temperatures and concentrations that have not been studied so far in the
literature with focus on high pressures, as high-pressure processes are
relevant to the offshore/subsea operations and long-distance transportation of
natural gas. When needed, methane will be used for pressurization, as it is the
main component present in natural gas. Therefore experimental data for the
system H₂S-CH₄-MDEA-H₂O will be also provided.
Such data can be used to validate and/or extend existing models in terms of
concentration and/or pressure and predict the thermodynamic behavior of the
system accurately.

APPARATUS

A high-pressure equilibrium cell described
by Dicko et al. (2010) is used for the experimental
measurements of vapor-liquid equilibrium (Figure 1). A static-analytic method
is employed utilizing two ROLSI® capillary samplers for fluid sampling (ARMINES, 2003), one for the vapor phase and
one for the liquid phase. The VLE apparatus is designed for measurements with
acid gases and organic sulfur compounds with different solvents and it allows
for measurements at a pressure range of 0 – 10000 kPa and temperature range of 243
– 473 K. The analysis of the vapor and liquid phase samples is performed by
means of gas chromatography. The apparatus is connected to a gas chromatograph
which allows the analysis of the two phases samples and so the estimation of
the respective molar compositions.

Figure 1: Flow diagram of the
equipment. d.a.u.: data acquisition unit; DDD: displacement digital display;
DM: degassed mixture; DT: displacement transducer; EC: equilibrium cell; GC:
gas chromatograph; LB: liquid bath; LS: liquid sampler; LV_i: loading
valve; MR: magnetic rod; P: propeller; PP: platinum probe; PN: pressurized
nitrogen; PT: pressure transducer; PTh: pressure transducer for high pressure
values; PTl: pressure transducer for low pressure values; SD: stirring device;
SM: sample monitoring; ST: sapphire tube; TR: thermal regulator; Th:
thermocouple; V_i: valve; VP: vacuum pump; VS: vapor sampler; VVCM:
variable volume cell for mixture.  

RESULTS

In
this paper, we are going to present experimental vapor-liquid equilibrium data
for the system H₂S-MDEA-H₂O and CH₄-H₂S-MDEA-H₂O
at different loadings of the solution and at conditions relevant to the
operating conditions of the absorber and the stripper (up to 100 bar and up to
120^oC). A 50 wt.% MDEA solvent will be used for validation by
comparing with available data from the literature, as well as two highly
concentrated solvents, i.e. 70 wt. % and 90 wt. % MDEA, will be studied. A
comparison of the new data and the predicted values from a thermodynamic model
will be also presented.

REFERENCES

ARMINES, 2003. Procédé et Dispositif Pour Prélever Des
Microéchantillons D’un Fluide Sous Pression Contenu Dans Un Container. 2 853 414.

Dicko,
M., Coquelet, C., Jarne, C., Northrop, S., Richon, D., 2010. Acid gases partial
pressures above a 50 wt% aqueous methyldiethanolamine solution: Experimental
work and modeling. Fluid Phase Equilibria 289, 99–109.
doi:10.1016/j.fluid.2009.11.01

Kohl,
A.L., Nielsen, R.B., 1997. Chapter 2 - Alkanolamines for Hydrogen Sulfide and
Carbon Dioxide Removal, in: Gas Purification. Gulf Professional Publishing,
Houston, pp. 40–186.

Versteeg,
G.F., Van Dijck, L.A.J., Van Swaaij, W.P.M., 1996. On the kinetics between CO2
and alkanolamines both in aqueous and non-aqueous solutions. An
overview. Chem. Eng. Commun. 144, 133–158.