1. Introduction - Firenze University Press

in which CO2 is separated from H2, but also in biogas upgrading processes, in which CO2 must be
separated from biomethane [1;9;11].
The basis of the separation is the selective partition of the target component between the hydrate
phase and the gaseous phase. It is expected that CO2 is preferentially encaged into the hydrate
crystal phase compared to the other components. For instance, the equilibrium pressure of N2
hydrate is three times greater than that of CO2. This difference allows to separate CO2 from treated
flue gas, that is a CO2-N2 mixture [12].
Flue gas from power plants usually contains from 15% to 20% mol. of CO2 and are released at
atmospheric pressure. The gas/hydrate equilibrium pressure for this kind of gas mixture is relatively
high. For example, the equilibrium pressures for a gas mixture containing CO2 at 17.61% mol. are
7.6 MPa and 11.0 MPa at 274 K and 277 K, respectively [13]. These pressures are not compatible
with the industrial reality, since the operative cost will be expensive if it is necessary to compress
the gas to the hydrate formation pressure.
In addition, evaluations of energy consumption for gas separation processes by the clathrate hydrate
formation indicate that hydrate separation process is competitive – compared to other conventional
separation processes - under lower pressure conditions, as well as in case of lower hydrate
formation heat [14].
Consequently, the main challenge is to obtain a decrease in the operating pressure. This task can be
achieved using specific compounds called promoters that allows to reach milder conditions for
hydrate formation. A suitable promoter is essential to help in reducing the hydrate formation
pressure and the energy consumption.
Conventionally, water-soluble additives are classified either as kinetic or as thermodynamic
additives. Thermodynamic additives consist of organic compounds and have the tendency to
displace the equilibrium conditions towards higher temperatures or lower pressures. Kinetic
additives consist typically of surfactant molecules and have the effect to accelerate hydrate
formation [10,12].
Several studies report a significant reduction of hydrate equilibrium pressures at a given
temperature by adding small amounts of tetrahydrofuran (THF) in the aqueous phase. Kang et al.
[13] and Linga et al. [15] found that the equilibrium pressure of hydrates in the presence of this
additive is considerably lower than the case without the additive.
Another promoter is Sodium dodecyl sulfate (SDS), which seems the best commercially available
surfactant to be used for enhancement of hydrate formation [16] and has already been investigated
in various works [17-20]. It was found that a small concentration of SDS added to the aqueous
phase drastically increases the kinetics of hydrate formation.
Recently, Liu et al. [21] and Torré et al. [22] showed that THF and SDS used in combination are
efficient additives for promoting CO2 hydrate formation.
According to previous works [23-26] a continuous production of hydrates is feasible, provided that
the technology assures an optimal contact between gas and liquid phases.
The choice of the correct gas-liquid mixing method, together with the proper promoter, is crucial
for producing hydrates in a continuous manner suitable for scale-up to industrial settings.
The apparatus described in the present work allows the use of aqueous solutions with additives for
rapid hydrate production.
In particular, the reactor was designed to maximize interfacial area between reactants. A first set of
hydrate formation experiments indicated that mass transfer barriers and thermal effects that
negatively affect conversion of reactants into hydrate are minimized, resulting in fast hydrate
production and good storage capacity [20].
In the present paper, an improved configuration of the apparatus and its application to CO2 hydrate
formation are presented and discussed. Experiments on formation of hydrates from pure CO2 are
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